Bearing Temperature

[itsme]

Hi,

Can anyone with previous rolling bearing experience please advise what a 'normal' running temperature should be for a ball bearing (60 x 95 x 18mm) with rubber seals, grease lubrication and standard clearance? It is running at 3850RPM, which is not too far off the published maximum speed of 4300RPM for this bearing. I am looking for a temperature based on no load.

The reason I ask this, is because I believe I am having overheating issues with one of these bearings. I have attached the temperature profile of the outer race of this bearing (which is pressed into a cast iron housing). The temperature was measured using a thermocouple. As you can see, it is getting to 80 degrees celcius after only 25 minutes of running. The inner race was at about 93 degrees celcius at that time. The photo of the bearing clearly shows heat damage on the inner race (the brown ring around the inside).

There was no load on the bearing when these measurements were taken. I currently suspect that there may be misalignment in the bearing assembly, and/or that the bearing clearance is too tight. I will probably give a C3 clearance bearing a try. Does anyone have any suggestions on the general subject?

Thanks

[Geof]

The brown ring is not from the temperature, you would need many hundreds of degrees to discolor the steel. In the picture it looks a bit like a fretting make; is this bearing just a sliding fit on the shaft and is the shaft ground to size?

Your temperature is not badly above what could be considered normal. A general rule of thumb for a sealed bearing is that provided you can hold your hand on the housing containing the bearing for 10 or 15 seconds without it feeling too hot it is okay; this means the housing is at between 60 to 65 degrees C.

However you do say it is pressed into a cast iron housing; how much press? A press fit that is too tight can often cause bearing overheating because it squeezes the outer race and reduces the clearance for the balls. Was the housing ground or turned and what was the press?

You can normally find the maximum allowable press in bearing catalogues along with the operating rpm and load. Rarely is it going to be more than a few ten thousandths of an inch; for the bearing in the picture it could be as low as 0.0001". Some bearings need a much larger press which must be tightly controlled because the press actually closes up clearance in the bearing to give the correct operating clearance.

It is important to check alignment and cleanliness also because a small amount of debris in a press fit can constrict the bearing and if the bearing is canted in the hole the outer race gets distorted.

[HimyKabibble]

Something is VERY wrong there! You should be able to run at those speeds all day long with virtually no heating. I run 8200RPM on my spindle, and after four hours of cotinuous runngin under heavy load the bearings are just warm to the touch - perhaps 35C. If they *ever* got to even 40C, much less 80C, I'd know something was terribly wrong. You must have either a serious misalignment, or, much more likely, WAY too much preload.

Regards,
Ray L.

[HuFlungDung]

Rubber seals generate heat, and if the heat cannot dissipate quickly enough, it will build up.
A shielded bearing will run cooler, because it does not actually make firm contact with the inner race. But, it is not a dust/moisture proof either.

If you have one side of the bearing closed within a housing, then you might remove the seal on that side, to decrease friction somewhat.

Machine spindles and high speed electric motors do not have contact seals. Instead, they use a baffle system to keep the grease in and the dirt out. The baffle is called a labyrinth seal system, which is not something you buy, but something you build. A labyrinth is a pair of disks with interpositioned grooves, creating a "square wave" path which the grease/dirt/air must pass through. But the one disk is fastened to the rotating member and the opposing disk is part of the housing. They run close together, maybe .010" apart.

[itsme]

Hi,

Thanks for the replies.

The shaft is ground, and the bearing is not a sliding fit on it. I haven't measured it, but I'd say it feels just about right. Again, the housing fits haven't been measured, but they feel about right.

As for the discolouration, on closer inspection it seems that the brown colour is only on the surface. There is a matching black/grey deposit on the shaft where the bearing sits. This black deposit scrapes off into a fine powder. Could it be an oil/grease film on the parts that has turned into a carbon deposit with the heat? I have also run a different bearing now with the same result, only worse, and the deposit on the surface was a lot more obvious.

I will probably go with shielded bearings on the next attempt because of the friction concern - thanks for that advice. As far as preload is concerned, these are not the main spindle bearings (they are on a sub shaft), and therefore don't have any preload applied.

The bearing bores are going to be measured up on a CMM today to determine if they are concentric/parallel etc.

[DMF_TomB]

For normal bearing tempertures

40 degree C temperature rise is normal for motors. So 25+40=65C
Factors to consider

1) bearing size and load. a undersized bearing for a certain load can run hot.

2) quantity of lubricant. Should be 1/3 to 2/3. Too much grease can cause problems. Wrong viscosity grease or old grease that is loosing oil from soap binder. Grease usually is good for 1 - 10 years depending on quantity and seals / shields preventing contamination from dirt and lose of lubricant. Grease is usually rated 100 to 150 degrees C before it breaks down. Some oil lubricated systems circulate oil specifically to keep it's temperature within reasonable limits.

3) Cooling ability, a system that is not exposed to ambient temperature (insulated) can build up heat.

4) alignment when system at running temperature. Most bearing pairs one is held and other bearing has room for shaft to expand in length from thermal growth. If both bearings are held tight then they will bind as shaft expands from heat buildup.

5) Bearing clearance or space between rolling elements and outer race. The tighter the bearing the hotter. Fan bearings that is large multiple ton fans usually have extra bearing clearance.

5) speed, vibration and balance can effect heat. 2000 rpm and 20,000 rpm obviously can run at different temperatures.

6) been my experience is oil or grease is smoking you got a problem. I have seen equipment with temporary out of balance loads have one or both bearing get hot. Sometimes very hot. When you spit on a bearing housing and the spit boils in general it's running hot.

Did the bearing actually fail ? I have seen people change a lot of bearings and actually none actually failed. When you see a 100hp motor with a bent shaft that turned blue from a locked up bearing now that's an actual failure.