Motor Bearing Temperatures

I have one kinda off the wall theory if you'll roll with me on this

"convection" that is, is the grease acting like an insulator preventing the end bell from acting as a heat sink?

I'd love to hear the results of your remedy

Stub

If in the bearing there is too much grease and if the grease has an oil with a higher viscosity then the temperature goes up due to internal friction between grease and bearing components.

The only thing I can think of is you adding more grease pushed more of the foreign grease from the grease point in to the bearing, they may be similar but not always compatible. Cleaning would appear to have solved that problem.

As to the noise, does the motor have earth insulated bearings?

A defect in a wound rotor winding can cause sparking and overheating at the bearings. The rotor to stator air gap needs to be even.

Was the noise you heard detected without the aid of an Ultra Sound device as in audible if it was I suggest listening with ultrasound detection devise such as UE or SDT you should be able to detect source of noise and make diagnosis much clearer. Also you will be able to trend the noise to determine condition and source. With ulra sound the condition of the bearing will be very clear and somewhat predictable.

For a noise analysis the best would be to make with adequate equipment an FFT and determine what is the origin.

Which type of bearing ?

what is the motor rpm and the bearing? what is the driven machine? How old is the motor (bearing)? Have you checked or what is the load (amp) of motor when the bearing temp rise?

1. excess grease can cause the temp rise.

2. if bearing life had been exceeded probably the clearance (internal) is too big.

3. for high speed - lubrication type must be consistent. avoid mixing

4. defective coupling can create sound too.

Even though, I am in operation side and mechanical branch,What I have learnt from E/M guys that this is possibly due to over greasing and they have told me that while putting the new grease, grease upto the point when all old grease is fully out and new grease has started to come out. We usually wait for the temp to subside on its own after usual two to three days

Secondly you can check the vibration, to see whther it is within limits. If that is Ok, no need to worry, as higher temp and vibration will damage the equipment & not the noise. In case of higher vibration, a frequency analysis of vibration monitoring can help to locate the defect.

Agree, a temperature rise of 20 odd degrees during re-greasing is quite common. It is important to have grease relief facilities if you are greasing using the method you advocate, otherwise overheating would be serious.

Cannot comment on the noise without hearing it, but monitoring any change in vibration for a few weeks is the way to go.

I agree with the comments that overgreasing increases the the bearing torque and hence power resulting in higher bearing temperatures. This can also be checked with the increase in power consumption of the motor.

Some greases are not compatible and can produce 'chalky insulating' material - it may be worth checking if the greases used are recommended by the bearing manufacturer. Just the viscosity of the grease/oil or mixture of greases can create insulation between races and balls. If the motor body and rotor shaft are not earthed it is possible for the static charge build to jump across and damage (by pitting) the balls or the races and produce noise. It may not be possible to detect the damage at slow speed as the arcing will only occur at some speed and the race to ball contact will depend on the axial and radial loads at that speed for a particular bearing. Does the noise start at a particular speed?

Other sources of noise may be seals or shields touching the rotating parts of the bearing.

Hi all...Thanks for the feed back..... I managed to get a vibration read out this week and vibration levels doubled from the previous month... I will be shutting down the machine to inspect the bearings, and motor... lets hope that its a simple easily fix... I will keep you in touch.

Now this is what I found... within a week after cleaning & regreasing with Shell Alvania RL3 grease ( a caramel coloured grease).... could this be a serrious case of electrical current passage through the bearing ??

It would be of help to see as well the other ring and the rollers. For me it seems to be but it is always possible to have other reasons too. With the other pictures it will be more precise. Anyway you have to change the bearing so that you can disassemble the rollers and guide ring to have a good image of the static ring. Could you count the rollers as number and give the diameter of rollers and internal ring ?

the picture shows that you got dried grease. you had used a thicker grease and most probably missed your greasing schedule. Again make sure to have the right bearing fitting clearance. The vibration indicates loose bearing fittings or damage bearing. another consideration is probably you inject a dirty or dusty grease.

If you are doing some welding works in the machine, the ground should be near to the welding area without crossing the bearing.

But as far i experience this is not a sign of arcing because the wear is uniform.

Here are other pictures of the bearing,

It looks suspiciously like the outer race is loose in the housing. The dark water mark like discolouration is indicative of fretting corrosion.

This extract from an SKF article may help you decide whether the fluting that you see is caused by vibration or electrical discharge.

"The passage of electric current frequently leads to the formation of fluting (corrugation) in bearing raceways. Rollers are also subject to fluting, while there is only dark discolouration of balls.
It can be difficult to distinguish between electric current damage and vibration damage. A feature of the fluting caused by electric current is the dark bottom of the corrugations, as opposed to the bright or rusty appearance at the bottom of the vibration induced fluting. Another distinguishing feature is the lack of damage to the rolling elements of bearings with raceway fluting caused by vibrations.
Both alternating and direct currents cause damage to bearings. Even low amperage currents are dangerous. Non-rotating bearings are much more resistant to electric current damage than bearings in rotation. The extent of the damage depends on a number of factors: current intensity, duration, bearing load, speed and lubricant.
The only way of avoiding damage of this nature is to prevent any electric current from passing through the bearing."