Guys I am looking for some help on this issue.

We recently had a bearing failure on this 1500 HP Siemens plain bearing motor subsequent to the baring failure the DE vertical was .023 i/ps @ 1X. The following day the bearing failed miiserably. When the motor was returned from the rebuild shop we decided to take readings on it uncoupled to establish a baseline. The lowest reading we obtained was .156 ips after the rebuild. We found severe angular soft foot and decided to mill the pedestal feet to clean them up and remove the angularity. Now the motor has been reinstalled and the DE vertical is .400 ips, we do not want to couple and run this motor and risk the potential of another bearing failure.

Could anyone suggest a course of action to find and solve this problem. Qny help would be greatly qppreciated.

Attached are some of the recorded vibration readings.

Motor_Unclpd.doc (181 Kb, 39 downloads)

Check for rigidity of the mounting beams or grouted plate. May be grout below the plate might have come loose. May be the foundation bolts have loosened and can't be further tightened due to grout coming loose. any spectrum and waveform may be more helpful. Also check phase readings to confirm soft foot. check phase between grouted plate and concrete foundation.

I would expect a resonant structure that was affected by the change in stiffness at the motor base when you "improved" the contact. This hypothesis could be tested by conducting a coastdown test, impulse-response test, and/or a ODS test. I found a 4500-hp 3575-rpm motor with an upper housing resonance. I proved it by placing lead bricks (nuclear plant) at a "sweet spot", and I solved it by welding stiffeners at the right locations. The vibration level on the bearing cap was reduced by a factor of 4. I always suspect resonance when high uni-directional vibrations are present.

Walt

How directional is the vibraiton? Has all the vibration been at 1x?

How did the bearing temperature look before/during those previous two failures? And how does the bearing temperature look now?

Since you seem to have repetitive problems since the first failure, I would check closely the repair details of what diametrical clearance was measured in the bearing, what viscosity of oil was used, are the clearances and viscosity appropriate, how carefully was bearing alignment checked.

The machine is vibrating at 0.4 ips uncoupled. Have you verified that the bearing is not contacting the shaft shoulder (motor is running within it's endplay, not at the edge)?

Have the previous bearings been inspected for clues? You also may have another oporutnity to inspect the presently installed beairng if the machine can be shut down- support the rotor and roll out the bearing for inspection is a fairly easy job. Check bearing wear pattern for contact area and alignment and other clues. While you're at it, double check the shaft/bearing clearances and bearubg/housing clearances, check bearing alignmetn by looking for uniform clearance at the four corners between bearing bottom half and shaft. Look for rubs at seals, baffles etc. Look at bearing/housing seat area for anything unusual. Check TIR on accessible portions of shaft. If drive end bearing is insulated from housing or housing insulated from frame, check for mechanical degradation of that insulation.

Does the vibration change over time? Although there is not much specific reason to expect it in your case, my last experience with high vibration and bearing failures on a Siemens motor ended up being a rotor bar problem.

Also, on the surface it appears things got worse after correcting the base - apparently either the base was not well restored or some problem was created during handling of the motor.

After repairing the base and reinstalling the motor, was soft foot checked by both methods (loosening and measuring deflection while the motor is stopped, loosening and checking vibration while running)?

Thanks for all of your suggestions, this was my first post, I will attempt to answer all questions asked aand provide more information about the vibration signature. We will be looking at the problem again in the morning, with the motor shop folks.

thanks again for your comments, I will follow up.