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