Balancing Without Phase

Dennis Shreve, CMRP

Sometimes it may be required to balance a rotating machine or part under conditions where a conventional 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 plotting techniques. A 10-step process is outlined below.

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.
Measure and record the original vibration amplitude as value O.
Place a trial weight at some arbitrary location on the rotor, restart the machine, and record vibration as amplitude T1.
Stop the machine and rotate the selected trial weight 90 degrees to 120 degrees as most convenient at a constant radius,restart the machine, and record vibration as amplitude T2.
Stop the machine and rotate the trial weight another 90 degrees to 120 degrees in the same direction at the same radius, restart the machine, and record vibration as amplitude T3.
From the foregoing data, the location and the amount of a corrective weight may be determined as will be discussed and illustrated below.
- First, record all pertinent data:
- Original unbalance measurement, O = 10 mils. Selected trial weight, TW = 50 grams.
- First trial reading, T1 = 7.0 mils with TW @ 0 degrees.
- Second trial, T2 = 12.0 mils with TW @ 120 degrees.
- Third trial, T3 = 18.0 mils with TW @ 240 degrees.
- Note: The angles refer to the trial weight positions on the rotor relative to an arbitrary zero point.
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, the angles of choice are 0, 120, and 240 degrees.)
Using the points marked on the circumference of the first circle as centers, draw arcs with radii equal to the respective vibration amplitudes measured with the trial weight at the three locations.
Draw a line from the origin of the original circle to the intersection of the three trial-weight circles. This is the angular position of the corrective weight relative to the trial weight positions (42 degrees in this example).
Measure the length of the line, F, drawn in Step 9. (It is 8.5 in this example.)

The corrective weight addition may now be calculated from the formula:

Corrective weight, C = TW * O/F = 50*10/8.5 = 59.0 grams

This is the amount of weight to be placed at an angle of 42 degrees. See graphical illustration below.

Fig 1

Fig 2

Dennis is Channel Support Engineer for the Channel Partner Sales organization with Commtest Inc. He has 40 years of experience in designing and developing electronics and software systems and leading projects for real-time industrial process monitoring and control applications. Over the past 21 years, he has specialized in predictive maintenance (PdM) technologies and vibration detection, analysis, and correction methods for maintaining machinery health. www.commtest.com

Comments (2)

your article on this subject is very helpful in such a way that we can be confident enough to apply it practically. we need more people like you who share their expertise for free. God bless you
1) Posted 1:45 am, 03 December 2010 by rene hinolan
i have an experience before that my balancing instrument was encounter a major abnormality and the in-place balancing process was delayed for 3 days, since we fix firts the unit. i think would not happen again because on this article, thanks Dennis on this very informative materials.
2) Posted 7:15 pm, 21 December 2010 by Antonio N. Felipe