Chapter 33 : Transmission of Power by Belts and Ropes 671
In Fig. 33.1 is shown an open belt drive consisting of pulleys A and B. The pulley A is keyed to
a rotating shaft, and is known as driver (since it drives the other pulley). The pulley B is keyed to a
shaft, intended to be rotated, and is known as follower or driven (since it is driven by pulley A).
When the driver rotates, it carries the belt due to grip between its surface and the belt. The belt,
in turn, carries the driven pulley which starts rotating. The grip between the pulley and the belt is
obtained by friction, which arises from the pressure between the belt and the pulleys. The friction
grip, if required, is increased by tightening the belt.
33.2. TYPES OF BELTS
( ) Flat belt.a ( ) V-beltb ( ) Circular belt.b
Fig. 33.2.
Though there are many types of belts used these days, yet the following are important from the
subject point of view :
- Flat belt. The flat belt is mostly used in the factories and workshops, where a moderate
amount of power is to be transmitted, from one pulley to another, when the two pulleys are
not more than 10 m apart. - V-belt. The V-belt is mostly used in the factories and workshops where a great amount of
power is to be transmitted, from One pulley to another, when the two pulleys are very near
to each other. - Circular belt or rope. The circular belt or rope is mostly used in the factories and work-
shops, where a great amount of power is to be transmitted, from one pulley to another,
when the two pulleys are more than 5 m apart.
If a huge amount of power is to be transmitted, then a single belt may not be sufficient. In such
a case, wide pulleys (for V-belts or circular belts) with a number of grooves are used. Then one belt
in each groove is provided to transmit the required amount of power from one pulley to another.
33.3.VELOCITY RATIO OF A BELT DRIVE
The velocity ratio of a belt drive may be broadly defined as the ratio of the velocities of the
driver and the follower or driven. Now we shall discuss the velocity ratios of :
1.Simple belt drive and 2. Compound belt drive.33.4.VELOCITY RATIO OF A SIMPLE BELT DRIVE
Consider a simple belt drive (i.e. one driver and one follower) as shown in Fig. 33.1.
Let d 1 = Diameter of the driver,
N 1 = Speed of the driver in r.p.m., and
d 2 , N 2 = Corresponding values for the follower.
∴ Length of the belt, that passes over the driver, in one minute = πd 1 N 1 ...(i)
Similarly, length of the belt that passes over the follower, in one minute = πd 2 N 2 ...(ii)