High axial vibration

I have a recently reconditioned 2 pole, sleeve bearing, 1500 HP induction motor that has high axial vibration levels. Drive end axial is 0.42 in/sec and opposite drive end is 0.32 in/sec. Spectrum are nearly equivalent peaks at 1x and 2x. Horiz and vert readings are acceptable (0.12-0.14 in/sec). I have not performed any phase testing yet.

I don't believe it is coupling alignment. Motor was hot aligned (Rotalign). Also, during test run in shop (uncoupled), axial readings were a little high (0.16 in/sec). (Above axial numbers are with motor fully loaded.)

The axial readings in the shop did appear to respond to voltage. At half voltage, the axial vibration reduced to 0.09 in/sec.

Question 1. Is there an air gap problem. I thought that electrically induced problems with a motor (air gap, dynamic eccentricity, rotor bars etc.) show up as primarily radial vibration forces. Yet I did observe a direct repsonse to axial readings in the shop with voltage. How can varying the excitiation (voltage) affect axial readings??

Question 2. Do I have a bearing alignment problem in the motor? The bearings were repoured during the recent recondition. One housing was also built up 0.004 in.

In the field, we found by removing shims at a opposite drive end foot, we lowered the vibration levels. I don't know if I was inproving bearing alignment or improving air gap uniformity or both. But it did have quite an impact (down from 0.55 in/sec drive end).

As a final field observation, you can hear the sound surging 44 times per minute. I assume this is twicw slip frequency (3575 RPM full load). Motor load is constant. It's this surging noise that makes it hard to walk away from this one.

Any thoughts/opinions/experiences are welcome.

Thanks in advance.
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Sounds like you are on the right track regarding electrical and magnetic center. Centering the bearing sounds like a good idea, there may be something with the coupling contributing. Testing will tell.

When you get phases you might want to fill out this form...


Hugh Hackett
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We use PdMA Motor Tester MCEmax for testing our motors.
As you are sure the alignment has been corectly done, you need to check if there is rotor or stator problem. We do Rotor Influnce Check (RIC) using MCEmax to isloate problems to either rotor or sator. As the motor is turned incrementally by hand, a plot of its influence on the ststor coils is developed using inductance measurements. If the influnce on the pole groups is uniform, the effects are caused by the rotor. If the influnce on the pole groups is non-uniform, the effects are caused bt the stator. Also using the same tester we can also do High and Low resolution Tests for rotor bar analysis and speed acquisition. We look at the current spectrum identify the pole pass frequency (FP) sidebands and their assoicated amplitude to determine the condition of the rotor. Cracked or broken rotor bars also result in excessive vibration.

Once you have eliminated the rotor or ststor problem, than check eccentricity or AirGap. If electrical problems are eliminated than you have to check for the mechanical causes. For information on MCEmax tests logon to WWW.pdma.com

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First we need to know the spectra content generating the vibration. What is 1X amplitude. What are the other peak frequencies.
Second, a new or reconditioned motor should run 0.05 ips peak setting rubber pads or properly shimmed and bolted down. 0.16 ips axial is very high for a shop test. How was the shop test conducted?
Adjusting the shims indicates soft foot which can create or agrivate internal bearing to bearing alignment.
Regarding the beat, if you zoom in with high resolution you should be able to see the two frequecies (2x run speed and 2x line frequency) beating in / out of phase.
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The lack of radial vibration and the sleeve bearing arrangement would indicate the coupling is trying to pull the rotor away from its magnetic centre. Check the magnetic centre and coupling gap/assembly is correct to negate this as the cause of the high axial vibration.
To check for soft foot(which is what you are indicating in Q2) do a running check with the motor running by sequentially loosening and retightening each individual hold down bolt in sequence while monitoring live vibration readings on your analyser. If there is any increase or decrease in these levels then a soft foot condition which is distorting the motor frame air gap exists. I have just last week tested this on a 2300KW 2 pole motor with one of our clients. The motor tested OK on the shop floor but when on site failed misserably. They assured me that they had checked for soft foot with both dial gauges and laser gear but when checked again after presenting the vibration data an angular soft foot was detected. Repairing the soft foot dropped the vibration levels by 28% and allowed the machine to pass its acceptance test.
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