Balancing large 8600hp electric motor with fluid film bearings

[Sparky4598]

Hello all, looking for some advice, tips, and suggestions here. We have two large 8600 horsepower motors where I work and we are having some vibration problems from them. Have been for a long time, but nobody has ever bothered to try to fix it because it worked good enough and ran without tripping vibration limits. The motors have oil journal bearings.

The motors have vibration sensors that read in velocity and we have a very good data acquisition system they are connected to. Everything is ISO calibrated as we are a testing facility. The mentality has been well it's good enough to run what we've been doing so we're not going to bother with it. But now we have a test coming up that is going to require higher rotational speeds and we tripped due to high vibrations when trying to reach those speeds. Forgot to mention the motors are variable speed, VFD controlled.

I have a bit of experience balancing rotors mounted with ball or roller bearings, but not any experience with fluid film bearings. We tried balancing the motor using a trial weight and the 4-run method. We only have a speed encoder on the motors, not a position or index encoder so we can't use the 2-run method. The results were lower vibrations, but not nearly as good as expected.

My suspicion with my limited knowledge and experience of journal bearings is the oil in the journal bearings is absorbing a considerable amount of the vibration. I am suspecting the vibration readings we are getting are significantly lower than they would be for the exact same equipment with roller bearings. Like I guess I'm saying I'm thinking the vibration with fluid film bearings doesn't necessarily correspond to the imbalance linearly as it would in a roller bearing.

So I guess my question is what would be the best method to balance the rotor without removing it from the motor housing?

I have thought about using lasers and monitoring shaft displacement, but I have read that displacement vibration monitoring is only good up to about 10Hz(600rpm) and these motors typically run at 20Hz but the test we have coming up will run at 30Hz(1800rpm). They are rated for up to 2000rpm.

Thanks for your time!

 

[GregLocock]

Frankly I'd be calling in a large motor balancing guy.

Did you 2 plane balance? Since automotive crankshafts are oil bearings I don't remember any problems balancing them.

Depending on the natural frequencies of the rotor you may be operating above the whirl speed. Certainly possible, definitely exciting.

Watch out for a couple of sub critical rpms, at half the critical rpm, due to oil film whirl and gravity 2/rev excitation. So mount an accelerometer to each bearing housing (if you can get to it) or on the frame in line with each bearing, all pointing the same way. Then for each plane add an imbalance at each bearing in separate runs.

This gives you the response at each plane to an oob at each plane. With just 2 planes it is easy to do by hand, more than that you probably want to build a matrix.

I prefer the 3 trial run method rather than just 1 trial run, for robustness, but my runs are cheap.

B&K do a nice series of free books, the blue vibration one covers balancing in some detail.

 

[Sparky4598]

Frankly I'd be calling in a large motor balancing guy.

Did you 2 plane balance? Since automotive crankshafts are oil bearings I don't remember any problems balancing them.

Depending on the natural frequencies of the rotor you may be operating above the whirl speed. Certainly possible, definitely exciting.

Watch out for a couple of sub critical rpms, at half the critical rpm, due to oil film whirl and gravity 2/rev excitation. So mount an accelerometer to each bearing housing (if you can get to it) or on the frame in line with each bearing, all pointing the same way. Then for each plane add an imbalance at each bearing in separate runs.

This gives you the response at each plane to an oob at each plane. With just 2 planes it is easy to do by hand, more than that you probably want to build a matrix.

I prefer the 3 trial run method rather than just 1 trial run, for robustness, but my runs are cheap.

B&K do a nice series of free books, the blue vibration one covers balancing in some detail.

Supposedly they called in a motor repair and maintenance company and they attempted to balance it after they did other maintenance like replacing the bearing journals and some tuning on the drives, but did not do it properly because the vibration were not any better. Or so I have been told, that was before my time here.

Normally I haven't had much trouble balancing things before so I figured we should have no problem doing it since we already have all the sensors, but I've never done oil bearings before.

We have only done the balancing of 1 plane so far. The other plane is going to require a lot more effort to install the trial and balance weights so we did the easy one first. When we did not get the improvement we expected, I started looking into why and if something may be different about journal bearings that I am not familiar with. Maybe we do just need to go ahead and do the 2nd plane and see how it is after that. Just didn't want to spend a lot of effort doing so if it needs to be done differently than I have done in the past.

I do believe the motor is running above the critical speed though, had not considered those effects yet. Good point.

 

[mfgenggear]

Supposedly they called in a motor repair and maintenance company and they attempted to balance it after they did other maintenance like replacing the bearing journals and some tuning on the drives, but did not do it properly because the vibration were not any better. Or so I have been told, that was before my time here.

Normally I haven't had much trouble balancing things before so I figured we should have no problem doing it since we already have all the sensors, but I've never done oil bearings before.

We have only done the balancing of 1 plane so far. The other plane is going to require a lot more effort to install the trial and balance weights so we did the easy one first. When we did not get the improvement we expected, I started looking into why and if something may be different about journal bearings that I am not familiar with. Maybe we do just need to go ahead and do the 2nd plane and see how it is after that. Just didn't want to spend a lot of effort doing so if it needs to be done differently than I have done in the past.

I do believe the motor is running above the critical speed though, had not considered those effects yet. Good point.

I never have been involved with balancing of electric motor.but did with aerospace gear shafts. Dynamic 2 plane balancing.
The machines used not that complex.
But it was permissible remove material with a hand grinder.

I had a 383 stroker Chevy engine. The crank shaft was from ac400 Cid Chevy engine. The crank have had to be dynamic balanced with heavy metal installed on the flywheel.
350 Chevy engine crankshaft was they called internal balancing. The crank shaft it self was added or stock remove. Hot rod only not sure about production. All bearings usage
It should make a difference.
But the work I was involved with was very stringent.
So based on that, the balance has to be with rotor removed. As said to verify contact the manufacture.

 

[GregLocock]

2 plane balancing is more complex than 2 1 plane balances, as you need to consider the influence coefficients from plane1 to plane 2 and vice versa.

 

[Sparky4598]

2 plane balancing is more complex than 2 1 plane balances, as you need to consider the influence coefficients from plane1 to plane 2 and vice versa.

I suppose that may be the issue then, just need to do a real 2 plane balance. It certainly makes sense how an imbalance on one end would show vibrations on the other end.

I will say my balancing experience has either been with a dynamic balancing machine where the calculations are done for you or overhung fan rotors where simgle plane balance is usually good enough.

So what we were doing is 2 - 1 plane balances? Balancing one end of the rotor and applying the correction weight and then doing the same on the other end? What is the difference between that and a true 2 plane balance? I assume the math and the procedure is quite different?

 

[mfgenggear]

I suppose that may be the issue then, just need to do a real 2 plane balance. It certainly makes sense how an imbalance on one end would show vibrations on the other end.

I will say my balancing experience has either been with a dynamic balancing machine where the calculations are done for you or overhung fan rotors where simgle plane balance is usually good enough.

So what we were doing is 2 - 1 plane balances? Balancing one end of the rotor and applying the correction weight and then doing the same on the other end? What is the difference between that and a true 2 plane balance? I assume the math and the procedure is quite different?

I recommend take it to a balancing company.
To verify max out of balance, then take advice from them for the next step then decide.

 

[GregLocock]

You do the 2 single plane balances by record the vibration or velocity at both planes. Same number of runs just more data.

Maths is here https://www.eng-tips.com/threads/diy-crankshaft-balancing-with-an-oscilloscope.511892/#post-8933361

 

[Sparky4598]

I recommend take it to a balancing company.
To verify max out of balance, then take advice from them for the next step then decide.

I appreciate the input, that is probably what we will end up doing if a second attempt does not go well.
But the intent of this post was to learn more about the process of balancing a rotor and to find out if balancing in position on fluid film bearings requires a different procedure or calculations.

 

[Sparky4598]

You do the 2 single plane balances by record the vibration or velocity at both planes. Same number of runs just more data.

Maths is here https://www.eng-tips.com/threads/diy-crankshaft-balancing-with-an-oscilloscope.511892/#post-8933361

Gotcha. So we have vibration recordings from both planes at the same time, that's no problem.
Thank you very much for the link to the other topic, that helps explain what is going on in the math some. And brings up one important point I hadn't thought of... So the trial weights needs to be indexed the same and placed in the same locations on both planes? Like 0deg on plane 1 must be 0deg on plane 2 and not arbitrary like 100deg on plane 2.

So from your remarks, it sounds like there is nothing special or different about fluid film bearings. The less than desirable results we got was just because the balance job is incomplete and we only did 1 plane instead of 2. That is good news.

My biggest concern was the bearing type would change the procedure.

And one last question, the trial weight needs to be placed on plane 2 before a correction weight is applied to plane 1 doesn't it? It wouldn't work right if you balance 1 plane and then attempt to do the same procedure and balance the 2nd plane independently because they are dependent on each other?

 

[mfgenggear]

I appreciate the input, that is probably what we will end up doing if a second attempt does not go well.
But the intent of this post was to learn more about the process of balancing a rotor and to find out if balancing in position on fluid film bearings requires a different procedure or calculations.

Your welcome
Then your best option even if you pay for it
Obtain training from the manufacture of the dynamic balanced.
It really is not that difficult.one - two hours of training will be very beneficial.
Even if it's Skype or zoom. They have all the computations. I subcontracted all my balancing.
If the out of balance is too much it can't be balanced to spec. At that RPM rotor have be precision machined. Can't imagine with the mass of a rotor with the wiring.
What is the balancing requirement?

 

[Sparky4598]

Your welcome
Then your best option even if you pay for it
Obtain training from the manufacture of the dynamic balanced.
It really is not that difficult.one - two hours of training will be very beneficial.
Even if it's Skype or zoom. They have all the computations. I subcontracted all my balancing.
If the out of balance is too much it can't be balanced to spec. At that RPM rotor have be precision machined. Can't imagine with the mass of a rotor with the wiring.
What is the balancing requirement?

There is no balancing grade specified, but the maximum bearing vibration is specified at 0.12ips peak. The rotor weighs about 10,000 lb and is rated up to 2,000 RPM.
We recorded a bearing vibration of 0.14ips for 2 seconds when we tried to spin up to 1,500 RPM before the PLC shut it down. At least we know our safety limits work now lol.

 

[GregLocock]

In theory it doesn't matter where the trial weights are located on each plane relative to the other plane, I think. But i haven't done this in a long time(25 years). I'd have thought so long as all measurements and locations were referenced to a single shaft reference marker it would be OK.

 

[mfgenggear]

There is no balancing grade specified, but the maximum bearing vibration is specified at 0.12ips peak. The rotor weighs about 10,000 lb and is rated up to 2,000 RPM.
We recorded a bearing vibration of 0.14ips for 2 seconds when we tried to spin up to 1,500 RPM before the PLC shut it down. At least we know our safety limits work now lol.

Wow that is big. My stuff is all small.
Balanced .020 in oz, and some if even .001
In oz. So call a supplier. Discuss the issue
And grab an hour of paid consultation.
In my honest opinion for that size , appears
To be pretty good, but iam guessing.
How difficult would it be to verify run out
On each end. Or some type electronic measure?
For dynamic balance components are located on each end with roller bearings.
On static balancing the component with be rotated by hand, but this rotor is heavy.
I never had to rotate any thing thus big.
A shop would have to have the capability
For big stuff like this. On small stuff a pulley
Wrapped is install to spin it 1000 rpm.
Then the equipment would calculate out of balance and depict the heavy and low points.
With with the fata given by calibrated balance, The operation then can add or subtract weight. In this believe it would be to add heavy weight metal.

 

[Sparky4598]

In theory it doesn't matter where the trial weights are located on each plane relative to the other plane, I think. But i haven't done this in a long time(25 years). I'd have thought so long as all measurements and locations were referenced to a single shaft reference marker it would be OK.

Okay, so looking back at the post you linked with the math, I see where my confusion is now. The math and the method you are talking about requires an index position of the rotor and phase information. We don't currently have that index marker, but can reconfigure our data system to include it if we need to.
We used the 4 run method with a trial weight at 3 equally spaced locations for what we have done so far since that method does not require an index mark and phase information. But I'm not sure what the math would be for that method on 2 plane balancing and how the influence vectors would play into that.
We may just have to change the data system to have the index mark and use the 2 run method.