DTS Quiz-7

The rollers of cycle chain are subjected to following type of stress

• compressive

• tensile

• bending

• fatigue

Answer explanation

Explanation:

The power rating of the roller chain is obtained on the basis of four failure criteria.

Wear

Fatigue

Impact

Galling

Wear:

The wear of the chain is caused by the articulation of pins in the bushings which results in elongation of the chain or chain length is increased. 

This makes the chain ride out on the sprocket teeth, resulting in a faulty engagement. 

When the elongation 1s excessive, 1t becomes necessary to replace the chain. 

The permissible elongation of the chain is 1.5 to 2.5%. 

Fatigue:

As the chain passes around the sprocket wheel, it is subjected to a tensile force, which varies from maximum on the tight side to a minimum on the loose side.

The chain link is subjected to one complete cycle of fluctuating stresses during every revolution of the sprocket wheel which results in fatigue failure.  

Impact:

The engagement of the rollers with the teeth of the sprocket results in impact which leads to breakage of roller or bushing.

By increasing the number of teeth on the sprocket or reducing chain tension and speed we can reduce the magnitude of the impact force. 

Galling:

Galling is a stick-slip phenomenon between the pin and the bushing

When the chain tension is high, welds are formed at the high spots of the contacting area.

Such microscopic weld immediately broke down due to relative motion of contacting surface and eventually leads to excessive wear even in presence of the lubricant. 

For changing the direction of motion of the belt

• For changing the direction of motion of the belt

• For applying tension

• For increasing velocity ratio

• All of the above

Answer explanation

Concept:

There are two advantages of using an idler pulley:

Initial tension in the belt can be increased from time to time by adjusting the position of adjustable weight or by increasing the adjustable weight value. Due to increase in initial tension, power transmitting capacity of belt drive gets increased.

Angle of contact on the smaller pulley increases which further increases the ration between friction tension on both the sides because `T1T2" tabindex="0">T1T2T1T2 = eθ" tabindex="0">eθeθ. Hence, in effect power transmitting capacity of belt drive gets further increased.

Which one of the following is NOT correct in respect of "V" belt drive

• It is compact

• It can be easily replaced and maintenance is easy

• It causes less noise and vibration

• It is used where distance between driver and driven pulley is more

Answer explanation

Explanation:

Belt Drive

The belt drive is the most common method of power transmission.

The belts are made from leather, cotton, rubber, and synthetic materials.

There are four types of belts:

Flat belt

V - belt and multiple V - belt

Ribbed belt

Toothed or timing belt

V - Belt drive

V - belt drives are generally used when the distance between the shafts is short for flat belt drives.

Owing to the wedge action between the belt and the sides of the groove in the pulley, the V belt is less likely to slip, hence more power can be transmitted.

Advantages of V -belt drive

It is compact, so installation is possible in limited space

It is used when the center distance between the drive and the driven pulleys is short.

Less vibration and noise

Cushions the motor and bearings against load fluctuation

Easy replacement and maintenance

In flat belt drive, the condition for maximum power transmission is given by (where T maximum tension and T_c centrifugal tension in belt)

• T = 3T_c

• T = 2T_c

• T = πT_c

• T = 3πT_c

Answer explanation

Explanation:

Centrifugal Tension: 

Since the belt continuously runs over the pulleys, therefore some centrifugal force is caused, whose effect is to increase the tension on both the tight as well as the slack sides.

The tension caused by the centrifugal force is called centrifugal tension (T_c) 

Condition for Maximum power transmitted by the belt:

T = 3TC 

i.e. power transmitted will be maximum when tension is equal to three-time centrifugal tension or it shows that when the power transmitted is maximum, rd of the maximum tension is absorbed as centrifugal tension.

Power transmitted by a belt is given by:

P = (T1 – T2) × v

Which one of the following drives is used for transmitting power without slip?

• Belt drives

• Rope drives

• Cone pulleys

• Chain drives

Answer explanation

Explanation:

The power is transmitted from one shaft to the other by means of belts, ropes, chains, and gears.

The belts and ropes are flexible members which are used where the distance between the two shafts is large.

The chains also have flexibility but they are preferred for intermediate distances.

The gears are used when the shafts are very close to each other.

A chain drive is also called a positive drive because there is no slip.

If the distance is slightly larger, a chain drive can be used for making it a positive drive.

Belts and ropes transmit power due to the friction between the belt or rope and the pulley.

There is a possibility of slip and creep in belts and rope drive and that is why this drive is not a positive drive.

Hence, the gear drive and chain drive are positive drives.