Field Balancing

The third method of balancing is generally called field balancing, because this is where it's often done. If it appears that major balancing is required, however, this method should not be used. Normally it is a trim, or so called touch-up balance. There is always some minor adjusting of the elements after coming up to temperature and speed. The minor movement is caused by the imperfect cooling and the failure of the component to come onto the shaft in a perfectly square condition while shrink fitting. This area of consideration, referred to as the element seating, may be enough to raise the residual unbalance, but not enough to remove the rotor and unstack, leaving more vibration than desired. On axial or centrifugal gas compressors, where there is normally no provision for field balancing, this can be time-consuming because the cover must be removed to make a trial weight change. Most balancing involves reading unbalance and phase, adding a trial weight, watching the resulting change, then, if the balancer is skilled, triangulating the last weight estimate from the vectors to bring in the correction. If all worked well, it still means two cover lifts. The method is more often used on steam turbines or air compressors where provision is sometimes made for field weights that are accessible without a cover lift. This technique is also used on a vendor test floor if it is believed that the out-of-tolerance compressor can be corrected in a less costly manner with a test floor trim balance. In a vendor shop, there are normally several skilled balancers available.

Two methods for determining the location of the unbalance vector are used. One is with portable vibration pickups and a strobe light. The pickup is attached to the bearing housing with a magnet or other temporary attachment. Numbers are painted on some rotating part, generally the coupling. The numbers identify evenly spaced marks on the circumference. If keys are used, the numbers start there to establish a physical mark. When the compressor comes up to speed, the vibration signal triggers the strobe light, theoretically at the maximum point of the vibration signal. The peak-to-peak reading of the probe forms the vector length. The numbers, frozen by the strobe light, give the vector direction. The technique proceeds as stated earlier with a trial weight.

The alternate method uses the proximity probes and an oscilloscope. A Lissajous figure is established on the oscilloscope. The orbit pattern and the keyphase mark are used to generate a vector. Weights are added or removed and the changes in the orbit are noted. Triangulation is used to anticipate the next move. For more complete information or technique, the reader is referred to a book on the subject by Jackson [1].

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