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Often by necessity, sometimes by preference, it may become necessary to balance a rotating machine or part under conditions where a phase measurement is either impossible or unavailable. In this situation, a four-run method can be used to arrive at an amount and position for a corrective weight.

To perform a four-run balancing procedure, one only needs a means to measure vibration amplitude and use polar graph paper.

1. With the machine in operation, locate and measure the point of highest vibration on a bearing. Either attach the pickup to this position or accurately mark its location so subsequent measurements may be taken from exactly the same place.

2. Measure and record the vibration amplitude.

3. Place a trial weight at some location on the rotor, restart the machine, and record vibration amplitude-T.

4. Stop the machine; rotate the trial weight 90 degrees to 120 degrees as most convenient at a constant radius, restart the machine, and record vibration amplitude-T2.

5. Stop the machine; rotate the trial weight another 90 degrees to 120 degrees in the same direction at the same radius, restart the machine, and record vibration amplitude-T3.

6. From the foregoing data, the location and the amount of a corrective weight maybe determined as will be illustrated below:

Original unbalance (O) = 10 mils; Trial weight, TW = 50 g
First trial, T, = 7.0 mils @ 0 degrees
Second trial, T2 = 12.0 mils @ 120 degrees
Third trial, T3 = 18.0 mils @ 240 degrees

Note: The angles refer to the trial weight positions on the rotor relative to an arbitrary zero.

1. On the polar graph paper, draw a circle with a radius of 10 mils. On the circumference of the circle, mark the trial weight positions. (for this example: 0, 120, and 240 degrees.)

2. Using the points marked on the circumference of the first circle as centers, draw arcs with radii equal to the amplitude measured with the trial weight at the location.

3. Draw a line from the origin of the original circle to the intersection of the three trial-weight circle; this is the angular position of the corrective weight relative to the trial weight positions (42 degrees in this example).

4. Measure the length of the line drawn in step 3, (F) (8.5 in this example). The correct weight addition may now be calculated from the formula: TW x (O)/(F) = 50 x 10/8.5 = 59.0 g.

Tip provided by Dennis Shreve, CMRP, Commtest, Inc.

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