You don't need an expensive impact hammer. I assume that since the plots are MasterTrend/RBMware that you have at least a single channel analyzer. A photo-tach and accelerometer can be used to measure the 1xSS amplitude and phase (Monitor Peak/Phas) at locations (primarily vertical direction) on machine that is running, other 2 machines on same skid not running and at other locations on skid. This will indicate the relative amplitude and whether in or out of phase to indicate forced deflection shape (like ODS test). Based on this test you can decide where to add mass or stiffeners. Rigid mass is added where motion amplitude is high. Stifferner(s) are added to restrain locations with high motion to rigid locations with low motion. An Excel line graph can be used to plot scaled machine dimensions and vibration (+/- values indicate in/out of phase) levels.

I have done structural modifications for years, and I certainly don't recommend wood (studs or wedges), clamps, chains, come-alongs, or turn-buckles. These items either don't work at all or only on very weak or compliant structures.

Correcting the resonant structure should be far more effective and easier than trying to balance a motor-pump.

quote:

If you have a natural frequency at running speed, I would say the influence is high. I'm not sure I've ever heard anyone suggest anything different.

The discussion earlier was specifically about irrelevant resonances.

My instinct tells me this resonance is the primary problem. But if there is a way to quantify it, I would like to do that before making any changes to the system.
So I am interested in this "Transfer Function" discussed by Steve:

quote:

Using an instrumented hammer allows you to measure the transfer function, which is the relationship between the force of the impact and the response of the system to that impact at a given frequency. You could accomplish the same thing by measuring vibration on a rotor, then installing a balance weight and measuring the vibration again. This gives an influence vector which is essentially a transfer function as well

How do we get to the influence of the resonance?

Walt - are you saying that you don't agree with trying out temporary bracing to see if it works? Or are you saying you would agree with it provided stronger materials were used.

The thing we are thinking of doing is going to a different (more rigid) coupling, assuming the rotor is resonant.

The current coupling is a rubber gear tooth type with little or no contact while the machine is off.
This afternoon we are going to see if we can dampen the resonance by putting a lever to the shaft of the center (non-running) motor.

If we dampen it, then I think we have a strong case for changing the type of coupling.

The reason we think the rotor is resonant, as opposed to the motor casing itself is this:

1. standing on the motor had zero effect on the vibration readings.

2. During the bump tests the rotor was very easy to excite..ie, very little energy was need to exite it, the motor required much more.

My sense is that the closer you get to the source of the resonance, the easier it is to exite.. ( or node VS antinode)

Does this make sense to you guys?

The way to qualify the effect of a natural frequency is how close is it to running speed. Preferred is 15-20% away in general (at least according to Ralph B.). You are somewhere within 1% away from running speed. That very strongly suggests that your 1x vibration is amplified by resonance and will be reduced if you can change the natural frequency.

"standing on the motor had zero effect on the vibration readings."
This proves absolutely nothing. It is not expected to influence the vibration regardless of the type of resonance as per previous discussion.

As far as the possibility of rotor resonance different than structural resonance, I don’t know much about it and I’m sure others can contribute more. But did you whack the stator radially on the endbell the same way you whacked the shaft extension radially? (it could be a twisting mode excited by whacking on the end but not in the center). My opinion would still be to examine structral resonances carefully before moving to rotor and coupling. In our plant we would not give much thought to adding structural bracing but we would stop and think very carefully before we changed a coupling style for fear of introducing some other problem.

Now I’m really going to shut up and get back to work. (although I'd still like to understand Walt's comments better)

Martin,

I am sorry, but your idea about the coupling is xxxxx (can't say it). You don't have any real data that indicates the shaft is bending at 3585 cpm. If you measure (all in vertical direction) ODS data or simple peak/phase data at the motor OB bearing housing, top center of motor case (lift lug), motor inboard bearing housing, and on shaft coupling and plot this to scale, it would help indicate whether shaft is bending and define the motion of motor case.

As Pete said, you have a resonant structure. Now what??

Walt:
A little more info about this problem is in order:

1. The vibration of the non-running cener motor is highest (.5"sec) at the coupling (.25 at the MIV) Mov is always lowest of the verticals. usually about 50% of MIV

2. We removed the coupling of the center motor during the exciter test, and the resonant response plummented.

While we have not yet done any phase analysis, we are planning to at the next opportunity

Martin,

I took your vibration levels, guessed the relative distance between motor bearings and coupling and plotted the data with Excel.
The result is that motion is nearly a straight line if the vibration at motor OB is out-of-phase with motor IB and coupling. Thus the shaft is not significantly bending. If this is the case, then there is little motion (less than .05 IPS) at motor center. If motion is in-phse, then vibration level at motor center would be .2 IPS. You really don't have to measure phase to confirm this.

I expect that you changed (raised) the natural frequency when you removed the coupling, because you removed mass from the structure. Without a picture to see, I would consider either adding mass or stiffness to the motor support in vicinity of motor IB bearing/foot.

Here is my simple Excel plot for vertical vibration at 1xSS:

This message has been edited. Last edited by: Walt Strong,

Motor_Vib.xls (15 Kb, 17 downloads) Excel Plot

FWIW, I agree with Walt.

A natural frequency is a result of the total system. Maybe you can change it by removing one part of the system (coupling) but that doesn't mean that one part is the cause of the problem and doesn't mean it can't be fixed by adjusting another part of the system (bracing).

Walt's explanation is certainly plausible. i.e. Maybe your 3600 resonant mode involves a pivot point somewhere near the center of the motor (or slightly toward the outboard end). This gives the magnitudes you described. Also the coupling mass does have the most effect on this mode because it has the biggest moment arm distance from the pivot point.

Side comment - If the machine has been idle for long With 0.25 ips on the bearing housing, I would say chances of false brinneling damage of the bearings are high (although not likely contributing to your resonant 1x vibration). Have you checked for bearing fault patterns or checked to see if bearings feel loose by pushing/pulling radially on the shaft.

Ok:
It seems to me that Walts description is the more likely scenario.. ie. the whole system is resonant.
I concur with Walt that it seems unlikely that the shaft & coupling of a 40hp motor is resonant @ 3600 cpm

We are going to do some testing today to see if we can effect this resonance.

It seems that my misunderstanding here is the idea that the highest amplitude at the resonant frequency is the problem rather than the symptom.
I was thinking in terms of a simple text book, node and antinode, where the antinode is the highest amplitude and securing the antinode has the greatest dampening effect on the resonance.


The other thing that kind of put us off track here is the assumption that standing on the motor or base plate had a reasonable chance of dampening this resonance.

Anyway, we are going back out there today, and I'll let the group know what we find.

Thanks for all the help!