Balancing three-bearing rotors
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Date
2020
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Turbomachinery Laboratory, Texas A&M Engineering Experiment Station
Abstract
Multistage cryogenic pump rotors are vertically oriented and supported radially by three bearings. However, while the rotors are oriented vertically and supported in three planes, they are balanced horizontally in two-plane balancing machines. Since the balancing machines are designed for two planes of support, a third support usually at the far end, is needed to support the rotor. There are two major issues that may affect the quality of balancing a three-bearing rotor as compared to a two-bearing rotor: The first issue is runout, which is essentially eccentricity that can contribute to unbalance. The shaft runout should be measured with the rotor supported at two locations. For rotors that will have short length ball bearings added after the balancing operation, the two support locations of the balancing machine is typically used, and the far end is left without support. The measurement on the far end could be large, and it is easily mistaken as sag and left without correction. The second issue is how to adjust the position of the third support when preparing to balance. The support is usually a two-roller device, and the regular adjustment method is to place it under the 3rd bearing location and raise it gradually to touch the rotor such that when the rotor is spun, both rollers are spinning. The result of this adjustment, however, is for the far end to be in a deflected sagged condition, which is different from the operating condition where all three bearings are aligned. This paper explains the difference between runout and sag, and presents a method to adjust the support such that the rotor can be correctly balanced. The influence of the support adjustment is shown with an example.
Description
Lecture