Unbalance - Improving Field Balancing of Overhung Rotors

Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 9, Improving Field Balancing of Overhung Rotors

Assume, as shown in Fig. 6, that the person performing field balancing has removed all the static unbalance and all that is left is a residual couple. Typically one couple trial weight is placed in one plane and the other couple trial weight is placed in the opposite plane, 180° opposite to the first weight. Look at the straight-on end view of the two weights. Notice that if the two forces are truly equal in oz•in or g•mm and are truly 180° opposite each other, then the two couple forces create no residual static unbalance. However, too often the positioning of the weight in the opposite plane is "eyeballed" rather than using shop tools or instruments to ensure its accurate location. Usually a point is located by eye, in the same plane as the first weight. This point is then extended by eye, to the opposite plane. The inaccuracies created by two sightings by eye usually cause the second weight to be placed several degrees off from 180° opposite the first weight. The typical five to ten degree error does not seem like very much, but this resultant becomes a small amount of static unbalance.

If the resultant static unbalance is on a rotor mounted between bearings, it will create very little "false couple." However, as illustrated Fig. 7, for an overhung rotor, depending on the extent of the overhang, the resulting couple can be small or very large. For most overhung rotors, the resulting couple unbalance will be considerable larger than the small amount of static that created it.

When balancing overhung rotors, always keep in mind the separation of static and couple components. Then remember that the measurement, preparation and placement of trial weights has to be very precise. Especially for correcting couple unbalance, use instruments or other careful means for accurately determining the precise location for trial or final balancing weights. Accuracy for the radial positioning of final weights is also very important. Any error that produces relatively small amounts of static unbalance, results in larger amounts of couple error.

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