1. Flexible Connectors
Light drives Medium drives Heavy drives
Power transmitted Low Medium Large
Belt speeds (V) V< 10m/s 10 < V< 22m/s V >22m/s
Applications Agricultural machines Machine tools Compressors & generators
d<<8m d<8m d>8m
Material:
High
Co-efficient
of Friction
Leather Cotton Rubber Balata
2. Thumb Rules
1. The arc of contact on the smaller pulley should be as large as possible, hence, the pulleys
should not be too close together.
2. The pulleys should not be too far apart, to avoid:
•the belt weighing heavily on the shafts, increasing the friction load on the bearings.
•sideward swinging of the belt, risking run out of the pulleys.
4. Types of flat belt drives (Configuration)
Parallel shafts, rotating in same direction
A pulls the belt from one side (RQ) and delivers it to the other side (LM). Thus:
• the tension in the lower side will be more than that in the upper side belt.
• lower side is known as tight side, while the upper side belt is known as slack side.
5. Flexible Connectors
A pulls the belt from one side (PQ) and delivers it to the other side (LM). Thus:
• the tension in the belt RQ (tight side) > that in the belt LM (slack side).
Parallel shafts, rotating in opposite direction
To prevent the belt
from leaving the pulley
The face width of the pulley
>=
1.4 * width of the pulley
10. Slip of the belt
Assumption: Firm frictional grip between the belts and the shafts
Realism: The frictional grip may be insufficient
Effects - Some forward motion of:
• the driver pulley without carrying the belt
• the belt without carrying the driven pulley
This is called: Slip of the belt (reflecting in reduction of the Velocity ratio: Out/In)
11. Creep of the Belt
When the belt passes from the slack to the tight side, a portion of the belt extends,
and it contracts again when the belt passes from the tight to the slack side.
Due to these changes of length, there is a relative motion between the belt and
the pulley surfaces. This is termed as Creep, whose effect is to slightly reduce the
Speed of the driven pulley.
16. Ratio of Driving Tensions for Flat Belt Drive
In terms of Coefficient of friction & Angle of contact
Resolving tensions
horizontally and
vertically
does the trick
18. Centrifugal tension
As the belt runs continuously, some centrifugal force is
caused, that tends to increase the tension on both, tight
as well as the slack sides. The tension caused by
centrifugal force is called centrifugal tension (Tc).
For V<10m/s, Tc is small but should be accounted for at
larger V.
How does Fc split into extra tension?
19. Condition for transmission of Maximum Power
When the power transmitted is maximum, 1/3rd of the maximum tension is absorbed as
Centrifugal tension
T
20. Initial Tension in the Belt
The motion of the belt from the driver & follower is governed by a firm grip, due to friction
between the belt and the pulleys. In order to increase this grip, the belt is tightened up. Even
when the pulleys are stationary, the belt is subjected to some tension: Initial Tension
When the driver starts rotating, it pulls the belt from one side (increasing the tension that
side) and delivers it to the other side (decreasing the tension that side).
Assuming the belt material is perfectly elastic, implying the length of the belt remains
constant, when at rest or in motion
21. Summary
•Velocity ratio of belt drive •Velocity ratio of compound belt drive
•Slip of the belt •Creep of the belt
•Power transmitted by the belt •Ratio of driving tensions
•Open belt drive • Crossed belt drive
Angle of Contact
Length
•Centrifugal tension •Condition for transmission of max power
Max T = T1 + Tc, where,
•Initial tension in the belt
22. Problem-1
A shaft which rotates at a constant speed of 160 rpm is
connected by belting to a parallel shaft 720 mm apart,
which has to run at 60, 80, and 100 rpm. The smallest
pulley on the driving shaft is 40 mm in radius. Determine
the remaining radii of the two stepped pulleys for:
(a) A crossed belt,
(b) An open belt.
Neglect belt thickness and slip.
