The procedure requires the operator to attach a trial mass to the rotor sequentially in 6 (or 12) equally spaced radial positions for each plane and record the unbalance results. The unbalance results must then be plotted in terms of amplitude versus angular location. This polar plot is then used to indicate the actual residual unbalance present on the rotor correction plane. Comparing the actual residual unbalance to the maximum allowable residual unbalance determines if the actual residual unbalance is acceptable.

Anyone familiar with the procedure will agree that while this is an effective tool to ensure that an impeller is within tolerance, it can also add considerable time to the balancing operation.

Averaging Over Runs and Marking of Measured Values Software is used to indicate up to 30 runs simultaneously on the display screen and then perform an averaged value. 

The software will save countless hours in performing this procedure and provide a quick return on investment. In addition, the software has the added benefit of reducing human errors associated with data entry, calculations, and plotting. It also allows the test data to be stored and transferred electronically.

This display tool can be a helpful aid to the operator when performing a residual unbalance test to mark the result of each run as the trial mass is traversed sequentially around the rotor, creating a circle of marked points.  The average of these points can be calculated by the software and displayed, providing a center of the circle of marked values.  This allows the operator to compare the actual residual unbalance of the rotor with the maximum allowable residual unbalance (rotor tolerance). 

Figure 1: Vector display of six trial weights traversed

Figure 2: Vector display showing the resultant residual unbalance (average).  Note the tolerance band given by the shaded area.

The distance between the centers of the circle created by the readings from the origin of the polar plot corresponds to the actual residual unbalance present in the rotor for that correction plane.  The API standards recommend that if the residual unbalance amount determined from the test is greater than the tolerance specified for that rotor, then the rotor is to be balanced more accurately by making the necessary corrections and the test be repeated.

More about:

Balancing Machines for Pump Impellers  l Instrumentation Features for Pump Applications
Correction l Balancing to API Standards