BELTS
Introduction
The belts or ropes are used to transmit power from one shaft to another by means of pulleys
which rotate at the same speed or at different speeds. The amount of power transmitted
depends upon the following factors :
1. The velocity of the belt.
2. The tension under which the belt is placed on the
pulleys.
3. The arc of contact between the belt and the smaller
pulley.
4. The conditions under which the belt is used.
It may be noted that
( a) The shafts should be properly in line to insure uniform tension across the belt section.
(b) The pulleys should not be too close together, in order that the arc of contact on the smaller
pulley may be as large as possible.
(c) The pulleys should not be so far apart as to cause the belt to weigh heavily on the shafts,
thus increasing the friction load on the bearings.
(d) A long belt tends to swing from side to side, causing the belt to run out of the pulleys, which
in turn develops crooked spots in the belt.
(e) The tight side of the belt should be at the bottom, so that whatever sag is present on the
loose side will increase the arc of contact at the pulleys.
( f )In order to obtain good results with flat belts, the maximum distance between the shafts
should not exceed 10 metres and the minimum should not be less than 3.5 times the
diameter of the larger pulley.
Selection of a Belt Drive
Following are the various important factors upon which the selection of a belt drive depends:
1. Speed of the driving and driven shafts, 2. Speed reduction ratio,
3. Power to be transmitted,
4. Centre distance between the shafts,
5. Positive drive requirements,
6. Shafts layout,
7. Space available, and 8. Service conditions.
Types of Belt Drives
The belt drives are usually classified into the following three groups :
1. Light drives. These are used to transmit small powers at belt speeds upto about 10 m/s, as
in agricultural machines and small machine tools.
2. Medium drives. These are used to transmit medium power at belt speeds over 10 m/s but
up to 22 m/s, as in machine tools.
3. Heavy drives. These are used to transmit large powers at belt speeds above 22 m/s, as in
compressors and generators.
Types of Belts
(a) Flat belt.
(b) V-belt.
(c) Circular belt.
Fig. 11.1. Types of belts.
Though there are many types of belts used these days, yet the following are important from
the subject point of view :
1. Flat belt.The flat belt, as shown in Fig. 11.1 (a), 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 8 metres apart.
2. V-belt. The V-belt, as shown in Fig. 11.1 (b), 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 very near to each other.
3. Circular belt or rope. The circular belt or rope, as shown in Fig. 11.1 (c), 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 more than 8 meters 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 a
belt in each groove is provided to transmit the required amount of power from one pulley to
another.
Types of Flat Belt Drives
The power from one pulley to another may be transmitted by any of the following types of
belt drives:
1. Open belt drive.
The open belt drive, as shown in Fig. 11.3, is used with shafts arranged
parallel and rotating in the same direction. In this case, the driver A pulls the belt from one side
(i.e.lower side RQ) and delivers it to the other side (i.e. upper side LM). Thus the tension in the
lower side belt will be more than that in the upper side belt. The lower side belt (because of
more tension) is known as tight sidewhereas the upper side belt (because of less tension) is
known as slack side, as
shown in Fig. 11.3.
Fig. 11.3.Open belt drive.
2. Crossed or twist belt drive.
The crossed or twist belt drive, as shown in Fig. 11.4, is used
with shafts arranged parallel and rotating in the opposite directions.
In this case, the driver pulls the belt from one side (i.e. RQ) and delivers it to the other side
(i.e. LM). Thus the tension in the belt RQ will be more than that in the belt LM. The belt
RQ(because of more tension) is known as tight side, whereas the belt LM(because of less
tension) is known as slack side, as shown in Fig. 11.4
A little consideration will show that at a point where the belt crosses, it rubs against each
other and there will be excessive wear and tear. In order to avoid this, the shafts should be
placed at a maximum distance of 20 b, where bis the width of belt and the speed of the belt
should be less than 15 m/s.
3. Quarter turn belt drive.The quarter turn belt drive also known as right angle belt drive, as
shown in Fig. 11.5 (a), is used with 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 begreater or equal to 1.4 b, where bis the width of belt.
In case the pulleys cannot be arranged, as shown in Fig. 11.5 (a ), or when the reversible
motion is desired, then a quarter turn belt drive with guide pulley, as shown in Fig. 11.5 (b),
may be used.
4. Belt drive with idler pulleys. A belt drive with an idler pulley, as shown in Fig. 11.6 (a ), is
used with shafts arranged parallel and when an open belt drive cannot be used due to small
angle of contact on the smaller pulley. This type of drive is provided to obtain high velocity ratio
and when the required belt tension cannot be obtained by other means.
When it is desired to transmit motion from one shaft to several shafts, all arranged in parallel,
a belt drive with many idler pulleys, as shown in Fig. 11.6 (b), may be employed.
5. Compound belt drive. A compound belt drive, as shown in Fig. 11.7, is used when power is
transmitted from one shaft to another through a number of pulleys.
6. Stepped or cone pulley drive.
A stepped or cone pulley drive, as shown in Fig. 11.8, is
used for changing the speed of the driven shaft while the main or driving shaft runs at constant
speed.
This is accomplished by shifting the belt from one part of the steps to the other.
7. Fast and loose pulley drive.
A fast and loose pulley drive, as shown in Fig. 11.9, is used when the driven or machine shaft is
to be started or stopped whenever desired without interfering 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. When the driven shaft is required to be stopped, the belt is pushed on to the loose
pulley by means of sliding bar having belt forks.
Velocity Ratio of Belt Drive
