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CHAPTER 10:

Springs

Rev: 2_Jan2011

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Springs

Springs are mechanical members, which

are designed to give a relatively large

amount of elastic deflection under the

action of an externally applied load.

Most springs are made of steel, although

phosphor bronze, silicon bronze, brass

and beryllium copper are also used.

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Springs are universally made by companies that

specialize in their manufacture.

The helical coil spring is the most popular type ofspring; torsion bars and leaf springs are also

widely used

Many different types of special springs areavailable to satisfy unusual requirements or

applications. These include Belleville springs,volute springs, constant force springs, powersprings, and garter springs.

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Purposes and Applications of springs

Control of Motion in Machines.

- such as providing operating forces in clutches and brakes

- used to maintain contact between two members such as a camand its follower, keys in typewriters

Reduction of Transmitted Forces as a Result of Impact or

Shock Loading.- Applications here include automotive suspension system

springs and bumper springs.

Storage of Energy.

- found in clocks, movie cameras, and lawn mowers having

recoil starters. Many parking meters use spring mechanismsthat perform the timing function.

Measurement of Force.

- Scales used to weigh people is a very common application forthis category.

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Types of ends used on

compression coil springs

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(a.) Plain End

least expensive to produce

tends to bow sideways under load

resulting in increased stresses on one side ofthe spring

(b.) Plain-and-Ground End

will seat against its mating machinery betterthan the plain end coil spring.

during storage, handling, and assembly, theends tend to become tangled.

leads to time-consuming sorting

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(c.) Squared End

does not tend to become tangled during themanufacturing process as readily as the plain-and-ground end

(d.) Squared-and-Ground End

enjoys the advantage of not becoming readilytangled during manufacturing

in use the load is transmitted in a perfectlyaxial direction (no sideways bowing).

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Spring Terminology

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Spring Terminology

(a) Outside Diameter, Do.

The outside diameterDo is the maximum diameter of the coilspring. This parameter is important when radial space isrestricted.

(b) Inside Diameter, Di.

The inside diameterDiis the minimum diameter of the coilspring. This parameter is of concern when a part such as a pinmust fit inside the spring.

(c) Mean Diameter, D.

The mean diameterD is the average diameter of the coil spring.

Stress and deflection analysis depends on the mean diameter.(d) Wire Diameter, d

This parameterdrefers to the diameter of the wire that is woundto create the coils.

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(e) Free Height, L

The term free height L refers to the overall length of the spring inthe free or unloaded configuration.

(f) Solid Height Lo.

The term solid height Lo refers to the length of the spring whenall the coils are touching.

(g) Deflection . The deflection refers to the change in length of a spring

resulting from an applied load.

(h) Direction of Coiling.

This term refers to the direction in which the coils are wound:either right-hand or left-hand. The springs of Figure 11.2 are

coiled right-hand.

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(i) Spring Index C.

The spring index Crefers to the ratio of the coil mean diameter

to the coil wire diameter: D/d.

(j) Active Number of Turns n.

This term refers to the effective number of coil turns supportingthe external load. It equals the total number of actual turns minus

the inactive turns at the ends.

Note:

When space is limited radially or when the maximum solidheight is restricted, the use of square or rectangular cross

sections permits greater energy storage in less space.

However, the use of square or rectangular wire should be

avoided when possible because of cost.

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Basic Spring Calculation

Shear Stress

38

dKPD

factorWahlCC

CK

615.0

)44()14(

xSpringInded

DC

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Wahl Factor K versus the Spring Index C = D/d.

As can be seen, the value of K becomes significant when

the Spring Index C is less than 18

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Spring Deflection

4

38

Gd

nPD

For steel G = 12 x 10 6 lb / in 2

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Linear relationship between P and

nC

Gd

nD

GdPk

33

4

88

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Sample Problem 1

Helical Coil Spring Stress

A helical coil spring has a 1 inch diameter wire and a mean coildiameter of 4 inches. The spring supports a 2000 lb load. Find the

maximum shear stress.

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Sample Problem 2

Spring Deflection

A compression coil spring must foreshorten 5 inches under a 50 lbload. The following parameters are known.

- Spring Index, C = 8

- G = 12 x 10 6 lb/in2

- max = 80 000 lb/in2 Find the number of active coil, n ?

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Other Common Spring Configuration

(a.) Leaf Spring

Originally called laminated or carriage spring

A simple form of spring, commonly used for the

suspension in wheeled vehicles

can be made from several leaves stacked on top ofeach other in several layers, often with systematically

shorter leaves.

can serve locating and to some extent damping as well

as springing functions.

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(b.) Helical Torsion Springs

widely used in door and cover hinges, automotive

starters and in various electrical mechanisms.

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(c.) Belleville Springs

a conical spring washer

have applications in buffers and cushions, where high

loads and small deflections are required. When a load is applied, the conical washer tends to

flatten out.

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(d.) Conical Springs special versions of the conventional compression coil spring.

can be designed so that each coil can fit into the adjacent larger

coil as shown

the solid height of a conical spring can be as small as one wire

diameter have an increasing stiffness as each turn becomes successively

inactive during compression.

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(e.) Volute springs operate like conical compression springs

the volute spring consists of tapered metallic strips wound in such

a way that each turn telescopes inside the preceding one

Since each coil contacts the preceding one, sliding friction occurs,which provides vibration dampening.

The solid height of a volute spring equals essentially the width of

the metallic strip.

As in the case of the conical compression spring, volute springs

provide an increasing stiffness as compression progresses

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(f.) Constant-Force Springs

one whose force does not change as it is extended

has a spring rate equal to zero

The spring metal is pressurised in a special way to maintain aconstant output force throughout its entire deflection range.

provides a much greater deflection than does a conventional

extension coil spring

Application :

- Feeding devices for vending machines

- Control of force of follower roller on a cam

- Powering of mechanical movie cameras, provide smooth

operation and longer running times per winding.

- Retracting of hose reels and electrical cords- Driving of toys as a long-running constant torque power source

- Electric motor brush springs

- Window or door counterbalances

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(g.) Power Springs

commonly called clock springs, store energy after winding and

then release the energy to provide torque through an output shaft

or drum Application :

- Seat belt retracting mechanisms

- Ordinary clock springs

- Power lawn mowers having rewind starters

- Timing mechanisms in parking meters

- Energy drive source for toys

- Hose retrieval reels on gasoline pumps

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(h.) Garter Springs

are coil extension or compression springs whose ends are

connected so that the spring forms a complete circle, or garter

garter springs exert radial forces similar to a rubber band common application is in oil seals where the inside diameter of

the spring is expanded to fit over a larger-diameter shaft. In this

way, the spring is actually stretched, and therefore it behaves as

an extension coil spring. As a result, a compressive radial force in

exerted on the shaft.

Other applications include small motor belts, electrical connectors

and piston ring expanders.

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