I've wanted to ask this question many times during the last 10 years or so but never dared by fear of being laughted at, but there it is !

Taking any piece of rotating equipment whose natural frequency unfortunately happens to match the rotating speed. If ! And I say if, assuming that the rotor was balanced to near perfection at operating speed on a balancing stand, would it be fair to state that there would not be any noticeable vibration to measure on that piece of equipment when being run by itself, such as in the case of an electric motor ?

Hypothetically when there are no exciting force applied to the equipment, then the equipment's natural frequency should not manifest itself ?

This is not a trick question, but a thought that has bothered me for a long time.

MarkoLeo

That's a fact.

It seems you knew the answer, yet asked anyway, which implies a reason not yet revealed. So the more interesting question to me is this:

What aspects of this scenario caused you to consider that the opposite might be true?

---pardon me for stirring the pot---
MMM

Dear MMM,

No hidden agenda, really.
well... perhaps one !

Why then, does a piece of equipment vibrated at 1 times RPM if there were no known exciting forces applied ?

I recall an incident at our shop that took all of us in disbelief of what was happening and we never did find the answer.

Mind you the vibration was not severe but it was there... so we shipped out the beast anyways.
And a large beast at that : 1,500hp - 600rpm - Synchronous Motor - Sleeve Bearing design on pedestals and huge steel base.

Regards,

Markoleo

Who said there was no 1X force present? That is not possible.

I don't think that you can achieve no 1 x rpm force in any rotating element, can you?

He did say "HYPOTHETICALLY"

Martin,

He did say that in his first post, but his follow-up implied that this was a real case with no force at 1 x rpm. (At least that is how I interpretted it).

It reminds me of the fate of an Optalign laser alignment system.

It was left to almost ruin in the basement of a very messy papermill after the mechanic using it became frustrated. When I asked him what went wrong, he said the he had spent 8 hours on a pump alignment and never did get it right. I aksed him what degree of alignment he was trying to achieve and he said "Zero".

My point now is the same as then-you cannot achieve zero force due to balance or alignment.
(Well maybe Sam can)

Dear all,

After having read all your inputs, this point came to my mind.

As I have said before, the rotor was balanced on a Shenk-Treble hard bearing cardan driven m/c to practically zero reading on the balancer's dial and at operating speed as well.

But, the final installation as a motor was on babbitted sleeve bearing pedestals lubricated by a typical oil ring.

Just wondering now if the different degree of damping or lack of in the 2 different ways of mounting might have something to do with this phenomena ?

Hard mounts on the balancer vs oil film on the motor bearings !

I remember this rotor being dismantled off the motor and remounted on the balancing m/c 4 to 5 times. Same results everytime.

We even balanced the cardan separately in between rotor balancing trials and noticed no difference in the readings on the balancer.

Some more food for thoughts...

MarkoLeo

P.S. By the way, the natural frequency of the system was determined by the 'famous' bump test method.

Was a residual unbalance test performed?

I must be missing something here. The rotor was removed from the motor,

quote:

the rotor was balanced on a Shenk-Treble hard bearing cardan driven m/c to practically zero reading on the balancer's dial and at operating speed as well

What are we referring to when we say "at operating speed as well", operating speed while on the balancing machine?

Are we saying after re-assembling the motor in its runable state, there was no longer a "near zero" 1X amplitude?

Aren't there more forces acting upon the rotor during powered running conditions other than imbalance that would or could cause a 1X to become noticable?

Has the rotor been "checked" on more than one balancing machine since this seems to be an issue of the 1X being there after assembly, that is not there on the balance machine.

I may just be mis-reading the whole thing. If so, I am sorry.

Could be a resonance of the rotor as installed into the machine.

Could be a thermal bow of the motor rotor. We have seen that manifest itself even during uncoupled runs.

Less likely some problem with the bearings like misalignment or excess clearance?

Probably some other possibilities.

Were the motor bearing surfaces checked for runout? As a consultamt, I once balanced a higher motor rotor with fluid film bearings for my local GE shop for which the rotor had been balanced at 300 RPM in a SChenck shop balancer. If there is runout, balancing in the Scheck balancer has the effect of building in an unbalance weight error comparable to the runout at the balancer speed. You are , in effect, balancing the rotor with the shaft off center in the balancer.

I don't know the Schenk shop balancer, but things that come to my mind:
1) Is the shaft of the balancing machine almost "exact" the same as the machine shaft?
2) Are the keys used the same?
3) Are the key slots the same?

I believe that a "perfect" balancing will be achieved in the field, correcting the unbalance of the total assembly. Impurities on the final shaft will be corrected by a field balance job.

There will always be a 1x exciting force on a rotating equipment, which is proportional to the square of the rotational speed.

F = m * e * w^2
where e is the offset of the center of gravity to the true rotating center.

Hello all,

Let me recap here in order to set things straight.

1- This Synchronous Motor was a large 1,500hp - 600rpm - Synchronous Motor - Sleeve Bearing design on pedestals and on a huge steel base.
2- Once assembled we found a borderline 1 x rpm vibration amplitude.
3- We then took the rotor off and put it back on a Schenk-Treble balancing m/c to see if we had missed something in the balancing operation. We found that it had been balanced to practically zero residual.
4- We re-assembled and found the same phenomena again.
5- We again took the rotor off and installed it on another Schenk-Treble m/c. No change from the previous time.
6- We then re-assembled the unit.
7- We did a 'bump' test using a small DC motor fitted with an unbalanced flywheel and fixed it solidly to the structure.
8- We slowly accelerated the small DC motor being used as an exciting force and discovered that the predominant unit resonant frequency was, to our dismay, 1 x rotating speed ( 600 rpm ).

Hence the question raised by my original message.

Assuming that the Synchronous Motor Rotor is nearly perfectly balanced, therefore not being the exciting force that triggers the resonant vibration, what possibly could be the culprit.

From what some of you guys are telling me... no rotor is ever prefectly balanced. OK, I'll accept that argument.

Also, as I proposed in my other messages, what appears to be a perfect rotor on a balancing m/c fitted with hard mounts is not really so when mounted on sleeve bearing's oil film.
In my opinion, I figured that the oil film 'allowed' the rotor to exhibit whatever was left of unbalance that was not detected by the balancing machine.

We did not attempt a trim balance on the unit as such... the levels were acceptable although we were questionning ourselves on what phenomena we were experiencing.

We tried stiffening the pedestals to the base by welding steel members to no avail. We concluded that it would take a lot more re-inforcement of the static portion of the base in order to raise its natural frequency well beyond 1 x rpm.

We wrote it off to a bad design of the entire stator, pedestals and base assembly although the overall vibes were just within limits.

So much for the recap... your interest in this post is really appreciated.

MarkoLeo

Woops, forgot the most important thing - the balancing speed was 610 rpm ( funny speed but that is Schenk-Trebel featured selection )

The diameter of the running balancing surfaces were .0005 TIR with the journal surfaces.
Hard to beat on a large and long shaft.

One thing tho' - A synchronous rotor is not really a 'solid' rotor as it is composed of individual wound pole pieces that are bolted onto what we call a 'spider drum' welded to the main shaft.

Thanks for your patience... gosh, I did not think that I would raise such interesting posts

MarkoLeo

If there are forces at other frequencies acting on the machine, and if these forces are changing in any way, there cannot be a perfectly constant "steady-state" response to that excitation. There will be transient motion going on, and this will be at system natural frequencies. The lighter the damping, the more persistent the transient motion will be.

Nothing is ever "perfectly" balanced, not even in a balancing stand. And when the piece is mounted in the field, other imbalance is going to be introduced (coupling, coupling keys). So it's not going to be "perfect."

But, for the sake of argument, assuming it IS perfect, then the "imbalance" will not excite a resonance, even if it is exactly at running speed. I did a bump test on a fan once that repeatedly showed a resonance at running speed + 1 rpm. The base was all broken up and had been repeatedly welded up, with all the welds broken... there were welds on top of welds..... a classic case of resonance. But I was able to balance the fan successfully.

Of course it didn't stay balanced because this blower was actually moving pieces of chopped up "trim" slit from the edges of packaging film. So they replaced it.

But it ran very smoothly until they started putting trim through it.

Markoleo,
I think you have two things here that could explain your situation. One, you balanced on a fit that was .0005 non-concentric to the bearing journal. This is called Mass Center Displacement, balancing with the center of rotation moved even as little as .0005 will result in an unbalance when run on the journals that was not there in the balance stand. Two, you were running at resonance which was amplifying your unbalance. I think you were luck it was still in tolerance even if just barely. I am going to attach part of a reply from a thread I started concerning a 3600 RPM rotor that was being balanced in a similiar manner as you described.

Hope this helps,
Ronnie

MCD_(Mass_Center_Displacement).doc (20 Kb, 11 downloads)

Many thanks to you all.

You've shed some light on my problem.

Markoleo