Grooved idlers should have the standard groove dimensions for optimum results. In case of long spans & the drives with severe vibrations, however, deep grooved pulleys are recommended Flat pulleys on the other hand should always be cylindrical & not crowned. Wherever Idlers are used as bell guides, It is suggested that a flanged pulley be used with sharp corners to avoid the running over of the belt over the flange causing turnover.
The distance between two flanges is governed by the following formula:
b = b2 + m
where b = required face width ? mm )
b2 = face width of grooved pulley (mm)
m = additional value (mm)
The calculations for the length required are same as for the drive with two pulleys. However the following details are required to be noted when designing the same.
1) The required belt length is calculated using the standard procedure.
2) If the belt has to be used with fixed centre, distance, then double the installation allowance be added to the pitch length calculated above, i.e.,
Ld = Lp + 2 ?
where, Ld is the required length
Lp is the pitch length
? is the installation allowance
3) The next largest standard length be chosen , however care be taken to check that the belt can be adequately tensioned with the idler the outermost position.
Ld = Lp + 2X
where, Ld is the length after maximum belt stretch
Lp is the pitch length chosen above
X is the take up allowance
Number of Belts required
The usage of idlers invariably decreases the service life of belts. To avoid a reduction in the belt service life, idler correction factor has also to be considered while calculating the number of belts required.