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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 14, Field Balancing Exercise

Developing Further Understanding of Balancing

In the early days of field (in-situ) balancing understanding, the principles in the following exercise were absolutely necessary. Now, there are readily available computer balancing programs that are sometimes included in the vibration instrument. Theoretically, blindly following the computer-led instructions for balancing should be enough. However, during actual balancing, difficulties do arise that result in far too many balancing runs or adjustments of balancing corrections.

When balancing to previously mediocre tolerances, understanding the process wasn't very important. It was enough to simply follow the instructions from the computer. However, when performing very close precision balancing, slight errors in counterbalancing weight or angle result in much larger, unexpected phase and amplitude changes. By combining basic understanding of the principles involved with the good instructions from the computer, precision balancing can be accomplished quickly and easily.

This exercise is performed using the example of single-plane unbalance. Practice is recommended with a relatively narrow rotor, such as a single-grooved sheave mounted on the end of a small well-balanced motor.

The table below is based on the operator obtaining an "original" reading before a trial weight is added and then adding a "guessed at" trial weight at any position on the rotor. Each line represents the possible resulting changes in amplitude and phase angle. They reveal whether the trial weight was light or heavy, compared to the original unbalance, or was placed at the correct angle or not.

Assume a trial weight that is lighter than finally required, except when otherwise indicated.



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