BELT DRIVES

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Driver:

The pulley A is keyed to the rotating shaft and is known as driver (since it drives the other pulley).
Driven or Follower:

The pulley B is keyed to the shaft, intended to be rotated, and is known as follower or driven pulley (since it is driven by pulleys).
Types:

1. Flat Belt:

- Moderate amount of power to be transmitted.
- Two pulleys are not more than 10m apart.

1. V-Belt:

- Great amount of power to be transmitted.
- It is of trapezoidal shape and taper is 30º to 40º.
- Two pulleys are very near to each other.(less than 2 m)

1. Circular Belt or Rope:

- Great amount of power to be transmitted.
- Two pulleys are more than 5m apart.
* If a huge amount of power is to be transmitted, then a single belt may be sufficient. In such a case, wide pulleys (for v-belts or circular belts) with a number of grooves are used.
* Then a belt in each groove is provided to transmit the required amount of power from one pulley to another.
Materials used for Belts:

1. Leather belts – Soft and flexible.
2. Cotton or fabric belts – Waterproof, cheaper, suitable in warm climates. Used in farm machinery, belt conveyors.
3. Rubber belts – Very flexible and quickly destroyed when contacted with heat, oil or grease. Saw mills and paper mills. Used in place where it is exposed to moisture.
4. Balata belts – Similar to rubber belts except that balata gum is used in place of rubber. Acid proof and water proof and not affected by animal oils or alkalies.

Types of Flat Belt Drives:

1. Open belt drive – Shafts arranged in parallel and rotating in same direction.
2. Crossed or twist belt drive – Arranged parallel and rotating in opposite direction. To avoid excessive wear shafts placed at a maximum distance of 20b, where b is the width of the belt.
3. Quarter turn belt drive
4. Belt drive with idler pulley
5. Compound belt drive – Transmit from one shaft to another through a number of pulleys.
6. Stepped or cone pulley drive
7. Fast or loose pulley drive

Quarter Turn Belt Drive:

- Shafts arranged at right angles and rotating in one definite direction.
- In order to prevent the belt from leaving the pulley, the width of the face of the pulley should be greater or equal to 1.4b, where b is the width of the belt.
- In case, pulleys cannot be arranged, or when the reversible motion is desired, then belt drive with guide pulley may be used.
Belt Drive with Idler Pulley:

- Shafts arranged in parallel and when an open belt drive cannot be used due to small angle of contact on the smaller pulley.
- Provided to obtain high velocity ratio and when the required belt tension cannot be obtained by other means.
- When desired to transmit motion from one to several shafts, all arranged in parallel, a belt drive with many pulleys are employed.
Stepped or Cone Pulley Drive:

- Used for changing the speed of the driven shaft while the main or driving shaft runs at constant speed.
Fast and Loose Pulley:

- Used when the driven or machine shaft is to be started or stopped whenever desired without interfacing with the driving shaft.
- A pulley, which is keyed to the machine shaft, is called fast pulley and runs at the same speed as that of machine shaft.
- A loose pulley runs freely over the machine shaft and is incapable of transmitting any power.
Velocity Ratio of Belt Drive:

- It is the ratio between the velocities of the driver and the follower or driven.
Velocity ratio, N₁/N₂ = (d₁ + t) / (d₂ + t)
Velocity Ratio of a Compound Belt Drive:

Speed of the last follower = Product of diameters of drivers
Speed of the first driver Product of diameter of followers
Slip:

- When the frictional grip between the belts and shafts is insufficient, it may cause some forward motion of the driver without carrying the belt with it. This may also cause some forward motion of the belt without carrying the driven pulley with it. This is called slip of the belt and expressed in percentage.
- The result of the belt slipping is to reduce the velocity ratio of the system.
Creep:

- When the belt passes from the slack side to the tight side, a certain portion of the belt extends and in contracts again when the belt passes from the tight side to slack side. Due to these changes of length, there is a relative motion between the belt and the pulley surfaces. The relative motion is termed as creep.
- The total effect of creep is to reduce slightly the speed of the driven pulley or follower.
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, tight as well as the slack sides. The tension caused by centrifugal force is called centrifugal tension.
- At lower speeds the centrifugal tension is very small, but at higher speeds its effect is considerable and thus should be taken into account.
Initial Tension in the Belt:

When the pulleys are stationary, the belt is subjected to some tension called initial tension, while the belt is tightened up to increase the grip.
Advantages of V-Belt drive over Flat Belt drive:

a. V-belt gives compactness due to the small distance between the centers of pulleys.
b. The drive is positive, because the slip between the belt and the pulley groove is negligible.
c. The operation of the belt and pulley is quiet.
d. The belts have the ability to cushion the shock when machines are started.
e. The high velocity ratio (maximum 10) may be obtained.
f. The wedging action of the belt in the groove gives high value of limiting ratio of tensions. Therefore the power transmitted by v-belts is more than flat belts for the same co-efficient of friction, arc of contact and allowable tension in the belts.
g. The v-belt may be operated in either direction, with tight side of the belt at the top or bottom. The centerline may be horizontal, vertical or inclined.
Disadvantages

a. The v-belt drive cannot be used with large center distances.
b. The v-belts are not so durable as flat belts.
c. The construction of pulleys for v-belts is more complicated than pulleys of flat belts.
Rope Drive:

- The ropes for transmitting power are usually made of manila, hemp and cotton.
- Circular in cross section.
- Preferred when the distance between the shafts is large.
- Frictional grip is more than in v-drive.
- A number of separate drives may be taken from the one driving pulley.
- Groove angle of the pulley for rope drives is usually 45^o.
- Grooves in the pulleys are made narrow at the bottom and the rope is pinched between the edges of the v-groove to increase the holding power of the rope on the pulley.
Chain Drives:

- In order to avoid slipping steel chains are used.
- Ensures perfect velocity ratio.
- Transmission of power in bicycles, motor vehicles, agricultural machinery, road rollers etc.
1. Bush roller chain
- Extremely strong and simple in construction.
- Little noise produced.
- Gives good service under severe conditions also.
- Used where there is little lubrication.
2. Inverted tooth chain or silent chain
- Runs smoothly and quietly.
Advantages of Chain Drives over Belt or Rope Drives:

a. As no slip takes place during chain drive, hence perfect velocity ratio is obtained.
b. A chain drive takes up less space than a belt or rope drive.
c. The chain drives may be used when the distance between the shafts is less.
d. The chain drives gives high transmission efficiency.
Disadvantages:

a. The production cost of chains is relatively high.
b. The chains require more careful maintenance.