Balancing Without Phase
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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Comments (7)
I have found an alternative on the Vibronurse website (http://www.vibronurse.com/NoPhaseBalance.html) it’s like drawing the plot but on the PC and it has some handy features like blade positions and split weights you can also save and print which makes it good for reports. It’s also free and you can download it if you don't have internet access onsite (http://www.vibronurse.com/Balance.zip).
1) Posted 6:45 am, 17 June 2010 by Steve Young
2) Posted 7:51 am, 17 June 2010 by Tom Burke
Great balancing tip, however calculating the mass of the trial weight is critical. The trial weight should not produce a force more than 10% of the static weight of the rotor.
Thanks,
George Sheppard
3) Posted 8:40 am, 17 June 2010 by George Sheppard
4) Posted 11:15 am, 17 June 2010 by Lyn Greenhill
5) Posted 2:54 pm, 17 June 2010 by Simon Hurricks
Over the years I taught many craft people to balance in the real world, on a real fan. They were started out on a little test rotor but once they thought they pretty much understood the process, I would turn them loose on a full size fan that I kept in running condition on a down line, just for their practice.
I stumbled into a good use for 4Run in that training. They often would find out that they didn’t understand it quite as well as they thought. So they would end up moving the TW all over the impeller, taking all kinds of data, trying to figure it out. When they started to ask questions, I would help them out and I found that I could often pull both a regular solution out of the mass of data they had already collected but also a 4 run that collaborated the regular. I would show them how I came up with that and it helped them to not only do both processes but more importantly to understand the necessity to study and understand the data they were taking rather than just taking more and more of it, hoping that something will eventually work.
Dave Williamson
6) Posted 10:11 am, 24 June 2010 by Dave Williamson
It's a really useful and easy method to do balancing of rotors specially small and low rpm ones. It would be more easy to understand if you explain it with diagram.
Thanks,
Rajeev
7) Posted 10:43 pm, 24 June 2010 by Rajeev Singhai