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# 2 plane balancing without trial masses2

## 2 plane balancing without trial masses

(OP)
Hi

Can someone explain how dynamic balancing machines determine the "trim weight" required without using trial weights? There is an example in this video http://erbessd-instruments.com/Eng/video.php (2 Planes Balancing of Turbocharger Rotor) that illustrates this. How does the machine accurately determine the location of the heavy spot and its location?

thanks

dynaman

### RE: 2 plane balancing without trial masses

Didn't watch the video. For a given rotor design the transfer function of vibration to balance mass and location is constant, therefore the manufacturer can calibrate the system once for a given rotor design. I do not know how two plane balancing on commercial tire balancers is done, because they can be used on a variety of wheel geometries and weights without recalibrating, although I have seen at least one that measures some geometry.

Cheers

Greg Locock

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### RE: 2 plane balancing without trial masses

(OP)
Thanks Greg. So if I was to do the same how would I determine the transfer junction?

### RE: 2 plane balancing without trial masses

accelerometer and trial masses and a trigger for phase angle reference (it is possible to do dynamic balancing without a phase ref but in my opinion it is much harder to understand what is going on). I'd use at least 6 locations on each balancing plane, and plot them out by hand. So that's be 13 trials altogether including the initial unbalanced run.

Using complex maths

response_at_plane1=TF11*UB1+TF12*UB2

response_at_plane2=TF22*UB2+TF21*UB1

note that TF12=TF21 by reciprocity

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: 2 plane balancing without trial masses

(OP)
Hi Greg

Sorry what are UB1 & UB2 exactly? Is there a reference that explains this method in detail?

thanks

Mark.

### RE: 2 plane balancing without trial masses

Unbalance at plane 1 and 2. That is the vector sum of the trial unbalance and the original unbalance.

Yes I think the Bruel and Kjaer blue book on vibration analysis has a pretty good section on this method. http://www.bksv.com/doc/bn1330.pdf

Incidentally I'm not entirely convinced that TF12=TF21, that'd be an interesting check. Specifically if the accelerometers are not pointing in the same direction then I can imagine the phase relationship between the two would change.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: 2 plane balancing without trial masses

Figures 11.9 and 11.13 are directly relevant but you probably need to read the whole chapter up to section 11.6. I'd skip the bits where they jump to the solution in one step, that is your eventual aim but you need to be pretty confident in the system and setup to do that. The vector diagrams are the key, if your head works like mine.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: 2 plane balancing without trial masses

(OP)
OK thanks, so I take Fig 11.9 is the key here. After the calibration runs we calculate the terms in the matrix. So when we carry out further balance runs all we are doing is calculating V10 & V20 and solving for MC1 & Mc2 again?

### RE: 2 plane balancing without trial masses

(OP)
Greg I have another question. When we use the 2 plane method WITH trial weights, and the solution says to add weight at 2 locations, is it equally correct to remove the same weights 180deg opposite? Silly question I know but have to ask.

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