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motor vibration non-repeatability

motor vibration non-repeatability

motor vibration non-repeatability

(OP)
Has anyone experienced non-repeatable motor vibration, with motor running solo? One symptom is that vibration is higher initially if the motor sits for a while. Motor is an 1800 rpm induction unit with pressurized hydrodynamic bearings.

RE: motor vibration non-repeatability

Sorry to answer a question with more questions, but what is the frequency where the changes take place?  If at 1X rotation, etc., did you monitor amplitude and phase during start-up and compare between runs?  

RE: motor vibration non-repeatability

(OP)
Vibration is almost all at running speed frequency. Don't know phase characteristics.

RE: motor vibration non-repeatability

I have seen a similar thing on a motor/fan combo.  Vibration was at 1x and phase was all over the place.  The solution on that one was to replace the vibration isolators supporting the equipment, not unbalance as one would first believe.

Therefore based on the limited information available,I would suggest that you look at soft foot, resonance, and the isolators if they are present.

RE: motor vibration non-repeatability

Right- any investigation should start with inspection of machine mounts and foundation. Be sure to check all mounting bolts, bearing attach bolts, etc.  Also ensure no abnormalities in bearing lube system.  Are there any adjacent machines running at same speed?  

Next, break the investigation into two parts- (1) source vibration (changes in rotor balance, etc.), and (2) response. I assume the vibration level is rough or severe.  Have you determined where vibration is strongest
(eg: drive end, opp drive end?) and direction?

(1) I recommend monitoring phase and amplitude at 1X rotation and comparing over several start-ups/coast-downs.  Consider doing a balance sensitivity test with a trial weight to see if you can duplicate the changes (probably not if you have non-repeatable results to begin with).  
If the phase varies significantly (with the same measurement point) between runs, then consider doing
an internal inspection (after looking at (2) below) for parts or assemblies on the rotor that can change
position and affect rotor balance. I don't know what that may be on your unit, but consider it anyhow.  

(2) Investigate for structural resonance, especially if the vibration is significantly directional. The changes could be more related to changes in stiffness that affect the structural response to a steady vibration level.  Depending on the arrangement and structural dynamics, small changes could have a significanteffect.

Beyond that, you could go goofy and consider conditions such as bent shaft/thermal bow combinations.  

Pls let us know if you find anything.

RE: motor vibration non-repeatability

(OP)
Latest data showed that non-repeatability was worse with higher amplitudes with cold oil, compared to normal DTE light turbine oil at 115 degree F. design temperature. Motor base vibration was over ½ amplitude at bearings in horizontal direction. As rotor vibration was much worse near the coupling end bearing, with a non-symmetrical orbit, bearing was taken apart and re-assembled. As impact tests on casing during coastdown at slow roll speed showed frequency near 1950 cpm, giving only about 10% margin, motor supports were also stiffened in the horizontal direction - insuring that the rotor/bearing first critical speed remained above operating speed. After re-installing bearing, and checking for soft foot after welding support braces, motor then ran with much lower vibration levels.

RE: motor vibration non-repeatability

I think this was a valuable thread showing the effect of resonance. All vibrations is the elementary result of impulse (in this case unbalance) and (actually "times") dynamic response characteristics. Fine, from the beginning this looked like a variable unbalance, and since time aspect was involved you could suspect a thermal effect. I do believe Franko in his recent post confirmed thermal influence was indeed the reason for unbalance change, but it was most probably a very small and normal one caused by for instance a slight normal rotor bend (only unbalance change) and/oroil viscosity change (affecting more the tuning of the foundation/rotor resonance (kind of a system resonace it seems rather than just a critical).
If the coupling is not a constantly soft one, but may get stiffer with time (ageing rubber inserts for example) you should keep an eye open for the future. If the rubber gets stiff, the node of tragged out towards the driven machine and you might slide down towards running speed again. This is an eternal struggle with feed water pumps in a nearby power station here. They are saving money to be able to buy a better dimensioned coupling that does not age that fast. Best regards Arne

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