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Possible causes of TEFC NDE brg spinning on shaft
5

Possible causes of TEFC NDE brg spinning on shaft

Possible causes of TEFC NDE brg spinning on shaft

(OP)
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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(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

With the shaft getting slightly longer when the rotor temperature comes up, something has to allow axial movement. Usually the bearing is allowed to slide in the end bell and perhaps a spring washer used to ensure preload on the bearing.  

If the bearing seizes in the end bell, then every thermal cycle would degrade the interferance fit on the shaft.    

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
Thanks.  What would cause bearing to seize in housing?  

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(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

Being installed dry and clean, without the slightest trace of grease, would certainly allow, if not cause, a bearing to freeze/rust into a housing.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
Mike
I'm not sure I understand.  Are you suggesting to apply grease to the seating surface of the housing (where outer ring od sits)? I haven't heard of that.

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(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

As far as grease goes you might look for mixing of incompatible grease... I have seen it cause bearing failures bad enough that it caused the inner race to spin on the motor shaft and was so bad the it bent the shaft. It too was on a 3600 Rpm motor. 300 Hp.

RE: Possible causes of TEFC NDE brg spinning on shaft

I think he means no grease on the races or the balls themselves??

RE: Possible causes of TEFC NDE brg spinning on shaft

Yes.  Apply grease, or at least leave the preservative grease on when you remove the wrapper from the new bearing.

I'm guessing nothing like that is mentioned in the maintenance manual, and probably not in the process sheets of the original manufacturer, either.

Without meaning any insult to sparkys, it's just silly to expect a clean steel bearing to slide, even slowly, even just a fraction of a mm, inside a clean ferrous housing absent some kind of lubrication.

Old-time motor shops probably didn't worry about it because all available hands and work surfaces were covered with grease anyway.  Nowadays, the lawyers who write standards expect workshops to be clean enough to not blemish their silk ties.
 

Mike Halloran
Pembroke Pines, FL, USA

RE: Possible causes of TEFC NDE brg spinning on shaft

This is from our repair spec.
 "To mount the endbell, it must be clean and should have a coat of light oil (711 or WD40) applied to its bearing surface, also, a coat of light oil may be applied to the cooled bearing's outer race."

Thanks

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
I can see light oil would be helpful.  I would be a little cautious in applying grease... might help during installation, but over the years as the oil bleeds out and the thickener is left... seems like it might actually make the situation worse.  

Bearing OEM's provide very specific bearing installation instructions regarding bearing housing fits,  deviation from roundness of the housing, I think I have even seen roughness specification for the housing.  Personally, I can't recall seeing bearing OEM guidance regarding this practice of lubricating the housing, which I would expect if it were critical.  I would be appreciative if someone can provide further references regarding this practice.  

=====================================
(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)

Quote (flandrax):

As far as grease goes you might look for mixing of incompatible grease... I have seen it cause bearing failures bad enough that it caused the inner race to spin on the motor shaft and was so bad the it bent the shaft. It too was on a 3600 Rpm motor. 300 Hp.  
Are you talking about a situation where the bearing binds internally such that inner ring can't move relative to outer ring?
 

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(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
I did find the SKF Bearing Installation and Maintenance Handbook states "The housing bore and bearing outside diameter should be coated with a thin coating of lightweight oil."

 

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(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)

Quote (electricpete):

I did find the SKF Bearing Installation and Maintenance Handbook states "The housing bore and bearing outside diameter should be coated with a thin coating of lightweight oil."
BUT thinking some more, it should be noted that the same reference also states: "To press bearing on shaft, first apply a thin coating of lightweight oil to the bearing seat and bore."
Clearly the reason we coat the bearing bore is NOT to faciliate movement of the bore on the shaft during subsequent operation (which would be undesirable), but simply to facilitate installation of the bearing.   So we have to consider the possibility that the lubrication of the outer ring /housing is stated for the same purpose (installation, not to facilitate movement during operation).   Along the same lines, we have to question whether the lubricated/non-lubricated condition of the bearing seat after 8 years of operation has any relationship to it's initial state.... that original light coating can certainly disappear... the vapors from the bearing grease might also replenish some lubrication.

In summary, on the subject of importance of lubricating the housing, I'd like to say it's a good practice, and I think most would do it simply for ease of installation, but I'm not convinced that failure to do it would be a fatal flaw.  I don't have a strong opinion one way or the other yet. I will say that the whole idea of bearings moving within the housing to faciliate thermal growth is fraught with many unknowns and uncertainties ... for example it can be faciliated by equipment vibration so a smooth well-aligned machine could be a liability. So again it certainly doesn't hurt to do what you can in terms of initial lubrication.

I think I will have a pretty good chance tomorrow to test the hypothesis of whether binding of the outer ring in the housing caused subsequent movement of the shaft..... I will clearly mark the orientation of both bearings before dismounting them, then cut them apart and check the direction of thrust loading on each bearing evidenced by the ball track.  If there has been enough thrust to force that bearing out, there should be clear evidence of that thrust loading on each bearing in the direction associated with restraining an expanding shaft

===========

Just thinking about one other thing... ccjersey already came up with a good hypothesis of how a stuck outer ring within housing could cause sliding inner ring on shaft.   Is there any way that sliding inner ring, resulting in a lot of heat, could end up binding the outer ring in the housing? (I can't think of any way, but maybe you guys can).

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(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

>>>BUT thinking some more,<<<

You gotta stop doing that; you're going to hurt yourself.  winky smile

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Possible causes of TEFC NDE brg spinning on shaft

>>>the whole idea of bearings moving within the housing to faciliate thermal growth is fraught <<<  

Agreed.  I prefer mounting the bearings in flexible diaphragms, or almost anything except expecting the bearing itself to move in the housing.

I'm sure it looked okay on the drawing board.  ... and I'm pretty sure that no M.E. was looking at it at the time.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
That drawing board has knuckle drag marks all over it winky smile

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(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

Pete, if the previous bearing replacement did not include a recording of the bearing journal and housing fits then you probably cannot determine the cause of failure with any certainty without the assistance of a bearing expert (ie. bearing failure analysis by the manufacturer)

My experience suggests that an improper bearing fit due to age related wear is more a more likely cause of failure than a perfectly fitted bearing with too much grease. Another likely cause is improper installation practice as discussed in previous posts.
 
I would suggest that your motor repair specification for any motor, regardless of size, must include checks of the bearing journal and housing fits at a minimum of 120 degree intervals. Any fit that does not meet specification must be repaired by plating, flame spray (if allowed), or sleeving.

PS: for a grease related failure, I would expect the stator to be literally filled with grease.  

RE: Possible causes of TEFC NDE brg spinning on shaft

I am with rhatcher. Probably the original shaft interference fit wasn't enough (we are talking less than a thou here, so the measurement and the measurer have to be accurate).

In my shop, we apply a light coat of oil on the housing bore and the bearing OD to ensure an easy mallet fit. The inner races are always induction heated to about 100 deg C to 110 deg C before fixing on the shaft, regardless of the size of the shaft.

And yes, we check & record all the fits on the shafts and the housings, before we fix the bearings. Sleeving is done only for the loose housings. For loose shafts, our SOP is to change the shaft.  

Muthu
www.edison.co.in

RE: Possible causes of TEFC NDE brg spinning on shaft

Pete, I will add that the ODE bearing on a motor like you describe will move in the housing to allow for thermal growth of the shaft length dimensions for the driven equipment and the motor.

Some ball bearing motors use a 'wavy washer' to presumably (?) resist this growth and to move the shaft back towards the 'cold' dimension during the motor cooling after the motor is stopped.

MikeHalloran referred to the 'wavy washer' as a 'flexible diaphram'. I believe these terms are synonymous.  

RE: Possible causes of TEFC NDE brg spinning on shaft

Uh, NO NO NO NO.  

A wavy washer is not a flexible diaphragm.

A wavy washer comes from a place like Smalley Steel Ring, and in this instance would be used to force the bearing's outer race to slide to one end or the other of its travel, or maybe to center the bearing within its travel range, sliding within the housing.

My reference to a flexible diaphragm means pretty much that; a motor NDE bell that is radially rigid, but axially flexible, like a flexure, so that if/when the bearing outer ring seizes in the housing bore, it doesn't matter, because the housing will flex to allow axial displacement of the bearing.  

Of course nobody actually builds motors that way.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Possible causes of TEFC NDE brg spinning on shaft

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

RE: Possible causes of TEFC NDE brg spinning on shaft

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.  

RE: Possible causes of TEFC NDE brg spinning on shaft

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.
 

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
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.  
 

=====================================
(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

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?

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
Thanks macmckim

Quote:

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?

Quote:

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.

Quote:

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.

Quote:

Was there any damange to the bearing balls?
No, races and balls in good condition.
 

=====================================
(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
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)

=====================================
(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

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

  
 

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
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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(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
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.  

=====================================
(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

(OP)
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.

=====================================
(2B)+(2B)'  ?

RE: Possible causes of TEFC NDE brg spinning on shaft

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
 

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