Al Musanna College of

Technology

Department of Engineering

Mechanical and Industrial Section

MIME 4222 – ENGINEERING DESIGN

II

Academic Year – 2015 - 16, Semester I

CONTENTS

CHAPTER 01: DESIGN OF WIRE ROPES AND PULLEYS

1.1. INTRODUCTION

1.2. CONSTURCTION OF STEEL WIRE ROPE

1.3. TYPES OF STEEL WIRE ROPES

1.4. WIRE ROPE CORE CONSTRUCTION

1.5. MATERIALS OF STEEL WIRE ROPE

1.6. WIRE ROPE CODING SYSTEM

1.7. ADVANTAGES OF STEEL WIRE ROPES

1.8. VARIOUS SIZES AND APPLICATIONS

1.9. SELECTION OF STEEL WIRE ROPE BASED ON STATIC STRENGTH

1.10. DESIGN PROCEDURE OF STEEL WIRES

SOLVED PROBLEMS

1.11. SIZING OF DRUMS AND SHEAVES

1.12. WIRE ROPE DEFECTS AND FAILURES

SAMPLE QUESTIONS

SAMPLE PROBLEMS

CHAPTER 3: DESIGN OG CLUTCHES

2.1. INTRODUCTION

2.2. FRICTION CLUTCHES

2.3. SINGLE PLATE FRICTION CLUTCH

2.4. MULTI PLATE CLUTCH

2.5. CONICAL CLUTCH

2.6. CENTRIFUGAL CLUTCH

SAMPLE QUESTIONS

SAMPLE PROBLEMS

CHAPTER 3: DESIGN OG BRAKES

3.1. INTRODUCTION

3.2. BLOCK BRAKE OR SHOE BRAKE

3.2.1. Pivoted lever with rigidly mounted shoe

3.2.2. Effective coefficient of friction

3.3. SIMPLE BAND BRAKE

SAMPLE QUESTIONS

CHAPTER 04: ERGONOMICS

4.1. INTRODUCTION

4.2. INTERACTION BETWEEN MEN AND MACHINE

4.3. ANTHROPOMETRICS

4.4. PHYISOLOGY

4.5. PSYCHOLOGY

4.6. VISUAL DISPLAYS

4.7. CASE STUDIES

4.7.1. DESIGN OF CONTROL PANEL OF A CLOTH DRYER

SAMPLE QUESTIONS

SAMPLE PROBLEMS

CHAPTER 05: HUMAN VALUES IN DESIGN

5.1. INTRODUCTION

5.2. CONTRACTS

5.3. TYPES OF CONTRACTS

5.4. GENERAL FORM CONTRACTS

5.5. DISCHARGE AND BREACH OF CONTRACTS

5.6. LIABILITY

5.7. PRODUCT LIABILITY

5.8. DESIGN ASPECTS OF PRODUCT LIABILITY

5.9. PROTECTING INTELLECTUAL PRORERTY

5.10. THE LEGAL AND ETHICAL DOMAINS

5.11. CODE OF ETHICS

5.12. PROFESSIONAL ETHICS

SAMPLE QUESTIONS

SAMPLE PROBLEMS

CHAPTER 06: DC MOTORS AND SELECTION

5.1. INTRODUCTION

5.2. CONTRACTS

5.3. TYPES OF CONTRACTS

5.4. GENERAL FORM CONTRACTS

5.5. DISCHARGE AND BREACH OF CONTRACTS

5.6. LIABILITY

5.7. PRODUCT LIABILITY

5.8. DESIGN ASPECTS OF PRODUCT LIABILITY

5.9. PROTECTING INTELLECTUAL PRORERTY

5.10. THE LEGAL AND ETHICAL DOMAINS

5.11. CODE OF ETHICS

5.12. PROFESSIONAL ETHICS

SAMPLE QUESTIONS

SAMPLE PROBLEMS

CHAPTER 01

DESIGN OF WIRE ROPES AND PULLEYS

1.1. INTRODUCTION

Wire ropes are intricate vital machine elements that are used to transmit FORCES or MOTION or

are simply used to support WEIGHTS/ LOADS. Historically, usages are starting out as ropes made up of

nylon or fibre but later on develop into strands of wires. Wire ropes were eventually invented by a German

engineer for mining haulage purposes. Before steel chains were commonly used for hauling purposes but

had numerous mechanical failures due to its design but not necessarily on the material used. Wire ropes on

the other hand, due to their design/ physical make-up and construction correct the flaws encountered by steel

chains.

Their service category could be classified into two.

1. STATICS (Stationary applications) - as tower supports, guy wires, suspension bridge supports, and

electrical power transmission lines.

2. DYNAMICS (Moving applications) - as for pulling or lifting applications. These include elevators,

cable cars, cranes, hoists, dredges, and control cables. When power source and load are located at extreme

distances (i.e. up to 150 meters apart) from one another, or loads are very large, the use of wire rope is

suggested. The wire ropes run on grooved pulleys but they rest on the bottom of the grooves and are not

wedged between the sides of the grooves. The wire ropes are made from cold drawn wires in order to have

increase in strength and durability. It may be noted that the strength of the wire rope increases as its size

decreases.

Ropes are used in elevators, mine hoists, cranes; oil well drilling, aerial conveyors, tram ways,

haulage devices, lifts and suspension bridges etc. Steel wire ropes play an important role in mining. In

underground mining for winding and haulage wire ropes are widely used. In surface mining shovel,

draglines, cranes, aerial rope ways, cable belt conveyors etc. use wire ropes.  This module discusses wire

ropes their classification and constructions, design calculation as well as their maintenance aspects.

1.2. CONSTURCTION OF STEEL WIRE ROPE

A wire rope has three basic components: the wires, strands and core as shown in the figure. The individual

wires are first twisted into strands and then the strands are twisted around a hemp or steel center to form the

rope. Often the central element is an independent wire rope core. The central part of the wire rope is called

the core and may be fiber, wire, plastic, paper or asbestos, may be a steel strand (Independent Wire Rope

Core, IWRC), a molded plastic extrusion or a natural or manmade fiber core.

The twisting or laying (lay) of the wire rope describes the manner in which either wires in a strand or

the strands in the rope are laid in a helix.

A wire rope is made is made up of strands and a strand is made up of one or more layers of wires as

shown in figure. The number of strands in a rope denotes the number of groups of wires that are laid over

the central core. For example a 6 X 19 construction means that the rope has 6 strands and each strand is

composed of 19 wires.

1.3. TYPES OF STEEL WIRE ROPES

I. ACCORDING TO CONSTRUCTION METHOD

1. PREFORMED WIRE ROPE

Has its wires and strands “PRESHAPED” to the curvature they are made as a ROPE.

2. NON-PREFORMED

It is the opposite of the preformed wire rope. There is no reshaping of the wires and strands. All wires in a

strand are twisted after which, the individual strands along with the core are again twisted to form the rope.

ADVANTAGE OF PREFORMING

1. Eliminates locked-up stress and strain existing in non-preformed wire ropes.

2. Prevents the rope from flying apart when “CUT or BROKEN” and resists “KINKING”.

3. Eliminates the tendency of the rope rotating about its axis.

4. Are easily “SPLICED”.

5. Are used as “OPERATING ROPES” (failure is due to fatigue) with small safety factors.

DISADVANTAGE OF PREFORMING

1. Difficulty of tucking.

2. Has no advantage when used for “stationary operation” (failure is due to corrosion)

3. Costs more.

II. ACCORDING TO LAY

1. RIGHT HAND LAY -- strands are twisted in a clockwise (CW) direction.

2. LEFT HAND LAY -- strands are twisted in a counter clockwise (CCW) direction.

III. ACCORDING TO THE LAYING OF WIRES THAT MAKE-UP THE STRAND

1. ORDINARY LAY (REGULAR LAY): The laying of wires in each strand is in opposite direction of the

laying of strands that form the wire.

RIGHT REGULAR LAY WIRE ROPE LEFT REGULAR LAY WIRE ROPE

2. LANG’S LAY (ALBERT’S LAY): The lay of wires in each strand is in the same direction as the lay of

the strands that form the wire.

RIGHT LANG LAY WIRE ROPE LEFT LANG LAY WIRE ROPE

Such type is able to flex over sheaves easier but has this disadvantage of untwisting or loosening-up when

subjected to high torques. This would be dangerous to its core as the load would be diverted into the core

itself instead of the strands primarily.

3. ALTERNATE LAY (REVERSE LAY):

Strands alternate equally between LANG’S LAY and ORDINARY LAY. Say a six (6) strand wire rope has

three (3) Ordinary Lay and three (3) Lang’s lay.

RIGHT ALTERNATE LAY WIRE ROPE

IV. ACCORDING TO STRAND: The design arrangement of a strand is called “CONSTRUCTION”. The

wires in the strand maybe of the same size or a mixture of sizes. The most common strand constructions are:

1. Ordinary Strand has wires making-up the strand are all of the same sizes.

2. Seale uses a large diameter of wires on the outside of the strand to resist abrasion and uses smaller wires

inside to provide flexibility.

3. Warrington: Alternates large and small wires to combine great flexibility with resistance to abrasion.

Warrington: Alternates large and small wires to combine great flexibility with resistance to abrasion.

Filler is where small wires fill the valleys between the outer and inner rows of wires to provide good

abrasion and fatigue resistance.

1.4. WIRE ROPE CORE CONSTRUCTION

The core is where the strands of wire are laid or twisted about and either allows flexibility or additional

strength combined with the strands.

Fiber core - There may be a hard fiber such as manila, hemp, plastic, paper, or sisal. It offers the

advantage of “great flexibility “ while serving as a cushion to reduce the effects of sudden strain as

well as an “oil reservoir” to lubricate the wires and strand. The fiber core is very flexible and very suitable

for all conditions.

Fiber cores are generally made of cotton twine for cables less than ¼ inch and hard fiber ropes (manila or

sisal) for the larger sizes. Fiber cores extend the life by cushioning the strands and reducing internal

abrasion, good for light crushing loads. Hard cores are impregnated with lubricant to deter rust and lubricate.

Wire core

Wire cores offer less stretch, have better resistance to heavy crushing loads. It resists more heat than a fiber

core while adding about 15% of additional strength. It has this disadvantage of making the rope less flexible

than a fiber core.

Independent Wire Rope Core

A separate wire rope strand over which the main strands are laid. The core strengthens the wire providing

support against, while being much more resilient to heat.

1.5. MATERIALS OF STEEL WIRE ROPE

Two types of ropes are commonly used. They are fiber ropes and metallic ropes. Fiber ropes are made of

Manila, hemp, cotton, jute, nylon, coir etc. and are normally used for transmitting power. Metallic ropes are

made of steel, aluminum, alloys, copper, bronze or stainless steel. The various materials used for wire ropes

in order of increasing strength are wrought iron, cast steel, extra strong cast steel, plough steel and alloy

steel. For certain purposes, the wire ropes may also be made of copper, bronze, aluminum alloys and

stainless steels.

There are three (3) primary grades / materials used in the construction of the wires in the strands of the wire

rope.

Mild Plow Steel (MPS): It is tough and pliable withstanding repeated strain and stress (tensile strength

of 200 – 220 ksi. These characteristic makes it desirable for cable tool drilling and other purposes where

abrasion is encountered.

Plow Steel (PS): Is unusually tough and strong with a tensile strength of 200 – 240 ksi and is suitable for

hoisting, hauling, and logging.

Improved Plow Steel: One of the best grades available and is the most common force used in Naval

Construction. This material is stronger, tougher, and more resistant to wear than either MPS or PS. It has a

tensile strength of 240 – 260 ksi and is therefore used for heavy duty service such as cranes.

Other Materials: Wrought Iron, Cast Steel, Alloy Steel, Stainless Steel, Copper and Bronze.

1.6. WIRE ROPE CODING SYSTEM

A wire rope uses an accepted coding system, consisting of a number of abbreviations for its material

composition and wire strand sizes and arrangements.

Characteristics Description Designation

Length Linear length Meters(feet)

Diameter Nominal diameter or size Centimeters (inch)

Construction Number of strands by number of wires per strand

Single layer – Uniform wire diameter

Warrington – alternating wire sizes in a single layer

Seale – alternate layers of different wire sizes

Filler wire – fine wires between layers

19 x 7

None

W

S

FW or F

Lay Right regular lay – strands laid right and strand wires laid left

Left regular lay - strands laid left and strand wires laid right

RRL or sZ

LRL or zS

Right lang lay - strands laid and strand wires laid right

Left lang lay - strands laid left and strand wires laid left

Alternate lay - regular and lang lay strand alternate

RLL or zZ

LLL or sS

RAL/ LAL

Grade Traction steel

Plow steel

Improved plow steel

Extra-improved plow steel

Extra-extra-improved plow steel

TS

PS

IPS

EIPS or XIPS

EEIPS or XXIPS

Finish Bright- uncoated, bare wires

Galvanized- zinc or zinc alloy coated wires

B

G

Core Fiber core

Wire strand core

Independent wire rope core

FC

WSC

IWRC

Lubrication Special requriements

1.7. ADVANTAGES OF STEEL WIRE ROPES

The wire ropes have the following advantages as compared to fiber ropes.

1. These are lighter in weight and high strength to weight ratio

2. These offer silent operation even at high working speeds

3. These can withstand shock loads less danger for damage due to jerks

4. These are more reliable in operation

5. These are more durable

6. They do not fail suddenly

7. The efficiency is high, and

8. The cost is low.

1.8. VARIOUS SIZES AND APPLICATIONS

Wire ropes are used on machines as well as for haulage and winding in underground mines. Wire ropes are

identified by classifications based upon the number of strands and nominal number of wires in each strand.

A 6 x 19 classification for example, includes six strands with each strand consisting of 15-26 individual

wires. The six strands of a 6 x 37 class wire rope are constructed of 27-49 individual wires. Other popular

classifications include 19 x 7, 7 x 19 and 8 x 19.

STANDARD

DESIGNATION

APPLICATION

6 x 7It is a standard coarse laid rope used as haulage rope in mines, tramways, power

transmission

6 x 19 It is a standard hoisting rope used for hoisting purposed in mines, quarries, cranes,

dredges, elevators, tramways, well drilling. It has excellent strength, flexibility, and

resistance to abrasion and fatigue.

6 x 37It is an extra flexible rope used in steel mill ladles, cranes, high speed elevators. More

flexible than 6 x 19, good for applications where pulleys are limited in size.

8 x 19 It is also an extra flexible hoisting rope

6 x 42 It is the most flexible of all standard cables, used for moderate loads.

1 x 19 Primarily used for stationary (non-flexible) applications.

19 x 7It is designed to resist the natural tendency of a cable to rotate when freely suspended

under load

7 x 7

It is the standard flexible aircraft cable. It has high strength and rugged construction,

used for towing and power transmission.

1.9. SELECTION OF STEEL WIRE ROPE BASED ON STATIC STRENGTH

There are several factors to consider in choosing the proper wire rope type to be used in a particular type

of operation such as if it could be able to withstand the type of wear and stress in that particular type

of operation. However selecting a wire rope is a matter of compromise. That is you have to sacrifice one

quality over the other to meet the urgently needed characteristic. In principle, small wire sizes are better

suited to being bent sharply over small sheaves (pulleys) while large wire ropes are preferred when

the cable/ wire rope will be rubbed or dragged through abrasives.

1. TENSILE STRENGTH

It is necessary to withstand the possible maximum load that could be applied. It includes a reserve of

strength measured in a so-called factor of safety.

Direct tensile stress in the wires of the rope, σt =

T = resultant tensile force, includes load to be lifted, weight of the rope and inertial effects due to

accelerating the load.

A = approximate cross-sectional area of the rope, a function of rope diameter, d.

2. CRUSHING STRENGTH

It is the necessary strength to resists the compressive or squeezing forces that distort the cross-section of a

wire rope as it runs over sheaves, rollers, and hoists drums when under a heavy load. A regular lay rope

distorts less in these situations than lang lay.

Bending stress, σb = Er ×

Er = Modulus of elasticity of wire rope, not the wire

3. FATIGUE RESISTANCE

It is the ability to resist constant bending and flexing that runs continuously on sheaves and hoists

drums. It is an important factor when a rope is running at high speeds.

4. ABRASION RESISTANCE

It is the ability to resist gradual wearing away of the outer material as the ropes run across sheaves and hoists

drums. The rate of abrasion depends upon the load carried upon by the rope and the running speed.

Wire ropes made from harder steel such as IPS has considerable resistance to abrasion. Ropes having larger

wires forming their outer strands are more resistant to wear that those having smaller wires that wear away

more quickly.

5. CORROSION RESISTANCE

It is the ability to withstand dissolution of the wire metal that results from chemical attack by moisture in the

atmosphere or elsewhere in the working environment. Ropes in static applications are protected by

insulating a special dressing or by galvanizing the wire itself. In dynamic applications, the ropes lubrications

also serve as their protection against corrosion.

1.10. DESIGN PROCEDURE OF STEEL WIRES

Let

d = diameter of rope; D = Diameter of sheave; dw = diameter of wire

Er = Modulus of elasticity of wire rope (not the wire)

H = Depth of mine or height of the building

W = static load or pay load (also add if bucket load is given)

WD = Direct load (W + Wr)

Wb = Equivalent bending load in the wire

Wr = Total weight of rope (Wt of rope per length multiplied by given rope length )

Ws = starting load

A = area of cross section of rope

Fa = allowable pull in the rope

Fu = ultimate or breaking load of the rope

n = Recommended factor of safety

1. Selection of suitable wire rope

First select the suitable type of wire rope for the given application from the following table.

Rope type Applications

67 It is standard coarse laid rope used as haulage rope in mines, tram ways, power

transmission

619 It is general purpose rope used for mines, quarries, dredges, hoists, cranes, well

drilling, elevators and tramways

637 It is an extra flexible hoisting rope used in steel mill ladles, cranes, high speed

elevators

819 It is an extra flexible hoisting rope

2. Calculation of design load

Calculate the design load by assuming a larger factor of safety. The assumed factor of safety is 2 to 2.5 times

the factor safety given in the following table.

Design load = Recommended FOS × Total load to be lifted

= (2 to 2.5) × Actual FOS from table × Total load to be lifted

Type of service Actual factor of safety

Elevators 8-12

Mine hoists 2.5-5

Cranes, motor driven 4-6

Cranes, Hand-driven 3-5

Derricks 3-5

3. Selection of wire rope diameter (d)

Find wire rope diameter (d) by equating the tensile strength or ultimate strength of the rope selected to the

design load.

Rope

type

Ultimate

strength, Fu

Wt per length,

N/m

Wire diameter,

dw,mm

Area, A

mm2

Average sheave

diameter, D mm

619 500.8d2 0.0363d2 0.063d 0.38d2 45d

637 473d2 0.0353d2 0.045d 0.38d2 27d

819 413d2 0.0343d2 0.05d 0.35d2 31d

67 473d2 0.0324d2 0.106d 0.38d2 72d

4. Calculation of sheave diameter (D)

Consulting the table, obtain the diameter of sheave. Always larger sheave diameter is preferred.

5. Selection of the area of useful cross section of the rope (A)

Consulting the table select the area of useful cross section of the rope.

6. Calculation of wire diameter (dw):

Calculate the diameter of wire using the relation, Dw =

where i = number of wires in the rope = number of strands × number of wires in each strand. Compare with

the values given in the following table.

7. Calculation of weight of rope (wr)

Obtain the rope weight (wr) per meter length from the table. Total weight of rope, Wr = wr × total length of

rope or depth of mining

8. Calculation of various loads

i. Direct load, WD = W + Wr

ii. Equivalent bending load, Wb = σb × A= Er × dw ×

A wire rope tension giving the same tensile stress as the sheave bending is called the equivalent bending

load. A wire rope may fail because the static load exceeds the ultimate strength of the rope.

iii. Inertia load (or) Acceleration load due to change in the speed of hoisting, WI = × a = × a

Where a =

iv. Starting or stopping load

a. When there is no slack in the rope: Starting load, Wst = 2. WD = 2(W + Wr)

b. When there is slack: Starting load, Wst = σst × A = (W + Wr) (1+√1+ 2 . as . h. Er / σd .l .g).

9. Calculation of effective loads

i. Effective load on the rope during starting, West = Wst + Wb = 2WD +Wb

ii. Effective load on the rope during acceleration of the load, Wea = WD + Wb + WI

iii. Effective load on the rope during normal working, Wen = WD +Wb

iv. Effective load on the rope during stopping, West = Wb + Wst

10. Calculation of working factor of safety (FOSw):

FOSw =

11. Check for safe design

Compare the calculated working factors of safety (FOSw) with the recommended factor of safety (n). If the

working factor of safety is greater than the recommended factor of safety. Then the design is safe. Suppose

design is not safe means change the other rope.

12. Calculation of number of ropes

Number of ropes = Recommended factor of safety / Working factor of safety.

SOLVED PROBLEMS

1. Find out the diameter of wire rope to lift 15 kN debris from a mine shaft 600 m deep. The

weight of the bucket is 3 kN. The maximum velocity of 24 m/s is attained in 6 seconds. Use

the following design data (do not assume any values) of some standard wire ropes. Actual

factor of safety as 5. (d is the diameter of the wire rope). Take Young’s modulus of rope as 0.84

× 105 N/mm2.

Rope type Ultimate strength, Fu Wt per length,

N/m

Wire

diameter,

dw,mm

Area, A

mm2

Average sheave

diameter, D mm

619 500.8d2 0.0363d2 0.063d 0.38d2 45d

637 473d2 0.0353d2 0.045d 0.38d2 27d

819 413d2 0.0343d2 0.05d 0.35d2 31d

67 473d2 0.0324d2 0.106d 0.38d2 72d

Given Data

Weight of debris = 15 kN

Depth of mine, H = 600 m

Weight of bucket (skip) = 3 kN

Velocity, v = 24 m/s

Therefore acceleration, a = V/t = 24/6 = 4 m/s2

Solution

Select 619 ropes from the table. For this rope

Ultimate strength, Fu= 500.8d2 N

Weight/m length, wr = 0.0363d2 N/mm

Wire diameter, dw = 0.063d mm

Area of rope, A = 0.38 d2 mm2

Average sheave diameter, D = 45d mm

Recommended FOS = 5

For safe design, Fa ≥ W + Wr + WI + Wb

Allowable load on the rope, Fa = = = 100.16 d2

Static load, W = weight of debris + weight of bucket = 15000 + 3000 = 18000 N

Total weight of rope, Wr = weight/m depth of mine = 0.0363 d2 600 = 21.78d2

Inertia of load, WI = a = 4 = 7339.45 + 8.88 d2

Bending load, Wb = = 84000 0. 38d2 = 44.59 d2

Substituting, 100.16 d2 ≥ 18000 + 21.78 d2 + 7339.45 + 8.88 d2 + 44.59 d2 or 24.91 d2 ≥ 25339.45

Standard diameter of the rope is d = 31.89 mm

PROBLEM –2: Select and design a wire rope for an elevator in a building 60 m height and

for a total load of 20 kN. The speed of elevator is 4 m/s and the full speed is reached in 10

seconds. Actual factor of safety is 8.

GIVEN DATA:

Weight = 20 kN = 20 000 N; Height of building, H = 60 m

Velocity, v = 4 m/s; Therefore acceleration, a = 4/10= 0.4 m/s2

TO FIND: Design a wire rope

1. Selection of wire rope: 619 (elevator purpose)

2. Recommended Factor of safety = (2 to 2.5) Actual FOS = 2.5 8 = 20 (say)

3. Design load for the wire rope = 20 20000 = 400 000 N

4. From table for 619 ropes, Ultimate strength, Fu= 500.8d2 N

Diameter of the rope: Tensile strength = Design load

(or) 500.8d2 = 400 000 or d = 28.26 mm. say 29 mm

5. From table for 619 ropes,

Weight/m length, wr = 0.0363d2 N/mm = 30.528 N/mm

Wire diameter, dw = 0.063d mm = 1.83 mm

Area of rope, A = 0.38 d2 mm2 = 319.58 mm2

Average sheave diameter, D = 45d mm = 1305 mm

6. Various loads

a) Static load, W = 20 000 N

b) Weight or rope, Wr = weight/m Height of building = 30.528 60 = 1831.68 N

c) Inertia of load, WI = a = 0.4 = 6132.81 N

d) Bending load, Wb = A = 319.58 = 26820.09 N

e) Impact load during starting, Wst = 2 (W + Wr) = 2 (20 000 + 1831.68) = 21831.68 N

6. Effective loads and Actual factors of safety

Effective load in N Actual factor of safety

Design load 400 000

During Starting Wst + Wb 21831.68 + 26820.09 = 48651.77 400000/48651.77 = 8.22

During Normal working

W + Wr + Wb

20000 + 1831.68 + 26820.09 = 48651.77 400000/48651.77 = 8.22

During acceleration 20000 + 1831.68 + 26820.09 + 6132.81 = 400000/54784.58 = 7.3

W + Wr + Wb + WI 54784.58

Since the actual factor of safety as calculated above are safe, therefore a wire rope of diameter 29 mm and 619 is satisfactory

PROBLEM –3: Select a wire rope for a vertical mine hoist to lift a load of 60 kN from a depth

of 600 meters. A rope speed of 600 m/min is to be obtained in 10 seconds. Actual factor of

safety is 6.

Rope

type

Ultimate or tensile

strength, FuN

Wt per meter

length, N/m

Wire diameter,

dw (mm)

Area,

A (mm2)

Average sheave

diameter, D (mm)

67 473d2 0.0324d2 0.106d 0.38d2 72d

619 500.8d2 0.0363d2 0.063d 0.38d2 45d

637 473d2 0.0353d2 0.045d 0.38d2 27d

819 413d2 0.0343d2 0.05d 0.35d2 31d

Given Data

Weight = 600 kN = 60 000 N; Depth of mine, H = 600 m

Velocity, v = 600/60 = 6 m/s; Therefore acceleration, a = 6/10= 0.6 m/s2

Solution

1. Selection of wire rope: 619 (mining purpose)

2. Recommended Factor of safety = (2 to 2.5) Actual FOS = 15 (say)

3. Design load for the wire rope = 15 60000 = 900 000 N

4. From table for 619 ropes, Ultimate strength, Fu= 500.8d2 N

Diameter of the rope: Tensile strength = Design load

(or) 500.8d2 = 900 000 or d = 42.39 mm. say 43 mm

5. From table for 619 ropes,

Weight/m length, wr = 0.0363d2 N/mm = 67.119 N/mm

Wire diameter, dw = 0.063d mm = 2.709 mm

Area of rope, A = 0.38 d2 mm2 = 702.62 mm2

Average sheave diameter, D = 45d mm = 1935 mm

6. Various loads

a) Static load, W = 60 000 N

b) Weight or rope, Wr = weight/m depth of mine = 67.119 600 = 40271.4 N

c) Inertia of load, WI = a = 0.6 = 6132.81 N

d) Bending load, Wb = A = 702.62 = 82628.11 N

e) Impact load during starting, Wst = 2 (W + Wr) = 2 (60 000 +40271.4) = 200542 N

6. Effective loads and Actual factors of safety (2 marks)

Effective load in N Actual factor of safety

Design load 900 000

During Starting Wst + Wb 200542 + 82628.11 = 283170.11 900000/283170.11 = 3.17

During Normal working

W + Wr + Wb

60000 + 40271.4 + 82628.11

= 182899.51

900000/182899.51 = 4.92

During acceleration

W + Wr + Wb + WI

60000 + 40271.4 + 82628.11 +

6132.81 = 189032.32

900000/189032.32 = 4.76

Since the actual factor of safety as calculated above are safe, therefore a wire rope of diameter 43 mm

and 619 is satisfactory.

1.11. SIZING OF DRUMS AND SHEAVES

Diameter of drums or sheaves in wire rope application is controlled by two main considerations:

1. The radial pressure between rope and groove

2. Degree of curvature imposed on the rope by the drum and the sheave size

Radial pressure can be calculated from, p = 2T/Dd; where p is unit radial pressure inn N/m2; T is the rope

load in N; D is the tread diameter of drum or sheave size; d is the nominal diameter of the rope.

1.12. WIRE ROPE DEFECTS AND FAILURES

Wire ropes are critical component and should not fail in its use. However, rope failure may take place if it is not properly maintained and lubricated as specified.  The typical defects of wire ropes are:

1. Wear of the external wires and wire coming out of strand2. Failure of a strand3. Rupture of the wire4. Excessive elongation and thinning at a specific location

Wire ropes must work with properly matched sheave and drum. Rope problem may occur when the rope jumps out of sheave or when the rope is used with a drum diameter smaller than the required diameter.

The rope failure is preceded by failure of the wires in the wire rope. The problems faced by the wires are:

1. Tensile failure due to overload

2. Bending fatigue3. Torsion break4. Sheared and cut5. Hammering fatigue (due to vibration)6. Severe corrosion and moderate load7. Plastic wear (jammed wire)8. Corrosion fatigue and abrasive wear.

To avoid the problems in wire rope the following must be avoided:

1. Twist, Loop or Kink of sire rope.2. Moisture, Dust and Acid or Sulphuric Hume gas.3. Overload.4. Crushing of hammering.5. Severe or reverse bending(S-Bending).6. Too small Sheaves, Drums and Guide Rollers.7. Hard rolling of Sheaves and Rollers.8. Worn Groove, Broken or Soft Sheaves and Rollers.9. Poor or No Lubrication.10. Heat Influence.11. Wrong Fitting and Spooling on the Drum.12. Excessive Fleet Angle.13. Vibration.14. Obstacles, Sand and Grit on the surface of operating line.15. Shock - Too fast start or Stop

To avoid wire rope problems it must be handled carefully. Care must be taken in unreeling or uncoiling wire ropes so that no kinks are formed. There is standard procedure for such jobs and the workers must be trained for that. Similarly while winding a wire rope on drum one must pay attention to correct method of winding and unwinding. Careless winding may lead to crushing and flattening of rope as successive layers are spooled. Winding must consider the lay of the rope and how it is being wound.

Rope terminations are another critical area to avoid rope problems. Depending on the usage different rope terminations are used.

SAMPLE QUESTIONS

1. Where wire ropes are used? Give two applications.2. Why cold drawn wires are used for wire ropes?3. Write the advantages of wire ropes.4. How to designate wire ropes?5. Classify wire ropes according to the direction of the twist of the individual wires and that of strands.6. Write the step by step design procedure for wire rope design7. What are the various loads acting on the wire rope?8. What condition is used for safe design9. What do you mean by fractional horse power motors? Where they are used?10. What causes bending stress in the wires?11. Write one application of 6 X 7 wire rope.12. How do you calculate working factor of safety?13. What are the two factors that control the diameter of sheave?

14. What causes hammering fatigue?15. What causes direct stress in the wires?16. Write one application of 6 X 19 wire rope.17. What is the condition for safe design of wire rope?18. How do you calculate the radial pressure between rope and groove?19. What do you mean by hammering fatigue?20. What are the advantages of pre-formed wires?21. Write one application of 6 X 37 wire rope.22. How do you calculate wire diameter from rope diameter and number of strands?23. Why we have to calculate inertia load on wire rope?24. What is the effect of using a rope with a drum of smaller diameter?25. What are the advantages of pre-formed wires?26. Write one application of 7 X 7 wire rope.27. How do you calculate wire diameter from rope diameter and number of strands?28. Why we have to calculate inertia load on wire rope?29. What is the effect of using a rope with a drum of smaller diameter?30. What are the advantages of fiber cores in wire rope?31. What type of rope is selected when rope rotates when freely suspended under load?32. What is the relation between starting load and direct load when there is no slack in the rope?33. How do you find effective load during normal working?34. Why tensile failure occurs in wire ropes?35. How do you improve the abrasive resistance of a given wire rope?36. Write one application of 1 X 19 wire rope.37. How do you calculate bending stress in the wire rope?38. How do calculate effective load on the rope during starting?39. What is the effect of too fast or stop of load on wire rope?

SAMPLE PROBLEMS

1. An 8 X 19 (9/9/1) steel wire rope is used to lift a load 15kN from a depth of 1000m. The maximum speed of rope is 2.5m/s and the acceleration is 1.5m/sec2 when starting under no slack conditions. Determine the size of the rope required.2. Select a wire rope for an elevation in a building where the total lift is 40 m. The rope velocity is100 m/min and fall speed is to be reached in 2 meter. The lifting sheaves are to be of the traction type. The elevation car weighs 10 kN and passengers weigh 20 kN. Assume a factor of safety =10.3. A workshop crane is lifting a load of 25 kN through a wire rope and a hook. The weight of the hook etc. is 15 kN. The rope drum diameter may be taken as 30 times the diameter of the rope. The load is to be lifted with an acceleration of 1 m/,s-2.. Calculate the diameter of the wire rope. Take a factor of safety of 6 and Young’s modulus for the wire rope 80 kN/,m-2.. The ultimate stress may be taken as 1800 MPa. The cross-sectional area of the wire rope may be taken as 0.38 times the square of the wire rope diameter.

CHAPTER 02

DESIGN OF CLUTCHES

2.1. INTRODUCTION

Clutch is a mechanical device used to engage or disengage a continuously rotating driving shaft with a

driven shaft (stationary or rotating), as a means of stopping or starting the driven shaft. It controls the flow

of mechanical power. For example, in an automobile, clutch transmits power or torque from the engine shaft

to the rest of transmission system, and the automobile driver uses a pedal mechanism to control the flow of

the power between them.

According to the way in which engagement of clutches takes place, they may be divided into two general

groups: positive type or jaw clutches and friction clutches as shown in Fig.2.1.

a) Jaw Clutch b) Friction clutch

Fig.2.1. Types of clutches

The following table summarizes the differences between the two types of clutches.

Positive type clutches Friction clutches

The maximum power transmitted is not limited by slip The maximum power transmitted is limited by slip

Shock accompanies engagement at any speed Because they can slip relative to each other, there is

very little shock during engagement.

No wear takes place and no heat generation Serious wear, heat generation and cooling required

It is used for low speed applications like presses and

house hold appliances

It is used for high-speed applications such as

automotive vehicles

Generally lighter and less costly than a friction clutches

of similar torque capacitylarge outline size; requires periodic replacement of

friction material and external cooling

2.2. FRICTION CLUTCHES

Friction clutches transmit power from the driving shaft to the driven shaft by means of plates, disks or cones.

It is not positive type clutch and so power transmission relies on the friction between the plates that are

pressed together as shown in Fig.2.2. The slip between friction surfaces limits the capacity of a clutch.

Fig.2.2. Friction clutch

During operation of the clutch engagement/disengagement is done gradually. At the beginning of the

engagement, one shaft may be rotating at its full speed while the other shaft may have less speed or not at all

rotating. At the end of engagement, the two shafts rotate together with the same speed. A definite time

period elapses before driving and driven shafts together begin to rotate at the same speed. During this

period, relative motion takes place and energy is dissipated.

Friction clutches are classified into:

2.1. Flat plate or disc clutch - a) Single plate clutch b) Multi plate clutch

2.2. Conical clutch

2.3. Centrifugal clutch

The theories of friction clutches (excluding centrifugal clutch) are based on the theories of pivot and collar

bearing.

2.3. SINGLE PLATE FRICTION CLUTCH

A single plate friction clutch consists of two flanges as shown in Fig.2.3. One flange is rigidly keyed to the

driving shaft, while the other is free to move along the driven shaft due to splined driven shaft. The actuating

force is provided by a spring, which forces the driven flange to move towards the driving flange. The face of

the dive flange is lined with friction materials such as cork, leather or ferodo.

Fig.2.3. Single plate clutch (one side effective)

a) Not engaged position b) Engaged position

Fig.2.4. Single plate clutch (both sides effective)

There are two theories concerning the maximum torque required to produce slip between the surfaces of

clutch.

UNIFORM PRESSURE THEORY AND UNIFORM WEAR THEORY

The face of a disc which has a contact surface beginning from inner radius (ri) and extending to outer radius

(ro) is shown in Fig.2.5. If it is assumed that the normal reaction N acts perpendicular to the plane of the

disc, then the frictional force produced in the plane of the disc

F = μ N (when the plates about to slip)

This friction force will be acting tangential to the circle of radius rm which can be called the friction mean

radius. The friction torque produced by force of friction about the center of the disc:

T = F rm = μ W rm (2.1)

Here μ is coefficient of friction between the discs.

The mean friction radius depends on the type of clutch used. In a newly assembled clutches, intensity of

pressure acting on the plate may be assumed uniform over the entire surface because of the perfect fit

between the two surfaces (since the machine components are manufactured to exact tolerance).

i.e. Uniform pressure, p = constant (2.2)

In old clutches or worn out clutches, uniform pressure theory is not valid because the friction surfaces tend

to wear by material removal. In order that the surface maintains contact all over area, the wear must be

uniform over the entire area of contact. The wear rate is proportional to frictional work that is proportional

to the product of normal pressure and velocity of rubbing. However, the rubbing velocity is the product of

radius and angular velocity (); and angular velocity of shafts and discs is constant, for uniform wear:

Uniform wear rate, pr = constant (2.3)

MAXIMUM TORQUE TRANSMITTED BY NEW CLUTCH

ELEMENTARY RING

Consider an elementary ring of thickness dr at radius r from the center. The axial thrust W provided by the

springs produces a pressure p at radius r on the contact surfaces. Relative rotation of these surfaces sets up

tangential friction forces dF distributed over the elemental area as shown in Fig.2.5.

From the symmetry of the element about the center, area of the elemental ring, dA = 2πrdr

Axial force or normal reaction on the element perpendicular to the plane of disc, dN = pdA = p (2πrdr)

Frictional force distributed over the entire differential element, dF = μdN

Frictional torque on the differential element, dT = dF r = μdN r = 2πμpr2dr

ANNULAR SURFACE

The actuating force or axial thrust that needs to be supplied by the spring to transmit the torque,

W = dN = p (2πrdr) = 2πprdr = 2πprdr = pπ (r– r)

or p =

Now, frictional torque produced on the entire friction surface, T =dT = 2πμpr2dr

= 2πμpr2dr = 2μp

= 2μ = μW = μW rm (2.4)

Where, frictional mean radius, rm =

MAXIMUM TORQUE TRANSMITTED BY OLD CLUTCH

Mean friction radius according to Uniform wear theory

Permissible pressure

One important factor in design of the clutch would be the permissible pressure on the friction surface. While

using uniform pressure assumption (or new clutch), the pressure is easily calculated as axial force/area of

clutch face; in case of uniform wear (as in the case of old clutch) it will be maximum at inner radius ri. The

permissible pressure then must not exceed the pressure at inner radius

pmax = pper (2.5)

where, pper is permissible value of pressure for the material of disc or lining placed on the disc.

Fig.2.5. Condition of disc

The actuating force or axial thrust, W = p (2πrdr) = 2πpr dr = 2πpr (ro – ri)

According to uniform wear theory pr is constant, so pr = C = (2.6)

Frictional torque on the entire surface, T = 2πμpr2dr= 2πμprrdr = 2πμrdr

= rdr = μW = μW rm (2.7)

Here, frictional mean radius, rm =

  Note   (i) In most of automobile clutches, both sides of friction plates are effective. Thus, the torque

generated will be two times the torque generated when only one pair of surfaces is in contact by an axial

thrust.

(ii) To calculate the radii of a clutch, students confuse whether to assume uniform wear theory or uniform

pressure theory. It is logical to note that in a new clutch which is just put in service the uniform pressure

theory is best option. But as the old clutch is used more and more, uniform wear rate theory is the best

option. Since one would like to design the clutch for long service life rather than for better service in the

initial stages only, the uniform wear theory is better proposition.

(iii) The coefficient of friction depends on the combination of metals as surfaces of contact. Metal to metal

contact surfaces such as cast iron on cast iron or steel and bronze on cast iron and steel are common. Now-

a-days, in automobiles, bronze and steel combination is replaced by asbestos based surface lining.

2.4. MULTI PLATE CLUTCH

A multi-plate system used for large transmission forces or limited-space applications. Multiple clutches are

more difficult to cool, so they are appropriate for high load, low speed applications.

Fig.2.6. Multi plate clutch

Fig..2.6 depicts a multi-plate clutch. The operation of multi-plate clutch is very similar to the single plate

clutch but there is an increase in the number of contact surfaces, so increase in effective friction torque. The

number of contact surfaces are one less that the total numbers of plates in the clutch. Let n1 is numbers of

driving discs and n2 is numbers of driven discs. Therefore numbers of friction surfaces, n = n1+ n2 – 1.

It should be noted that as the discs are free to slide axially under the spring pressure, each pair of contact

surfaces is subjected to the same full axial load.

Frictional torque on the entire surface, T = nμWrm (2.8)

Where, frictional mean radius, rm = (According to uniform pressure theory)

or frictional mean radius, rm = (According to uniform wear theory)

In earlier multiple-clutch, the plates were made alternately of bronze and steel but now all are made in steel.

Alternatively one set of plate maybe lined with friction material. With regard to the size, this type of clutch

has reduced size in radial direction but its axial dimension is increased. Apparently in such cases where

radial space is restricted and axial space is available this clutch will be suitable. Two wheelers are example.

SAMPLE PROBLEMS

Example 2-1   A single plate friction clutch of both sides effective has 320 mm outer

diameter and inner diameter equal to half of the outer diameter. The coefficient of friction

0.25 and it runs at 1000 rpm. Find the power transmitted for uniform wear and uniform

pressure distributions cases if allowable maximum pressure is 0.08 MPa.

Given data: ro=160 mm; ri = 80 mm; n = 2; μ = 0.25

Parameters uniform pressure theory uniform wear theory

Angular velocity, = 104.72 rad/s = 104.72 rad/s

Axial force provided

by the springs

W = pπ (r– r) = 4825.5 N W = pmax2π ri(ro – ri)= 3217 N

Frictional mean

radius, rm

= 124.44 mm = 120 mm

Friction torque

(both sides are

effective), T

nμWrm = 2 0.25 4825.5

124.44 = 300242 N.mm

nμWrm = 2 0.25 3217 120

=193020 N.mm

Power transmitted,

P

T 104.72

= 31.44 kW

193020 104.72 = 20.21 KW

Example 2-2     A single plate clutch has dimensions 300 mm outside diameter and 100 mm inside

diameter. Both sides of the plate are effective. Assuming uniform wear theory and coefficient of friction

of 0.35, determine the maximum power that can be transmitted at 1500 rpm, if the maximum pressure is

not to exceed 1 MN/m2. Find also the minimum intensity of pressure and its location.

Given Data: Type of clutch = single plate clutch; Outer radius, ro = 0.15 m; Inner radius, ri = 0.05 m;

No of effective sides, n = 2; Coefficient of friction, μ = 0.35; Speed of clutch, N= 1500 rpm; Max pressure,

pmax = 1 106 N/m2;

 Solution: uniform wear theory

According to uniform-wear theory, maximum pressure occurs at inner radius. Hence, axial force provided

by the springs,

W = pmax 2π ri (ro – ri) or W = 11062π 0.05 (0.15 – 0.05) = 31.42 103 N

Frictional mean radius, rm = = = 0.1 m

Friction torque (both sides are effective), T = 2μW rm = 2 0.35 31.42 103 0.1 = 2199.4 N.m

Angular velocity of driver shaft and clutch, = = 157.08 rad/s2

Power transmitted by clutch, P = T 2199.4 157.08 345.48 kW

Minimum intensity of pressure occurs at the maximum radius. i.e,

pmin = = = 0.333 106 N/m2

Example 2-3     A clutch is required to transmit 10 kW at 3000 rpm. It is of single plate type, both sides

being effective. The coefficient of friction is 0.25 and the axial pressure is limited to 8.5 N/cm 2. Determine

the dimensions of the plate, assuming that the external diameter is 1.4 times the inner diameter.

Given Data: Type of clutch = single plate clutch; Power to be transmitted, P = 10103 Watts; speed of

clutch, N = 3000 rpm; effective surfaces, n = 2; coefficient of friction, μ = 0.25; Max pressure, pmax =

8.5104 N/m2; ro = 1.4 ri.

 Solution: Assume uniform wear theory

Angular velocity of flywheel, = 314.16 rad/s2

Torque transmitted by shafts, T = 31.8 N.m

Torque transmitted by clutch by means of friction = Torque transmitted by shafts (no slip)

According to uniform-wear theory, maximum pressure occurs at inner radius. Hence, axial force provided

by the springs,

W = pmax 2π ri (ro – ri) or W = 8.5104 2π ri (1.4ri – ri) = 213.63103 rN

Frictional mean radius, rm = = = 1.2 ri

Friction torque on the clutch, T = 2μW rm (both sides are effective)

31.8 = 2 0.25 213.63 103 r 1.2 ri = 128.18103r

Inner radius, ri = 0.0628 m = 62.83 mm

Outer radius, r0 = 0.08797 m = 87.97 mm

2.5. CONICAL CLUTCH

A conical clutch transmits power from one shaft to another through the friction forces on the conical

surfaces. It is able to transmit a larger torque than disc clutches with the same outside diameter and actuating

force, because of increase in frictional area and the wedge action that takes place. It is most suitable for low

speed applications such as synchromesh device of gearboxes.

In this clutch as shown in Fig.2.7, the cone with friction on the outer surface is placed on driving shaft and

the cup is placed on the driven shaft. The two inclined surface are pressed against each other by a helical

spring.

Fig.2.7. Cone clutch

For calculating frictional torque or frictional radius in a cone clutch the same assumptions as made in case of

plate clutch are valid in respect of pressure. It may be either a uniform pressure between the contact surfaces

or a uniform wear of contact surfaces.

Fig.2.8. Geometry of Cone clutch

The clutch, in fact, is a frustum of cone bounded by inner radius ri and outer radius ro as shown in Fig.2.8.

An elementary area on surface of cone frustum is chosen and this element is bounded by inner radius r

and outer radius r + dr. Thus

The sloping length =

Surface area of elemental ring = dA =

Surface area of cone clutch = A =dA =

If p denotes the uniform pressure between contact surfaces as shown in Fig.2.8,

Normal force on elemental area, dWn =

Axial force on elemental area, dWa = sinα = 2πprdr

Total axial force, Wa = dWa = 2π prdr (2.9)

Friction force due to normal force acting on the elemental area, dF = μ dWn =

Friction torque to due to force of friction acting on the elemental area, dT = dF r = p

Total torque to due to force of friction, T = dT = p = pr2dr (2.10)

Torque transmitted according to Uniform pressure theory

If p denotes the uniform pressure between contact surfaces, then the normal reaction between them,

Total axial force on the entire contact area, Wa = dWa = 2πp rdr = pπ (r– r)

Intensity of pressure, p = (2.11)

The above equation indicates which is same as in case of disc or plate clutch, that normal pressure intensity

on contact surfaces is independent on inclination of contact surfaces.

Total torque to due to force of friction, T = pr2dr = r2dr =

Substitute p = in the above equation, we have:

Friction torque, T = = μWarm cosecα (2.12)

Where, frictional mean radius, rm =

Torque transmitted according to Uniform wear theory

According to uniform wear theory, pr = C, where C is constant.

Axial force on the entire contact area, Wa = dWa = 2πprdr = 2πCdr = 2πC(ro – ri)

pr or C = (2.13)

The above equation indicates which is same as in case of disc or plate clutch, that normal wear on contact

surfaces is independent on inclination of contact surfaces.

Total torque to due to force of friction, T = pr2dr = rdr =

Substitute C = in the above equation

Friction torque, T = = μWacosecα

= μWarm cosecα (2.14)

Here, frictional mean radius, rm =; α is semi-apex angle of the cone; and n = 1 because only one pair of

driving surfaces is possible. The following geometric relations are also derived from geometry of the clutch.

Mean radius, rm =

Face width of cone surface = b =

Outer Diameter, Do = Dm + b sinα

Inner diameter, Di = Dm – b sinα

Even though a cone clutch is compact in size and need a less strong spring to maintain contact, compared to

a plate clutch of same capacity, it is not much preferable because of the problems caused by effect of wear

on cone surface. Firstly a small amount of wear affects the perfect fit between the cone and the cup and it

requires a considerable axial movement. Secondly for smaller cone angle (less than 100) the cup and cone

lock together and becomes difficult to draw out or disengage.

SAMPLE PROBLEMS

Example 2-4     A cone clutch is to transmit 7.5 kW at 900 rpm. The cone has a face angle of 12 0. The

width of the face is half of the mean radius and the normal pressure between the contact faces is not to

exceed 0.09 N/mm2. Assuming uniform wear and the coefficient of friction between contact faces as 0.2,

find the main dimensions of the clutch and the axial force required to engage the clutch.

Given Data: Type of clutch = Cone clutch; Power to be transmitted, P = 7.5 103 Watts; speed of clutch,

N = 900 rpm; coefficient of friction, μ = 0.2; Permissible pressure, pmax = 0.09106 N/m2; face angle, α =

120; width of face, b = =

 Solution: Assume uniform wear theory

Face width of cone surface, b = = or 2(r – r) = (r+ r) sin120 or r = 1.235 ri

Angular velocity of clutch, = 4.25 rad/s2

Torque transmitted by shafts, T = 79.58 N.m

According to uniform-wear theory:

Torque transmitted by clutch by means of friction, T = μWacosecα

Or 0.2Wacosec120 = 79.58 N.m or Wa ri = 74.03 N.m (i)

Maximum pressure occurs at inner radius so axial thrust, Wa = pmax 2π ri (ro – ri)

Wa = 0.09 106 2π ri (1.235ri – ri) = 0.133 106 r (ii)

On solving (i) and (ii) 0.133 106 r = 74.03 or ri = 82.26 mm and ro = 101.59 mm

Example 2-5     A leather faced conical clutch has a cone angle of 300. If the intensity of pressure

between the contact surfaces is limited to 0.35 N/mm2 and breath of the conical surface is not to exceed

1/3rd of the mean radius, determine the dimensions of the contact surfaces to transmit 22.5 kW at 2000

rpm. Assume uniform wear rate and coefficient of friction as 0.15.

Given Data: Type of clutch = Cone clutch; Power to be transmitted, P = 22.5 103 Watts; speed of

clutch, N = 2000 rpm; coefficient of friction, μ = 0.15; cone angle, α = 300; width of face, b = rm

 Solution: Assume uniform wear theory

Angular velocity of clutch, = = 209.44 rad/s2

Torque transmitted by shafts, T = 107.43 N.m (i)

Torque transmitted by clutch by means of friction, T = μWrm cosecα = μ rm

= 0.9Wb (ii)

From equations (i) and (ii) 0.9Wb = 107.43 or Wb = 119.37

rm = 3b = = ; sin α = ; and W = p 2π ri (ro – ri)

According to uniform-wear theory, maximum pressure occurs at inner radius. Hence, axial force provided

by the springs,

W = pmax 2π ri (ro – ri) or W = 0.35 1062π ri (ro – ri) = 0.57106ri (ro – ri)

But sin α =

0.15 0.57106ri (ro – ri) = 107.43

b = (or) ri b = 1.25610-3

2.6. CENTRIFUGAL CLUTCH

Majority of small engines such as squirrel cage motor, lawn mowers, chainsaws, go-karts uses centrifugal

clutch.

It engages automatically when the shaft speed exceeds a certain magnitude.Fig.2.9 depicts a spring

controlled centrifugal clutch. It comprises two or more friction shoes that, when the driving shaft on which

they are mounted has reached a certain speed, overcome the force of restraining springs by the action of

centrifugal force and move outwards to press against the inner surface of the rim mounted on the driven

shaft. In this way, the transmission of power to the driven shaft is gradually and automatically increased, so

that smooth engagement is possible.

Fig.2.9. Centrifugal clutch

If W be the weight of the shoe and r be the radius of the centre of gravity from the axis of rotation and N be

the speed of the driver in rpm, then centrifugal force F is given by:

F = rω2

If P be the spring force provided by restraining springs opposing the centrifugal force, the radial force

between shoe and pulley is:

F – P = rω2

If R be the internal radius of the pulley; n be the number of shoes; and μ the coefficient of friction between

the shoe and the drum, then the torque transmitted:

The maximum torque transmitted, T = μnR rω2 – P = Aω2 – P .Where A is constant for a given centrifugal

clutch.

SAMPLE QUESTIONS

1. Define clutch. Give two examples of its uses

2. What are the types of clutches and compare them

3. How power is transmitted in friction clutches? Classify them

4. What is uniform pressure theory? In which types of clutch it is applicable?

5. What is uniform wear theory? In which types of clutch it is applicable?

6. Write the expression for mean friction radius according to uniform pressure theory and uniform

wear theory.

7. Write the expression for axial thrust according to uniform pressure theory and uniform wear theory.

8. What are the advantages of multi-plate clutch over single plate clutch? Where it is mainly used?

9. How do you calculate numbers of friction surfaces in multi-plate clutch?

10. Why the driven shaft in a clutch mechanism is splined shaft?

11. Why a conical clutch is able to transmit a larger torque than disc clutches with the same outside

diameter and actuating force?

12. What are the disadvantages of conical clutch

CHAPTER 03

DESIGN OF BRAKES

3.1. INTRODUCTION

A brake is a machine element that is used to either slow down the motion of a moving member or bring it to

rest. The braking action relies on friction between a moving member and stationary member. During its

performance, either kinetic energy of the moving member or potential energy of falling objects is absorbed.

The energy absorbed by brakes is dissipated in the form of heat and this heat should be dissipated to the

surrounding air (or water which is circulated through the passages in the brake drum) so that the excessive

heating of the braking lining does not take place.

Brakes are commonly used in automotive vehicle, lifts, cranes and mine hoists. In vehicles the two main

functions are:

a) To control or stop the vehicle in the shortest possible time at the time of need

b) To control the speed of vehicle at turns and also at the time of driving down a hill slope

They can be also used as dynamometer for measuring power in power generating and power consuming

machines.

3.2. BLOCK BRAKE OR SHOE BRAKE

A block brake is shown in Fig.3.1. It consists of a block or shoe normally made of a softer material riding on

a metallic brake drum keyed to the shaft. When brake shoes press against external surface of a brake

drum, such brakes are called external closing or contracting brakes. These types of brakes are normally used

on lift, elevator, or crane.

To obtain braking action the block is pressed against the drum under a force obtained from a lever. The

action generates a reaction between the shoe and the drum and hence a tangential friction force on the drum

to cause retardation of the drum.

3.2.1. Pivoted lever with rigidly mounted shoe

Fig.3.1 shows a rigidly mounted shoe on a hand lever which is pivoted at the left end and actuating effort

acts at the right end of the lever. Let this effort be P and the drum rotates in clockwise direction dragging the

shoe. The friction force thus acts to the right on the shoe as shown. The equal and opposite force will act on

the drum to retard its motion.

Fig.3.1. Shoe brake with FBD

Condition (a) Fulcrum of lever below the line of action of friction force

Consider equilibrium of shoe-lever assembly under the action of following forces:

a) Effort P acting downwards at free end of the lever

b) Friction force F at the centre of the shoe to the right or in the direction of rotation, and

c) The normal reaction N between the shoe and the drum acting upward at the centre of the shoe

As shown in Fig.3.1 the fulcrum is at a horizontal distance a and vertical distance c from the centre of the

block and lever arm is l. The fulcrum is below the central point of the shoe contact.

Now consider the equilibrium of the lever under three forces P, F, and N, the summation of moments about

the fulcrum should add up to zero. Hence

Na – Fc = Pl or Na – μNc = Pl or N =

Now consider the equilibrium of the braking drum

TB = Fr = μNr

or TB = Pl

As indicated by the above equation, if fulcrum is below the central point of shoe contact, the frictional force

is helping effort.

Condition (b) Fulcrum of lever on the line of action of friction force

If fulcrum is on the line of action of friction force, the contribution of friction force will vanish. In this case,

c is zero and the braking torque becomes:

TB = Pl

Condition (c) Fulcrum of lever above line of action of friction force

If fulcrum is above the line of action of friction force, the friction force opposes the effort to break. In this

case, c is negative and the braking torque becomes:

TB = Pl

3.1.2. Effective coefficient of friction

Uniform pressure condition

If the angle of contact between shoe and drum 2 is less than or equal to 600, the condition is known as

uniform pressure condition and effective coefficient of friction is equal to given coefficient of friction.

μ′ = μ.

Uniform wear condition

If the angle of contact between shoe and drum 2 is greater than 600, the condition is known as uniform wear

condition and the effective coefficient of friction is given as:

μ′ =

SAMPLE PROBLEMS

Example 3-1     The block-type hand brake shown in the figure has a face width of 30 mm

and a mean coefficient of friction of 0.25. For an estimated actuating force of 400 N, find the

maximum pressure on the shoe and find the braking torque.

1. Draw the free body diagram

2. Effective coefficient of friction is given as: μ′ = = = 0.275

3. Normal reaction: N 200 – 400 500 = 0 or N = 1000 N and force of friction, F = 0.275 1000 = 275

N

4. Friction torque, TB = 275 150 = 41250 N.mm

5. Maximum pressure on the shoe, pmax = = = 0.16 N/mm2

Example 3-2    The block type hand brake shown in figure has a face width of 45 mm. The

friction material permits a maximum pressure of 0.6 MPa and a coefficient of friction of0.24.

Determine: 1. Effort, 2. Maximum torque, and 3. Heat generated if the speed of the drum is

100 rpm and the brake is applied for 5 sec at full capacity to bring the shaft to stop.

Given Data: μ = 0.24; 2 = 900; N = 100 rpm; r = 150 mm; w = 45 mm; p = 0.6 N/mm2; d = 300 mm

SOLUTION

1. Draw the free body diagram

2. Effective coefficient of friction is given as: μ′ = = = 0.264

3. Allowable pressure on the shoe, pmax = (or) 0.6 =

Normal force, N = 5727.56 N

4. Tangential friction force, F = μ′ N = 0.264 5727.56 = 1512.1 N

The various forces acting on the shoe are shown in figure. From the figure a = 200 mm, b = 300 mm and c =

0. The tangential force passes through the fulcrum.

Effort, P = = = 2291.1 N

5. Torque on the drum, TB = 1512.12 150 = 226815 N.mm = 226.815 N.m

6. Power absorbed, Q = = 2.375 kJ/s and in 5 seconds, 5 2.375 = 11.875 kJ

Example 3-3     A 400 mm radius brake drum contacts a single shoe as shown in figure and

sustains 200 N.m torque at 500 rpm. Take a coefficient of friction 0.25.

a) Determine for clockwise rotation of drum: (i) Normal force on the shoe (ii) Required force

F to apply for the brake. (iii) Heat generated

b) Determine for counter clockwise rotation of drum: (i) Normal force on the shoe (ii)

Required force F to apply for the brake. (iii) Heat generated

SOLUTION

A. DRUM ROTATION IS CLOCKWISE

1. Draw the free body diagram

2. Friction torque, TB= 200 N.m

3. Tangential friction force, F = = 200000/400 = 500 N

4. Normal force, N = = 500/0.25 = 2000 N

5. Actuating force: – 2000 350 + F 1000 + 500 40 = 0 or F = 680 N

6. Heat generated = = 10472 kJ/s

B. DRUM ROTATION IS COUNTER - CLOCKWISE

1. Draw the free body diagram

2. Friction torque, TB= 200 N.m

3. Tangential friction force, F = = 200000/400 = 500 N

4. Normal force, N = = 500/0.25 = 2000 N

5. Actuating force: – 2000 350 + F 1000 – 500 40 = 0 or F = 720 N

6. Heat generated = = 10472 kJ/s

3.3. SIMPLE BAND BRAKE

A band brake consists of a flexible band of leather, one or more ropes, or steel lined with friction

material, which embraces a part of the circumference of the drum. A band brake, as shown in Fig., is called

a simple band brake in which one end of the band is attached to a fixed pin or fulcrum of the lever while the

other end is attached to the lever at a distance b from the fulcrum.

When a force P is applied to the lever at C, the lever turns about the fulcrum pin O and tightens the band on the drum and hence the brakes are applied. The friction between the band and the drum provides the braking force. The force P on the lever at C may be determined as discussed below:Let T1 = Tension in the tight side of the band,T2 = Tension in the slack side of the band, = Angle of lap (or embrace) of the band on the drum,μ = Coefficient of friction between the band and the drum,r = Radius of the drum,t = Thickness of the band, andre = Effective radius of the drum = r + t / 2.

We know that limiting ratio of the tensions is given by the relation, = eμ

and braking force on the drum = T1 – T2∴ Braking torque on the drum, TB = (T1 – T2) r (Neglecting thickness of band) = (T1 – T2) re (Considering thickness of band)

Now considering the equilibrium of the lever OBC. It may be noted that when the drum rotates in the clockwise direction as shown in Fig. (a), the end of the band attached to the fulcrum O will be slack with tension T2 and end of the band attached to B will be tight with tension T1. On the other hand, when the drum rotates in the anticlockwise direction as shown in Fig. (b), the tensions in the band will reverse, i.e. the end of the band attached to the fulcrum O will be tight with tension T1 and the end of the band attached to B will be slack with tension T2. Now taking moments about the fulcrum O, we have P.l = T1.b (for clockwise rotation of the drum)and P.l = T2.b (for anticlockwise rotation of the drum) where l = Length of the lever from the fulcrum (OC), and b = Perpendicular distance from O to the line of action of T1 or T2.Notes: 1. when the brake band is attached to the lever, as shown above in Fig. (a) and (b), then the force (P) must act in the upward direction in order to tighten the band on the drum.2. Sometimes the brake band is attached to the lever as shown below in Fig. (a) and (b), then the force (P) must act in the downward direction in order to tighten the band. In this case, for clockwise rotation of the drum, the end of the band attached to the fulcrum O will be tight with tension T1 and band of the band attached to B will be slack with tension T2. The tensions T1 and T2 will reverse for anticlockwise rotation of the drum.

3. If the permissible tensile stress (σt) for the material of the band is known, then maximum tension in the band is given by T1 = σt w twhere w = Width of the band, and t = Thickness of the band.4. The width of band (w) should not exceed 150 mm for drum diameter (d ) greater than 1 meter and 100 mm for drum diameter less than 1 meter. The band thickness ( t) may also be obtained by using the empirical relation i.e. t = 0.005 d for brakes of hand operated winches.

SAMPLE QUESTIONS

1. Define mechanical brake. How it differs from a dynamometer?

2. What are the uses of mechanical brakes in automobiles?

3. Draw the free-body-diagram of the lever and drum separately.

4. When the frictional force is helping effort? When the friction force opposes the effort to break?

5. What are the uniform pressure condition and uniform wear condition in brakes?

6. What do you mean by effective co-efficient of friction? When it is used?

CHAPTER 04

ERGONOMICS

4.1. INTRODUCTION

Any engineer designing a product or system will require exact information about materials,

structures, tolerances, power and the capacities of various components, and how to combine them when

trying to meet a specification. However in the past designers relied on common sense when considering the

needs of the people who would use and operate the products and systems they designed. Ergonomics is a

relatively new science and can be described as 'The science study of looking at human beings in their work

environment.' It helps to minimize stress in work setting and maximize efficiency.

Ergonomics is the study of how people work (rest and play) in their environment, which could be an

office, school, factory or even at home. In simple terms ergonomics is about how to make people more

efficient at what they do. Effective Use of ergonomic principles improves:

Productivity,

Morale and,

Stress Reduction

It studies the interactions or relations between MAN-MACHINE and MAN-ARTIFACT.

A good example of how a product is ergonomically designed is a mobile phone. The phone has

rounded edges to make it comfortable, the distance between the microphone and the speaker fits the

distance between the average adults ear and mouth and the buttons are well spaced and easy to use. Notice

also that the buttons use a bold typeface that is easy to read.

4.2. TYPES OF INTERACTION BETWEEN MEN AND MACHINE

Most machines work in coordination with people. There are four ways a person interacts with the product: as

occupant of work space, as power source, as sensor, and as a controller. It forms the basis of the study

of the human factors that play a major role in the design of a device.

Products are perceived to be best if they are comfortable to use (there is a good match between the

device and the person in workspace), they are easy to use (minimal power required), their operating

conditions is easily sensed, and their control logic is natural, or used friendly. Of equal importance is the

concern for safety. Although not listed as one of the factors in the survey, it is readily assumed that an

unsafe design will never be perceived as quality product. Customers assume that neither they nor others will

be injured, and that no property will be destroyed (obvious exceptions are products that are designed to

destroy or injure)

Consider the types of interactions you have with a standard gas-powered lawn mower. First, in

starting and pushing the mower you occupy a work space around the mower. You have to bend in this

space to reach the starting of mechanism, then you have to position yourself while holding your arms at a

certain height to push and steer the mower. Second you provide a source of power to the mower to start it

and to push it. (Even if electrically started you have to push a button or turn a key). Additionally, it takes

muscle power to steer it, whether you are walking behind or riding it. Third you act as a sensor, listening to

determine if anything is stuck in the mower, and feeling with your hands any feedback motion through

seeing whether you are going so that you can steering that might give you information on how well you are

guiding the mower. Fourth, based on the information received by the sensory inputs, you act as a

controller. You determine how much power to provide and in what direction the mower under control.

Beyond these four basic types of interactions between the product and person, there are further human

interactions issues that must be considered during design. First, even those devices that spend their operating

life remote from all human interaction, at the bottom of a well or in a deep space, must first be assembled.

The assembler must interface with the device in the same four ways as described in the lawn-mower

example. Second, most devices have to be maintained, which presents yet another situation for the

consideration of human interaction in the design of a product.

Human factors must be taken into account for every person who comes into contact with the product,

whether during manufacturing, operation, maintenance and repair or disposal.

4.3. ROLE OF ERGONOMICS IN WORK PLACE

Ergomonics is about making things the right shape, size and weight for humans. But what if the

room that you are working in is too hot or too cold? People work best at ‘room temperature’ which is about

200 C. You cannot work efficiently if you are too hot or too cold. Ergomics also considers noise,

vibration, light, and smell. In fact if your senses are uncomfortable you will not work efficiently. Some of

the most important principles of ergonomics are:

The designer should aim for sitting position of work when it is not possible due to nature of work

then only standing position should be considered

Any unnatural position of the body should be avoided for reduction of body fatigue

The working area should be properly designed both for sitting and standing postures of the body

The most frequent movement of the arms should be close to the body as possible so that a person can

use implements without stretching them

The system requiring the use of knobs, levers, hand grips and push buttons should be properly

designed and located for efficient control as they directly influence the efficiency of the operator.

It is very important to ensure that people who spend a long time in the same position do not develop painful

and crippling problems such as repetitive strain injury (RSI). Computer operators, for example sit for very

long periods repeating very simple movements. One way of solving the problem may be to design a better

chair. Most chairs are like the ones you sit on at school, they cannot be adjusted. We have to adjust

ourselves to suit the chair which results in fidgeting, discomfort and loss of attention. Ergonomic designers

believe that adjustable chairs would be better. If the operators were more comfortable, efficiency would be

improved and there would be less chance of injury.

Take a look at the picture below. The person is sitting at a work desk that has been ergonomically designed.

The heating, lighting and noise are carefully controlled in offices so that people are comfortable and work at

their best. Offices are carefully designed so that people can work efficiently.

ILLUSTRATIONS ON APPLICATION OF ERGONOMIC PRINCIPLES

SITTING AND STANDING POSTURES

MATERIAL HANDLING

TOOL DESIGN

CONTROL PANEL

4.3. DISCIPLINES THAT CONTRIBUTE TO HUMAN FACTORS

The basic human sciences involved are anatomy, physiology and psychology. These sciences are

applied by the ergonomist towards two main objectives:

the most productive use of human capabilities and,

the maintenance of human health and well-being.

1) Anatomy or Anthropometrics

İt ıs basıcally the study of the structure of the human body. The study of human measurements such as

height, arm length, reaches, etc. helps engineers to design for varıous crıterıa demanded by the human body.

The contribution of anatomy lies in improving physical ‘fit’ between people and the things they use, ranging

from hand tools to aircraft cockpit design.

2) Physiology

Gıves informatıon about the functioning of the human body. The study of bodily strength, fatigue,

reaction times etc.

a) Work physiology addresses the energy requirements of the body and sets standards for acceptable

physical workrate and workload, and for nutrition requirements.

b) Environmental physiology analyses the impact of physical working conditions – thermal, noise and

vibration, and lighting – and sets the optimum requirements for these.

3) Psychology

It is concerned with human information processing and decision-making capabilities. In simple terms, it is

the cognitive ‘fit’ between people and the things they use. Relevant topics are: perception, long and short-

term memory, decision-making and action.

a) Physiological Psychology: Deals with the functioning of the brain and the nervous system. eg. When a

sound or smell comes from the environment, the brain reacts and a decision is made.

b) Experimental Psychology: Deals with the parameters of human behaviour.

4) Management: Deals with the organization of the work and the work space.eg. Where you should put the

things in a kitchen or in an office.

5) Engıneerıng: Gives information about the environment the human is within. This is important because,

for example, the temperature or the amount of humidity in a room has to be known to obtain human comfort.

6) Desıgn: Is the way of representation of any artifact.When we design any artifact we must consider human

health, safety, and comfort. We use ergonomic principles to do this.

4.3. ANTHROPOMETRICS

The challenge for designers and engineers is to design things which can be used by the majority of the

population. Because we are all different this often means providing a limited form of adjustment. The

driver’s seat in a car has a number of adjustments which allow it to be customized by each driver. It is only

Formula One drivers who have cockpits tailor-made to their own measurements! Knowing the

measurements of the person or persons for whom you are designing is the key to successful design.

The science of anthropometrics provides data on dimensions of the human body in various postures and

other physical characteristics. The application of this information is useful to the design of things they use

from something as simple as a pencil to something as complex as a car.

This is the branch of ergonomics that deals with body shape, size, weight, strength, proportions, and

working capacity of the human body.

It is the technology of measuring human physical traits such as size, reach, mobility and strength.

It is the study of human body measurement for use in anthropological classification and comparison.

Anthropometric side of ergonomics is matching the physical form and dimensions of the product or work

space to those of it’s user; and Matching the physical demands of the working task to the capacities of the

work force.

ANTHROPOMETRIC DATA AND ITS TYPES

However, anthropometric data differs between races, and changes with time. For example, some

Asian races were traditionally smaller than western races. British manufacturers exporting beds to Japan had

to make smaller beds than those sold in Europe. However with improved diet and an increased protein

intake, these races are quickly catching up. Most races are gradually getting bigger because of both better

diet and better health care. Look at the doorways in old houses - nowadays many people have to bend down

to get through them.

Structural anthropometrıc data Measurement of the dimensions in static positions

Functıonal anthropometrıc data Data that define the movements of a part of the body in reference to a

point.

Newtonıan anthropometrıc data Used for the mechanical analysis of the loads on the human body.

Body segment measurement for use in biomechanical analyses

These types of measurements are classified as:

1. STATIC MEASURES - These measures are passıve measures of the dımensıons of the human body.

They are used to determine sıze and spacıng requırements of work spaces such as: Height, Weight, Wing

span, Seat – elbow height.

2. DYNAMIC MEASURES - Measures of the dynamıc propertıes of the human body such as strength and

endurance. These measures are used to match the dynamıc characterıstıcs of controls to user. Eg. Range of

motion for various joints, force of leg pushes, strength of fingers.

If a graph is plotted of the height of any population, it will look like the one shown below.

This is known as the normal distribution curve. The line through the middle of the graph is known as

the 50th percentile or means (average) value. If height were being measured, the 50 th percentile would be the

height that occurred most often. People whose height falls on the 50th percentile line are often said to have

average height. People whose height falls on the 5th percentile can be said to be small people, while people

whose height falls on the 95th percentile can be said to be tall people.

Percentiles are shown in anthropometry tables and they tell you whether the measurement given in the

tables relates to the 'average' person, or someone who is above or below average in a certain dimension.

If you look at the heights of a group of adults, you'll notice that most of them look about the same height. A

few may be noticeably taller and a few may be noticeably shorter. This 'same height' will be near the

average (called the 'mean' in statistics) and is shown in anthropometry tables as the fiftieth percentile, often

written as '50th %ile'. This means that it is the most likely height in a group of people.

If we plotted a graph of the heights (or most other dimensions) of our group of people, it would look similar

to this:

The graph is symmetrical – so that 50% of people are of average height or taller, and 50% are of average

height or smaller.

The graph tails off to either end, because fewer people are extremely tall or very short.

To the left of the average, there is a point known as the 5th percentile, because 5% of the people (or 1 person

in 20) is shorter than this particular height.

The same distance to the right is a point known as the 95th percentile, where only 1 person in 20 is taller

than this height.

THERE ARE SOME STEPS A DESIGNER MUST TAKE:

we also need to know whether we are designing for all potential users or just the ones of above or below

average dimensions. When selecting optimum sizes it is a common mistake to always design for the average

person.

For example, if we were designing a doorway using the height, shoulder width, hip width etc., of an average

person, then half the people using the doorway would be taller than the average, and half would be wider.

Since the tallest people are not necessarily the widest, more than half the users would have to bend down or

turn sideways to get through the doorway. Therefore, in this case we would need to design using dimensions

of the widest and tallest people to ensure that everyone could walk through normally.

The following points should be kept in mind when designing for ergonomics.

Decide who you are designing for

Anthropometric tables give measurements of different body parts for men and women, and split into

different nationalities, and age groups, from babies to the elderly. So first of all, you need to know exactly

who you are designing for. The group of people you are designing for is called the USER POPULATION.

If you were designing an office chair, you would need to consider dimensions for adults of working age and

not those for children or the elderly. If you were designing a product for the home, such as a kettle, your user

group would include everyone except young children

Decide which body measurements are relevant

You need to know which parts of the body are relevant to your design. For example, if you were designing a

mobile phone, you would need to consider the width and length of the hand, the size of the fingers as well as

grip diameter. You wouldn't be too interested in the height or weight of the user (although the weight of the

phone might be important!)

Decide whether you are designing for the 'average' or extremes

Nobody is 'average' in all body dimensions. Someone might be of average height but have a longer than

average hand length.

Deciding whether to use the 5th, 50th or 95th percentile value depends on WHAT you are designing and

WHO you are designing it for. Usually, you will find that if you pick the right percentile, 95% of people

will be able to use your design. For instance, if you were choosing a door height, you would choose the

dimension of people's height (often called 'stature' in anthropometry tables) and pick the 95th percentile

value – in other words, you would design for the taller people. You wouldn't need to worry about the

average height people, or the 5th percentile ones – they would be able to fit through the door anyway.

At the other end of the scale, if you were designing an aeroplane cockpit, and needed to make sure everyone

could reach a particular control, you would choose 5th percentile arm length – because the people with the

short arms are the ones who are most challenging to design for. If they could reach the control, everyone

else (with longer arms) would be able to.

What is it that you are aiming for with your

design?Design examples:

Examples of measurements to

consider:

Users that your design should accommodate:

Easy reach Vehicle dashboards, Shelving

Arm length,Shoulder height

Smallest user: 5th percentile

Adequate clearance to avoid unwanted contact or trapping

Manholes,Cinema seats

Shoulder or hip width,Thigh length

Largest user: 95th percentile

A good match between the user and the product

Seats,Cycle helmets,Pushchairs

Knee-floor height, Head circumference, Weight

Maximum range: 5th to 95th percentile

A comfortable and safe posture

Lawnmowers,Monitor positions,Worksurface heights

Elbow height,Sitting eye height,Elbow height (sitting or standing?)

Maximum range: 5th to 95th percentile

Easy operation Screw bottle tops,Door handles,Light switches

Grip strength,Hand width,Height

Smallest or weakest user: 5th percentile

To ensure that an item can't be reached or operated

Machine guarding mesh,Distance of railings from hazard

Finger width

Arm length

Smallest user: 5th percentileLargest user: 95th percentile

Sometimes you can't accommodate all your users because there are conflicting solutions to your

design. In this case, you will have to make a judgment about what is the most important feature. You must

never compromise safety though, and if there is a real risk of injury, you may have to use more extreme

percentiles (1%ile or 99%ile or more) to make sure that everyone is protected (not just 95% of people).

It is important to take the strength of your users into account, as well as the environmental conditions

and the space they have to perform tasks.

If you were designing tools for changing car wheels, for example, it's more than likely that they would have

to be used in cold and wet weather. People need to grip harder if their hands are wet and cold, and they need

to exert more force to carry out tasks than they would if they were warm and dry.

You may also need to consider people's eyesight and hearing abilities. Can they read the small labels on the

remote control that you've designed? Is there enough light to read them by? Can they hear an alarm bell

above the general noise in the room?

4.4. PHYISOLOGY

Physiology is the science of how living things work. This subject is of interest to designers so that they can

design products or systems within the limitations of the human body.

A car braking system must be designed in such a way that any driver can easily exert a force on the pedal

and bring the car to rest. Thus information needs to be gathered on the strength of peoples’ legs and then

lever and hydraulic systems designed to suit such forces. The designer must also consider which part of the

body is most suited to performing a specific task. Legs are stronger than arms and are more suited to simple

repetitive tasks involving large forces such as applying the brakes. Hands and fingers are more nimble and

are better suited to finer controls such as a steering wheel or adjusting the volume on a radio.

The shape and size of hand - grips vary tremendously and depend on the tasks that they are used for. The

picture below shows two grips, one is for a gas cooker ring whilst the other is for adjusting the height of an

office chair. The cooker control has a smooth texture and no grip as it is easy to turn and is used for fine

adjustment however the chair adjuster has to be gripped firmly to enable the user to tighten it; therefore it

has been serrated to provide plenty of grips.

To input information into the product, there must be controls that readily interface with the human.

Following figure shows 18 common types of controls and their use characteristics; it also gives dimensional,

force, and recommended use information. Note that the rotary selector switch is recommended for more than

two positions and is rated between ‘acceptable’ and ‘recommended’ for precise adjustment. Thus the rotary

switch is a good choice for the time control of the dryer. Also, for rotary switches with diameters between

30 and 70 mm, the torque to rotate them should be in the range from 0.3 to 0.6 N.m. this is important

information when one is designing or selecting the timing switch mechanism.

Humans often have to supply some force to power a product or actuate its controls. Human force-generation

data are often included with anthropometric data. This information comes from the study of biomechanics

(the mechanics of human body). Listed in above figures is the average human strength for different body

positions. In the data for “arm force standing “we find that the average pushing force 40 inch off the ground

(the average height of the mower handle) is 73 lb, with a note that hand forces of greater that 30 to 40 lb are

fatiguing. Although only averages, these values do give some indication of the maximum forces that should

be used as design requirements.

4.5. PSYCHOLOGY

Psychology is the study of the mind and the way it works. Using your five senses you transmit

information from the world around you to your brain. The brain interprets this information and provokes a

reaction. For example, a sudden loud noise will prompt you to cover your ears with your hands.

All aspects of the environment affect the way you behave i.e. if it is sunny you may feel happy and if it is

cloudy you may feel sad. In the same way a bright room will heighten your senses whereas a dull room will

make you subdued. There are a variety of aspects of product design which will affect your behavior and

having an understanding of how the mind works is important when designing the human/product

interface.

Designers can improve the human/product interface by making a product easy to use. The user must be able

to easily sense important information be it through touch, sight or sound and then react accordingly. For

example the on/off button must be easy to find and symbols for each of the different functions of a product

should be easy to understand.

The shape of a product can also suggest its function and dictate the way in which used - this is called

product semantics. The picture below shows an inkjet printer. The position of the input and output trays,

combined with the rounded form, suggest the path of the paper through the printer.

The display of information especially in a plane cockpit or power station control room presents a challenge

for a designer. Displays showing rates of change such as speedometers can be either digital or analogue. A

digital display is better for accurate measurements when the rate of change is slow whereas an analogue

display is better for showing faster rates of change and giving an overall picture of what is happening. In

practice a combination of both is used.

Look at a typical car dashboard. Identify which displays are analogue and which are digital and try to

explain why they were chosen.

On complex control panels important information such as warnings have to be relayed quickly to the

operator and this is when more than one sense may have to be called on. For instance a flashing light may

not be enough to attract the pilot’s attention in the cockpit so a warning sound may also be necessary.

One recent development in microprocessor technology is the membrane switch panel. These are often

found on products used out of doors such as mobile phones and cash dispensers. One problem with such

panels is that the switches don't move so you are often unsure if you have pressed the button or not. The

solution is to use a bleep which sounds as you press the button thus confirming that it has been pressed. If

one sensation is reinforced by another then you feel as if you have more control over the product or system.

4.6. VISUAL DISPLAYS

In general, when designing controls for interface with humans, it is always best to simplify the structure of

the tasks required to operate the product. Recall the characteristics of the short-term memory of human

beings. We learnt there that humans can deal with only seven unrelated items at a time. Thus, it is important

not to expect the user of any product to remember more than four or five steps. One way to overcome the

need for numerous steps is to give the user mental aids. Office reproducing machines often have a clearly

numbers sequence (symbol display) marked on the parts to show how to clear a paper jam, for example.

In selecting the type of controller, it is important to make the actions required by the system match

the intentions of the human. An obvious example of mis-match would be to design the steering wheel of a

car so that it rotates clockwise for a left turn-opposite to the intention of the driver and inconsistent with the

effect on the system. This is an extreme example; the effect of controls is not always so obvious. It is

important to make sure that people can easily determine the relationship between the intention and the action

and the relationship between the action and the effect of the system.

A product must be designed so that when a person interacts with it, there is only one obviously

correct thing to do. If the action required is ambiguous, the person might or might not do the right thing. The

odds are that many people will not do what was wanted, will make an error, and, as a result, will have a low

opinion on the product.

4.7. CASE STUDIES

4.7.1. DESIGN OF CONTROL PANEL OF A CLOTH DRYER

Most interfaces between humans and machines require human sense the state of the device and based

on the data to control it. Thus products must be designed with importance features readily apparent, and they

must provide for easy control of these features.

Consider the control panel from the clothe dryer. The panel had three controls, each of which is

intended both to actuate two toggle switches. The top switch is a three-position switch that controls the

temperature setting to either “low”, “permanent press”, or “high”. The bottom switch is a two-position

switch that is automatically toggles to off at the end of the cycle or when the dryer door is opened. This

switch must be pushed to start the dryer. The dial on the right controls the time for either the no-heat cycle

(air dry) on the top half of the dial or the heated cycle on the bottom half.

The dryer controls must communicate two functions to the human: temperature setting and time.

Unfortunately, the temperature settings on this panel are hard to sense because the ‘temperature’ rocker

switch does not clearly indicate the status of the setting and the air-dry setting for temperature is on the dial

that can override the setting of the ‘temperature switch’. There are two communication problems in the time

setting also; the difference between the top half of the dial and the bottom half is not clear and the time scale

is the reverse of the traditional clockwise dial. The user must not only sense the time and temperature and

must regulate them through the control. Additionally, there must be a control to turn the dryer on. For this

dryer, the rocker switch does not appear to be the best choice for this function. Finally, the labeling is

confusing.

This control panel is typical of many that are seen every day. The used can figure out what to do and what

information is available, but it takes some conjecturing. The more guessing required to understand the

information and to control the action of the product; the lower perceives quality of the product. If the

controls and labeling were as unclear on a fire extinguisher, for example, it would be all but useless-and

therefore dangerous. There are many ways to communicate the status of a product to a human. Usually the

communication is visual; however, it can also be through tactile or audible signals. The basic types of visual

displays are shown in figure.

When choosing which of the displays to use, it is important to consider the type information that needs to be

communicated. Figure.6 relates five different types of information to the types of displays.

Comparing the clothes-dryer control panel of figure to the information of figure, the temperature control

require only discrete settings and the time control a continuous (but not accurate) value. Since toggle

switches are not very good at displaying information, the top switch of the panel of figure, should be

replaced by any of the displays recommended for discrete information. The use of the dial to communicate

time setting seems satisfactory.

An alternative design of the dryer control panel is shown in figure. The functions of the dryer have been

separated, with the temperature control on one rotary switch. The ‘start’ function, a discrete control action, is

now a button, and the timer switch has been given a single scale and made to rotate clockwise. Additionally,

the labeling is clear and the model number is displayed for easy reference in service calls. In addition, note

that for the rocker switch, no more than two positions are recommended. Thus the top switch on the dryer,

figure.4 is not a good choice for the temperature setting.

SAMPLE QUESTIONS

1. What is ergonomics?2. What are the variables of mankind?3. What are the disciplines that contribute to ergonomics?

CHAPTER 05

HUMAN VALUES IN DESIGN

5.1. INTRODUCTION

Engineering is not only applying scientific laws and principles to technical problems. It is focused on

improving the lot of society, and as such, it brings engineers into the mainstream of business and industry.

Almost all entry-level engineers become involved, at least tangentially, with situations that call for some

understanding of the law and situations that call for ethical judgments. Therefore, this chapter presents a

brief overview of some legal and ethical issues in engineering, with topics as broad as law and ethics we can

only scratch the surface, so we have chosen to focus on those issues that are most pertinent to engineering

design. The followings are examples of where a design engineer might be concerned with legal and ethical

issues:

Preparing a contract to secure the services of a product data management firm

Reviewing a contract to determine whether a contractor who built an automated production facility

has satisfactorily fulfilled the terms of a contract.

Deciding whether it is legal and ethical to reverse engineer a product

Managing a design project to avoid the possibility of a product liability suit.

Protecting the intellectual property created as part of a new product development activity.

Deciding whether to take a job with a direct competitor that is bidding on a contract in the area

where you are not working.

5.2. CONTRACTS

A Contract is a promise by one person to another to do or not to do something. Only promises that the

law will enforce are contracts. The three elements of a contract are: offer + acceptance + consideration.

An offer is an expression made by one person that leads another person to reasonably expect that the

promisor wishes to create an agreement. The offer must be clear, definite, and specific, with no room for

serious misunderstanding. An acceptance of the offer is necessary to make a contract legally binding. Both

the offer and the acceptance must be voluntary acts. A contract cannot be forced on anyone, A contract is

not enforceable by laws unless it contains an agreement to exchange promises with value, the consideration.

For example, if A and B enter into a contract in which A promises to pay B 1000 dollars for modifying a

CAD software package, both the money and the service are considerations.

5.3. TYPES OF CONTRACTS

An express contract is the contract in which all of the terms are agrees upon and expressed in words,

either written or oral. An oral contract, once made, can be just as legal as a written contract, but it is much

more difficult to prove and enforce. Moreover, many states have statutes of fraud that requires writing for

certain contracts to be enforceable.

An implied contract is a contract in which the agreement between parties is enforced by the legal system

wholly or in part by their actions. For example, Jim goes to the local convenience store, where he has an

account. He picks up a Sunday New York times and holds it up so the clerk sees him take it and the clerk

nods in return as he leaves the store with the paper. Jim has made an implied contract to pay for the

newspaper.

A bilateral contract is a contract in which two parties have both made a promise to each other. A promise is

made in return for a promise. Each party is both a promisor and a promise.

A unilateral contract is one in which the promisor does not receive a promise as consideration for her

promise but instead agrees to pay if she receives an act or service. For example, Mrs. Jones says to John

smith, “I promise to pay you 100 dollars tomorrow if you will clean out my basement and garage today”.

John is immediately goes to work. This constitutes acceptance of the offer and creates a unilateral contract.

An Engineer will have to deal with contracts in a number of different situations. Contracts for the purpose or

sale of property are common. On taking a job you may be asked to sign a contract stating that all technical

ideas that you develop belong to the company, even those conceived will not in the job. These contracts are

often negotiable at the time of employment and are something to consider when you are looking for

employment. In technical dealings between companies, one of the parties may be asked to sign a

confidentiality agreement. This is a contract in which one of the parties agrees to not disclose, make use of,

or copy a design or product that the other party is about to disclose.

Types of contracts when there is more than one promisor or promisee

Types of contracts Numbers of parties Liability

Joint Two or more persons promise the

same performance as a single party

All promisors are liable for complete

fulfillment of the contract

Several Separate promises made by more than

one promisor

Each promisor is liable for his or her

individual promise

Joint and several Two or more parties make a joint

contract but also state that they are

individually liable for completion of

contract

All promisors face cumulative liability

5.4. GENERAL FORM CONTRACTS

In general, every business contract should contain the following information:

1. Introduction to the agreement. Includes title and date

2. Name and address of all parties. if one of the parties is a corporation, it should be so stated.

3. Complete details of the agreement. State all promises to be performed. Include such details as

specifications and expected outcomes. Give details on promises of payments, including amounts,

timing of payments, and interest.

4. Include supporting documents such as technical information, drawings, specifications, and

statements of any conditions on which the agreement depends.

5. Time and date of the start of the work and of excepted completion.

6. Terms of payment

7. Damages to be assessed in case of nonperformance. Statement of how disputes are to be arbitrated.

8. Other general provisions of the agreement

9. Final legal wordings. Signature of parties, witnesses, and notary public.

5.5. DISCHARGE AND BREACH OF CONTRACTS

A contract is said to be discharged when the agreement has been performed to the satisfaction of both

parties. The contracting parties can agree at any time that the contract has been discharged. It can be

discharged if it becomes impossible to perform due to circumstances outside the control of the contracting

parties. e.g., force majeure. However, extreme difficulty in executing the contract does not discharge it even

if it becomes more costly to carry out than originally anticipated.

A breach of contract occurs when one party fails to perform his or her part of the contract. A legal injury

is said to have occurred, and the injured party can sue in court for damages. General or compensatory

damages are awarded to make up for the damage that occurred. Special damages are awarded for the direct

financial loss due to the breach.

5.6. LIABILITY

Any party to a contract must be clear on the potential liability he or she is incurring. Liability means

being bound or obligated to pay damages or restitution. Two ways to incur liability are breaking a contract

or committing a tort, such as fraud or negligence.

A breach of contract refers to violating a contract’s promise. Failure to deliver details drawings of a nes

machine by the date specified in the contract is a breach of contract. It makes no difference whether this was

done intentionally or not.

Fraud is intentional deceitful action aimed at depriving another party of his or her rights or causing

injury in some respect. Examples would be doubling billing a client or falsely certifying that a component

has passed ASME pressure vessel code.

Negligence is failure to exercise proper care and provide expertise in accordance with the standards of

the profession that result in damage to property or injury to person. This is the most common way for an

engineer to incur liability to the public. For example, an engineer fails to include a major source of loading

in design calculations for a public product so that the design fails. Note that being honest and well-

intentioned does not absolve the engineer from a legal charge of negligence.

5.7. PRODUCT LIABILITY

Product liability refers to the legal actions by which an injured party seeks to recover damages for

personal injury or property loss from the producer or seller of a product. Product liability suits are pursued

under the laws of tort.

5.8. DESIGN ASPECTS OF PRODUCT LIABILITY

The following aspects of the design process should be emphasized to minimize potential problems from

product liability.

1. Take every precaution to assure that there is strict adherence to industry and government standards.

Conformance to standards does not relieve or protect the manufacturer from liability, but it certainly

lessens the possibility of product defects.

2. All products should be thoroughly tested before being released for sale. An attempt should be made

to identify the possible ways a product can become unsafe and tests should be devised to evaluate

those aspects of the design. When failure modes are discovered, the design should be modified to

remove the potential cause of failure.

3. The finest quality control techniques available will not absolve the manufacturer of a product

liability if in fact the product being marketed is defective. However, the strong emphasis on product

liability has placed renewed emphasis on quality engineering as a way to limit the incidence of

product liability.

4. Make a careful study of the relationships between your product and upstream and downstream

components. You are required to know how malfunctions upstream and downstream of your product

may cause failure to your product. You should warn users of any hazards of foreseeable misuses

based on these system relationships.

5. Documentation of the design, testing, and quality activities can be very important. If there is a

product recall, it is necessary to be able to pinpoint products by serial or lot numbers. If there is a

product reliability suit, the existence of good, complete records will help establish an atmosphere of

competent behavior. Documentation is the single most important factor in winning or losing a

product liability lawsuit.

6. The design of warning labels and users instruction manual should be an integral part of the design

process. The appropriate symbols, color, and size and the precise wording of the label must be

develops after joint meeting of the engineering, legal, marketing, and manufacturing staffs. Use

international warning symbols.

7. Create a means of incorporating legal developments in product liability into the design decision

process. It is particularly important to get legal advice from the product liability angle on innovative

and unfamiliar designs.

8. There should be a formal design review before the product is released for production.

5.9. PROTECTING INTELLECTUAL PRORERTY

The protection of intellectual property by legal means has become a topic of general interest and

international diplomatic negotiations. There are two conflicting motivations for this:

1. Creations of the mind are becoming more valuable in the information age, and

2. Modern information technology makes it easy to transfer and copy such information.

We saw that intellectual property is protected by patents, copyrights, trademarks, and trade secrets. These

entities fall within the area of property law, and as such they can be sold or leased just like other forms of

property.

The functional features of a design can be protected with utility patent. A utility patent protects not only the

specific embodiments of the idea shown in the patent application but functional equivalent as well. A well-

written patent is the best protection for a valuable data. If an idea is worth patenting, it is worth hiring an

experienced patent attorney to do the job well.

A different type of patent, the design patent, covers the ornamental aspects of a product such as its shape,

configuration, or surface decoration. Design patents are easier to obtain than utility patents, and they are

easier to enforce in court. If a competitive design has essentially the same overall appearance, then it is in

violation of your patent. A design patent can have only one claim, which is a serious disadvantage, because

it means that every unique aspect of a product’s design requires a separate patent. This can be expensive.

A copyright has only limited usefulness in protecting product designs. This form of intellectual property is

primarily intended to protect writing.

Trademarks are used to protect the names or symbols (logo) of products. A related from of protection is

known as trade dress. This consists of distinctive features of a product like its color, texture, size, or

configuration. Trademark and trade dress are intended to protect the public about the source of a product-

that is, to protect against cheap ‘ knock-offs’. Trademark protection is achieved by registration with the

patent and trademark office or by actual use of the trade mark in the market place such that it achieves

market recognition. Obviously, it is easier to defend against a competiting trademark if it is registered. A

registered trademark is issued for 20 years and can be renewed every 20 years as long as the product remains

in the market place.

An innovation becomes a trade secret when a company prefers to forgo legal protection for the intellectual

property. The reason for doing this is often a feeling that patents are difficult or costly to defend in the

particular area of technology, or an unwillingness to let the public know what the company is doing. If the

company takes active steps to protect the trade secret, then the courts will protect it as a form of intellectual

property. Process innovations are more often protected by trade secrets than product innovations. Companies

sometimes require nondisclosure agreements from their employees and may attempt to legally prevent an

employee who leaves their employ with sensitive trade knowledge from working for a competitors in order

to protect a trade knowledge from working for a competitor in order to protect a trade secret.

5.10. THE LEGAL AND ETHICAL DOMAINS

We move now from considerations of the law to a discussion of ethics, and how ethical issues affect

the practice of engineering design. Ethics is the principles of conduct that governs the behavior of an

individual or a profession. It provides the framework of the rules of behavior that are moral, fair, and proper

for a true professional. Ethical conduct is behavior desired by society and is above and beyond the minimum

standards of the law.

Quadrant 1, legal and ethical behavior, is where you should strive to operate at all times. Most design and

manufacturing activities fall within this quadrant. Indeed, a good case can be made that quality id dependent

on ethical behavior. ’Doing what is right in the first place and doing what is best for all involved, when done

at every level of the organization and in every work process, has proven to be the most efficient way of

conducting a business.

Quadrant 2, legal and unethical, is the concern of the rest of this chapter. The goal is to explain how to

identify unethical behavior and to learn what to do about it when it occurs. There is a feeling that unethical

behavior in the workplace is increasing because of increasing workplace pressures and changing societal

standards. Most corporations have adopted codes of ethics. Many have established an ethics office and

offering ethics training to their personnel. It is interesting that the prevailing view about ethics instruction

has changed substantially. Throughout most of the 20th century the common view about ethics was that you

either or when you are growing up, it was too late. This is changing today to a view that ethics is a teachable

subject that can be learnt by just about everyone.

Quadrant 3, illegal and unethical, is the sector where ‘go-to-jail’ cards are distributed. In general most

illegal activities are unethical.

Quadrant 4, illegal and ethical, is a relatively rare event. An example could be an engineer who had signed

a secret agreement with an employer, but then found that the employer has been engaged in producing a

product that was very hazardous to the general public. Unable to get attention focused on the problem within

the company, the engineer goes to the press to warn the people. The engineer has breached a contract, but in

what is believed to be a highly ethical cause. Such a person would be called a whistle blower.

5.11. CODE OF ETHICS

We start by making a distinction between morality and professional ethics. Morality refers to those

standards of conduct that apply to all individuals within society rather than only to members of a special

group. These are the standards that every rational person wants every other person to follow and include

standards such as the followings:

Respect the rights of others

Show fairness in your dealings with others

Be honest in all actions

Keep promises and contracts

Consider the welfare of others

Show compassion to others

Note that each of these standards of conduct is based on the italicized values.

5.12. PROFESSIONAL ETHICS

By professional ethics we mean those standards of conduct that every member of a profession expects every

other member to follow. These ethical standards apply to members of that group simply because they are

members of that professional group. Like morality, standards and ethical conduct are value-based. Some

values that are professional ethics include:

Honesty and truth

Honor- showing respect, integrity, and reputation for achievement

Knowledge- gained through education and experience

Efficiency- producing effectively with minimum of unnecessary effort

Diligence- persistent effort

Loyalty- allegiance to employer’s goals

Confidentiality- dependable in safeguarding information

Protecting public safety and health

Note that some of these values are directed toward the employer (eg: diligence), some toward the customer

(e.g: confidentiality), some towards the profession (e.g. honor).

TYPICAL ETHICAL QUESTIONS ASSOCIATED WITH PRODUCT DESIGN

STEPS IN PRODUCT DESIGN POSSIBLE ETHICAL QUESTIONS

Market study Is the study unbiased, or has it been embellished to attract investors or

management support?

Conceptual design Will the product be useful or will it be just a gimmick?

Embodiment design Does the design team have sufficient expertise to properly judge whether

computer programs are giving reliable results? Have any patents been

violated?

Detail design Has checking of results been done?

Manufacturing Is the workplace safe and free of environmental hazards? Is enough time

allowed to do quality work?

Product use Is the product is safe to use? Are users informed of possible hazards?

Retirement from service Has the design allowed for recycle or reuse?

CHAPTER 06

ELECTRICAL MOTORS: DC MOTORS

6.1. INTRODUCTION

In this chapter we will study about how to select a motor to drive a mechanical system. The system may be a

simple fan or a complex printing press or textile machine. Selection of motor consists of:

1. Determining the starting torque of the driven machine

2. Determining running torque as a function of the operating speed of the driven machinery

3. Selecting a motor that can deliver the starting torque and running torque.

Electric motors are classified into:

Alternate current motors (AC motors) where Voltage varies sinusoid

Direct current motors (DC motors) where Voltage is constant

An electric motor is a machine which converts electrical energy into mechanical energy. Its action is based

on the principle that when a current carrying conductor is placed in a magnetic field, it experiences a

mechanical force whose direction is given by “Fleming’s left-hand rule”.

There is no constructional difference between DC generator and DC motor. In fact, the same DC machine

can be used interchangeable as a generator and motor. When a generator is in operation, it develops voltage.

This voltage can be sending a current through a load resistance. When a motor is in operation, it develops

torque. This torque can produce mechanical rotation. DC motors are also like generators classified into shut

wound, series wound and compound wound motors.

6.2. PRINCIPLE OF OPERATION OF DC MOTOR

Figure 6.2 (a) shows a uniform magnetic field in which a straight conductor carrying no current is placed.

The conductor is perpendicular to the direction of the magnetic field.

In figure 6.2 (b), the conductor is shows as carrying a current away from the viewer, but the field due to N

and S poles has been removed. There is no movement of the conductor during the above two conditions. In

figure 6.2 (c) the current carrying conductors is placed in the magnetic field. The field due to the current in

the conductor supports the main field above the conductor, but opposes the main field below the conductor.

The result is to increase the flux density in to the region directly above the conductor and the flux density in

the region directly below the conductor. It is found that a force acts on the conductor, trying to push the

conductor down as shown by the arrow.

Fig.6.2.

If the current in the conductor is reversed, the strengthening of flux lines occurs below the conductor and the

conductor will be pushed upwards.

Now consider a single turn coil carrying a current as shown in figure 6.3. In view of the reasons given below

above the coil side A will be forced to move downwards, whereas the coil side B will be forced to move

upwards. The forces acting on the coil sides A and B will be same magnitude. But their direction is opposite

to one another. As the coil is wound on the armature core which is supported by the bearings, the armature

will now rotate. The commutator periodically reverses the direction of current flow through the armature.

Therefore, the armature will have a continuous rotation.

A simplified model of such motor is shown as below. The conductor are wound over a soft iron core DC

supply is given to the field poles for producing flux. The conductors are connected to the DC supply through

brushes.

Fig.6.3. Simplified version of the DC motor

6.3. TYPES OF DC MOTOR

In the same way as generators DC motors are also classified into three types. This classification is based on

the field wind connection with the armature.

They are:

1. Shunt DC motor: The rotor and stator windings are connected in parallel.

2. Separately Excited motor: The rotor and stator are each connected from a different power supply; this

gives another degree of freedom for controlling the motor over the shunt.

3. Series motor: the stator and rotor windings are connected in series. Thus the torque is proportional to I2 so

it gives the highest torque per current ratio over all other dc motors. It is therefore used in starter motors of

cars and elevator motors.

6. Permanent Magnet (PMDC) motors: The stator is a permanent magnet, so the motor is smaller in size.

Dis adv.: only used for low torque applications.

5. Compound motor: the stator is connected to the rotor through a compound of shunt and series windings, if

the shunt and series windings add up together, the motor is called cumulatively compounded. If they subtract

from each other than a differentially compounded motor results which is unsuitable for any application.

6.4. DISADVANTAGES OF DC MOTORS

1. Brush wear: Since they need brushes to connect the rotor winding. Brush wear occurs, and it increases

dramatically in low‐pressure environment. So they cannot be used in artificial hearts. If used on aircraft, the

brushes would need replacement after one hour of operation.

2. Sparks from the brushes may cause explosion if the environment contains explosive materials.

3. RF noise from the brushes may interfere with nearby TV. sets, or electronic devices, etc.

6.5. DC SERIES MOTOR

The characteristics of these motors are:

1. Variable speed. Speed can be controlled.

2. High starting torque. They offer large starting torque and high torque output per ampere at the expense of

good control.

In DC series motors, the field winding is connected in series with the armature as shown in fig 6.6. The

series field winding carried the input current. The conductors of the series field winding have large cross

sectional area. It has a few numbers of turn per pole. Because of its large cross sectional area and less

numbers of turns, the series field winding has low resistance.

Let Rse = Resistance of serried field; Ra = Resistance of armature; Eb = Back emf induced; The relationship

between V, Eb, and Ia is given below. V = Eb + Ia Ra + Ia Rse

Suited for traction motors in electric and diesel-electric rail road locomotives and underground mine cars

because they can deliver large torque at low speeds as in starting or in pulling a heavy load up an inclined

plane and run at high speed using only the torque required to overcome wind and bearing resistance.

Applications includeelectric locomotives rapid transit systems trolley cars, etc. Cranes and hoist conveyors.

SPEED CONTRONL OF DC SERIES MOTORS

Speed of a DC series motor can be controlled by the following methods

1. Field diverter method

2. Armature diverter method

3. Tapped field control

4. Variable resistance in series with motor

6.6. DC SHUNT MOTOR

Approximately constant speed can be controlled. It has medium starting torque (up to 1.5 full load torque).

In DC shunt motors, the field winding is connected in parallel with the armature as shown in fig.6.9. The

field winding has a large number of turns and relatively smaller cross sectional area. Since the field current

is small the field power loss is also small. The relationship between V, Eb, and Ia is given below.

V = Eb + Ia Ra

Armature current, Ia = IL - If

For driving constant speed line shafting lathes, centrifugal pumps, machine tools, blowers and fans, reciprocating

pumps

SPEED CONTRONL OF DC SHUNT MOTORS

Different ranges of speeds are required for different applications. A single motor can be used for different speeds for

various works. Smooth speed control is possible in DC shunt motors.

The speed of DC motor is directly proportional to the voltage impressed across the armature and inversely

proportional to the flux. Hence the speed of a DC motor can be controlled by varying voltage of flux. The two

methods are known as: Armature control and Field control

6.7. DC COMPOUND MOTOR

In compound motors, both series field and shunt field winding are connected with the armature. The diagram

of connections of long-shunt and short-shunt compound motors is shown in figure.6.16.

In large shunt compound motors, the series field winding is connected in serried with the armature. But in short shunt

compound motors the series field winding is connected in series with the parallel combination of armature and shunt

field windings.

For intermittent high torque loads, for shears and punches, elevators, conveyors, heavy planners, rolling mills, ice

machines, printing presses and compressors

6.8. SPEED TORQUE CURVES FOR DC MOTORS

CHAPTER 07

ELECTRICAL MOTORS: AC MOTORS

7.1. INTRODUCTION

AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of

two basic parts, an outside stationary stator having coils supplied with alternating current to produce a

rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic

field. The rotor magnetic field may be produced by permanent magnets, reluctance saliency, or DC or AC

electrical windings.

Less commonly, linear AC motors operate on similar principles as rotating motors but have their stationary

and moving parts arranged in a straight line configuration, producing linear motion instead of rotation.

7.2. TYPES OF SINGLE PHASE MOTOR

Single phase induction motors

1. Split phase motors

2. Capacitor type motors – Capacitor-start, induction-run motors, capacitor-start, capacitor-run motor

Single phase Commutator type motors

1. Repulsion type motors

2. Shaded pole motors

3. Universal motors

4. Sub merge motors

Three phase induction motors

a. Single squirrel cage induction motor

b. Double squirrel cage induction motor

Induction Motor: So called because voltage is induced in the rotor (thus no need for brushes), but for this to happen,

the rotor must rotate at a lower speed than the magnetic field to allow for the existence of an induced voltage.

Therefore a new term is needed to describe the induction motor: the slip.

Synchronous Motor: So called because rotor tries to line up with the rotating magnetic field in the stator. It has the

stator of an induction motor, and the rotor of a dc motor.

7.3. AC SINGLE PHASE MOTORS

Single phase motors perform a great variety of useful services at home, office, farm, and factory and in business

establishments. Single phase motors are generally manufactured in fractional HP rating below 1 HP for economic

reasons.

Hence those motors are generally referred to as fractional horse power motors with a rating of less than 1 HP. Most

single phase motors fall into this category. Single phase motors are also manufactured in the range 1.5, 2, 3 and up to

large as a special requirement.

A single phase induction motor is similar in construction to that of a poly phase induction motor with a different that

its stator has only one winding. If such a stator is supplied with single phase alternating current, the field produced by

its changes in magnitude and direction sinusoidal.

Such an alternating field is equivalent to two fields of equal magnitude rotating in opposite directions at equal speed

as explained below:

7.4. DOUBLE FIELD THEORY OF SINGLE PHASE INDUCTION MOTOR

Consider two magnetic fields represented by quantities OA and OB of equal magnitude revolving in opposite direction

as shown in the following figure.7.1.

The result of the two fields of equal magnitude rotating in opposite direction is alternating. Therefore an alternating

current can be considered as having two components which are equal in magnitude and rotating in opposite directions.

From the above, it is clear that when a single phase alternating current is supplied to the stator of a single phase motor,

the field will be of alternating nature which can be divided into two components of equal magnitude one revolving in

clockwise and other in counter clockwise direction.

If a stationary squirrel cage rotor is kept in such a field equal forces in opposite direction will act and the rotor will

simply vibrate and there will be no rotation.

But if the rotor is given a small jerk in any direction in this condition, it will go on revolving and will develop torque

in that particular direction. It is clear from the above that a single phase induction motor when having only one

winding is not a self-starting. To make it a self-starting anyone of the following can be adopted

1. By splitting in phase (called as split phase motor)

2. By shading the poles (known as shaded pole motor)

The resultant of the two fields of equal magnitude rotating in opposite direction is alternating. Therefore an

alternating current can be consideres as having two components which are equal in magnitude and rotating

in opposite directions.

From the above it is clear that when a single phase alternating current is supplied to the stator of a single

phase motor, the field produced will be of alternating in nature which can be divided into two components of

equal mangnitude one revolving in clockwise and other in counter clockwise direction.

7.5. SPLIT PHASE INDUCTION MOTOR

PRINCIPLE

The basic principle of operation of a split phase induction motor is similar to that of a polyphase induction

motor. The main difference is that the single phase motor does not produce a rotating magnetic field but

produces only a pulsating field.

Hence to produce the rotating magnetic field for self starting, phase splitting is to be done to make the motor

to work as a two phase motor for starting.

WORKIING OF SPLIR PHASE MOTOR

In split phase motor two windings as main winding and starting winding are provided. At the time of

starting, both the main and starting winding should be connected across the supply to produce the rotating

magnetic field.

The rotor is of a squirrel cage type and the revolving magnetic field sweeps part the stationary rotor,

inducing emf in the rotor. As the rotor bars are short circuited, a current flows through them producing a

magnetic field.

The magnetic field opposes the revolving magnetic field and will combine with the main field to produce a

revolving field. By this action, the rotors startes revolving in the same direction of the rotating magnetic

field as in the case of a squirrelt cage induction motor.

Hence, once the rotor starts rotating, the starting winding can be disconnected from the supply by some

mechanical means as the rotos and stator fields form a revolving field. There are several types of split

motors.

TYPES OF SPLIT PHASE MOTOR

1. Resistance-start, induction-run motors

2. Capacitor-start, induction-run motors

3. Capacitor-start, capacitor-run motors

4. Shaded pole motors

7.5.1. RESISTANCE-START, INDUCTION-RUN MOTOR

As the starting torque of this type of motor is relatively small and its starting current is high, these motors

are most commonly used for rating upto 0.5 HP where the loas could be started easily. The essential parts

are shown below.

Main winding or running winding

Auxiliary winding or starting winding

Squirrel cage type motor

Centrifugal switch

The stating winding is designed to have a higher resistane and lower reactance than the main winding. This

is achieved by using small conductors in the auxiliary winding than main winding. The main winding will

have higher inductance when surrounded by more iron, which could be made possible by placing it deeper

into the stator slots, it si obviously that the current would split as shown in figure.7.2 (b).

The starting current I start will lag the main supply voltage V line by 15 degree and the main winding

current. I main lags the main voltage by 80 degrees. Therefore, these currents will differ in time phase and

their magnitude fields will combine to produce a rotating magnetic field.

When the motor has come up to 75 to 80% of synchronous speed, the starting winding is opened by a

centrifugal switch and the motor will continue to operate as a single phase motor.

At the point where the resisting winding is disconnected, the motor develops nearly as much torque with the

main winding alone as with both windings connected. This can be observed from the typical torque-speed

characteristics of this motor as shown in figure.7.3.

The direction of rotating of a split-phase motor is determined by the way the main and auxiliary windings

are connected. Hence, either by changing the main winding terminals or by changing the starting winding

terminals, the reversal of direction of rotating could be obtained.

APPLICATIONS

These motors are used for driving fans, grinders, washing machines, and wood working tools.

7.5.2. CAPACITOR-START, INDUCTION-RUN MOTOR

A drive which requires a large starting torque may be fitted with a capacitor-start, induction-run motor as it

has excellence starting torque as compared to the resistance-start, induction-run motor.

CONSTRUCTION AND WORKING

The schematic diagram of a capacitor-start, induction-run motor is shown below. As shown, the main

winding is directly connected across the main supply whereas the starting winding is connected across the

main supply through a capacitor and centrifugal switch.

Both these winding are placed in a stator slot at 90 degree electrical apart, and a squirrel cage type rotor is

used.

As shown above, at the time of starting, the current in the main winding lag the supply voltage by 90

degrees, depending upon its inductance and resistance. On the other hand, the current in the starting winding

due to its capacitor will lead the applied voltage by say 20 degrees.

Hence the phase difference between the main and starting winding becomes near to 90 degrees. This in turn

makes the line current to be more or less in phase with its applied voltage, making the power factor to be

high, thereby creating an excellent starting torque.

However, after attaining 75% of the rated speed, the centrifugal switch operates opening the starting

winding and the motor then operates as an induction motor, with only the main winding connected to the

supply.

CHARACTERISTICS

As shown in figure.7.5, the displacement of current in the main and starting winding is about 80/90 degrees

and the power factor angle between the applied voltage and line current is very small.

This results in producing a high power factor and excellent starting torque, several times higher than the

normal running torque as shown in the curve.

REVERSING THE DIRECTION OF ROTATION

In order to reverse the direction of rotation of the capacitor-start, induction-run motor, either the starting or

the main winding terminals should be changed.

This is due to the fact that the direction of rotation depends upon the instantaneous polarities of the main

field flux produced by the starting winding. Therefore, reversing the polarity of one of the field reverse the

torque.

APPLICATIONS

Due to the excellent starting torque and easy directional-reversal characteristics they are used in: 1. Belted

fans; 2. Blower dryers; 3. Washing machines; 4. Pumps and compressors

7.5.3. CAPACITOR-START, CAPACITOR-RUN MOTOR

As discussed earlier one capacitor-start, induction-run motors have excellent starting torque, say about 300%

of the full load torque and their power factor during starting is high.

However, their running torque is not good, and their power factor while running is low. They also have

lesser efficiency and cannot take overloads

These problems are eliminated by the use of a two valve capacitor motor in which one large capacitor of

electrolytic (short duty) type is used for starting whereas a smaller capacitor of oil filled (continuous duty)

type is used for running, by connecting them with the starting winding as shown in fig.7.7. A general view

of such two valve capacitor is shown in fig.7.7.

This motor also works in the same way as a capacitor-start, induction-run motor, with exception, that the

capacitor C1 is always in the circuit, altering the running performanc to a great extent.

The starting capacitor which is of short duty rating will be disconnected from the starting winding with the

help of a centrifiugal switch, when the starting speed attains about 75% of the rated speed.

CHARACTERISTICS

The torque-speed characteristics of this are shown in fig.7.7, this motor has the following advantages:

The starting torque is 300% of the full load torque

The starting current is low, say 2 to 3 times of the running current

Starting and running power factor are good

High efficient running

Extremely noiseless operation

Can be loaded up to 125% of the full load capacity

APPLICATIONS

Used for compressors, refrigerators, air-conditioners, etc

Higher starting torque

Higher efficiency, higher power factor and over loading

Costlier than the capacitor-start, induction-run motors of the same capacity

7.5.4. SHADED POLE MOTOR

The motor consists of a yoke to which salient poles are fitted as shown in fig.7.8 (a) and it has a squirrel

cage type motor.

CONSTRUCTION

A Shaded pole made of laminated sheets has a slot cut across the lamination at about one third the distances

from the edge of the pole. Around the smaller portion of the pole, a short-circuited copper ring is placed

which is called the shading coil and this part of the pole is known as the shaded part of the pole. The

remaining part of the pole is called the un-shaded part which is clearly shown in fig.7.8 (b).

Around the poles, exciting coils are placed to which an AC supply is connected. When AC supply is affected

to the exciting coil, the magnetic axis shifts from the un-shaded part of the pole to the shaded part as will be

explained in details in the next paragraph. This shifting of axis is equivalent to the physical movement of the

pole.

This magnetic axis, which is moving, cuts the rotor conductors and hence, a rotational torque is developed in

the rotor.

By this torque the rotor starts rotating in the direction of the shifting of the magnetic axis that is from the un-

shaded part to the shaded part.

As the shaded coil is of thick copper, it will have very low resistance but as it is embedded in the iron case,

it will have inductance. When the exciting winding is connected to an AC supply, a sine wave current passes

through it.

Let us consider the positive half cycle of the AC current as shown in figure.7. 9.

When the current rises from zero value of point to a point a the change in current is very rapid. Hence, it

reduces an e.m.f in the shaded coil on the basis of Faraday’s law of electromagnetic induction.

The induced emf in the shaded coil produces a current which, in turn, produces a flux in accordance with

Lenz law. This induced flux opposes the main flux in the shaded portion and reduces the main flux in that

area to a minimum value as shown in figure.7.9. This makes the magnetic axis to be in the centre of the un-

shaded portion as shown by the arrow in part of figure.7.9. On the other hand as shown in part 2 of 3 when

the current rises from a point a to point b the change in current is slow the induced emf and resulting current

in the shading coil is minimum and the main flux is able to pass through the shaded portion.

This makes the magnetic axis to be shifted to the centre of the whole pole as shown in by the arrow in part 2

of figure.7.9.

In the next instant, as shown in part 3 of figure.7.9, when the current falls from b to c the change in current

is fast but the change of current is from maximum to minimum.

Hence the large current is induced in the shading ring which opposes the diminishing main flux, thereby

increasing the flux density in the area of the shaded part. This makes the magnetic axis to shift to the right

portion of the shaded part as shown by the arrow in part.

From the above explanation it is clear the magnetic axis shifts from the un-shaded part to the shaded part

which is more or less a physical rotary movement of the poles.

Simple motors of this type cannot be reversed. Specially designed shaded pole motors have been constructed

for reversing operations. Two such types:

a. The double set of shaded coils method

b. The double set of exciting winding method

Shaded pole motors are built commercially in very small sizes, varying approximately from 1/250 HP to 1/6 HP.

Although such motors are simple in construction and cheap, there are certain disadvantages with these motors as

stated below:

Low starting torque, very little overload capacity, and low efficiency.

APPLICATIONS: Record players, Fans and Hair driers.

7.6. COMMUTATOR TYPE SINGLE PHASE MOTOR

This type of motors have a wound rotor with brush and commutator arrangement like a DC motor. They

consists of two classes, namely, those opearting on the principle of repulsion and those operating on the

principle of series motors.

7.6.1. REPULSION MOTOR

Repulsion motors, though complicated in conmstruction and higher in cost, are still used in certain industries

due to their excellent starting torque, low starting current, ability to withstand long spell of srarting currents

to drive heavy loads and their easy method of revesal of direction.

Now there is a condition that the rotor north pole will be repelled by the main north pole and the rotor south

pole is repelled by the main south pole, so that a torque could be developed in the rotor. Now due to

repulsion action between the stator and the rotor poles, the rotor will start rotating in a clock wise direction.

As the motor torque is due to repulsion action, this motor is named as repulsion motor.

DIRECTION OF ROTATION

To change the direction of rotation of this motor the brush axis needs to be shifted from the right side as

shown in figure.7.11 to the left side of the main axis in a counter clockwise direction.

CHARACTERISTICS

As explained earlied, the torque developed in a repulsion motor will depend upon the amount of brush shaft

as shown in figure.7.11, whereas the direction of shift decides the direction of rotation.

Further the speed also depends upon the amount of brush shift and the magnitude of the load. Relationship

betweem the torque and brush position angle in a repulsion motor.

Though the starting torque from 250 to 400% of the full load torque, the speed will be dangerously high

during light loads.

This is due to the fact that the speed of the repulsion motor does not depend on frequency or number of

poles but depends upon the repulsion principle.

Further, there is a tendency of the sparking in the brush at heavy loads, and the PF will be poor at low

speeds. Hence, the conventional repulsion motor is not much and the other three improved types are popular.

7.6.2. UNIVERSAL MOTOR (SERIES MOTOR)

It is also commutator type motor. A universal motor is one which operates both on AC and DC supplies. It

develops more horsepower per kg weight that any other AC motor mainly due to its high speed.

The principle of operation is same as that of a DC motor. Though a universal motor resembles a DC series

motor it required suitable modification in the construction, winding and brush grade to achieve sparkles

commutation and reduces heating when operated on AC supply, due to increases inductance and armature

reaction.

A universal motor could therefore be defined as a series or a compensated series motor designed to operate

approximately the same speed and output at either direct current or single phase alternating current of a

frequency not greater than 50 Hz, and of approximately the same RMS voltage. Universal motors are also

named as AC single phase series motor.

The main parts of a universal motor are an armature, field winding, stator stamping, frame and plates and

brushed. The increased sparking at the brush position in AC operation is reduced by the following means:

Providing commutating inter poles in the stator and connecting the inter pole winding in series with the

armature winding. Providing high contact resistance brushed to reduce sparking at brush positions.

OPERATION

A universal motor works on the same principles as a DC motor. i.e, force is created on the armature

conductors due to the interaction between the main field flux and the flux created by the current carrying

armature conductors. A universal motor develops unidirectional torque regardless of whether it operated on

AC or DC supply.

Figure.7.12 shows the operation of a universal motor on AC supply. In AC operation, both field and

armature current changes their polarities, at the same time resulting in unidirectional torque.

CHARACTERISTICS

The speed of a universal motor is inversely proportional to the load. i.e, speed is low at full load and high on

no load.

The speed reaches a dangerously high value due to low field flux at no loads in fact the no load speed is

limited only by its own friction and windage losses. As such these motors are connected with permanent

loads or gear trains to avoid running at no load thereby avoiding high speeds.

Figure.7.13 shows the typical torque-speed relation of a universal motor, both for AC and DC operations.

This motor develops about 450% of full load torque at starting, as such higher than that of any other type of

single phase motor. Universal motors are used in vacuum cleaners, food mixers, portable drills and domestic

sewage machines.

CHANGE OF ROTATION

Direction of rotation of a universal motor can be reversed by reversing the flow of current through either the

armature or the field windings. It is easy to interchange the leads at the brush holders.

However, when the armature terminals are interchanged in a universal motor having compensating winding,

care should be taken to interchange the compensating winding also to avoid heavy sparking while running.

7.7. AC THREE PHASE INDUCTION MOTORS

The most common type of AC motor being used throughout the work today is the induction motor.

Application of three-phase induction motor of size varying from half a kilowatt to thousands of kilowatts are

numerous, They are found everywhere from a small workshop to a large manufacturing industry.

The advantages of three-phase AC induction motor are listed below:

Simple design

Rugged construction

Reliable operation

Low initial cost

Easy operation and simple maintenance

Simple gear control for starting and speed control

High efficiency

Induction motor is originated in the year 1891 with crude construction. Then an improved construction with

distributed stator winding and a cage rotor was built.

The slip ring rotor was developed after a decade or so. Since then a lot of improvement has taken place on

the design of these two types of induction motors. Lots of research work has been carried out to improve its

power factor and to achieve suitable methods of speed control.

PRINCIPLES OF 3 PHASE INDUCTION MOTOR

Induction motors work on the same principle as a DC motor, that is, the current carrying conductors kept in

magnetic field will tend to create a force.

However the induction motor differs from the DC motor in the fact that the rotor of the induction motor is

not electrically connected to the stator, but induces a voltage/current in the rotor by the transformer action,

as stator magnetic field sweeps across the rotor.

The induction motor derives its name from the fact that the current in the rotor is not drawn directly from the

supply, but is induced by the relative motion of the rotor conductors and the magnetic field produced by the

stator currents.

The stator of the three-phase induction motor is similar to that of a 3 phase alternator, a revolving field type.

If a three phase supply is connected to the three phase winding in the stator that produces rotating magnetic

field in the stator core. The rotor of the induction motor may have either shorted rotor conductors in the form

of squirrel cage or in the form of a three phase winding to facilitate the circulation of current through a

closed circuit.

Let us assume that the stator field of the induction motor is rotating in a clock-wise direction as shown in

figure.7.14. This makes the relative motion of the rotor in an anti-clockwise direction as shown in

figure.7.14 (b).

Applying Fleming’s right hand rule, the direction of emf induced in the rotor will be towards the observer as

shown in figure.7.17. As the rotor conductor have a closed electric path, due to their shorting a current will

flow through them as in a short circuited secondary of a transformer. The magnetic field produced by the

rotor current will be in counter-clockwise direction as shown in figure.7.17.

According to Maxwell’s corkscrew rule, the interaction between the stator magnetic field and the rotor

magnetic field results in a force to move the rotor in the same direction as that of the rotating magnetic field

of the stator as shown in figure.7.17. As such of the rotor follows the stator field in the same direction by

rotating at a speed lesser than the synchronous speed of the stator rotating field.

Al higher speeds of the rotor nearing to synchronous speeds, the relative speed between the rotor and the

rotating magnetic field of the stator reduces and results in a smaller induced emf in the rotor. Theoretically,

if we assume that the rotor attains a speed equal to the synchronous speed of the rotating magnetic field of

the stator field and the rotor and thereby no induced emf or current will be there in the rotor.

Consequently, there will not be any torque in the rotor. Hence, the rotor of the induction motor cannot run at

a synchronous speed at all. As the motor is loaded, the motor speed has to fall to cope up with the

mechanical force, thereby the relative speed increased, and the induced emf and current increases in the

rotor resulting in an increased torque.

ROTATING MAGNETIC FIELD

A Magnetic field is that which rotates in space at synchronous speed, inside an induction motor starter.

ROTATING MAGNETIC FIELD FROM A 3 PHASE STATOR

The operation of the induction motor is dependent on the presence of a rotating magnetic field in the stator.

The stator of the induction motor contains 3 phase windings placed at 120 degree electrical apart from each

other. These windings are placed on the stator core to form non-salient stator field pole when the stator is

energized from the three phase voltage supply each phase winding will step up a pulsating field, however,

by virtue of the spacing between and the windings and the magnetic fields combine to produce a field

rotating at a constant speed around the inside surface of the stator core. This resultant movement of the flux

is called “rotating magnetic field” and its speed is called the “synchronous speed”.

The manner is which the rotating field is setup by may be described by considering the direction of the

phase currents at successive instants during a cycle.

Figure.7.17 shows a simplified star-connected, three phase stator winding. The winding shown is for a two-

pole induction motor. Figure.7.17 (b) shows the phase current for the three phase windings.

The phase current will be 1200 apart as shown in figure.7.17 (b). The resultant magnetic field produces the

combined effect of the three currents is shown at increments of 600 for the cycle of the current.

CONSTRUCTIONAL DETAILS OF INDUCTION MOTOR

Three phase induction motors are constructed into two major types:

1. Squirrel cage induction motors

2. Slip ring induction motors

7.7.1. SQUIRREL CAGE MOTOR

A) Stator construction

The induction stator resembles the stator of a revolving field, three phase alternators. The stator or the

stationary part consist of three phase winding held in place in the slots of a laminated steel core which is

enclosed and supported by a cast iron or steel frame as shown in figure.7.18.

The phase windings are placed 1200 apart and may be connected in either star or delta externally, for which

six leads are brought out to a terminal box mounted on the frame of the motor. When the stator is energized

from a three phase voltage it will produce a rotating magnetic field in the stator core.

ROTOR OF A SQUIRREL CAGE INDUCTION MOTOR

The rotor of the squirrel cage motor shown in figure.7.19 consists no windings. Instead it is a cylindrical

core constructed of steel laminations with conductor bars mounted parallel to the shaft and embedded near

the surface of the rotor core.

These conductor bar are short circuited by an end rings at both end of the rotor core. In large machines, these

conductor bars and the end rings are made up of copper with the bars brazed or welded to the end rings

shown in figure.7.19.

In small machines the conductor bars end rings are sometimes made of aluminium with the bars and rings

cast in as part of the rotor care.

The rotor or rotating part is not connected electrically to the power supply but has voltage induced in it by

transformer action from the stator.

For this reason, the stator is sometimes called the primary and the rotor is referred to as the secondary of the

motor since the motor operates on the principle of induction and as the construction of the rotor with the

bars and end rings resembles a squirrel cage, the squirrel cage induction motor is used.

The rotor bars are not insulated from the rotor core because they are made of metals having less resistance

than the core. The induced current will flow mainly in them. Also the rotor bars are usually not quite parallel

to the rotor shaft but are mounted in a slightly skewed position. This feature tends to produce a more

uniform rotor field and torque. Also it helps to reduce some of the internal magnetic noise when the rotor is

running.

The function of the two end shield is to support the rotor shaft. They are fitted with bearings and attached to

the stator frame with the help of the studs or bolts action.

7.7.2. SLIP RING INDUCTION MOTOR

A. Stator construction

The construction of the slip ring induction motor is exactly similar to the construction of squirrel cage

induction motor. There is no difference between squirrel cage and slip ring motors.

B. Rotor construction

The rotor of the slip ring induction motor is also cylindrical or constructed of lamination. Squirrel cage

motors have a rotor with short circuited bars whereas slip ring motors have wound rotors having ‘three

windings’ each connected in star.

The winding is made of copper wire. The terminals of the rotor windings of the slip ring motors are brought

out through slip rings which are in contact with stationary brushes as shown in figure.7.20.

The advantages of the slip ring motor are:

It has susceptibility to speed control by regulating rotor resistance

High starting torque of 200 to 250% of full load value

Low starting current of the rotor of the order of 250 to 350% of the full current

Hence slip ring motors are used where one or more of the above requirements are to be met.

COMPARISON OF SQUIRREL CAGE AND SLIP RING MOTOR

S. No property

Squirrel cage motor Slip ring motor

1 Rotor construction

Bars are used in rotor. Squirrel cage motor is very simple, rugged and long lasting. No slip rings and gear need frequent maintenance

Winding wire is to be used

2 Starting Can be started by DOL, star delta, auto transformer starters

Rotor resistance started is required

3 Starting torque

Low Very high

4. Starting current

High Low

7. Speed variation

Not easy, but could be varied in large steps by pole changing or through smaller incremental steps through thyristors or by frequency variation

Easy to vary speed but speed change with change in pole changing is not possible. Speed change is possible by inserting rotor resistance using thyristors or by using frequency variation injecting emf in the rotor circuit cascading

7. Acceleration on load

Just satisfactory Very good

7. Maintenance Almost nil maintenance Requires frequent maintenance

8. Cost Low

7.7.3. DOUBLE SQUIRREL CAGE MOTOR

In order to overcome the disadvantages of the cage motor, and to avoid having to the use the more expensive slip ring motor and its associated gear, increasing attention is being given to the use of the double cage rotor is increased temporarily while starting.

The double cage rotor in its simple form consists of two separate cages. The outer or starting cage is made of high

resistance material and is arranged to have the smallest possible reactance.

The inner cage is of the ordinary low resistance type, and since it is sunk deep into the iron, has a high reactance of the

inner and outer cage can be varied in an indefinite number of combinations and many shapes of speed torque curve

can be obtained.

At starting, the frequency of the currents in the rotor conductors is the same as the supply frequency. Thus the high

reactance of the inner cage produces a choking and reduces the current flowing in this winding.

Most of the starting current is confined to the outer cage despites its high resistance. The outer cage being of high

resistance develops a high starting torque depending largely on the value of its resistance.

A punching of such a double cage rotor lamination is shown in figure.7.21. As the rotor speed increases and

approached synchronism, the frequency of the e.m.f on its conductor falls and the choking effect in the inner cage is

reduced.

The inner cage now carried practically all the current until finally the rotor operates with the characteristics of an

ordinary low-resistance rotor. The general result is to provide a machine having a high starting torque and a high

running efficiency, with reasonably small value of starting current.

7.8. MOTOR SELECTION: SPEED - TORQUE CURVES FOR AC MOTORS

Most industrial motors of one horse power or larger are three-phase motors because they are similar and

cheaper than single-phase motor of equal power. They range in capacity from less than one horse power to

10000 hp. In spite of their size advantage, fractional horse power three-phase motors are not used in

appliances and homework equipment because of added expenses of changing from single phase to three-

phase house wiring.

These motors are classified into: A, B, C, D, and F on the basis of their speed-torque characteristics.

Universal motors are series-wounded motors which operate at about the same speed and power on direct

current or single-phase alternating current at approximately the same root mean square voltage. They have

commutators or compensated series-wound.

Determining the power required to drive a machine is only the first step in sizing the motor. The next step is

to examine the motor performance curves to see if the motor has enough starting torque to overcome

machine static friction, to accelerate the load to full running speed, and to handle maximum over load.

Basic motor speed performance information is contained in the speed vs torque curve. Speed-torque curves

for both single-phase and poly phase AC motors are of general form as shown in Fig. It shows how the

output torque varies as speed increases from zero to synchronous speed with rated voltage and frequency

applied to the motor.

Some of the common terminology used in describing the AC motor characteristics is included.

Capacitor- start induction–run motors Gives a high starting torque which continues until the operating speed is attained

Permanent-split capacitor motors Slightly higher starting torque, equal full load torque, and slightly higher efficiency

Repulsion-start induction motor Similar to that of capacitor-start type except that the torque drops off faster as the design speed in reached.

Split-phase motors Relatively smaller starting torque, they still have adequate starting torque for many applications.

Shaded-pole motors Least expensive type and at the low end of the starting torque range. They are used for fans, blowers, and similar easily started equipment.

7.9. SPEED-TORQUE CURVES FOR THREE-PHASE MOTORS

DESIGN A Starting torque and break down (or pull up) torque is larger. They are capable of greater load acceleration.

Normal starting torque motor

DESIGN B Standard, general purpose, with low starting current, normal torque, and normal slip. Normal starting torque motor

DESIGN C Larger break away, starting torque, low starting current, and low slip

Plunger pumps, piston compressors, and conveyors which are hard to start

DESIGN D Large break away torque and high slip.

Punch presses, shears, oil well pumps, and similar high-inertia machinery where the rotor speed fluctuates, or slip, during working cycle. They are also used in multi motor drives that operate in parallel mechanically as on conveyors and draglines.

DESIGN F Rarely used because of low starting torque. Low slip and low pull-up torque.

Low starting current justifies their use in remote places or on small ships

Centrifugal pumps, blowers, and similar low inertia easily started machines when used without clutch. Machines with slightly greater inertia may be driven using a clutch..

7.10. SPEED-TORQUE CURVES FOR SINGLE PHASE SHADED POLE MOTORS

Single-phase shaded-pole motors have relatively low efficiency and low starting torque. These motors are

used for light duty appliances such as direct drive fans, phonograph turn tables, and blowers.

7.11. SPEED-TORQUE CURVES FOR CAPACITOR START MOTORS

These motors have greatest starting torque than shaded-pole motors but require a centrifugal switch to

remove the capacitor when 65 to 70% of the synchronous speed is achieved.

7.12. SPEED-TORQUE CURVES FOR REPULSION-START MOTORS

Repulsion-start motors are also called as universal motors. They are characterized by moderate to large

starting torque and use a centrifugal switch to short-circuit the commutator at about 75% of the synchronous

speed.

Repulsion-start motors have high starting torque so they may be preferred for applications that requiring

starting torque at full load, such as conveyors and stokers.

(A)Capacitor motor (B) Two value capacitor (C) Permanent split capacitor induction motor

7.13. SPEED-TORQUE CURVES FOR SPLIT-PHASE MOTORS

These motors are generally considered to be low to moderate starting torque motors as illustrated below.

Starting torque would be zero if it were not for the auxiliary windings which cancels a phase shift and

thereby provides the starting torque. Because this winding may seriously overheat if used for more than a

few seconds these motors are limited to about 1/3 hp to small appliances and office machines that require

only short acceleration times.

Repulsion-start induction motor

Repulsion-induction motor

Split-phase induction motor

7.14. SPEED-TORQUE CURVES FOR UNIVERSAL MOTORS

Small universal motors, which can operate either AC or DC power, are widely used in small machines, such

as vacuum cleaners, sewing machines, food mixers, electric drillers and hedge trimmers.

Universal motors at 60 Hz, 25 Hz, and DC

Large universal motors are not constant speed motors because the speed increases as the torque decreases.

Consequently they cannot be used where part failure, such as breakage of belt could cause the motor to lose

its load and over speed.

7.15. SPEED-TORQUE CURVES FOR SYNCHRONOUS MOTORS

Synchronous motors with zero slip are available for either poly phase or single supply. All of them operate

at precisely the frequency of the supply voltage as long as their torque limitations are not exceeded. They

require more maintenance than induction motors because of commutators and brush wear. Other

characteristics depend upon the power range and supply.

Two types of synchronous motors are reluctance type and hysteresis type

7.18. SPEED-TORQUE CURVES FOR HYSTERESIS MOTOR

These motors have common application in electric clock motors.

7.19. SPEED-TORQUE CURVES FOR RELUCTANCCE MOTOR

Although speed-torque curves discussed here may be obtained from motor manufacturers, they are not the

curves usually supplied to the buyers. The more common curves are the so called performance curves which

shows the speed in rpm, the efficiency, the current in amperes, the power factor, the kilowatts required, and

the torque as a function of horsepower.

7.20. SPEED CONTROL CURVES FOR DRIVEN MACHINES

Proper matching of motors to a driven machine can be accomplished only after the speed-torque curve of the

driven machine is known. Unfortunately, many manufactures have not conducted experiments to find the

speed-torque curves for their product. In the absence of this information, an estimate of the speed-torque

curve may be obtained using data at one point, such as the rated speed at full torque, and placing a typical

curve for machines of this type through that point.

7.21. MOTOR SELECTION: MATHCING THE MOTOR TO THE DRIVEN MACHINE

As noted earlier, choosing an electric motor to drive a particular machine is largely a matter of comparing

their speed-torque curves to assure that adequate torque is available over the operating speed of the machine.

Two factors are of particular importance

1. The capability of motor to handle changes in the load without danger to the motor itself

2. Its ability to accelerate the load up to its accelerating speed, or speeds, in a reasonable amount of

time.

1. Selection of motor for an industrial floor buffer

It is necessary to use a motor with a large starting torque, such as represented by curve A (super imposed

here) for a capacitor start motor.

As indicated by the above figure, the torque available for accelerating to operating speed during start-up is

the difference between the two curves and that the power required is proportional to the area between them.

Suppose a type C motor is selected, the combination will result in buffer that would never start unless lifted

from floor because the initial torque provided by the motor is less than the torque required to rotate the unit,

even without considering that the static friction is less than the dynamic friction.

2. Selection of motor for an industrial fan

Motor with curve C is suitable for driving a fan because of lower initial torque and will definitely gives

better performance at operating speed because of its greater torque at the higher speed range. In figure curve

A represents the speed-torque for the induction motor and curve B represents the speed-torque curve for the

fan.

Transformation of power vs. depth of cut data to torque-speed requirements

SAMPLE QUESTIONS

1. How do self-start a single phase motor?

2. Explain the working of a split phase induction motor? Write the applications

3. What are the types of split phase motors? Draw the torque-speed curve of this motor.

4. Explain the construction, working and applications of capacitor-start induction run motor.

5. Draw the characteristics curve of a capacitor-start induction run motor and explain it.

6. Explain the construction, working and applications of capacitor-start induction run motor.

7. Draw the characteristics curve of a capacitor-start capacitor run motor and explain it.

8. Explain the construction, working and applications of shaded pole motor.

9. Draw the characteristics curve of a capacitor-start capacitor run motor and explain it.

10. Write briefly about repulsion motor.

11. Explain the construction, working and applications of universal motor.

12. Draw the characteristics curve of a universal motor and explain it.

13. What are the advantages of three phase induction motor?

14. Explain the construction, working and applications of squirrel cage induction motor.

15. Explain the construction, working and applications of slip ring induction motor.

16. Write the comparison of squirrel cage and slip ring motor.

17. Write in details about speed-torque curves for AC motors.

18. Write in detail about speed-torque curves for driven machines.

19. Write in detail about selection of motors for driven machines.