Practical Approach to Precision Balancing

7- In-Place Two Plane Balancing of Overhung Rotors

Ralph Buscarello developed this method of balancing when conducting balancing training. One participant couldn’t visualize how vibration on an overhung rotor (Fig. 1) could be about zero at one bearing, and the other bearing still having large amplitude. They were discussing a situation regarding an overhung fan rotor that had very small amplitude at bearing #1 and a large amplitude at bearing #2 (Fig. 2).

For Fig. 2, assume that the amplitude at bearing #1 is almost zero, and the amplitude at bearing #2 is beyond acceptable limits, notice the line that represents the motion of the shaft axis. Trying to show this with a long blackboard pointer, Mr. Buscarello held the center of the pointer in one hand, restraining it so that there would be no motion at bearing #1. Motion was shown at bearing #2. As the pointer was long and its center was positioned at bearing #1, the pointer extended further to the right (Fig. 3). Notice that the pointer now indicates a rocking motion, which is characteristic of couple unbalance.

Recognizing that a machine with almost no amplitude at one bearing and a much larger amplitude at the other would most likely be the result of couple unbalance, it was determined that a static/couple method of balancing an overhung rotor would work. (Others who taught balancing soon copied the method, but some important details were left out. Therefore, this article will provide those details.)

Regardless of which bearing has the highest amplitude, focus only on bearing #1 and ignore bearing #2.

Fig. 3B. Apply the transducer at bearing #1. Measure amplitude and phase.

C. Using any single plane balancing method (computer or vectorial), apply balancing corrections in plane A, until the amplitude at bearing #1 is reduced to the proper tolerance.Sometimes, a single plane balance in plane A virtually eliminates the vibration at bearing #1, and also reduces or eliminates the vibration at bearing #2. However, the amplitude at bearing #2 could instead remain the same or actually increase. Whether it increases or not, simply go on to the next step, which is to make balancing corrections that reduce the amplitude at bearing #2 while not increasing the vibration at bearing #1.

Review again the motion of Fig. 3 that reveals motion due to couple unbalance. Remembering that corrections for couple unbalance can be applied in any two planes and have the same counterbalancing effect, pure couple corrections are to be applied to both planes A and B. However, the method will use the same type of thinking as for single plane balancing.

Use the following procedure:

A. After the vibration at bearing #1 is reduced to its proper tolerance, apply the transducer to bearing #2 and obtain amplitude and phase.

B. Using the same thinking as for the single plane method, apply a trial weight/balance correction in Plane B and an equal and 180o opposite correction in plane A. The two corrections have to be applied during the same balancing run.

Focusing only on bearing #2, continue to use single plane balancing reasoning, but using one correction in plane B and another, 180o opposite, in plane A. If perfectly performed, as the amplitude at bearing #2 decreases, the amplitude at bearing #1 should not increase. In order to not increase the amplitude at bearing #1, the two correction weights should be very carefully applied in such a way that they are truly equal in magnitude (oz·in, g·in, g·mm).

It is very important that the corrections be carefully applied 180° opposite each other. If not equal in magnitude, or not 180° opposite each other, a small static unbalance will result. As the small static unbalance would be acting in a plane that is away from the total rotor’s center of gravity plane, it results in a new couple unbalance (see details in the chapter on balancing overhung rotors).

For the final trim balance, sometimes the balancing specialist would find it easier to resort to the old “back and forth” method. However, with good tools and careful measurements of weights, distances and angles, overhung rotor balancing should become easier and easier.

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