GE GEH-5304A User Manual
Page 10
GEH-5304A
10
The radial load, W, produced by the belts when tightened just enough to transmit the load without
slipping is given by the relation:
126,000 x HP
W
=
x K
b
, lbs
D x RPM
Where:
D
=
Sheave pitch diameter in inches for V-belt application
HP
RPM
=
Maximum ratio of horsepower, including overloads, to the minimum speed at which that
power occurs.
K
b
=
Belt tension factory from table below:
Belt Tension Factor, K
b
K
b
Description
1.0
Chain and Sprocket Drive
1.2
Timing Belt
1.5
V-Belt, 1:1 Ratio
1.8
V-Belt, 2:1 Speed Decreased Ratio
2.0
Flat Belts
The curves that follow can be used to determine the anticipated L10 life, which is the life in hours that
90% of bearings with this load would be expected to exceed without failure. The standard ball bear-
ing and standard shaft option will be the most economic, if acceptable life is obtained from the curve.
A good commonly used design figure is 20,000 hours. However, applications with a calculated life of
as low as 5,000 hours have sometimes been necessary to limit belt speeds to 6,000 feet per minute.
The curves are drawn for 1750 RPM average speed. If the application has some other average speed,
the life can be adjusted by multiplying by the bearing life factor.
It is important to know that bearing life for V-belt applications is independent of the motor load.
Once the belts have been tightened just enough to prevent slipping when the maximum torque is being
delivered by the motor the radial load, W, on the shaft and bearing is there and remains constant re-
gardless of whether the motor is even turning. For timing belts and chain drives, the radial load, W,
does not vary somewhat with motor load. Therefore, the motor load duty cycle as well as the average
speed should be considered to estimate bearing life.
Belt tension should be checked and adjusted following the belt manufacturers’ recommendations.
If slippage occurs after the belt tension has been correctly adjusted, the belts and pulleys have not been
chosen properly for the application.
Over-tightening to avoid this slippage may result in early failures of belts, shafts
and bearings.
There is normally a drop in tension during the first 24 to 48 hours of operation. During this “run in”
period, the belts seat themselves in the sheave grooves and initial stretch is removed. Belt tension
should be re-checked after a day or two of operation.
Matched belts run smoother and last longer. Longer belt life results if the belts and sheaves are kept
clean and the belts are prevented from rubbing against the belt guards or other obstructions.