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 High axial vibration on motor
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| Author | Topic: High axial vibration on motor |
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clay Junior Member |
posted 12-19-2001 01:18 PM
   
I am at a nuclear power station in the Southeast USA. We have a 125 HP motor driving a building exhaust fan AHU. The unit is belt driven with 5 belts. The motor is bolted to an adjustable skid base that is bolted to an in-bed plate mounted on top of a concrete pedestal about 1 foot off the floor. The motor was replaced due to bearing fatigue. Vibration testing was performed on the replacement motor properly mounted in-place on its support pedestal, un-belted and without the motor sheave. All motor hold-down bolts were torqued to 75 ft-lbs. Vibration levels on the motor were at 0.6 ipsp axial, 0.4 ipsp vertical and 0.2 ipsp horizontal. All vibration is dominant at 1X which is 30 Hz. We slightly loosened one motor hold-down bolt and the vibration levels jumped to 1.5 ipsp. Vibration levels on the replacement motor with the fan belts exceeded 1.5 ipsp. All vibration levels were dominant at 1X. We checked run-outs on the motor shaft and found 2 mil radial run-out and 5 mil axial run-out on the face of the shaft. We failed to get phase data on the motor. We are in process of obtaining a new motor and plan on removing the motor support structure for inspection. I have never experienced high axial vibration levels on a motor with no belts and with the sheave removed. Any advice would be greatly appreciated. Thanks. IP: Logged |
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J-Marc Member |
posted 12-20-2001 08:42 AM
The changes happenning by loosenning bolts clearly tell you that you have a softfoot problem. J-Marc IP: Logged |
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Yvan A. Lejeune Member |
posted 12-20-2001 09:44 AM
You don't necessarily have a soft foot problem as air gap eccentricity created by the soft foot would cause vibration at 120 Hz. If 1x vibration is high, should you not consider imbalance as your primary suspect? A phase analysis across the motor would certainly have brought you that much closer to correctly diagnosing the problem. Don't feel bad though, it is rare to see posts where phase information is included. A shame, it sure would save a lot of trial and error type diagnosis. IP: Logged |
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J-Marc Member |
posted 12-20-2001 10:18 AM
High 1X-axial is often see in softfoot condition rotor being seeking for is magnetic center at each turn. I agree that you normally suspect unbalance and bow rotor with these specs but, Yvan, how imbalance could be influence by loosenning a bolt? Could it increase mobility that much? In my opinion, I don't think so. How do you explain? J-Marc IP: Logged |
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clay Junior Member |
posted 12-20-2001 11:00 AM
Thanks for the responce. My concern is with the axial vibration. I would expect imbalance to influence the radial vibration levels. I originally suspected soft-foot,(although there is no 120Hz indicator) that is why we loosened one motor hold-down bolt. Although, here again, I have never experienced HIGH axial direction vibration due to soft-foot ( maybe elevated but not dominant). The elevated vibration levels are with the belts and sheave removed. Also, the motor has roller element bearings and the mag-center is held in place. There is no visible thrusting of the shaft and no hunting noise during operation. This motor was recently rebuilt. Maybe the bearing(s) are cocked in-place or not in alignment?? Also, in-shop testing indicated axial vibration levels at .2 ipsp and horizontal vibration levels at .5 ipsp. It is possible that the technician entered the wrong values and that the axial vibration was at .5 ipsp. Sorry, I know that this only adds to the confusion. I am basically searching for other sources of axial vibration similar to my problem that someone else has experienced.
IP: Logged |
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Ralph Stewart Member |
posted 12-20-2001 11:53 AM
Is it possible the shaft is bent under the bearing? Thanks, IP: Logged |
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W Ron Brook Member |
posted 12-20-2001 01:51 PM
Clay, If I understand you correctly, this motor also ran very poorly at the repair facility? If they bolted the motor down to an inertial mass, and it still ran this poorly, then they are not repairing the motor correctly. I suspect this motor has a serious problem that would be caught while disassemble, probably a bent shaft. A two pole motor, operating uncoupled on a good solid foundation should yield levels in the 0.05 in/sec velocity range. IP: Logged |
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kenrr Member |
posted 12-20-2001 02:19 PM
Clay What is the motor rpm.?? (May have missed it somewhere) Does the motor have a roller bearing in the drive end of the motor?? ken IP: Logged |
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electricpete Member |
posted 12-20-2001 03:00 PM
Maybe the imbalance is at the pulley-wheel thingy (forgot the technical term), creating overhung imbalance which will create axial vibration? I would think that cocked bearings would give other clues in the vibration spectrum. IP: Logged |
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clay Junior Member |
posted 12-20-2001 03:22 PM
UPDATE, HOT OFF THE PRESS!! Our shop is in the process of disassembling the motor. So far we have confirmed that the drive end bearing should be a roller type bearing ( 90RNO ). The actual bearing installed during previous rebuilding is a ball bearing that is also undersized ( ball size ) by 0.005". Maybe we are on to something. Maybe this would lead to the bearing binding theory and motor problem that would be caught during disassembly. The nominal motor speed is 29.66Hz. All vibration was at 1X. Thanks for all the interest in our problem. IP: Logged |
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W Ron Brook Member |
posted 12-20-2001 04:47 PM
"pulley-wheel thingy"!!!!!!!!!!! Precious electricpete!!!!! Sorry, I meant to say four pole motor, not two pole. Two poles, as most of us are well aware are not as well behaved. IP: Logged |
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David Harmon Member |
posted 12-21-2001 06:06 AM
Looks like you are on the way to finding the problem. We had an electric motor that was exhibiting the same symptoms at one time. It turned out that a spring washer next to the bearing was installed at the wrong location. This forced the motor of off magnetic center resulting in high vibration, dominant in the axial direction, and increased significantly when put under load. IP: Logged |
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Yvan A. Lejeune Member |
posted 12-22-2001 08:11 AM
Reading back some, electricpete mentions that possibly the "sheave" may be imbalanced, actually it would likely cause high axial vibration if it were cocked on the shaft - however vibration is high withouth the sheave anyway. A cocked bearing or bent rotor would be my guess. But I still maintain that A PHASE ANALYSIS would have resolved this a long time ago. Clay, you mentioned that the drive end bearing was a ball bearing and should be a cylindrical roller bearing (I assume). Fine, especially if the motor is to be used on a belt drive. But if the motor was originally designed for a ball bearing, I dont' see how this is a contributor to high axial vibration. You also indicate that the bearing ball are 0.005" undersize - I've never had the misfortune to see that much tolerance discrepancy from any bearing manufacturer, then again - I've never had to measure any balls or should I say "rolling elements". Oh - there is one other thing that has come to my mind, this is a problem that we have seen quite often, but always with 2 pole motors - end bell resonance. Who knows, maybe a bump test with the motor fully assembled on the end bell in the direction of highest vibration may show something. IP: Logged |
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Ralph Stewart Member |
posted 12-22-2001 09:15 AM
Hi Clay, Isn't this an awful high amplitude for no sheave on the motor?
quote: Thanks and Merry Christmas, IP: Logged |
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vfsorensen Junior Member |
posted 01-04-2002 08:31 AM
Not knowing the entire condition of the motor or it's configuration, you can get a high axial from a motor where the bearing is skewed on the shaft. This occurs on press fit bearings which are commonly found in motors. IP: Logged |
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Rusty Castleman Member |
posted 01-12-2002 12:02 AM
Kind of hard to cock a bearing on a shaft if it's a press fit. More likely the bearing is square on the shaft, but the endbell was cocked as it was bolted up -- very easy to do. Or there could be a burr in the endbell, or a piece of trash trapped inside. If the endbell is cocked, the bearing won't slip in the endbell as it should when the rotor starts to heat up and grow. The longer it runs, the worse it gets. Look closely for outer race defect frequency in your data to confirm this (the inner race defect will also show up, but at a lower amplitude). And as pointed out, we should ALWAYS do a phase analysis when trying to troubleshoot ANY vibration problem. And bearing O.D. and fit I.D. should ALWAYS be miked, but very seldom ever are. A high 1x vibration will nearly always get worse when you loosen a foot bolt -- it wants to MOVE and you just gave it some room to. IP: Logged |
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Mubariz Junior Member |
posted 01-13-2002 03:27 AM
Clay, Hi.. Did you try to switch off the motor and see if the vibration disapear?? Any how, I recommed you to see the codition of the rotor. May rotor is loos on the shaft. Also, see the condition of the stator if any damage. I recommed as well to clearify the problem of bow or cracked shaft. Please Clay, let us hear from you soon ------------------ IP: Logged |
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zamir malik Member |
posted 01-14-2002 06:15 AM
in this type of equipments of AHU, normally vibration is due to loseness of bolts and structure/foundation. you have to properly adjust the foundation/structure. IP: Logged |
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