Belt Frequency

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Belt drives are commonly used to transmit power. They are cheap and easy to maintain. However they are often a source of vibration due to misalignment, belt wear, and belt resonance. 

Belt misalignment can be identified from frequency spectrum, 1X vibration in both the radial and axial directions.

The belt forcing frequency is known as belt pass frequency or belt rate, which is the rate at which a point on the belt passes a fixed reference point. It is always less than the speed of either sheave/pulley. The belt frequency can be calculated as follows:

Belt frequency or rate = (π x sheave RPM x sheave diameter)/belt length

If a belt is worn or loose there will be a peak at the belt rate, and harmonics, with twice this frequency,  2X.