Omsakthi

Department of Mechanical Engineering

Department of Mechanical Engineering

Question Bank

Subject Code / Subject : ME2203 / Kinematics of Machinery

Year / Sem : II / III

Question Bank consists of Part A

UNIT – 1 BASICS OF MECHANISMS

1. Differentiate between Machine and Mechanism.

Machine

Mechanism

Machine is a mechanism or collection of mechanism which transmits force from the source of power to the resistance (load) to overcome and thus performs useful mechanical work.

Combination of rigid or resistant bodies connected that they move upon each other with definite relative motion

E.g Lathe, Shaping Machine etc.

E.g single slider mechanism in IC engine

All machines are mechanism

All mechanisms are not Machine

2. Write down Kutzbach criterion to find the mobility of a planar mechanism.

The Kutzbach criteria which calculates the mobility.

F = 3(n – 1) – 2j

Where, F- Degrees of freedom

n – number of links

J – number of joints

3. Define transmission angle and its significance.

The acute angle between the coupler and the driven link.

In Figure , if AB is the input link, the force applied to the output link, CD, is transmitted

through the coupler link BC. (That is, pushing on the link CD imposes a force on the link AB, which is transmitted through the link BC.) For sufficiently slow motions (negligible inertia forces), the force in the coupler link is pure tension or compression (negligible bending action) and is directed along BC. For a given force in the coupler link, the torque transmitted to the output bar (about point D) is maximum when the angle between coupler bar BC and output bar CD is /2. Therefore, angle BCD is called transmission angle.

(5-3)

Figure - Transmission angle

When the transmission angle deviates significantly from /2, the torque on the output bar decreases and may not be sufficient to overcome the friction in the system. For this reason, the deviation angle =|/2-| should not be too great. In practice, there is no definite upper limit for , because the existence of the inertia forces may eliminate the undesirable force relationships that is present under static conditions. Nevertheless, the following criterion can be followed.

4. Enumerate the difference between a Machine and a Structure.

Machine

Structure

Machine is a mechanism or collection of mechanism which transmits force from the source of power to the resistance (load) to overcome and thus performs useful mechanical work.

Structure is the assemblage of resistant bodies without any relative motion between the links

E.g IC engine

E.g Bridges

All machines are mechanism

5. List out the inversions of a double slider crank chain.

Inversions of Double Slider Mechanism

First Inversion – Scotch Yoke mechanism

Second Inversion – Oldhams Coupling

Third Inversion – Elliptical trammel

Fourth Inversion – Hand Pump

6. Define Degree of Freedom and give the DOF for a shaft in a circular hole.

The number of independent input parameters which must be controlled independently so that a mechanism fulfills its useful engineering purpose.

7. State Grashof’s law for a four bar linkage.

It states that for a planar four bar linkage, sum of the shortest and longest link – lengths must be less than or equal to the sum of the remaining two link-lengths, is there is to be a continuous relative motion between two members

S + L ≤ P + Q

Where, S – Length of shortest link

L – length of longest link

P and Q – remaining two link lengths

i) If L + S < P + Q, then we call this a Grashof Mechanism

G.1 = crank-rocker if S is the crank and either of the

adjacent link is the fixed link

G.2 = double-crank if S is the fixed link

G.3 = double-rocker if the link opposite S is the fixed link

ii) If L + S > P + Q, then we call it non-Grashof mechanism

only double-rocker: no link is capable of making a complete revolution

iii) If L + S = P + Q, it can have G.1~G.3 and

parallelogram form (colinear)

8. Define degree of freedom.

It is defined as the minimum number of input parameters which must be independently

Controlled, inorder to bring the mechanism into a useful engineering purpose.

9. Define kinematic pair and illustrate any two types of constrained pair

When two kinematic links are connected in such a way that their motion is either completely or successfully constrained, these two links are said to form a kinematic pair. E.g foot step bearing, circular shaft in the circular slot

10. Define Pantograph

A pantograph is a mechanism used to produce paths on an enlarged or reduced scale as exactly as possible the path described by a given point. It is based on four bar kinematic chain

11. Define Link and List the various type of link

Link is a resistant body is one which is capable of transmitting the required motion and force with negligible deformation in the direction of force transmission.

Types of links : 1)Rigid Link 2) Flexible link 3) Fluid link

12. Define the kinematic chain

Kinematic chain is defined as the combination of kinematic pairs in which each link forms a pair of two kinematic pairs and the relative motion between the links is either completely constrained or successfully constrained.

When a number of links connected in space make relative motion of any point on a link with respect to any other point on the other link follow a definite law it is known as kinematic chain.

13. Name the inversions of four bar mechanisms

Inversions of four bar mechanisms

First Inversion – Coupled wheels of locomotive – double crank

Second Inversion – Beam Engine -Crank and lever mechanism

Third Inversion – Watt’s Engine Indicator – Double lever mechanism

14. Name the inversions of single slider mechanism

Inversions of Single Slider Mechanism

· First Inversion – Reciprocating engine mechanism

· Second Inversion – Gnome Engine or Rotary Engine – Whitworth quick return mechanism

· Third Inversion – Quick return mechanism – Crank and slotted lever – Oscillating cylinder engine

· Fourth Inversion – Hand Pump

15. Name some straight line generating mechanism

· Peaucellier mechanism

· Scott Russel mechanism

· Hart’s mechanism

16. Write down the Grubler’s criteria for planar mechanism

The following equation is used to describe mobility in 2D or planar systems:

M = 3(N-1) – 2 f1 – f2

Where,n = total number of links

M = DOF

f1 = number of 1DOF joints

f 2 = number of 2 DOF joints

This is known as GRUBLER’S EQUATION and is for mobility of planar systems.

M = 0 ( Motion impossible - statically determinate

M = 1 (Single input /monitoring necessary

M = 2 (Double input/output necessary

M = -1 (Statically indeterminate structure

17. Define Kinematic Pair

When two links are in contact with each other it is known as a pair.If the pair makes constrain motion it is known as kinematic pair.

18. Classify the kinematic pair based on the various characteristics

Kinematic pairs are classified on the basis of the following characteristics

1) Type of relative motion between contacting elements

2) Type of contact between contacting elements

3) Number of degrees of freedom

4) Type of closure

19. Define Higher and lower pair

Kinematic pairs in which there is a surface (area) contact between the contacting elements. All revolute pairs, sliding pairs, screw pairs, globular pairs, cylindrical pairs and flat pairs fall in this category.

Kinematic pairs in which there is a point or line contact between the contacting elements are called as higher pair. Meshing gear teeth, cam and follower pair, wheel rolling on a surface, a ball and roller bearings and pawl and ratchet are of higher pair.

When the two element of a pair have a line or point contact when the relative motion takes place and the motion between the two elements is partly turning and partly sliding, then the pair is known as higher pair

20. Define kinematic inversion

A mechanism is formed by fixing one of the links of a chain. The process of choosing different links of a kinematic chain for becoming frame.

21. Define Mechanical Advantage

It is defined as the ratio of the output torque, supplied by the driven link, to the input torque, required to be supplied to the driver link.

22. What is meant by planar mechanism

A planar mechanism is one in which all particles describe plane curves in space and all these curves lie in parallel planes: that is, the loci of all points are plane curves parallel to a single common plane. It utilizing only lower pairs is called planar linkages, they include only prismatic and revolute pairs.

23. What is meant by spatial mechanism

If there is any relative motion that is not in the same plane or in parallel planes, the mechanism is called spatial mechanism. Spatial mechanisms are three dimensional.

24. What are the methods for determining the velocity of a point on a link?

Instantaneous centre method,

Relative velocity method

25. State the Property of Inversion

26. Write down the Kutzback criteria for spatial mechanism.

According to Kutzback criterian,

Mobility, n = 6(N-1)-5PI-4P2-3P3-2P4-1p5

UNIT – II KINEMATIC ANALYSIS

1. How will you determine the magnitude and direction of Coriolis component of

acceleration ?

The direction of the Coriolis component of acceleration can be determined by rotating the velocity of a sliding vector Vs through 90˚ in the direction of rotation of angular velocity ω

The magnitude of Coriolis component of acceleration can be determined by

F = 2 Vs ω

(=Angular velocity

Vs =Linear velocity

2. Explain normal component of acceleration.

3. Define rubbing velocity.

The links in a mechanism are mostly connected by means of pin joints. The rubbing velocity is defined as the algebraic sum between the angular velocities of the two links which are connected by pin joints, multiplied by the radius of the pin.R = w.r.

4. Define Corioli's component of acceleration.

The corioli’s component of acceleration has two components of acceleration i.e. the vector sum of tangential acceleration and centripetal acceleration.

5. Define Instantaneous center of rotaion

Instantaneous center of rotation (Kinematics), in a plane or in a plane figure which has motions both of translation and of rotation in the plane, is the point which for the instant is at rest.

Instantaneous axis of rotation (Kinematics), in a body which has motions both of translation and rotation, is a line, which is supposed to be rigidly united with the body, and which for the instant is at rest. The motion of the body is for the instant simply that of rotation about the instantaneous axis

6. Illustrate the space centrode and body centrode

The trace of the locus of the center of rotation on the body is the body centrode and in space is the centrode.

7. A four bar mechanism has coupler pin centres at A and B and fixed pivot centres at A0 and B0. Write the two vector quantities involving the output velocity vector of B – It will be given in class

8. How will you determine the total acceleration of a point on a link, when the normal component of acceleration and the tangential component of acceleration are known - It will be given in class

9. State the properties of instantaneous centre method

· An Instantaneous centre of rotation is a point common to two links having relative motion.

· IC is a point about which one body can be assumed to rotate with respect to the oother

· It is an imaginary point at which the two bodies have same absolute velocity, It follows that the two bodies have zero relative velocity at the coincident points I.

· IC is in general not a stationary point, because the mechanism moves from one position to another, the velocities of points like A and B keep on changing.

10. Name the various components of acceleration

Radial component of acceleration

Tangential component of acceleration

11. State and explain the kennedy’s theorem

It states that if three bodies are in relative motion with respect to one another, the three relative instantaneous centres of velocity are collinear.

12. List the various types of instantaneous centres. What is the formulation to calculate the no of instantaneous centres are in a mechanism

The various types of instantaneous centres are

1. Fixed

2. Permanent

3. Neither fixed or permanent

Formula to calculate the no of instantaneous centre is

n (n-1) / 2

13. What are the expression for radial and tangential component of acceleration?

Radial component

arOB=(OB*OB

Tangential component

arOB=(OB*OB

Where, (OB=Angular velocity of link OB

(OB=Angular acceleration of link OB

OB=Length of link OB.

UNIT – III KINEMATICS OF CAMS

1. List the classifications of cam followers based on shape.

Cam followers are classified based on shape as follows

1. Knife edge follower

2. Roller follower

3. Flat follower

4. Spherical follower

2. What are the various types of motions of follower

The cam rotates at a uniform angular velocity, the follower may have the following motions

1. uniform velocity or uniform motion

2. simple harmonic motion SHM

3. Uniform acceleration and retardation

4. Cycloidal motion

3. What are the classifications of cams based on contact surfaces?

Cylindrical cam and Radial or Disc cam

4. State the basic requirements for high speed cams.

5. What are the necessary elements of a cam mechanism?

Cam-The driving member is known as the cam

Follower-The driven member is known as the follower.

Frame-It supports the cam and guider the follower.

6. State the expressions for maximum velocity and acceleration of a follower moves with Cycloidal motion. –

7. What is prime circle of a cam? What is the radial distance between the prime circle and base circle for a cam with knife edge follower?

Prime circle is the smallest circle drawn to the pitch curve from the centre of rotation of cam.

8. State the expressions for maximum velocity and acceleration of a follower moves with cycloidal motion. – will be given in class.

9. Define Cam

A cam may be defined as a machine element having a curved outline or a curved groove, which, by its oscillation or rotation motion, gives a predetermined specified motion to another element called the follower . The cam has a very important function in the operation of many classes of machines, especially those of the automatic type, such as printing presses, shoe machinery, textile machinery, gear-cutting machines, and screw machines

10. Define pitch curve of the cam.

The path generated by the trace point at the follower is rotated about a stationary cam. i.e., BY holding the cam fixed and rotating the follower in a direction opposite to that of cam, then the curve generated by the locus of the trace point is called the pitch curve.

11. Define Radial follower

When the motion of the follower is along an axis passing through the centre of the cam it is known as radial follower.

12. Define offset follower.

When the motion of the follower is along as axis away from the cam centre it is called offset follower.

13. Briefly explain the features of mushroom followers

Mushroom followers is used where the space is limited

14. Why roller follower is preferred to knife edge follower

Excessive wear of the knife edge follower is reduced by roller follower

15. Define pressure angle

Pressure angle represents the included angle at any point on the pitch curve between the line of motion of follower and normal to that point on the cam profile. This angle is of great importance in designing the cam profiles.

16. Define undercutting in cam. How it occurs? How can you prevent undercutting in cam ?

The cam profile must be continuous curve without any loop. If the curvature of the pitch curve is too sharp, then the part of the cam shape would be lost and thereafter the intended cam motion would not be achieved. Such a cam is said to be undercut. Undercutting occurs in the cam because of attempting to achieve too great a follower lift with very small cam rotation with a smaller cam.

Prevent undercutting in cam

· By decreasing the follower lift.

· By increasing cam rotation angle.

· By increasing the cam size' (i.e., Base circle).

UNIT – IV GEARS

1. Define (a) Module (b) Diametral Pitch of gears.

Module (m): is the ratio of pitch diameter to the number of teeth on the gear. m = D/T, where D- pitch circle diameter, T – no of teeth

Diametrical Pitch : It is the number of teeth per unit pitch circle. = T/D

2. What is axial pitch of a helical gear?

It is the distance, parallel to the axis, between similar faces of adjacent teeth. It is same as circular pitch and is therefore denoted by pc. The axial pitch may also be defined as the circular pitch in the plane of rotation or the diameteral plane.

3. List out the applications of epicyclic gear train.

Epicyclic gear trains are used in the differential gear box of an automobile, wrist watches, hoists, pulley blocks, back gear of lathe machine, in aircrafts etc.

4. Define epicycles gear train.

In a gear train when the axes of shafts over which the gears are mounted move

relative to a fixed axis is called epicyclic gear train.

5. Define velocity ratio.

Velocity ratio of a simple gear train is defined as the ratio of the angular velocity of

the first gear in the train to the angular velocity of the last gear.

6. Define gear train.

A combination of gears that is used for transmitting motion from one shaft to another

shaft is known as gear train. E.g. spur gear, spiral gear.

.

7. What is the principle reason for employing non standard gears?

a) To eliminate the undercutting.

b) To prevent interference.

c) To maintain reasonable contact ratio

8. What is the principle reason for employing non standard gears?

a) To eliminate the undercutting.

b) To prevent interference.

c) To maintain reasonable contact ratio

9. When in volute interference occurs.

If the teeth are of such proportion that the beginning of contact occurs before the

interference point is met then the involute proportion of the driven gear will mate a

non in volute portion of the driving gear and involute interference is said to occur.

10. Define cycloid.

A cycloid is the curve traced by a point on the circumference of a circle which rolls

without slipping on a fixed straight line.

11. Define gear tooth system.

A tooth system is a standard which specifies the relationship between addendum,

dedendum, working depth, tooth thickness and pressure angle to attain interchangeability of gears of tooth numbers but of the same pressure angle and pitch

12. What are the conditions to be satisfied for interchangeability of all gears.

For interchangeability of all gears, the set must have the same circular pitch, module,

diameter pitch, pressure, angle, addendum and dedendum and tooth thickness must be

one half of the circular pitch.

13. List out the characteristics of involute action.

a) Arc of contact.

b) Length of path of contact.

c) Contact ratio.

14. Define I) path of contact. II) Length of path of contact.

Path of contact: It is the path traced by the point of contact of two teeth from the

beginning to the end of engagement.

Length of path of contact: It is the length of common normal cut- off by the

addendum circles of the wheel and pinion.

15. Define circular pitch.

It is the distance measured on the circumference of the pitch circle from a point

of one tooth to the corresponding point on the next tooth. It is denoted by Pc

Circular pitch Pc= π/DT

Where D = Diameter of pitch circle.

T = Number of teeth on the wheel.

16. List out the function of differential gear used in the rear drive of an automobile.

a) To transmit motion from the engine shaft to the rear driving wheels.

b ) To rotate the rear wheel of different speeds while the automobile is taking a turn.

17. What is reverted gear train?

A reverted gear train is a compound gear train in which, the first and last gears are coaxial with each other. E.g In clocks and simple lathes where back gear is used to impart slow speed to the chuck

18. Define undercutting in Gears.

The under cutting concept in gearing is, when the two gears mates, pinions whose base circle is more than the dedendum circle, therefore the profile of tooth below the base circle is non– involute. In this case the profile of wheel and pinion will not be tangent to each other and the tip of the wheel will dig out or interfere with the flank of the pinion and remove the part of material called under cut and the process of removal of material is under cutting of gears. The teeth of pinion will become weak due to undercutting.

19. What is meant by Angle of dwell?

It is the angle through which the cam rotates while the follower remains stationary at the highest or the lowest .

\

20. What are the properties of involute tooth profile

a) A normal drawn to an involute at pitch point is a tangent to the base circle.

b) Pressure angle remains constant during the mesh of an involute gears. 

c) The involute tooth form of gears is insensitive to the centre distance and depends only on the dimensions of the base circle.

d) The radius of curvature of an involute is equal to the length of tangent to the base circle. 

e) Basic rack for involute tooth profile has straight line form.

21. What is meant by contact ratio

It is defined as average number of pairs of teeth which are in contact. It is the ratio of length of arc of contact to circular pitch.

22. Define pressure angle and explain the effect of different pressure angle.

The pressure angle is the angle which the common normal to the contacting tooth profiles, at the point of contact, makes with the common tangent to the two pitch circles at the pitch point.

23. Discuss the advantages of involute gear tooth profile.

a) It is easy to manufacture and the center distance between a pair of involute gears can be varied without changing the velocity ratio. Thus close tolerances between shaft locations are not required. The most commonly used conjugate tooth curve is the involute curve.

b) In involute gears, the pressure angle, remains constant between the point of tooth engagement and disengagement. It is necessary for smooth running and less wear of gears.

c) The face and flank of involute teeth are generated by a single curve where as in cycloidal gears, double curves (i.e. epi-cycloid and hypo-cycloid) are required for the face and flank respectively. Thus the involute teeth are easy to manufacture than cycloidal teeth.

24. Describe the advantages and applications of helical, bevel and worm gears

Helical gear :

Applications : These are highly used in transmission because they are quieter even at higher speed and are durable. The other possible applications of helical gears are in textile industry, blowers, feeders, rubber and plastic industry, sugar industry, rolling mills, food industry, elevators, conveyors, cutters, clay working machinery, compressors and in oil industry.

Advantages:

25. Define gear tooth system.

A tooth system is a standard which specifies the relationship between addendum,

dedendum, working depth, tooth thickness and pressure angle to attain

interchangeability of gears of tooth numbers but of the same pressure angle and pitch

26. What is meant by arc of contact

It is the path traced by a point on the pitch circle from the beginning to the end of engagement of two meshing teeth.

27. State and prove law of gearing

The common normal at the point of contact between a pair of teeth must always pass through the pitch point for all positions of the mating gears.

28. How the gears are classified?

a) Based on teeth: Straight teeth- Spur, Helical- Helical and Herring Bone, Inclined Teeth – Bevel Gears, Curved teeth – Spiral Gears

b) Based on engagement: External Gear, Internal Gear, Rack and Pinion Common type – Spur, Helical, bevel, Worm and worm wheel

29. Define angle of obliquity or Pressure angle

Angle between the common normal of two gear teeth profile at the point of contact and the common tangent at the pitch point.

30. What is the commonly used Gear Profile?

14 involute profile, Cycloidal involute, 20 full depth involute.

31. What is corrected Gear?

In many cases it is advantages to correct the gear by shifting the profile of the cutter by a small amount. The gear tooth profile remains as a part of involute curve as in the case of standard in corrected gearing, but a different portion of involute gear of the same base circle is used as active profile. These gears are called corrected gears.

32. What are advantages of toothed gears over other type of transmission system?

a) Positive drives – No slip, no variation in velocity ratio

b) Suitable for short distances

c) High power transmission

d) Change of speed is easy

33. What is meant by interference of Gears?

Gear profile usually starts from base circle and ends with tip circle gear teeth and made in such a way that their contact is along the profile. Since the top surface of teeth is made flat the tip of the teeth of one gear tends to dig into the bottom flank of mating gears. This action is called interference. It can be overcomed by undercutting the tooth of gears i.e. to remove some of the material in the root of the gear teeth

34. What are the advantage of Helical gears over Spur Gears

a) Noise less operation

b) High power transmission

c) Fully engaged

d) High speed operation

35. What are the two important failure modes in Gears

Tooth Breakage, Pitting of Tooth Surface, Abrasive Wear, Seizing of teeth.

36. Why helical gear tooth is stronger than spur gear?

The teeth of helical gear are inclined to axis of gear. During meshing the helical gears are made to contact in point whereas spur gears are in line contact. Therefore helical gear tooth are stronger than spur gear tooth.

UNIT – V FRICTION

1. Distinguish between sliding and rolling friction.

Sliding Friction – When two dry surfaces have a sliding motion relative to each other, then it is called as sliding friction e.g friction between nut and bolt.

Rolling friction – When two dry surfaces have a rolling motion relative to each other, then it is called as rolling friction E.g – friction in ball and roller bearings.

2. State the condition for transmission of maximum power in belt drives.

The velocity of the belt v = √ (Tmax / 3m)

3. What is bearing?

A bearing is a machine element which supports another moving machine element known

as journal. It permits a relative motion between the contact surfaces of the member while

carrying the load.

4. What are the factors to be considered when designing a disc or plate clutch?

When there is a uniform pressure

When there is a uniform axial wear.

5. What are the functions of clutch?

To engage or disengage the rest of transmission as required.

To transmit the engine power to rear wheels when the rear wheels without shock.

To enable the gear to get engaged when the vehicle is in motion.

6. What is clutch?

A clutch is a machine member used to connect a driving shaft to a driven shaft so

that the driven shaft may be stared or stopped at with or with out stopping the driving shaft. It is used in automobile.

7. Where is a V belt used?

The V belt is mostly used in the factories and workshops where a great amount of power is to be transmitted from one pulley to another when the two pulleys are very near to each other.

8. What are the factors that have to be considered for the selection of a belt drive?

Speed of the driving and driven shafts.

Speed reduction ratio

Power to be transmitted

Shaft lay out

Space available

9. List out any four desirable characteristics of brake lining material.

a)It should have low wear rate

b) High heat resistant

c) It should have high coefficient of friction with minimum fading

d) It should have adequate mechanical strength and high heat dissipation capacity

e) E.g materials – bronze, steel, wood on cast iron and fiber, asbestos, leather, cork on metal,

10. What is creep in the case of belt

The relative motion between belt and pulley surface due to unequal stretching of the two sides of drive. The effect of creep slow down the speed of the belt on the driving pulley than the peripheral velocity of pulley.

11. Discuss the advantages of V belts

It provides longer life 3 to 5 years

It can be easily installed and removed

High velocity ratio

Drive is positive.

12. What is meant by self locking and over hauling screw

If Φ < α, then the torque required to lower the load will be negative i.e load will start moving downward without applying any torque. Is known as overhauling of screws.

If Φ > α, the torque required to lower the load will be positive i.e some of torque is required to lower the load such a screw is know as self locking screw

13. Name the various types of pivot bearing

Based on the shape of the end of shaft and the shape of bearing surface,

1. Flat pivot bearing

2. Flat collar bearing

3. Conical pivot bearing

4. Truncated conical or trapezoidal bearing

14. Define brake and name its various types

The frictional force is used to absorb the energy possessed by a moving member.

Various types of brake

1. Block or shoe brake

2. Band brake

3. Band and block brake

4. Internal expanding shoe brake

15. Define centrifugal clutch

It works on the principle of centrifugal force i.e the centrifugal force is nicreases with the increase in speed. It is used when it is required to engage the driven member automatically after the driving member has attained certain speed.

16. List out the commonly used breaks.

1. Hydraulic brakes: e.g., Pumps or hydrodynamic brake and fluid agitator.

2. Electric brakes: e.g., Eddy current brakes.

3. Mechanical brakes: e.g., Radial brakes and Axial brakes

17. What do you mean by a brake?

Brake is a device by means of which motion of a body is retarded for slowing down (or) to bring it to rest which works on the principle of frictional force, it acts against the driving force.

18. Explain. Self-energising.

When moments of efforts applied on the break drum and frictional force are in the same direction, the breaking torque becomes maximum (frictional force aids the braking action). In such a case the brake is said to be partially self-actuating or self energising.

19. When is the intensity of pressure acting brake shoe is assumed to be uniform ?

The intensity of pressure is assumed to be constant when the break shoe has small angle of contact. For large angle of contact, it is assumed that the rate of wear of the shoe remains constant.

20. Where does the P.I.V. drive system used?

PIV. (Positive Infinitely Variable) drive is used in an infinitely varying speed systems.

21. Why lubrication reduces friction?

In practical all the manting surfaces are having roughness with it. It causes friction. If the surfaces are smooth then friction is very less. Lubrication smoothens the manting surface by introducing oil film bet_een it. The fluids are having high smoothness than solids and thus lubrication reduces friction.

22. What you meant by 'Crowning in pulley' ?

The process of increasing the frictional resistance on the pulley surface is known as crowning. It is done in order to avoid slipping of the belt.

23. What is meant by initial tension in belts?

In order to' increase the frictional grip between the belt and pulleys, the belt is tightened up. Due to this the belt gets subjected to some tension even when the pulleys are stationary. This tension in the belt is called initial tension (To).

24. State the law of belting?

Law of belting states that the centre line of the belt as it approaches the pulley must lie in a plane perpendicular to the axis of the pulley or must lie in the plane of the pulley, otherwise the belt will runoff the pulley.

25. What is meant by angle of contact? (Lap angle)

It is the angle made by a common normal drawn to the tangent line at the point of engagement and at the point of disengagement of the belt on a pulley, at its centre.

26. What is the centrifugal effect on belts?

During operation, as the belt passes over a pulley the centrifugal effect due to its self weight tends to lift the belt from the pulley surface. This reduces the normal reaction and hence the frictional resistance.

27. What are the disadvantage of V-belt drive over flat belt?

1.V- belt cannot be used in large distance..

2. It is not as durable as flat belt.

3. Since the V belt subjected to certain amount of creep therefore it is not suitable for

constant speed applications such as synchronous machines, and timing devices.

4. It is a costlier system.

28. When is the cross belt used instead of open belt?

1. Cross. belt is used where the direction of rotation of driven pulley is opposite to driving pulley.

2. Where we need more power transmission there we can use cross belt drive.

29. What is wipping ? How it can be avoided in belt drives?

If the centre distance between two pulleys are t9° long then the belt begins to vibrate in a direction perpendicular to the direction of motion of belt. This phenomenon is called as wipping. Wipping can be avoided by using idler pulleys.

30. What will be the effect on thee limiting ratio of tensions of a belt if the coefficient of friction between the belt and rim of pulley is doubled while angle of lap remains same?

The ratio of tension will be squared.

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ME 2203 / Kinematics of Machinery