SRI KRISHNA COLLEGE OF ENGINEERING & TECHNOLOGY, COIMBATORE – 08

DEPARTMENT OF MECHANICAL ENGINEERING

DESIGN OF TRANSMISSION SYSTEMS

2 – MARKS QUESTIONS – FLAT BELT DRIVES

1. What is a power drive? Mention their types.

The power drive is a set of machine members employed to transmit power or energy produced in one machine to

another machine. Their main types are

1. Mechanical 2. Hydraulic 3. Pnewmatic and 4. Electrical drives.

2. State the law of belting.

Law of belting states that the centre line of the belt when it approaches the pulley must lie in

the mid plane of that pulley which should be perpendicular to the axis of the pulley. Otherwise the belt will run off the

pulley.

3. Explain the term Crowning of pulley.

Pulley are provided a slight conical shapes (or) convex shapes in their rim’s outer surface in order to prevent

the belt from running off the pulley due to centrifugal force. This is known as crowning of pulley. Usually the

crowning height may be 1/96 of pulley face width.

4. Why are belt drives called as flexible drives?

In belt drives, slight deviation of shaft axes from their parallel position will not affect their function much

whereas this is a very dangerous fault as in gear drives. Also the belt pulleys are freed from some critical parameters

similar to gear module and hence the slight change of pulley diameters will have no effect on engagement. So belt drives

are considered as flexible drives.

5.What are the types of belts used for transmission of power?

1. Flat belts

2. V- Belts

Single V belt

Multiple V belt

Ribbed Belt

3. Toothed or Timing Belt

4. Round Belts.

6.List the materials used for belt drives.

Leather, cotton fabrics, rubber, silk, nylon, etc.

7. List the advantages and disadvantages of belt drives.

Advantages: -Used for long distance power transmission.

-Operations are smooth and silent.

-Ability to absorb the shocks and damp vibration

-Transmit only a definite load, which if exceeded, will cause the belt to slip over the pulley, thus protecting the

parts of drive against overload

-Simple design and manufacturing cost is lower.

Disadvantages: Have large dimensions and occupy more space. Velocity

ratio is not constant due to belt slip Impose heavy loads

on shafts and bearings

Loss of power resulting in low efficiency. Have

comparatively short service life.

8. What is the use of idler pulleys in a belt drive?

To obtain high velocity.

Many idler pulleys are used when it is desired to transmit motion from one shaft to several parallel shafts.

9. Define velocity ratio.

It is defined as the ratio between the velocity of the driver and the driven.

10. What is meant by the term ply of the belt?

Flat belts are made of thin strips and laminated one over the other in order to get thick belt. These thin strips or

sheets are called as plies of belt. Usually flat belts are made of 3 ply, 4 ply, 5 ply,

6 ply and 8 ply etc. and 4 ply belt is thicker than 3 ply belt and so on.

11. List the different types of joints employed for joining the flat belts.

Cemented Joints, Laced Joints, Crest Joints, Hinged Joints.

12.Why tight side of the belt should be at the bottom of side of the pulley?

The driving pulley pulls the belt from the lower side and delivers it to the upper side. So the lower side of the

belt is tight.

13. What is the difference between the open belt and cross belt drive?

In open belt drive the direction of rotations of driving and driven pulleys are same whereas in cross belt drive

they are opposite to each other.

14. What is belt rating?

It is defined as the power transmitting capacity of unit size flat belt or a particular grade single

V belt.

15. What is initial tension in belts?

The motion of belt and pulleys is governed by a firm grip between the belt and pulley. In order to increase this

grip for proper power transmission, the belt is tightened up. At this stage, even the pulleys are not running i.e., at rest the

belt is subjected to some tensions called as initial tension.

16. List the factors to be considered during the selection of a belt drive.

1. Amount of power to be transmitted.

2. Centre distance between shafts.

3. Space available.

4. Speed ratio.

17. List the factors on which the co-efficient of friction between the belt and pulley depends?

1. Belt material

2. Pulley material

3. Slip of the belt

4. Speed of belt.

18. List the losses in belt drives OR What are the various losses in the power transmission by belts.

1. Frictional losses.

2. Losses due to slip and creep.

3. Loss in bearings.

4. Windage losses or air resistance to the movement of pulley and belt.

19. A longer belt will last more than a shorter belt. Why?

The life of the belt is a function of the centre distance between the driven and driver shafts.

The shorter the distance, more often the belt will be subjected to bending stress while running around the pulleys. Hence a

longer belt will last more than a shorter one.

20. List the commercial types of flat belts.

HISPEED, FORT

21. Define slip.

It is defined as the relative motion between the belt and the pulley. The difference the linear speeds of pulley

rim and belt is the measure of slip. By roughening the belt or by crowning the slip can be avoided.

22. What is wipping and how it can be avoided?

If the centre distance between the belts are too long then the belt begins to vibrate in a direction

perpendicular to the direction of motion of belt. This phenomenon is termed as wipping.

Wipping can be avoided by using the idler pulleys.

23.Brief about the centrifugal tension.

As the belt moves round the pulley it would experience a centrifugal force which has a tendency to separate the

belt from the pulley surface. To maintain contact between pulley and belt, the centrifugal force produce additional tension

in the belt, which is known as centrifugal tension. Tc = mv2

24. What is initial tension of the belt?

the tension of the belt when a belt is fitted to a pair of stationary pulleys is termed as the initial tension of the

belt. To = T1+T2

----------- 2

25. Formula to find out the power transmitted by a belt P = (T1 – T2)v.

26. Ratio of driving tensions tensions for flat belt drive.

T1/T2 = eµØ

T1 - mv2

/T2 - mv2

= eµØ

27. Condition for the transmission of maximum power.

T = 3Tc

V = (T/3m)1/2

28. List the desirable properties of belt materials.

- Belt material should have high coefficient of friction with the pulleys

- Belt material should have high tensile strength to withstand belt tension

- Belt material should have high wear resistance

- Belt material should have high flexibility and low rigidity in bending in order to avoid

bending stresses while passing over the pulley.

29. List the various applications of belt drives.

Mainly used in electric motors, automobiles, machine tools and conveyors.

30. List the advantages of Flat belts over V belt drives.

- Cheap and easy to maintain.

- Different velocity ratios can be obtained by using a stepped pulley, where the belt is shifted from one step to

another, having different diameters.

- Can be used in dusty and abrasive atmosphere and require no closed casing.

- Design of flat drive is simple and inexpensive.

- They can be used for long center distances, even upto 15 m.

31.List the disadvantages of flat belt drive over V belt drives.

- Power transmission capacity of flat belt drive is low.

- Velocity ratio of flat belt drive is lower than V belt drive.

- Flat belt drives have large dimensions and occupy more space compared with V belt drives.

- Flat belts generate more noise than V belts.

- Flat belts are horizontal in general, and not vertical.

32. List the applications of flat belt.

Used in belt conveyors, baking machinery, brick and clay machinery, crushers, saw mills, textile

machinery, line shafts and bucket elevators.

33. List the advantages of V belts over the Flat belt drives.

- The force of friction between the surfaces of the belt and V grooved pulley is high due to wedge action.

Results in increase in the power transmitting capacity.

- V belts have short centre distance, that results in compact construction.

- Permit high speed reduction even upto seven to one.

- Smooth and quiet operation, even at high operating speeds.

- Drive is positive because the slip is negligible due to wedge action.

- V belts can operate in any position, even when the belt is vertical.

34. List the disadvantages of V belt drives over flat belt drives.

- The efficiency of the V belt is lower than that of he flat belt and the creep is also higher.

- The construction of V grooved pulleys is complicated and costlier compared with pulleys of the flat belt

drives.

35. List the applications of V belts.

- V belts are very popular where electric motor is used as the prime mover to drive compressors, pumps, fans,

positive displacement pumps, blowers and machine tools. They are also popular in automobiles to drive accessories on

petrol or diesel engines.

36. List the advantages of fabric rubber belts.

- Have high load carrying capacity.

- Have long service life

- Can operate at high operating speeds upto 300 m/s

37. List the disadvantages of fabric rubber belts.

- They cannot operate on small diameter pulleys.

- They are subjected to destruction in an environment of mineral oil, gasoline and alkalis.

2 – MARKS QUESTIONS – V - BELT DRIVES

1. List the advantages of V belt over flat belt.

Power transmitted is more due to the wedging action. More

compact.

Smooth and noiseless operation.

Able to obtain higher velocity ratio (10m/s)

Positive drive (no slip)

2. How a V belt can be specified or designated?

V belts are designated by its grade letter followed by its inside length in mm or in inches. Ex. D – 3048.

3. List the types of grades available in V belt.

A, B, C, D, E.

4. Ratio of driving tensions for V belt

T1/T2 = eµ . cosec

5. No. of belts required = Total power transmitted / power transmitted per belt.

6. In what ways timing belts or ribbed V belts or synchronous belt are superior than ordinary belts V

belts?

Since the timing belts possess toothed shape in their inner side, their engagement with toothed pulley will provide

positive drive without any slip. Whereas in the case of ordinary belt possibility of

the slip is more. In this manner toothed belts are superior.

7. List the advantages of ribbed V belts or timing belts.

- It is a positive drive; there is no slip and no vibration in output speed.

- It has a high strength to weight ratio, which allows for high pitch line velocities up to 80 m/s.

- The belt is thin and flexible, which permits the use of small diameter pulleys as small as 15 mm diameter.

- The ribbed V belt drive does not require any lubrication like chain drive.

- Ribbed V belt does not require initial tension like flat belt. This reduces bearing reactions.

8. List the disadvantages of ribbed V belt or timing belt drives.

- It is costly compared with flat or V belts

- Construction of sprocket is difficult compared with the pulleys for flat or V- belts

- It is more sensitive to misalignment than flat or V belts.

9. List the applications of ribbed V belts or timing belts.

Used in automobiles for driving camshaft from engine crankshaft. Also used in business machines,

sewing machines, timing devices and power transmission units..

10. What are the information required to select a V belt drive?

- Type of driving unit

- Type of driven machine

- Operational hours per day

- Power to be transmitted

- Input and output speeds

- Approximate centre distance depending upon the availability of space.

2 – MARKS QUESTIONS – CHAIN DRIVES

1. Write the features of a chain drive.

A chain drive is considered to be intermediate between belt drive and gear drive and it has the features common

to both. Chains are suitable for long as well as short distance drives and compact than belt drives. Chain drives are

similar to belt drives in which the chains are operated between toothed wheels called as sprockets.

2. List the applications of chain drives.

Used in transportation machineries like motor cycle, bicycles, automobiles and in technological machineries like

agricultural machines, metal and wood working machines.

3. List the advantages of chain drives.

1. More power transmitting capacity.

2. Higher efficiency and compact size.

3. No initial tension is applied on the sprocket shafts so it exert less load on sprocket shafts.

4. List the disadvantages of chain drives.

1. Design is complicated.

2. Noisy operation.

3. Production cost is high.

4. Require more accurate assembly of shafts than for belts.

5. List the types of chains.

1. Driving Chains

2. Hauling Chains

3. Loading Chains.

6. List the types of driving chains.

Roller Chains, Bush Chains, Silent (or) incerted toothed chains. Simplex,

Duplex, Triplex Chains.

7. In what way the silent chain is better than ordinary driving chain?

A silent chain consists of a series of toothed plates pinned together in rows across the width of the chain. Even

though the structure of silent chain is more complicated, its engagement with

sprocket is very smooth and hence we get soundless operation. That is why, silent chain is preferred.

8. List the components of a chain drives.

Chains, Sprockets, Chain housing, Slack adjusters.

9. What are the possible ways by which a chain drive may fail?

Wear in the joints which leads to the elongation of chain resulting improper engagement with the sprockets.

Wear of the sprockets.

Fatigue failure of the plates at the eyes. Poor

lubrication and improper maintenance.

10. What is the difference between bush chain and roller chain?

Even though the bush and roller chains seem to be similar, roller chains are provided with cylindrical plates

over the chain pins which are not available in bush chains. Hence wear due to engagement can be reduced much in

roller chains.

Also roller chains are heavier than bush chains.

11.List the materials the chain components are made of?

Chain plates – Medium carbon or alloy steels.

Chain pins, bushing, rollers – carburising steels and hardened to 50 – 65 RC. Chain

sprockets – cast iron or hardened steels.

12. What is slack adjusters?

Slack adjuster is a movable bearing which is employed for regulating chain sag and tensions in order to get proper

alignment. The movable bearings are in the forms of slides in which the sprockets shafts are installed.

13.In chain drives sprocket has odd no. of teeth and the chain has even no. of pitches. Why?

In order to balance and correct engagement of chain with the sprockets, length of chain should be corrected to

even no. of pitches.

14. What is chordal action in chain drives?

When chain passes over a sprocket, it moves a series of chords instead of a continuous arc as in the case of belt

drives. It result in varying speed of the chain drive. This phenomenon is known as chordal action. It results in pulsating

motion and jerk motion of the chains.

15. List the factors which affect the working conditions of a chain drives.

Type of loading (variable or constant), Distance between the driving and driven sprockets, Position of

driving shafts such as horizontal or inclined, Lubrication, Service, Environmental conditions such as corrosive and

dust atmosphere.

16. Mention the applications of hauling and hoisting chains.

Used in material handling devices such as cranes, elevators, winches and so on.

UNIT – II TWO MARKS QUESTIONS – SPUR GEAR DRIVES

1. What are the advantages of gear drives over other types of drives?

1. High load carrying capacity 2. High efficiency

3. Compact layout

4. Transmit power from very small value to thousands of value.

5. Exact velocity ratio is obtained – no slip.

2. Mention some applications of gear drives.

Automobiles, hoisting machineries, rolling mill, machine tools such as lathe, drilling machine etc.

3. How are gears classified?

1. Based on axes of gear shafts.

Parallel – Spur, Helical

Intersecting – Bevel gears

Non parallel non intersecting – worm gears

2. Profile of gear tooth

Involute, cycloidal gears.

4. What are the materials used for gear manufacturing?

1. Ferrous metals – carbon steels, alloy steels of nickel, chromium and vanadium.

2. Cast iron of different grades.

3. Non ferrous metals such as brass, bronze etc.

4. Non metals – phenolic resins, nylon, bakelite etc.

5. What factors influence backlash?

Module, Pitch line velocity, errors in tooth thickness, pitch, profile.

6. Define transmission ratio.

Ratio of speed of driving gear to the speed of driven gear.

7. Define module.

Ratio of pitch circle diameter to the number of teeth.

8. At what occasions non metallic gears are used?

Employed where silent operation and low power transmission is required.

9. Specify the types of gears failure.

Tooth breakage Pitting of tooth surface

Abrasive Wear Seizing of teeth

10. Tip Circle or Addendum Circle – The circle which coincides with the tops of all teeth.

11. Root Circle or Deddendum Circle – The circle which coincides with the roots or bottom of all the teeth.

12. Pitch Circle – It is the imaginary circle in which the pair of gears roll one over the other. This circle can be

visible when the pair of gears are fastly rotating. This will lie between tip circle and root circle.

13. Pressure angle – It is the angle making by the line of action with the common tangent to the pitch circles of

mating gears.

14. Circular Pitch – It is the distance along the pitch circle between corresponding profiles of adjacent teeth.

15. Diametral Pitch – It is the ratio of number of teeth on the gear to the unit length of pitch circle diameter.

16. What is working depth of a gear tooth?

It is the radial distance from the addendum circle to the clearance circle.

17. How number of teeth affects the design of gears?

Because most of the design parameters of gears such as module, centre distance, bottom clearance, face

width, tooth depth, pitch circle diameter, tip diameter and root diameter are based on the number of teeth of

the gears.

18. State Law of Gearing.

Law of gearing states that for obtaining a constant velocity ratio, at any instant of teeth the common normal at

each point of contact should always pass through a pitch point(fixed point), situated on the line of joining the

centers of rotation of the pair of mating gears.

19. What is the effect of increasing and decreasing the pressure angle in gear design?

- Increasing the pressure angle will increase the beam and surface strengths of tooth. But gear becomes noisy and

shortens the duration of contact.

- Decreasing the pressure angle will increase the minimum number of teeth required on the pinion to avoid

interference/undercutting.

20. Write short notes on backlash of gears.

It is defined as the ply between a mating pair of gear teeth in assembled condition. It is the amount, by which the

width of tooth space exceeds the thickness of meshing tooth, measured on the pitch circle. Proper amount of

backlash ensures smooth running of the gear set. The main purpose of providing backlash is to prevent jamming

and to ensure that no contact is made on both sides of teeth simultaneously.

21. What factors influence backlash?

The factors like errors in tooth thickness, pitch, tooth spacing, profile, etc influence the backlash.

22. What is interference in gears?

The phenomenon when the tip of a tooth undercuts the root on its mating gear is known as interference.

23. What stresses are induced in gear tooth?

1. Surface compressive stress. 2. Bending stress

24. AGMA stands for what.

The American Gear Manufacturers Association

25. List the standard systems for the shape of gear teeth.

- 14.5 ° Full depth involute system

- 20° Full depth involute system

- 20 ° Full depth involute system

26. List the methods to manufacture the gears.

- Milling

- Hobbing

- Rack generation

- Gear shaper

27. List the advantages of involute gears.

- Centre distance for a pair of involute gears can be varied within limits without changing the

velocity ratio.

- The pressure angle, from the start of the engagement of teeth to the end of the engagement,

remains constant.

- Face and flank of involute teeth are generated by a single curve whereas in cycloidal gears,

double curves (i.e. epicycloids and hypocycloid) are required. Thus the involute teeth are

easy to manufacture than cycloidal teeth.

28. List the advantage of cycloidal gears.

- Cycloidal teeth have wider flanks, therefore the cycloidal gears are stronger than the

involute gears for the same pitch.

- In cycloidal gears, the contact takes place between a convex flank and concave surface, whereas

in involute gears, the convex surfaces are in contact. This condition results in less wear in

cycloidal gears as compared to involute gears.

- The interference does not occur at all.

29. What are the advantage of 20 ° pressure angle system over 14.5 ° pressure angle system?

- It reduces the risk of undercutting

- It reduces the interference

- Due to the increased pressure angle, the tooth becomes slightly broader at the root. This makes

the tooth stronger and increases the load carrying capacity.

- It has greater length of contact.

30. List the functions of idler gear.

- Idler gears fill the space between the driving and driven gears.

- Idler gears change the direction of rotation of the last driven shaft relative to the first driving

shaft.

31. What are the methods to eliminate interference?

- Increase the number of teeth on the pinion.

- Increase the pressure angle

- Use long and short addendum gearing

32. List the objectives for providing backlash.

- Backlash prevents the mating teeth from jamming together.

- Backlash compensates for machining errors.

- Backlash compensates for thermal expansion of teeth.

33. What are the methods to provide backlash?

- The teeth of the gear are cut slightly thinner. This is obtained by setting the cutting tool

deeper into the blank resulting in thinner tooth and wider space.

- The centre distance between mating gears is slightly increased.

34. How does the number of teeth affect the design of gears?

- As the number of teeth decreases, a point is reached where there is interference

and the standard tooth profile requires modification.

- As the number of teeth increases, the pitch circle diameter and the size of the gear wheel also

increase, thus increasing the cost. Therefore the number of teeth on the pinion as well as on

the gear should be kept as small as possible.

2 MARKS – HELICAL GEARS

1. What is a helical gear?

A helical gear is a cylindrical gear similar to spur gear except that the teeth are cut at an angle, known as helix

angle, to the axis of the gear shaft, whereas in spur gear, teeth are cut parallel to the axis.

2. In what ways helical gears are differed from spur gears?

Spur gears Helical gears 1. Teeth are cut parallel to the axis Teeth are cut inclined to the axis

2. Rough and noisy operation Smooth and silent operation 3. Less power is transmitted More power can be transmitted.

3. What are the advantages of helical gears?

Provide smooth and soundless operation High load carrying capacity than spur gears

Used for high speed and high velocity ratio processes

4. What is helix angle?

The angle between the axis of the gear and the line passing through tooth face is called helix angle.( it is 8

– 25 degrees)

5. Brief the advantages of double helical gears.

In single helical gears, during power transmission some axial thrust will be produced between the teeth which

reduces the life of teeth whereas in double helical gears,

since they have two helical gears, whose teeth are cut in the opposite directions, the

thrust produced in one gear is nullified by the thrust produced in another gear and hence the resulting axial

thrust will become zero which result more life than single helical gear. Also double helical gear transmit

more power than single helical gear.

6. What is a herringbone gear?

A herringbone gear is made of two single helical gears attached with each other hence called as double helical

gear in which the teeth of one gear will be set in the opposite direction to the teeth of another gear. By this

arrangement the axial thrust produced

in one gear will be nullified by the thrust produced in another gear, and the resultant thrust is zero which improves

the life of the gear. Sometimes, a single cylindrical block

is employed for making herringbone gear.

7. Give a brief account of skew gears (crossed helical gear/spiral gears).

Skew gears are the helical gears in which the helix angles are more than that of ordinary helical gears and the axes

of pinion and wheel are arranged in such a way that their axes are non parallel and also non intersecting and hence

skew gear drive is called as crossed helical gear drive. They can be used for small power transmission such as in

instruments and distributor drive of automobile engine etc.

Skew gears are used to connect and transmit motion between two non parallel and non intersecting shafts.

8. What is meant by virtual number of teeth?

In order to simplify the design of helical gear, an equivalent spur gear is formed whose teeth will produce

same effect as that produced by the helical gear. Those

teeth of equivalent spur gear are called as virtual teeth and it is calculated as Zv = Z/cos3

9. Why is the crossed helical drive mostly not used for power transmission?

As the contact between the mating teeth of crossed helical gears is always a point,

these gears are suitable only for transmitting a small amount of power. That’s why

mostly these gears are not used for power transmission.

10.List the disadvantage of helical gear.

There is some axial thrust between the teeth, which is a disadvantage.

11.What is the difference between the herringbone gear and double helical gear?

-There is a groove between two helical gears in case of double helical gear. Double helical gears are cut on a

single gear blank

- While a gear without a groove is called herringbone gear. Herringbone gear is cut by two cutters.

12. List the applications of double helical and herringbone gears.

- Used in high power applications such as ship drives and turbines.

13. List the advantages of herringbone and double helical gears.

- They develop opposite thrust reactions and thus cancel out the thrust force within the gear itself. The net

axial force that acts on the bearings is zero.

- The power transmitting capacity is high.

- High pitch line velocities can be attained with herringbone and double helical gears.

14. List the disadvantages of herringbone and double helical gears.

- Herringbone and double helical gears are very expensive compared with helical gears.

- Balancing of thrust forces depends upon the equal distribution of load between the

right and left part of the gear. Therefore, a high degree of precision is required to locate herringbone and double

helical gears axially on the shaft.

15. Where do we use skew helical gears? What are the applications of skew helical gears?

UNIT – III TWO MARKS – BEVEL GEARS 1. What is a bevel gear?

Bevel gear is the type of gear for which the teeth are cut on conical surface in contrast with spur and helical gears

for which the teeth are cut on cylindrical surfaces. The structure of bevel gear is similar to an uniformly serrated

frustum of a cone.

2. When do we use bevel gears?

When the power is to be transmitted in an angular direction, i.e., between the shafts whose axes intersecting at

an angle, bevel gears are employed.

3. Differentiate a straight bevel gear and a spiral bevel gear.

When teeth formed on the cones are straight, the gears are known as straight bevel gears and when inclined,

they are known as spiral or helical bevel gears.

4. What is a crown gear?

A crown gear is a type of bevel gear whose shaft angle is 90 degree and the pitch angle of pinion is not equal

to the pitch angle of gear. (or) In a pair of bevel gears, when one of the gear has a pitch angle of 90 degree,

then that gear is known as crown gear.

5. What is zerol bevel gear?

A spiral bevel gear with curved teeth but with a zero degree spiral angle is known as zerol bevel gear.

6. What is the specific feature of miter gear?

Miter gear is the special type of crown gear in which the shaft angle is 90 degree and the pitch angles of pinion

and gear are equal and each angle is equal to 45 degree.

(OR) When two identical bevel gears are mounted on shafts, that are intersecting at right angles, they are called

miter gears.

7. Pitch angle: It is the half of the cone angle subtended by a bevel gear at its apex.

8. Shaft angle: It is the sum of the pitch angles of pinion and gear of a bevel gear pair.

9. Tip angle: It is the angle between the axis of gear and the line passing through the top surface of tooth.

10. Root angle: It is the angle between the axis of gear and the line passing

through the root surface of tooth.

11. Cone distance or pitch cone radius: It is the length of the pitch cone element in bevel gears.

12. Face angle: It is the angle subtended by the face of the teeth at the cone

center. It is equal to the pitch angle plus addendum angle. It is also called tip angle.

13. What is skew gears?

When two straight bevel gears are mounted on shafts, which are non parallel and non intersecting, they are called

skew bevel gears.

14. What are the various forces acting on a bevel gear?

- Normal Force - Radial Force

15. Mention the characteristics of miter gear.

- The pitch angles of pinion and gear of miter gears are same and each is equal to 45 degree

- The pinion and gear of miter gears rotate at same speed.

- The pinion and gear have same dimensions namely, addendum, dedendum, pitch circle diameter, number of

teeth and module.

- The pinion and gear of miter gears are always mounted on shafts, which are

perpendicular to each other.

16. Mention the characteristics of crown gear.

- The pitch angle of gear is 90 degree.

- The bevel pinion and crown gear are always mounted on shafts, that are intersecting at angle more than 90

degree.

17. What is internal bevel gear?

- When the teeth of bevel gear are cut on the inside of the pitch cone, it is called internal bevel gear.

Internal gears are used in planetary gear trains.

18. Mention the characteristics of skew bevel gears.

- Skew bevel gears have straight teeth. - Skew bevel gears are mounted on non-parallel and non-intersecting shafts.

19. What is hypoid gears?

- Hypoid gears are similar to spiral bevel gears that are mounted on shafts, which are non parallel and non

intersecting.

20. Mention the characteristics of hypoid gears.

- Hypoid gears have curved teeth

- Hypoid gears are mounted on non-parallel and non-intersecting shafts.

21. Where hypoid gears are used?

It is mainly used in automobile differentials.

- These gears allow the drive shaft to be placed well below the centerline of the rear axle and thereby lower the

centre of gravity of vehicle.

2 MARKS – WORM GEARS

1. What is a worm gear drive?

Worm gear drive is a type of gear drive in which pinion is made as lead screw shape and the wheel is made as

helical gear structure and it is employed for transmitting

power between the shafts whose axes are at right angle to each other. Also this gear drive is used for high velocity ratio of about 100, sometimes up to 500.

2. When do we use worm gears?

When we require to transmit power between non parallel and non intersecting shafts and very high velocity

ratio of about 100, worm gears can be employed.

3. What kind of contact occurred between worm and wheel? How does this differ from other gears?

In worm gear drive, sliding contact is occurred between the worm and wheel whereas other gears such as spur,

helical and bevel gears transmit power by rolling contact.

4. What is irreversibility or self locking in worm gears?

It means that the motion cannot be transmitted from worm wheel to the worm. This property of

irreversible is advantageous in load hoisting

applications like cranes and lifts.

5. Write some applications of worm gear drive.

Worm gear drive find wide applications like hoisting equipments, milling machine indexing head, table

fan, steering rod of automobile and so on.

6. Pitch: Pitch of worm is the distance, measured axially, from a point on one tooth to the corresponding point on

an adjacent tooth.

7. Lead: Lead of the worm is defined as the distance, measured axially, between the corresponding points of

adjacent teeth for the same helix. Based on the number of

starts, lead can be varied from pitch.

Usually Lead = No. of starts x Pitch

8. What are the commonly used materials for worm and wheel?

Worm Material: Steel, case hardened steel, hardened molybdenum steel.

Wheel Material: Cast iron and phosphor bronze

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

The efficiency of the worm depends mainly on pressure angle(also known as pitch angle of the worm). For a

single start worm this pressure angle will be less. In a

multistart worm, this pressure angle can be increased( of the order 45 degree). That’s

why multistart is more efficient.

10.Why is the efficiency of worm gear drive comparatively low?

Because of power loss due to friction caused by sliding.

11. When the number of start of a worm is increased in a worm gear drive, how it affects the other

parameters and action of the drive?

The increase in number of starts on the worm will increase the lead and lead angle of the worm. This results in

higher friction losses and hence the lower efficiency.

12. Why phosphor bronze is widely used for worm gears?

It have high antifriction properties to resist seizure. Because in worm gear drive, the failure due to seizure is

more.

13.List out the main types of failure in worm gear drive.

Seizure, pitting and rupture.

14. In worm gear drive only the wheel is designed. Why?

Since always the strength of the worm is greater than the worm wheel, therefore only the worm wheel is

designed.

15. Give the merits and demerits of worm gear drive.

Merits: 1. Compact layout for very high velocity ratio of about 100

2. Smooth and noiseless operation 3. Self locking facility is available.

Demerits: 1. Low efficiency. 2. More heat will be produced.

3. Low power transmission.

16. What are the forces acting in worm and worm wheel?.

- Tangential force on the worm

- Axial force on the worm

- Radial force on the worm.

17. Name the two classes of worm gear drives.

- Single enveloping worm gear drive

- Double enveloping worm gear drive

18. What is single enveloping worm gear drive?

- Single enveloping worm gear set is one in which the gear wraps around or partially encloses the worm.

This results in line contact between the threads of worm and the teeth of worm wheel.

19. What is double enveloping worm gear drive?

- A double enveloping worm gear set is one in which the gear wraps around the worm and worm also

wraps around the gear. This results in area contact between the threads of worm and the teeth of worm

wheel. This drive is also called cone gearing.

20. What are the advantages and disadvantages of double enveloping worm gear drive?

Advantages

- The contact pressure between the threads of worm and the teeth of worm wheel is low. This reduces

wear.

- The drive occupies less space for a given capacity.

Disadvantage - Requirement of precise alignment.

UNIT –IV TWO MARKS – GEAR BOX

1. On what situations gear boxes are used?

Gear boxes are used whenever the variable spindle speed is necessary. Since the gear box is provided with number

of gears of different sizes, arranged in different forms,

we can get number of output speeds by operating a motor at single speed.

2. Why G.P. series is selected for arranging the speeds in gear box?

The speed loss is minimum.

The no. of gears to be employed is minimum. The layout is

comparatively very compact.

It provides a more even range of spindle speed at each step.

3. List the methods used for changing speeds in gear box.

Sliding mesh gear box Constant mesh gear box

4. What are preferred numbers?

Preferred numbers are the conventionally rounded off values derived from geometric series. There are five basic

series, denoted by R5, R10, R20, R40 and R80 series.

5. What is step ratio? Or Define progression ratio.

When the spindle speeds are arranged in geometric progression, then the ratio between the two adjacent

speeds is known as step ratio or progression ratio.

6. What is a speed diagram? Or What is the structural diagram of a gear box?

Speed diagram or structural diagram is the graphical representation of different speeds of output shaft,

motor shaft and intermediate shafts.

7. What does the ray diagram of gear box indicate?

The ray diagram or kinematic arrangement of a gear box indicates the arrangement of various gears in various

shafts of the gear box in order to obtain the different output speeds from the single speed of the motor.

8. What is the function of spacers in a gear box?

Spacers are sleeve like components which are mounted on shafts in between gears and bearings or one gear and

another gear in order to maintain the distance between them so as to avoid interruption between them.

9. What are the main components of gear box?

Gears, Shafts, Bearings, Spacers, Levers for changing the position of gears for engagement, gear box

housing, oil seals for lubrication.

10.What purpose does the housing of gear box serve?

Gear box housing or casing is used as container inside which, the gears, shafts bearings and other components are

mounted. Also it prevents the entry of dust inside the

housing and reduce noise of operation. That is, the housing safe guard the inner components.

10.What are the methods of lubrication in speed reducers?

1. Splash or spay lubricating method

2. Pressure lubrication method

11. Give some applications of constant mesh gear box.

Constant mesh gear boxes are employed in various machine tools like lathe, milling machines etc., to provide

a wide range of spindle speeds.

12.What are the basic rules to be followed while designing a gear box?

- transmission ratio is limited by ¼ ≤ i ≤ 2

- In all stages except in the first stage, Nmax ≥ Ninput > Nmin

UNIT – V TWO MARKS – CLUTCHES

1. What is the function of a clutch?

The clutch is a mechanical device which is used to connect or disconnect the source of power at the operator’s will.

2. Specify the types of clutches.

1. Positive clutches or jaw clutches

2. Friction clutches

1. Disc clutches

2. Cone clutches

3. Centrifugal clutches 3. Magnetic clutches

4. What is meant by positive clutch?

A clutch which transmits power from driving shaft to the driven shaft by means of jaws or teeth is called positive

clutch. Jaw clutch are superior to friction clutches in that they ensure exact contact (without slipping), produce

equality in angular velocities of the connected shafts.

5. Define coefficient of friction.

Coefficient of friction is defined as the ratio of the force required to cause uniform sliding velocity of the block on

the surface to the normal pressure between the surfaces in

contact.

6. In what way clutches are differed from flange couplings?

Flange couplings are used as permanent connecting elements between two power transmitting elements whereas

clutches are used as temporary connecting elements and hence periodical engagement is possible in clutch

connection.

Flange couplings are having less number of parts such as flanges, bolts, keys etc. whereas the clutches

comparatively of large number of parts such as friction discs,

springs, keys, supporting plates and so on.

7. What are the properties required for the material used as a friction surface? The properties required

for the friction materials are

1. A high and uniform coefficient of friction

2. The ability to withstand high temperatures, together with heat conductivity. 3. High resistance to wear

8. Name few commonly used friction materials.

Wood, cork, leather, asbestos based friction materials.

9. Why are cone clutches better than disc clutches?

Since the cone discs are having large frictional areas and wedging action, they can transmit a larger torque than disc

clutches with the same outside diameter and actuating force and hence cone clutches are mainly used in low

peripheral speed applications.

10.Distinguish between dry and wet operation of clutches.

-When a clutch operates in the absence of a lubricant, then that clutch is known as dry clutch. In dry clutch the

torque capacity is high but the heat dissipating capacity is low

- When a clutch operates with lubrication, then that clutch is known as wet clutch. In wet

the torque capacity is low but the heat dissipating capacity is high.

11.What is the difference between cone and centrifugal clutches?

Cone clutches works on the principle of friction alone. But centrifugal clutch uses principle of centrifugal force in

addition to friction.

2 MARKS – BRAKES

1. What is the function of a brake?

Brake is a mechanical device by means of which motion of a body is retarded for slowing down or to bring it to rest,

by applying artificial frictional resistance.

2. Differentiate a brake and a clutch.

A clutch connects two moving members of a machine, whereas a brake connects a moving member to a

stationary member.

3. Why in automobiles, braking action when traveling in reverse is not as effective as when moving forward?

When an automobile moves forward, the braking force acts in the opposite direction to the direction of motion of the

vehicle. Whereas in reverse traveling the braking force acts

in the same direction of motion of the vehicle. So it requires more braking force to apply brake.

4. What is fade?

When the brake is applied continuously over a period of time, the brake becomes overheated and the coefficient of

friction drops. This results in sudden fall of efficiency of

the brake. This phenomenon is known as Fade or fading.

5. What is meant by self energizing brake?

When the moment of applied force and the moment of frictional force are in the same direction, then frictional force

helps in applying the brake. This type of brake is known as a self-energizing brake.