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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 5, Static Unbalance in a Non-Uniform Rotor

For a non-uniform rotor with unequal weight at each bearing, the plane for the lateral CG would no longer be equidistant from each bearing. Instead, it would be as shown in the plane of the imaginary fulcrum.

The static unbalance in the plane of the CG can be resolved to the two measuring planes, proportional to the rotor weight at each end. Therefore, the two static weights or forces will not be equal. For example, assume a rotor that has a total weight of four units, with its weight distributed so that the left bearing supported three units and the right bearing supported one unit. In order to remain in "pure" static unbalance (no couple present) at each measuring plane adjacent to the bearings, the left plane static unbalance force will be 3/4 that of the original total static unbalance. The right plane static unbalance force will be 1/4 of the original static unbalance.


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