Dear Samin,

There are three major factors that have guided engineers to balance pulleys:

1. improve pulley bearing life

2. eliminate unacceptable belt vibration

3. eliminate unacceptable structural oscillation

In most cases, only the third point is relevant, due to todays accuracy of manufacturing.

Often pulleys revolve at .5 to 2 hz. This can be in the same domain of long span and cantilevered trusses. The latter more specifically has a very low natural first / fundamental modal frequency. When so, undesirable oscillation can cause excessive structural stresses, and can cause sympathic belt beat vibrations.

In rare occasion, the bearing life will be reduced or belt vibration will become unacceptable. Usually, this is in part due to the use of a heavy backing bar used to control the longitudinal shell seam, misplaced heavy balanced weights, off-center machining of the shell OD, or poor rolling of the shell to a concentric shape. ■

More to your request, CDI calls for dynamic balancing of all pulleys on cantilevered structures, long span steel trusses, and on steel frames which connect or are a part of elevated steel when the spin frequency is above 1 hz.

Pulley specifications calling for static balancing, is applied to elevated steel for spin frequencies greater than .5 hz and when the pulley mass exceeds 2 tons.

You can make the imbalance force estimate. Most often you will find the use of balancing to be an unnecessary cost.

More often than not, pulley manufacturers will place welded weights in the highly stressed end disk region or on the shell ID inside such that a stress riser is formed that will lead to a fatigue metal failure. Better to eliminate them or to direct their application to the shell ID at its outer end. ■