Message Board Thread - "Motor outboard bearing case."

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Motor outboard bearing case. D_King 3/27/2006
Interesting case this motor was originially diagnosed with a bad motor base and alignment. The base was deemed to be sufficient for now and the alignement was performed.

Following the alignment more vib's where taken and in addition to it thermography. Motor A and B are located side by side in the same room. Both were ran using the same flow/time/procedure. During the intial starting sequence I monitored temperatures via IR. As plane as day you could see the outer race heat up followed by the inner race. (the inner race is covered by a plastic cap so took longer for the inner race to heat so show.) I actually suspect the inner race to be slightly higher than the outer race. There were also BSF/BPFI/BPFO on the bearing although at low amplitude and oddly enough in the axial direction.

Both motors taken at the same flow rate with approximately same run time. Same with the third row. It was my assessment that the bearing was either:

a) Axially loaded.

b) Over greased.

c) Housing defective.


With these results an inspection was done on the bearing. The endbell was found to be out of tollerance and was replaced, the bearing replaced, and a final alignment performed.

After bearing replacement another test was performed. The temperature actually went up from ~175F --> ~181F. Another item to note is the installed bearing was a C3 clearance (looser than normal clearance typically used on electric motors) the new bearing was a normal clearnce bearing.

But.... I'm back at square one. It seemed to me that during the heat up of the motor/bearing my first initial sources of heat were the bearing. I suppose that an excesively hot rotor could also help heat up the bearing but this is a fairly dramatic increase in comparison to the same rated/loaded motor next to it.
 
Re:Motor outboard bearing case. D_King 3/27/2006
The other motor with same operating design characteristics.
 
Re:Motor outboard bearing case. Pete 3/28/2006
D_king,

Is there any way you can attach the vib spectra when you post the I/R image?

There are several conditions with a motor that can cause vibes to show up in the axial direction. If you need a second opinion I can put you in touch with one of our vibration Guru's who might be able to offer up some possible causes.

Let me know if we can be of assistance. pte1@pge.com
 
Re:Motor outboard bearing case. D_King 3/28/2006
Heheheh having problems even attaching the thermals! Either way quick update.

The images hopefully will be up soon but in the mean time.

It was confirmed that the wrong bearing was installed on the replacement so that was the source of the aditional heat, by about 20F.

We opened up thee old bearing for analysis and as suspected the load zone on the inner and the outer race was shifted from the center of the race to the outboard 1/3 of the race. The bearing was infact axially loaded. There was also early signs of fluting. I should have clarified the vib's.

The BSF was only present on the axial the BPFI/BPFO were present in the horizontal but were also predominately in the axial. Those readings where low amplitude the overall energy was realitively low. Thermo was definetly hot though.

With the new bearing installed vibration showed zilch, nadda, nothin empty as my checking account. So for the non belivers out there your thermal anomally will DEFINETELY show before your vibration. This was a new bearing with great vibration characteristics. However, through thermography we were able to see the result of an axial load on a bearing not designed to see anything but a radial load.

We are currently doing an investigation now to determine the cause. I've got it set up for another test run to capture 20 sec snapshots of the heat up process from start to finish. It is quite a site to see the outer race and the inner race thermal signatures show up in the early run. The two races become plain as day through the motor housing just before the end bell temperature heats up rapidly bluring the races from view. Hopefully I'll get a good shot of it.
 
Re:Motor outboard bearing case. electricpete 3/28/2006
You asserted that thermography will definitely show a problem before vibration. This would generally be true for a lubrication-related problem or any other problem which tends to make the bearing run hot (too much interference between shaft and inner ring, not enough internal clearance, too much load etc).

I can also guarantee you that a tiny spall on a race (with normal lubrication conditions and loading) will show in the vibration very early and will not show up in temperature until it progresses very close to failure (if at all).

"normal" internal clearance is not correct for electric motor bearings. You need C3 ("looser than normal"). The lack of internal clearance associated with the "normal" internal clearnace might be a cause of your temperature problem.

Of course there are a lot of possibilities.
 
Re:Motor outboard bearing case. electricpete 3/28/2006
Just trying to understand some of what you saw.

Regardless of whether bearing inner ring or outer ring is actually hotter, you should see the heat appear as if coming from the outer ring because that is all that is in contact with the motor endbell. I have a very hard time understanding how heat would get transferred fro inner ring to endbelll any way other than through bearing and then through outter ring.

This is a greased bearing, right? I'm used to the grease cavity being outboard of each bearing (the side of the bearing away from the winding). Is that where it is in this motor?
 
Re:Motor outboard bearing case. D_King 3/29/2006
You are correct the heat did appear to be comming from the outer race. I was inferring that the inner race was hotter bassed on the fact that it is carring the load. I can't prove that via thermography. And the clearance was a problem the second time around. Which accounted for an additional temperature rise of 20-40F. We didn't run it long enough to stabalize out.

What can ya do.. /shrug you tell them that they are putting in the wrong bearing and they do it anyways. They believe us now btw. However, it still doesn't explain the axial loading with the original C3 bearing in.
 
Re:Motor outboard bearing case. D_King 3/29/2006
Old bearing: 2Z/C3 Polyurea

New bearing: 2Z Polyurea

Next new bearing: 2Z/C3 Polyurea

The rework could have been prevented if the job was not rushed. We brought up the bearing clearance problem before the endbell was put back on!!! Either way, still doesn't resolve the original problem.

We have several theory's as to why the loading exists but they are a little more involved. There is a mass on top of the motor (don't ask) that is acting as a heat sink as well as adding an additional weight to the casing of the motor. The thermal growth of the top of the motor and the bottom of the motor can not be anywhere close to the same.

Shoot me an e-mail if you would like some more details: David.King47@gmail.com

 
Re:Motor outboard bearing case. D_King 3/29/2006
Thermo on this motor had never been done before. Or rather... if it had it was not documented anywhere. This problem could have existed for years, since being installed even. Vibration monitioring didn't pick up the problem and the motor is only ran an hour or so at a time and only every few months. I'm really interested to see if the shoulder for the bearing is at the wrong depth. And... if i can get the bearing back from show and tell I want to take a better look at it. I would like to see if the load zone was at the same position all the way around or in an orbit of sort like the bearing was cocked.

The motor is coupled with a sufficent gap so there should be zero thrust load on the motor. That gap was verified.

So basically where my theroies take me now are:

1) Bearing shoulder being at the wrong depth.

2) Bearing installation wrong. Unlikely since it happened on two different bearing instalations.

3) Thermal growth being so severe that it distorted the bearing housing. Not likely but a possibility

4) The mass on the motor over time has distorted the casing.

 
Re:Motor outboard bearing case. Bob Berry 3/29/2006
2) Bearing installation wrong. Unlikely since it happened on two different bearing instalations.

Not true, consistant bad bearing installation is possible if you are not using a bearing heater. Simply tapping a bearing into place can cause damage. That damage may not be picked up early with vibration, but can be found very early with other methods like spike energy, shock pulse or acoustic emissions.
 
Re:Motor outboard bearing case. D_King 3/29/2006
Heater is being used.
 
Re:Motor outboard bearing case. electricpete 4/8/2006
I would agree mechanical damage such as from hammer (!) would lead to high vibration but not high temperatures until the very end.

I think your initial assessments are right - it's either a lub issue or a loading issue.

Subset under loading - have you check the shaft and housing fits? Excessive tightness of fits can result in negative internal clearance.

Also all bearings have speed limits published in the catalogue (for example skf interactive engineering catalogue). For example 6313 double-shielded limit is around 3,800 rpm if I'm not mistaken. What is the bearing size and machine speed?

Double shielded bearing (2Z) comes with grease from the manfucturer. If you are adding grease to the housing 1/2 full during installation (which I believe is standard practice to provide some degree of sealing...someone tell me if I am wrong), is it compatible with greased used by bearing manufacturer?
 


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