Possible causes of TEFC NDE brg spinning on shaft 5

[electricpete]

This is a 100hp, 3600rpm TEFC motor, with 6313 bearings.

In 2003, the motor bearings were replaced. Motor operated satisfactorily from 2003 until January 2011, when it started developing noise and increasing vibration (mostly broadband… like a “raised floor”).

We removed the motor and sent it to a repair shop.

The shop reports that for the outboard (NDE) bearing, the bearing inner ring is spinning on the shaft (a very undesirable condition, should be interference fit). Additionally, the outer ring is “locked in the housing”.

Additionally, their preliminary view of the cause of the event is that excessive greasing caused overheating which caused the bearing to grow enough to spin on the shaft.

I don’t know how much grease is present or what degree of overheating evidence is present.

Since it is a somewhat unusual failure, I am going to the shop tomorrow to take a look for myself. Before I do that, I want to think through what the possible causes are in general, and what I should be looking for during inspection of the disassembled motor.

My initial thought is that any indication of overheating doesn’t prove anything about the cause of the bearing spinning on the shaft… after all it will likely overheat once it starts spinning regardless of why it started spinning. Maybe the location of overheating will provide some clues, along with quantity of grease.

I also think that in general the most likely cause of any bearing spinning on shaft would generally be insufficient interference fit when bearing was last replaced. Shaft dimension and bearing housing dimension were not checked when bearings were replaced in 2003 (not by this motor repair shop).

Are there any other thoughts on what the possible causes of a bearing inner ring spinning and the shaft are..., and what I should be looking for during inspection?

How would outer ring locked in the housing relate to any of these scenarios?


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[starkopete]

The wavy or spring washer is primarily for pre-loading the bearings in electric motors. The washer is installed in the bearing housing and works by maintaining a force on the outer face of the ball bearing, supposedly to ensure smooth and quiet running, which will result in extended life.

Thanks

 

[automatic2]

Bearing shaft/bore/housings are lubricated on assembly to prevent picking up or galling of metal surfaces. Dry surfaces, no matter how clean can potentially gall. Your bearing fit can be substantially altered by rocking the bearing on or off. Do not allow a drift to be used in either direction. Tempertaure profiling of the motor on a somewhat regular basis adds to a predictive approach.

 

[Pumpsonly]

The bearing could have started failing months ago without you knowing it since it has been running more than 7 years.If condition monitoring is being carried regularly you would have pick up the early sign. Over greasing will cause the bearing to run hot initially but cool down as soon as some of the grease melted and run out of the housing. When the bearing heats up, the shaft and the housing also heat up in tandem. So it will not cause the inner race to dislodge from the shaft.For properly fitted bearing assembly, the interference fit between the shaft and the inner race is very much stronger than the friction torque between the bearing balls and the races. For the shaft to spin in side the bearing, the balls must failed badly first to stop rolling.
I had seen bearing failures that the balls were crush but the inner race did not come loose.

AS for the application of light oil onto the shaft, housing and bearing, it is normal practice to coat the shaft and housing with light oil to prevent rusting and the bearing is always coated in oil.

 

[electricpete]

Attached are my notes. I think it makes for an interesting case study.

Slide 1 – you can see vibration was really taking off… lucky that we were watching it.

Slide 2 – 5 vibration data. Increased broadband. Pulsing at pole pass frequency and half pole pass frequency.

Slide 5 (left photo) shows that when outboard bearing was removed, the bearing came with it due to the loose shaft fit. This is what they meant when they told me “the bearing stuck in the housing”. However the bearing then came out of the housing easily by hand. I had misinterpreted the shop’s comment about ‘stuck in the housing” (sorry), it appears it was not stuck at all.

Slide 5 (right photo) shows bearing cavities were full of grease. Grease appears otherwise in good condition, not much discoloration or sign of heating. Slide 6 shows grease within the bearing.

Slide 7 is view of shaft seat for outboard bearing.

Slide 8 is results of measurements of bearing seating surfaces. Notably, outboard bearing shaft seat is 8.4 mils too small and bearing ID is 2.5 mils to large…. Both of which presumably indicate wear from movement. (1 mil = 0.001”)

Slides 9 and 10 show the ID of the outboard bearing. There is not dramatic visual evidence of movement (I expected something like smearing). But instead there is something resembling pitting.

Slide 11 is a view of bearing races. Outboard bearing looks fine. Inboard bearing shows more discoloration (presumably ran hotter as expected for TEFC) and shows some evidence of thrust load (no thrust load evident from outboard bearing ball track pattern).

Slide 12 is a view of bearing OD’s. Both a little odd, but I tend to think it’s not significant.

Slides 13 – 16 discuss volume of cavity and greasing strategy. There are some questions in there…. Feel free to comment.

In the end there are 2 scenarios:
1 – Shaft was undersized to begin with at time of last bearing replacement. It wore over time.
2 – Possibly in combination with 1, the full grease cavity caused overheating of the bearing, which expanded the inner ring and facilitated spinning of the inner ring on the shaft.

Our shop tended not to agree with #2. Interested in your comments.

Regardless of whether full grease cavity contributed to this particular problem, I tend to feel that full grease cavity represents end of life for a bearing that has high D*N *( 3600rpm machine with relatively large 6313 bearings.). IF there is not significant grease shrinkage expected (question), then it is not suprising the cavity filled up given our greasing schedule.


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[macmckim]

FWIW - my two cents goes with " 1 – Shaft was undersized to begin with at time of last bearing replacement. It wore over time.

I was a bit surprised to see the NDE without any internal bearing
caps on a 2P-100HP motor. Was there a pre-load wavy washer in the NDE. Your analysis did not state if one was found.

The deep pitting in one raceway may be grease in-compatablity as some point in time in past 7 years. I have seen similar issues
when two in-compatable greases are used - almost acts like an acid inside the bearing. Was there any damange to the bearing balls?

 

[electricpete]

Thanks macmckim

I was a bit surprised to see the NDE without any internal bearing caps on a 2P-100HP motor.

That is observant. I guess you could tell by lack of cap-mounting holes in the slide 5 left photo? The NDE has no bearing cap, ODE does have a bearing cap. I don’t know why the difference. What is the advantage of internal bearing cap… particularly if you have shield on that side of bearing?

The deep pitting in one raceway may be grease in-compatablity as some point in time in past 7 years.

The pitting was actually not the raceway, but the surface of the ID of the inner ring that seats on the shaft. I probably threw you off with the word “spalling”, which normally applies to races …. might not be the correct term here on the inner ring seat.

Was there a pre-load wavy washer in the NDE.

None found. However I don’t think it caused any problem because there is no evidence of skidding. I asked the shop and they said they would add one during reassembly as long as they had enough endplay.

Was there any damange to the bearing balls?

No, races and balls in good condition.


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[electricpete]

I noticed slides 9 and 10 headings talk about pitting on the inboard bearing id... that is incorrect and should (obviously?) be the outboard bearing id (the one that spun on the shaft). The red labels IB and OB on slide 9 are correct. (Sorry)

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[starkopete]

Hi,
e-pete:
"What is the advantage of internal bearing cap..."

From my experience, both ends of motors this size have internal caps. The functions of the internal bearing caps include: locating bearing, directing grease flow, retaining grease and directing thermal growth. Based on the information that you provided (ball bearings on both ends) leads me to believe that the original design of this motor was that any thermal growth of the shaft was intended to go towards the ODE. To achieve that, the DE (drive end) bearing housing should have minimal depth clearance because it is the locked bearing. The only clearance in the depth of the bearing housing (on the DE) is to accommodate the lateral growth of the bearing race. The ODE (or NDE) housing has depth clearance to allow for the thermal growth of the shaft. The idea is to direct the thermal growth of the shaft to the ODE (NDE) of the shaft so as not to put an axial thrust load on the driven equipment and/or affect alignment.
LPS for you for starting and continuing a thread that is really educational ('cause I need educating).

Thanks

 

[electricpete]

Thanks starkopete.

Good point about fixed inboard / floating outboard configuration, that explains why an inner bearing cap is mandatory for inboard bearing (since it plays a role in clamping the bearing), but less important for outboard. As long as we have bearing shield facing winding, I don't think an inner cap would be necessary for preventing winding contamination.... on this particular motor without outboard bearing inner cap we had had no grease in winding even though the cavity on other side of outboard bearing (endbell side) was completely full. I guess the one advantage of having an inner bearing cap on outboard bearing would be that we keep a small extra reservoir of grease there which assists in keeping bearing lubricated since oil vapor can still migrate through small clearances around the shield. (Although the inner cap reservoir does not get replenished, so not as effective as the end-bell-side reservoir)

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[electricpete]

Attached I have revised my powerpoint to incorporate the above correction (5 Apr 11 18:17) and fix a few other typo’s.

I also added slide 16 explaining why movement at inner-ring/shaft is worse than at outer-ring/housing. I know this is obvious to you guys, but I added it because there was one person in our plant that expressed an opinion that this condition did not seem severe because he often sees evidence of movement at outer-ring / housing.

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[electricpete]

Another odd aspect that I hadn't noticed before is that the outboard bearing bore (the one that spun) has a smoother mirror-like surface compared to the other one. Apparently the relative movement acted to polish the surface.

This is hard to show on a photo, but I tried to show it in attached.

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[macmckim]

ElectricPete

In regards to the internal bearing caps comment - if the DE had
the internal brg cap, means the DE bearing is locked and thermal growth goes towards the NDE. As for the NDE not having internal bearng cap, looking at the picture of the NDE shielded bearing - it was caked in dust/grease crud. A steel shield will still allow contamination into the bearing. If the bearing had a neoprene shield on one side, the potential brg contamination would be lower in my opinion.
Also as you noted there was no preload wavy washer on NDE, but your repair shop will install one. Dollars to donuts, I bet there
was one originally and somebody forgot to re-install 7 years ago.

Mac