I have recently noticed quite a lot of bearings (C3 and C4) skidding in the non drive end of large motors. My main question is what is the cause of skidding is it linked to grease types, cross contamination, heat or lack there of? In some instances I will get 6g's overall in a 10K acceleration spectrum which is quite high as normal can be .8 - 1g. I feel as though if visually inspected there will be no defect / markings to the naked eye as the acceleration waveform is ok. Is this state Ok? The common fix is adding grease but the skidding remains slightly? So the "problem" if it is one persists. Any help will be greatly apperciated.

I have same problem with a motor in the non drive end (NU type bearing C3).If we add grease to the bearing it stops skidding for a short time (10-30 minutes) and the starts again.
We are using alvania 2 grease.
When it does not skid no vibration is registered. However when it skids 6g´s rms and up to 30 g's MPwave is registered.
I am thinking about changing grease type to a NLGI3 grease.
Did you solve your problem?

Thanks for your reply,
We are having the same results.
We are also using Alvania 2.
I have been looking to the grease side of things and have found that Alvania is a Li complex type & is compatible with Calcium, Lithium, Lithium - Ca , Poly-Urea
Incompatible with:
Aluminium & Aluminium comples, Barium, Calcium complex and Clay (bentonite, Hectorite)
The other grease we use is Castrol APXT.
I am currently trialing and comparing results will post if anything comes up out of it.

We have tried alvania 3 (NLGI 3) instead of alvania 2 (NLGI 2) but skidding is still present. Maybe we have not taken out the old grease completely and we have a mixture.
I think that noise and vibration is due to the high internal clearance C3.
Possible solutions
1- Change the bearings to a C2 clearance. I do not dare to do that because maybe other problems appear.
2- Change to a grease with more consistency NLGI3 instead of NLGI2 (it seems not to work)
3- I am thinking also about changing to a grease with less viscosity but also NLGI3 consistency. Less viscosity so the temperature in the bearing increases, the bearing expands and internal clearance reduces. However I do not know if this will work either, because increasing the temperature, the consistency decreases. do you know a grease compatible with alvania and that meets this features.

This is a very common problem in my experience and the g's are very similar to mine and the waveform looks continous with no regular spikes, ie no ball pass impacts.

I suggest the cause is metal to metal rolling contact due to lack of lubrication at the rolling surfaces, not skidding, as suggested, unless you have evidence that it is.

The effect of poor lube is metal to metal contact and the surfaces micro weld together and break as they roll causing a "frosted" appearance (like pulling an arc welding rod of the job after it has stuck) which has surface roughness greater than the normal grease film thickness of no more than several polymer layers.

Greasing temporarily increases the lube film thinkness and the contact ceases. You can estimate the surface roughness by how long it takes for the vibration to return, several days is quite long, several seconds is quite short.

This type of fault is often caused when a motor is first started and the grease is squeezed out of the rolling elments very quickly, but it's too cold to "slump" back into the rolling elements and stays stuck to the housing walls, so the bearing is running dry, and the frosting begins right then. Once started it may reduce if you put a Permalube on, but will never smooth over to as new condition.

For long life it's absolutely critical to use a grease that slumps properly at operating temperature of the inside of the bearing housing. Put a dob of grease on the outside and see if it slumps ? If not use a lighter one.

The micro welds form the initital surface stress concentrations which begin to spall in time, so these bearing have a short life in terms of a few years, rather than many B10 lives.

A B10 life is the time 10% of a large group of bearings will fail within, and for 50 KW motors it's usually 3 years.

Hope this helps.

John

Hi John,
Thanks very much for your reply and information, Your explanation makes a lot of sense and is more than likely what is happening, I am going try a few different greases but I have not seen the problem for a little while so it is tough to trial things.

Cheers.

Hello John;

First of all thank you very much for your help.

I understand from your comments that the bearings should show a frosted appearance in the rolling elements. However we removed one of the noisy bearings (NU 240) and we did not see anything strange on it. We even sent it to the bearing manufacturer and he did not find anything on it either. We must also say that that bearing only worked for a month or less before removing it.

should frosted appearance appear since the beginnig or after a time?

In the following months we will remove other noisy bearings and we will check again.

It also sounds very extrange to me because we use shell alvania 2 an 3 in all our smaller motors and only this one make noise.

You are advising us to use a grease that slumps properly at operating temperature. As you know when starting the motor grease temperature is ambient temperature. Afterwards when running temperature increases. I do not understand well why when the grease is hot after running for a while the noise stands. Do you mean that the failure occurs when starting the first time and then nothing can be done (appart from using perma).That changing the grease after the first starting failure does not improve the situation?

Another question; what do tou mean by a slumping grease? a grease with an ISO VG oil with less viscosity? or you mean a grease with less consitency NLGI 1

Thank you in advance for your help.

Best regards;