Bearing Overheating: Causes, Vibration & Heat Relationships

[Cessh]

Dear All,

I'm doing an FMEA for Bearing that uses grease as the main lubrication.

I've identified several failure causes for bearing overheating (above 90 degrees Celsius):

1) Overgreasing
2) Undergreasing
3) Loose Fits
4) Loss of Lubrication (leakage)
5) Wrong Type of grease used

6) Vibration??

The assumption number 6, I want to ask:

1) in your experience, does vibration in bearing normally causes overheating?

2) In bearings, How is the vibration and heat relates to one another in the process of overheating? When the vibration goes above recommended limits, does the temperature also rises above the normal value (less than 90 degrees Celsius)? or when the bearing overheats does that mean it must also be vibrating above normal limits?

3) In what ways does vibration causes overheating? Because it is known that vibration is caused by many things such as misalignment, unbalance, mechanical looseness, etc.

Please share your thoughts regarding this.

NB: Would there be any other cause for bearing overheating other than those stated above? Excluding environmental cause and human error.

Best Regards,

 

[Nidum]

Too big a subject to try and cover in postings but here are a few notes :

(1) Two cases of vibration in bearings : Coming from external sources or generated within bearing .

There are standard ways of including external source vibration as part of the bearing load .

Vibration generated within the bearing usually means that the thing has or is about to self destruct .

(2) Bearing temperature is not a very reliable indicator as to whether a bearing will fail under a particular loading condition . Generally there is a safe zone of acceptable bearing temperatures . Inside the zone is ok . Outside the zone is not .

(3) Greased bearings which are basically adequate for their duty sometimes fail prematurely shortly after start up . This is a particular problem at low temperatures .

(4) There is a natural pumping action of lubricant in a rolling contact bearing . Part hydrodynamic , part displacement and part squeeze film . Any interruption of the natural pumping action will generally cause premature bearing failure .

(5) Lubrication quality is one of the factors which you can include when doing bearing life calculations by standard methods .

 

[jim hardy]

what kind of machine ?

If an electrical machine that's larger than ~500 KW or used with VFD or SCR drives, study up on 'shaft currents' .http://www.vibescorp.ca/learn-about/shaft-currents/

 

[Nidum]

If an electrical machine that's larger than ~500 KW or used with VFD or SCR drives, study up on 'shaft currents' .

Hi Jim ,

Another anecdote :

A long gone local engineering company once had a semi automatic weld deposition machine specially built for hardplating the insides of large hot hoppers used on blast furnaces .

Radial welding arm picked up current via a mercury pool in base of machine . Pool was supposedly sealed .

Bearings on that machine were generously big by any standards but failed monotonously . Examination showed distinct arc tracks on races and arc pits on the bearing balls .

Nobody ever worked out exactly what caused the problem .

 

[jim hardy]

A long gone local engineering company once had a semi automatic weld deposition machine

Any Rectifiers in it ? I'd be not a bit surprised if shaft current were the culprit.

We too failed twenty year bearings in ten months.

Harmonics aggravate the condition. It was a little known phenomenon until VFD's showed up in home appliances - suddenly washing machine manufacturers learned of it. There's much more published on it now than in 1970.

Sleeve bearings develop pits. Ball bearings develop regular marks and your immediate reaction is "What in the world made such a regular pattern on that race ? It's striped just like a raccoon's tail ! "
Of course as damage progresses it completely destroys the bearing race and the evidence, so maintenance tries different grease types while procurement blacklists bearing vendors one by one..

Fix is to insulate the bearing housing, as is done in big utility generators.

 

[Q_Goest]

Hi Jim,

... Ball bearings develop regular marks and your immediate reaction is "What in the world made such a regular pattern on that race ? It's striped just like a raccoon's tail ! "

We had one that looked like your description. Let's see if this picture works...

This is of the inner race of a cylindrical roller bearing. The race of course is pressed onto the shaft in this photo. I was told by the bearing mfg. that this was due to the bearing sitting idle with water present. The bearing is oil lubricated and had sat for some number of months. We believe the oil had drained away and water condensed on the bearing. The areas where there are dark spots were areas between the cylindrical rollers where oil more easily drained away. Between the roller and race, the oil couldn't drain as easily so it didn't rust at that spot.

The bearing in the photo runs at very high speed, around 12,000 RPM. It made a very loud noise when operating, something like a siren. Later I discovered the outer race housing was not perfectly round. It was a few ten thousandths out of round, but I don't think that had anything to do with the racoon tail. It is worth inspecting parts however to verify the bearing mounting locations are perfectly round to eliminate one potential source of vibration.

Regarding vibration, another issue regards rotating parts being out of balance.

I don't believe that vibration alone however, would create any additional heating though I suppose that's possible.

 

[Nidum]

Do any of you know how to listen to a bearing with a screwdriver ??

 

[Q_Goest]

I've used a mechanic's stethoscope and bits of rubber tubing but haven't tried the screwdriver trick. :)

 

[jim hardy]

This is of the inner race of a cylindrical roller bearing.

That photo in post 6 looks a lot less regular than the raccoon tail stripes on my ball bearing, but it could be just farther along.
Do those spots continue all the way around the race?

www.eaton.com/ecm/idcplg?IdcService=GET_FILE&allowInterrupt=1&RevisionSelectionMethod=LatestReleased&Rendition=Primary&dDocName=AP040061E

Mine had stripes about 4x further apart than this bearing

IF yours is shaft current fluting, i'd suspect a magnetic effect at a low multiple of line frequency.
Honestly though, it looks to me a bit too irregular for that.

Do any of you know how to listen to a bearing with a screwdriver ??

I press the handle right against a tragus so as to close off my ear, press tip near the bearing housing

 

[Cessh]

We had a case in our plant, it was after regreasing of the bearing (roller bearing), when the equipment (crusher) started running, the bearing temperature (bearing #4) increases to 90 degrees celcius. I causes panic to the operation staff. We held a meeting trying to find the cause of the problem... but then just after two days of the equipment running, the temperature went back to normal around 70 degrees celcius on its own without maintenance team having to do anything.

The maintenance team said it was a normal thing and that it happens frequently like that, after greasing, the temperature went high for two days and then it goes down on its own...

Is it really normal something like this to happen? Why is it happening like this?