Design of Transmission Systems 2 Marks

7/29/2019 Design of Transmission Systems 2 Marks

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INFANT JESUS COLLEGE OF ENGINEERING AND TECHNOLOGY

Department of Mechanical Engineering

Sixth Semester

Design of Transmission System

Unit I

Belt Drive

1. Define Belt.

Belts or connectors are used for power transmission between two parallel shafts, by means of friction

2. What are the commonly used materials for flat belts?

a. Leather beltsb. Fabric belts

c. Rubber belts

3. What are the factors on which the coefficient of friction between the belt and a pulley depends?

The coefficient of friction between the belt and a pulley, depends upon the following factors,

1. Belt material

2. Pulley material

3. Belt Slip4. Belt speed

4. Differentiate between open belt drive and crossed belt drive.

When the driver and driven pulleys rotate in the same direction, the drive is said to be an open belt

drive. In the crossed belt drive, the directions of rotation of the driver and driven pulleys are in opposite

directions.

5. What are the advantages of flat belt drive?

The following are the advantages of a fat belt drive:

a. The flat belt are cheap and easy to maintain.

b. Flexible nature of the belt offers protection to the machinery against shock loading.

c. By using fast and loose pulley arrangement, the belt drive can be used for obtaining different

velocity ratios.

d. By using stepped cone pulleys, the same belt can be used for obtaining different velocity ratios.

e. The drive can be used for long center distances. There is virtually no limit to the centre distance.


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6. What are the disadvantages of flat belt drive?

a. It is not a positive drive and so constant velocity ratio cannot be obtained.

b. The drive requires pre-tensioning to avoid slip, resulting in more bearing reactions and more

stresses in the belt cross section.

c. The drive is not compact, as it occupies more space. This also makes it difficult for shielding the

drive with guards.d.

7. Define centrifugal tension.

The belt is having some mass, and as it passes over the pulley around a circular path, it is subjected

to centrifugal force. The effect of this is to increase the tension on both the tight and slack sides. The

tension caused by centrifugal force is called centrifugal tension.

8. Explain the term crowning of pulley.

The provision of a slight convex surface for the flat belt pulley rim is known as crowning. Thecrowning prevents axial slipping of the belt during operation. The amount of crowning varies from 0.3

mm to 6 mm for the pulleys sizes ranging from 40 mm to 2000 mm. Obviously, the amount of crowning

increases with the increase in width of the pulley.

9. What is the effect of crowning a flat belt pulley?

The provision of a slight convex surface for the pulley rim is known as crowning. The crowning

prevents the axial slipping of the belt during operation. However, it induces non- uniform stress across

the cross section of the belt.

10. What is meant by slip and what is its effect?

The power transmission in a belt drive is by means of friction. But generally, the frictional grip may

not be effective, causing some forward motion of the driver surface, with respect to the belt. This

phenomenon is called belt slip and is generally expressed in percentage. Due to slip, the belt speed is

less than the peripheral speed of the driving pulley and is slightly more than the peripheral speed of the

follower pulley.

11. What is meant by belt creep and what is its effect?

The difference in length of the belt that is being received and delivered by a pulley is called creep.

Due to creep, the average velocity of the belt is slightly lower than that of the driver pulley surface. At

the follower pulley, the average velocity of the belt is slightly more than that of the pulley surface.

12. What is slack adjuster?


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After some period of service, a belt may undergo a permanent stretch, reducing the initial tension

The initial tension may be represented either by increasing the center distance or by using an idler

pulley. The idler pulley is called slack adjuster.

13. How is belt drive better than a gear drive?

As the belt is elastic in nature the belt drive can damp-down shocks and vibrations. Hence, when

provided with the belt drive, the machine tools are protected better quality surface for work pieces when

compared to that produced by machine tools provided with gear drive.

14. Differentiate between flat and V belts.

Depending upon the shape of the cross section, belts are classified as: flat belts and V belts. Flat

belts have a narrow rectangular cross section, and V belts have a trapezoidal section. A flat belt operates

on pulleys with smooth surfaces, whereas pulleys with wedge shaped grooves are used with V belts.

15. What are the applications of a V belt drive?Automotive field: In this, V- belts are used to drive fans, water pumps, generators, and air conditioning

compressors.

Industrial field: In this, V belts find application in machine tools, and for driving compressors, fans,

conveyors, etc.

16. When are split pulleys preferred over solid type pulleys?

The split pulleys are used for intermediate positioning on longer shafts or when it would be

inconvenient to mount a pulley because of other mountings and bearings on the shaft. The two halves of

the split pulley are bolted together.

17. What are the disadvantages of V belt drive?

a. V grooved pulleys are costlier compared to flat belt pulleys.

b. The efficiency of the V belt drive is lower than that of the flat belt drive.

c. The creep of V belt is more than that of the flat belt drive.

18. What are the advantages of a V belt drive?

The following are the advantages of a V belt drive:

1. The drive can be used with shot centre distance, resulting in a compact drive.

2. The drive can be run at high speeds with velocity ratios up to 1:10.

3. On account of wedging effect of the belt in the pulley groove, the drive causes less pull on the

shaft than flat belt of the same characteristics.


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4. Power transmission capacity of V belt drive is more than that of a flat belt drive, for the

same characteristics.

ROPE DRIVE

1. Define rope drive.

A rope drive is used for transmitting large amount of power over large distances which can be

much more than that possible with flat belt drives.

2. What are all the materials are used in fibre rope?

Cotton, manilla and hemp are the materials used for fibre ropes.

UNIT II

SPUR AND HELICAL GEARS

1) Differentiate between a gear and belt drive.

Gear drive is used for transmission of large amount of power at a constant velocity ratio, as the

drive is positive. Belt drive is a flexible drive, used for low and medium power transmission

between two parallel shafts, by means of friction. Owing to the inevitable slip and creep of the

belt, the velocity ratio obtainable from the belt drive is not constant.

2) Classify gears.

Based on the shape of the gears and relative positions of shafts between which the power is

transmitted, gears are classified as:

1. Spur gears (gearing)

2. Helical gears (gearing)

3. Bevel gears (gearing)

4. Hypoid gears (gearing)

5. Worm gearing


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3) What are the applications of spur gearing?

Spur gearing is used for connecting shafts, whose axes are parallel. Spur gears may be made to mesh

externally or internally; the former being the mostly used one. Rack and pinion also come under spur gearing.

Since a rack is defined as a spur gear with infinite radius. Rack and pinion are used for converting rotary motion

into reciprocating motion and vice versa.

4) What is working depth of a gear tooth?

Working depth of a gear tooth refers to the distance of engagement of two mating teeth, and is equal to

the sum of the addendums of the mating teeth of the gears.

5) What factors influence backlash in gear drives?

While designing, it is the practice to assume that the thickness of the tooth measured along the pitch

circle, is equal to one half of the circular pitch .However, it is necessary to cut the teeth slightly thinner to

provide some clearance so that the gears will not bind but will roll together smoothly. This clearance is called

backlash. It is defined as the amount by which a tooth space exceeds the thickness of the engaging tooth

measured on the pitch

Backlash is usually obtained by setting cutting tool deeper into the blank so that a thinner tooth and

wider space are produced. Tooth thickness is usually reduced on both the gears on the gearing; although it is

possible to obtain the backlash by making the teeth thinner on one of the gearing. Backlash can also be

increased or decreased by giving a small variation to the center distance at which the gears are mounted.

6) Why is dedendum value more than the addendum value?

Working depth of a tooth is the maximum depth to which a tooth penetrates into the tooth space of the

mating gar. It is equal to the sum of addendums of the two gears. Full depth of tooth is the radial depth of the

tooth space It is equal to the sum of addendum and dedendum. For smooth functioning of a gearing, there

should be some clearance between the meshing tooth and the space into which it enters. This is possible when

the full depth of teeth space is more than the working depth of a tooth. Therefore , dedendum value should be

more than the addendum value.

7) How number of teeth affects the design of gears?


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The part played by the number of teeth in the design of gears can be understood from the following;

1) The strength factor of a gear depends upon the form factor, which in turn depends upon the number

of teeth, apart from the tooth profile.

2) Number of teeth also decides the number of pairs of teeth in contact. Number of pairs of teeth in

contact, greater than 1, ensures continuous smooth transmission of motion.

3) Undercutting can be prevented by using more and smaller teeth in a gear.

8) Define the terms,(i) pitch circle (ii) base circle (ii) pressure angle (iv)backlash, as applicable to spur

gears.

1. Pitch circle This is the circumference of an imaginary cylinder which rolls without slipping, when in contact

with another such cylinder, as in friction drive. The size of the gear is denoted by the diameter of the pitch

circle.

2. Base circle This is the circle, which is associated with only involute system of gears. This is the circle from

which the involute tooth profile is developed.

3. Pressure angle This is the angle between the line of action and the common tangent to the pitch circles.

4. Backlash It is the difference between the thickness of a tooth and the space into which it meshes and is

measured on the pitch circles. Theoretically, it should be zero; but from practical point, some backlash is

provided for smooth meshing between the teeth

9) Differentiate between involute and cycloidal gears.

1. Involute gearing permit variation in center distance within certain limits, without affecting the

velocity ratio. With cycloidal gearing, exact center distance must be maintained for constant velocity

ratio.

2. In involute gearing, the pressure angle is contact from the beginning to the end of engagement

resulting in smooth running and less wear, In cycloidal gearing, the pressure angle is maximum at

the beginning and end of engagement, and zero at the pitch point, resulting in less smooth running

3. The face and flank portions of involute teeth are generated as a single curve, facilitating easy

manufacture with simple tools. In cycloidal system, the face and flank portions are formed as double


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curves. i.e., epi-cycloidal and hypo-cycloid respectively, resulting in the usage of complex shaped

tools for the manufacture.

10) Briefly describe the stub tooth, 20 deg involute system.

In stub tooth, the whole depth of the tooth is less then that with full depth, 20 deg involute system. This

system gives better results, when the pinion has less than 25 teeth. This system is preferred in automotive

transmission, since it results in smaller gears with greater strength, for a given module and material. This is

because of shorter moment arm of the tooth, resulting in less bending moment.

11) What is interference in involute profile?

The only disadvantage of the involute gearing is the occurrence of interference, with the pinion

having lesser number of teeth. The phenomenon when the tip of a tooth undercuts the root on its mating

gear known as interference. Interference can be avoided by changing the heights of addendum and

dedendum of the mating teeth

12) What is meant by composite gear tooth profile and what are its characteristics?

Composite gear tooth profile is a combination of involute and cyclodial forms. It is used to avoid

interference. In the composite form, approximately the middle one third of the profile is in the form of involute

while the reminder is cycloidal

13) What are the methods to be followed for the prevention of interference?

Following are the methods to be followed for prevention of interference:

1. Adoption of generation process to produce gear teeth

2. Adoption of larger pressure angle

3. Using more number of teeth Stubbing teeth

14) What are the advantages of cycloidal gearing?

The following are the advantages of cycloidal gearing:

1. Cycloidal tooth is stronger than involute tooth for the same pitch, as it gives more root area.


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2. Interference is inherently absent in cycloidal gearing.

15) Name some of the commonly used gear materials.

The materials for gears are decided based on the strength and other service requirements. Both metallic and

non-metallic materials are used for the purpose.

Cast iron is used for light duty and cast steel for medium duty. For very high strength requirements,

hardened and tempered alloy steels as well as case hardened steels should be used. Bronze as a gear material,

displays high strength and good wear resistance. Gears made of phenolic resins and nylon are good for high

speed duty, including noiseless running.

To save alloy steels, Large gears are made with fretting rings. For this, the center portion of the gear is made

of cast iron and the ring portion, from the steel of the required grade.

16) What are the points to be considered, while designing a gear drive?

The following points must be considered, while designing a gear drive :

1. The gear shoule have sufficient strength so that it does not fail under starting torque, and dynamic

loading.

2. The gear teeth must have good wear characteristics.

3. Suitable material combination must be chosen for the gearing, so that it gives good wear

characteristics and the drive is silent

4. The drive should be compact.

17) Discuss the importance of velocity factor.

Allowable static stresses for gear materials are valid for the design of gears, working at very very slow

speeds, like hand operated gears. These values are obtained by dividing the ultimate strength of the material by

suitable factor of safety.

The allowable stresses used in practice, decrease with increase in velocity. To account for the dynamic effects,

which will become more severe, as the pitch line velocity increases, a factor known as velocity factor is

considered for determining the design or allowable stresses.


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18) What are the contributory factors for dynamic loading of spur gearing?

The following may be the contributory factors for dynamic loading of spur gears :

1. Inaccuracies in teeth spacing

2. Elements of the teeth surfaces, not parallel to the gear axis

3. Deflection of teeth under load

4. Deflection and twisting of shaft under load

19) What is meant by wear load of a gear tooth? What are the factors on which it depends upon?

The maximum load that the gear tooth can resist, without premature wear, it called wear load of a gear tooth.

It is the limiting or permissible load, on the basis of wear. It depends upon curvature of the tooth surface,

elasticity, and surface fatigue limit of the material.

20) What is hunting tooth and what for it is used?

If the number of teeth in a gear is in simple multiple of teeth in the pinion, the same pair of teeth will engage

during every revolution of the gear. If there is a defective tooth in the pair, then wear will be concentrated

between specific pair(s) of teeth. To distribute the wear more evenly, a tooth may be added to one of the gears

in the pair. If the resulting velocity ratio is permissible. This tooth that is added is known as the hunting tooth.

21) On what way, helical gearing is superior to spur gearing?

Helical gears differ from spur gears in that they have teeth that are cut in the form of helix on the gear blank,

instead of parallel to the gear axis. In spur gearing, contact extends across the entire tooth width, along a line

parallel to the gear axis. This sudden application of load results in noisy operation. In helical gearing, contact

begins at one end of the meshing tooth and gradually extends along a diagonal line. across the tooth face as the

gear rotates. Further, in helical gearing,number of pairs of teeth in contact is more than that found with spur

gears. This, associated with gradual engagements, leads to gradual tooth loading, reducing the dynamic load and

so the operation is relatively silent. Hence, higher pitch line velocities can be used with helical gears.

Further, as the contact occurs along a diagonal line, the effective is less than the actual tooth height, and

therefore it can resist greater load than the straight tooth of a spur gear


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22) What is a herringbone gear and what is the special feature of it?

A herringbone gear is a double helical gear and it is equivalent to two helical gears of opposite hand, cut on

the same blank. They develop opposite thrust forces and thus cancel each other. This gear is more efficient,

permit higher velocity ratios, and high pitch line velocities, compared to even single helical gear

23) Where do we use skew helical gears?

Skew helical gears or crossed helical gears are used for transmitting power between non-parallel shafts. In

this gearing, the teeth have only point contact, which does not shift axially along the teeth during operation.

This severely limits their power-carrying capacity. For this reason, skew helical gears are used only, for

applications requiring the transmission of relatively small loads.

24) Define the terms, (i) helix angle (ii) axial pitch (iii) normal pitch, as applicable to helical gears.

i) Helix angle This is the angle made by the helices with the axis of the gear.

ii) Axial pitch It is the distance measured between similar faces of adjacent teeth along the pitch

circle, in the plane of rotation or diametral plane. It is the same as the circular pitch

iii) Normal pitch It is the distance between similar faces of adjacent teeth, measured normal to the

teeth elements

UNIT III

BEVEL AND WORM GEARS

1) Name the various types of bevel gears or gearings and explain.

The following are the various types of bevel gears:


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Mitre gears when bevel gears of the same size are used on intersecting shafts at right angle, they are

called mitre gears. Obviously these gears are used power transmission at right angle, without any change

in speed.

Angular bevel gears when bevel are mounted on intersection shafts, making an angle, other than 90

deg they are called angular bevel gears.

Crown gear when the pitch angle of a bevel gear is 90 deg, it is called a crown gear.

2) Define the following terms: (a) Cone distance and (b) face angle.

Cone distance This is the length of the pitch line, from the apex to the base. It is also called pitch cone

radius.

Face angle It is the angle subtended by the face of the tooth at the cone center. It is equal to the pitch

cone angle plus addendum angle.

3) State true or false and justify.

Mitre gears are used connecting non-intersecting shafts

False. This is because , mitre gears are bevel gears. And bevel gears are used for power transmission

between two intersection shafts. In fact, when bevel gears of the same size are used on intersection shafts

at right angle, they are called mitre gears. These gears are used for power transmission at right angle,

without change in speed.

4) Define the terms, (i) pitch cone (ii) pitch cone angle, and cone length, as applicable to bevel gears.

i) Pitch cone It is a cone which rolls without slipping, when it is in peripheral contact. It is

analogous to pitch cylinder of a spur or helical gear.

ii) Pitch cone angle This is the angle subtended at the apex, by the gear axis and a pitch cone

generator

iii) Cone length This is the length of the pitch line, from the apex to the base.


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5) What are the advantages of worm gear drive?

The following are the advantages of worm gear drive:

1. High velocity ratios, as high as 100 can be obtained with a single worm gearing.2. The worm gear drive unit will be compact, compared to spur or helical gear drive units, for the same

velocity ratio.

3. The operation is smooth and hence silent.

4. Provision can be made for self-locking of the drive. This feature is advantageous in hoisting

applications.

6) Write short notes on worm gearing.

Worm gearing is used for transmitting power between two non-intersecting, non-parallel shafts which

are in general at right angle to each other. The worm gearing consists of a worm and a worm wheel or

gear. The worm resembles a threaded screw, while the worm wheel is a toothed gear (helical gear).

Worm gearing combines features of both a conventional gear drive and a power screw. The worm may

be left handed, or right handed, and single threaded or multiple-threaded. In worm gearing, usually the

worm is the driver. As the worm rotated, its threads exert pressure on the gear teeth, and thus impart

rotational motion.

7) Define the terms,(i) axial pitch of the worm (ii) lead of the worm, and (iii) lead angle of the worm, as

applicable to worm gearing.

1. Axial pitch of the worm This is the distance measured axially, from a point on one thread to the

corresponding point on the adjacent thread of the worm. It may be noted that the axial pitch of the

worm is equal to the circular pitch of the mating worm wheel.

2. Lead of the worm This is the distance that a point on the pitch circle o the worm w3heel will

advance during one revolution of the worm. For the worm with single start threads lead is equal to

the axial pitch; with double start threads, it is equal to twice the pitch, and so on.

3. Lead angle of the worm This is the angle between a tangent to the thread helix on the pitch cylinder

and a plane, normal to the axis of the worm.


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8) In which gear drive, self-locking is available?

In worm gearing, self-locking is available. If the efficiency of the worm gearing is less than 50%, then it

is said to be under self-locking. When the drive is under self-locking, the worm gear cannot drive the

worm.

9) Why is multistart worm more efficient than single start one?

The efficiency of a worm gearing is more if the lead angle of the worm is more. Lead angle of the worm

is proportional to number of starts in the worm. Therefore, multistart worm is more efficient than single

start worm.

10) Worm gearing is not interchangeable. Explain.

The elements of a worm gearing are not interchangeable, because a worm wheel cut with a hob of onediameter will not mesh properly with a worm of different diameter, even if the pitch of the thread is the

same. This is because of the fact that the teeth on the worm wheel, envelop the threads on the worm.

UNIT IV

GEAR BOXES

1) Explain the reasons for opting for stepped regulation of speeds and feed rates in machine tools

In any machining operation, optimum tool life can be achieved only by working at optimum

values of cutting speed, feed, and depth of cut. For machining a component of any arbitrary diameter, the

machine tool should have stepless regulation so that the spindle can be run at any desired speed,

corresponding to the optimum cutting speed. By a similar logic, the machine tool should also have

provision for stepless variation of feed rate. Incorporation of these facilities is not economical, and so

majority of the machine tools are provided only with stepped regulation of speeds and feed rates.


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2) What are the properties (advantages) of geometric progression ?

The following are the properties (advantages) of geometric progression:

1. If, from a G.P having progression ratio, members are removed in such a manner that only every X th

member remains, the remaining members form a new G.P with the ratio,

2. If the members of a G.P are multiplied by a factor,then the whole series is shifted by y members

3. If a G.P with a progression ratio, is multiplied by a constant,c,then the resulting series is also a G.P

with the same progression ratio, but whose members are c times greater.

3) What are the characteristics of logarithmic progression (L.P)?

With logarithmic progression in the low as high r.p.m range, the values lie between the values of

geometric progression and harmonic progression. It may thus be concluded that from the point of view

of the operational efficiency of the machine tool, L.Pis the most suitable. The efficiency of G.P is poorer

in the low r.p.m range , whereas that of the H.P is poorer in the high r.p.m range.

4) What are the points to be considered while designing a sliding mesh type of multispeed gear box?

Following are the points to be considered while designing s sliding-mesh type of multi-speed gear box :

1. Entire range of speeds should be obtained without stopping the motor.

2. Any desired speed should be obtained, without making all the intermediate changes between the

present and desired speeds.

3. Only those gars required for obtaining the desired speed should be engaged at a time.

4. Least possible gears, shafts, and levers should be used.

5. Sliding cluster should not have more than 3 gears.

6. As far as possible, same module should be used for all the gears.

5) What are the points to be considered while designing gear boxes, incorporating sliding cluster gears?

Following are the points to be considered while designing gear boxes, incorporating sliding cluster gears


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1. One`set of gears must be completely disengaged before the other set begins to come

Into mesh.

2. The sum of the teeth of mating gears in a given set, must be the same for any assumed module in a

clustered set.

3. The minimum difference between the numbers of teeth of adjacent gears must be 4.

4. The minimum number of teeth in a set of gears should be greater than 17 in order to avoid

interference.

6) What is a ray diagram and what for it is used?

Ray diagram displays exact location of a particular speed, and helps in calculating gear ratios in gear

box design.

7) What is the function of spacers in a gear box?

In a sliding mesh type of gear box, only the gears required for obtaining the desired speed, should be

engaged. The concerned gears are made to slide to engage with the required gears. In this process, the

concerned gears are should be totally disengaged with the previous meshing before the actual meshing to

take place with the required gears. To ensure this, spacers are used between the gears requiring sliding.

Further , to create the spaces for accommodating the gear shifting lever ends between the gears requiring

sliding, spacers are used.

8) What are the methods of lubrication in speed reducers?

When the gears run in an enclosed casing, as in a speed reducer, the larger gears can be made to dip into

a bath of lubricating oil so that the lubricant can be carried to the surfaces needing lubrication

Sometimes, the gears are lubricated by spraying a jet of oil on the working surfaces as they move

towards each other.

9) What are rules to be followed while constructing acceptable ray diagram?

The following are rules to be followed while constructing acceptable ray diagram :


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1. Each ray must connect follow-up rays until the output stage is reached.

2. All the output level, one and only one ray must connect an output point. Two or more rays should not

lead to one output point. However ,any number of rays can emerge from an input point.

3. All the output points must be connected by rays.

UNIT V CAM

1) What is a cam and what for it is used?

The demand for the present day machine components to move in a pre-determined motion and path is

rarely fulfilled by the connected members. This necessitated the use of members with miscellaneouscontour surfaces called cams A cam is a mechanical member used for transmitting a desired motion to a

follower by direct contact. Complicated output motions which are otherwise difficult to achieve can easily

be produced by using cams.

2) Name the parts of a mechanism.

The three parts or links of a cam mechanism are: (i) the cam which is the driving link and has curved or

straight contact surfaces, (ii) the follower, which is the driven link and gets its motion by contact with the

surface of the cam, and (iii)the frame, which is used to support the cam and guide the follower.

3) Classify the cam based on shape.

The types of cams based on shape are:

Wedge or flat cam

Disc or radial cam

Cylindrical or drum cam

4) Classify the cam followers, based on the shape.

The types of cam followers, based on the shape:

Knife-edge follower


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Roller follower

Flat-face (mushroom) follower

5) Define pressure angle, as applicable to cams.

It is the angle at any point on the pitch curve, included between the normalto the curve at thet point and

the line of action of the follower. This angle is very important in cam design, as it refers to the steepness

of the cam profile.

6) What are the parameters about which the pressure angle depends upon?

The pressure angle at any point on the cam profile depends upon the following parameters :

1. Sum of the radii of the base circle and roller follower,

2. Lift of the follower

3. Angle of ascent

4. Type of follower motion

5. Follower off-set

7) What are the various measures by which pressure angle can be reduced on a cam?

pressure angle can be reduced by one or more of the following:

1. Increase the diameter of the base circle

2. Increase the angle of cam rotation for a given follower displacement.

3. Decrease the total displacement of the follower.

4. Increase the amount of follower off-set

8) What is the requirement of pressure angle for a cam?

Pressure angle is an important parameter used in cam design. It is associated with the distribution of forces

in the follower. It is the maximum value of the pressure angle that is of concern. The pressure angle

dictates the cam size, torque, loads accelerations, wear etc. the pressure angle should be kept as small as

possible, and in general, should not exceed 30 deg.


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9) What is the effect of base circle size on pressure angle?

The size of the base circle controls the pressure angle. For the same lift and angle of ascent, a smaller base

circle will give a larger value of pressure angle. Also if the base circle diameter is too small , all the points

of the follower may not be able to touch the cam surface. It is therefore preferable to use the base circle

diameter as large as possible depending upon the availability of space.

10) Name the various possible follower motions that are used in practice.

During working of a cam mechanism; the follower can have one of the following motions;

1. Simple harmonic motion

2. Uniform velocity

3. Uniform acceleration and retardation

4. Cycloidal motion

CLUTCHES

1) Classify the clutches based coupling methods.

Based on he coupling methods, clutches are classified as: positive and friction clutches.

2) What is a clutch?

A clutch is a device used to connect a driving shaft to a driven shaft so that the driven shaft may be started

or stopped at will, without stopping the driving shaft. It is fitted between the driver(source of power) and

driven shafts, for example between the engine shaft and gear box of an automobile. It permits easy

connection or dis-connection of the engine shaft and the gear box driving shaft. It thus facilitates the

change of gears, and sudden dis-connection of the drive in case of emergency

3) What is a positive clutch? Explain.

When the driving and driven shafts are connected by the engagement of inter-locking claws or jaws of the

clutch, it is said to be a positive clutch. The positive clutches give a sudden starting action and so these are

used only when the inertia of the driven parts is small of for small power transmission under slow speeds.

Otherwise the clutch should be engaged when the shafts are stationary and unloaded. It may be noted that

the clutch cannot be engaged at rest unless the jaws are aligned. However these clutches do not slip and

hence heat is not generated at the clutch surfaces


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The jaw clutches have a very important application where synchronized operation is required as for

example in power presses and punching machines.

4) Name the types of friction clutches.

The following are the types of friction clutches:

1. Disc or plate clutches. These are further sub-divided into single and multiple-disc clutches

2. Cone clutches, and

3. Centrifugal clutches

5) Differentiate between single disc clutch and multi-disc clutch.

A single disc clutch consists of a single clutch plate or disc whose both sides are covered with some

friction material. This makes both the sides of the disc effective. Multiple-disc clutch consists of two sets

of discs, made of different metals.

In general, the torque transmission capacity of a multiple disc clutch is more. In otherwords, for a given

power, the size of a multiple disc clutch is smaller the that of a single clutch

Heat generation is more in multiple disc clutches and hence they are usually operated under coolant oil

and so called as wet clutches. Single plate clutches are operated under dry conditions and so these are

called as dry clutches.

Single plate clutches are used where large radial space is available as in trucks and cars multiple disc

clutches are used where compactness is required as in scooters

6) Name four materials used for lining of friction surfaces in clutches.

1. Woven asbestos friction disc it consist of asbestos fibre woven around brass, copper or zinc wires

impregnated with rubber or asphalt.

2. Moulded asbestos friction disc it is made from the wet mixture of brass chips and asbestos which

poured into a mould followed by heating and pressing.

3. Bronze base sintered metal

4. Iron base sintered metal


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7) What is the principle of operation of centrifugal clutch?

A centrifugal clutch is used when it is required to engage the load after the driving shaft has attained a

particular speed. After attaining the specified speed the clutch is automatically engaged bringing the

driven shaft to the operating speed.

The centrifugal clutch works on the principle centrifugal force, which increase with the speed.

8) What are the desirable properties to be possessed by friction materials used in clutches?

Friction materials suitable for a clutch must possess the following properties:

1. High coefficient of friction and remain unaffected by moisture and oil.

2. Resistance to wear scoring and galling.

3. Ability to withstand high temperatures together with good heat conductivity

BRAKES

1) What for a clutch and brake are used in sequence in an automobile?

Clutches and brakes are similar in their basic function, as both are used to control the power transmission

within a machine. To stop an automobile for a while, the driving shaft is disconnected from the driven

shaft while the engine is still running and then the brakes are applied on the wheels.

2) What are the factors on which the capacity of a brake depends upon?

In the process of regulation the motion the energy of the moving parts is converted into heat energy which

must be dissipated to the surroundings the capacity of a brake depends upon the pressure between the

braking surfaces the coefficient of friction between the surfaces brake drum velocity heat dissipation

capacity of the brake,etc

3) What is the difference between the function of a brake as applied to an automobile and an elevator?

An automobile brake is used for reducing the speed of the automobile or for bringing it to rest. The

automobile brake absorbs the kinetic energy of the moving vehicle. In elevators, the brake absorbs the

potential energy released by the objects during the braking period.

4) How does the function of a brake differ from that of a clutch?


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Clutches and brakes work on the same principle, however, it may be noted that a clutch connects one

moving member to another moving member, whereas brake connects one moving member to another

stationary part.

5) Classify the mechanical brakes.

Depending upon the shape of the friction material, the mechanical brakes are classified as :

1. Block or shoe brake,

2. Band brake,

3. Band and block brake, and

4. Internal expanding shoe brake.

6) What is meant by a self energizing brake?

When the frictional force helps to apply the brake, the brake is then said to be self energizing brake

however when the frictional force is great enough to apply the brake with no external force the brake is

then said ot be self locking brake.

7) Where does a block or shoe brake is commonly used?

The block or shoe brake is commonly used in locomotives. The friction between the block and wheelinduces a tangential braking force on the wheel retarding the rotation of the wheel.

8) Where a self locking feature does is desirable?

Self locking feature is desirable in hoisting applications

9) What are the desirable properties of materials used for brake lining?

The materials used for a brake lining should posses the following properties:

1. High coefficient of friction

2. Less wear rate

3. High heat resistance

4. High heat dissipation capacity


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5. Low coefficient of thermal expansion

6. Adequate mechanical strength

10) Why should the temperature rise be kept within the permissible range in brakes?

Among many things the capacity of a brake depends upon heat dissipation capacity of the brake. If the heat

dissipation capacity is not much the temperature of the brake increases affecting its performance hence the

temperature rise of he brake must within the permissible range.

11) What is fade?

One of the desirable properties of brake lining is high coefficient of friction which should not change

however coefficient of friction of the brake lining changes due to change in temperature of the brake drum.

To begin with the coefficient of friction gradually rises with the drum temperature reaches a peak value

and then start falling rapidly as the drum temperature is increased. This rapid fall is known as fade.

12) What are the thermal considerations to be considered in brake design?

The energy absorbed by the brake is converted into heat. The heat must be dissipated to the surrounding

air. Otherwise the temperature of the brake lining increases decreasing the coefficient of friction and so

affecting the torque transmission capacity of the brake. Further at high temperatures the friction lining will

be worn-out rapidly reducing the life of the lining therefore the temperature rise should be kept within the

permissible range.

Documents

Design of Transmission Systems 2 Marks