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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 22, Non-Square Spacers

Another source for a rotor to be positioned off-center are the spacers on long, multi-stage pumps. A major, unscheduled shutdown once occurred when the bearings in a newly installed pump wore out after three days. Well aware of the precise balancing required in such pumps, the machinist carefully balanced each of the individual stages before balancing the total assembly. After the pump had again run for three days, its bearings wore out.

During rebalancing, it was noticed that the shaft could be bent slightly by applying different axial clamping pressures. This was due to the slightly off-square spacers. The resulting bent shaft offset the center rotors from their true axis of rotation, causing an unbalance. This small amount of unbalance resulted in a centrifugal force that deflected the shaft further, creating more unbalance, etc.

This was corrected by having the ends of the spacers squared on a grinder to within a tolerance, at their periphery, of +/- 2 mils.


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