Greasing a motor bearing (single shielded and open bearing) 2

[Aptx4869]

Greetings

My question is regarding the single shielded and open bearing types. Are they designed to run with the cavity being full of grease or with a specific amount ?

Now if the manufacturer is recommending to put only a 5 gram of grease when greasing. wouldn't greasing the bearing on a time basis will end up filling the cavity although the recommended is only 5 grams ?

some practices will add new grease on a time basis and this will end up filling the cavity. Is this a good approach ? will it not cause bearing over greasing ? If this is not a good act, then what is the right time to grease the bearing ?

Thanks

 

[electricpete]

These are good questions. I'd say the answers vary quite a bit depending on specific application and also depending on who you ask!

My question is regarding the single shielded and open bearing types. Are they designed to run with the cavity being full of grease or with a specific amount ?

For my application electric motors, typically we would spread some grease into the bearing itself and then fill the cavity about half full of grease.

Now if the manufacturer is recommending to put only a 5 gram of grease when greasing. wouldn't greasing the bearing on a time basis will end up filling the cavity although the recommended is only 5 grams

One one hand, there is a certain amount of oil that may seep out of the grease and disappear onto the surfaces or leave as vapor especially when bearing is hot. Considering grease is 80% oil, it is possible in theory that a large portion of the grease volume would seems to "disappear" over time. On the other hand, I've seen plenty of full grease cavities in my time. I think it is more common / more realistic that the grease doesn't go anywhere and you're going to have to decide what to do if/when the cavity becomes full. You have to take more care during your greasing that afterwards you run the machine drain plug removed (or grease relief cleaned/installed) in order to allow thermally expanding grease to expel grease without presurizing the cavity. In general full cavity is more problem on large / high-speed bearings which run hot anyway as compared to small / low-speed bearings.

some practices will add new grease on a time basis and this will end up filling the cavity. Is this a good approach ? will it not cause bearing over greasing ? If this is not a good act, then what is the right time to grease the bearing ?

I dunno. Most of this is repeat of the previous. Ultrasound vendors will say you should walk around listening to the bearing to figure out how much grease to add and perhaps also when it's time to grease. It doesn't seem to have caught on much. I'm a little skeptical (if it's really effective than why don't more people do it).


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(2B)+(2B)' ?

 

[Aptx4869]

Thank you electricpete

One small question, based on your experience does over-greasing causes the single shielded bearings to have their shield "caved-in" and contact the balls ? or just this happens on double shielded bearing (although I read it is not recommended to re-grease a double shielded one) ?

 

[romke]

most bearings will not reach their design life due to overgreasing....

bearing manufacturers specify the amount of grease depending on bearing type and d_m*n (average bearing diamtre times rpm). when d_m*n is low a larger percentage of the available space may be filled with grease then with a higher d_m*n value. for electric motors a value of 30 to 50 percent is usual.

when regreasing open bearings you need to make sure that excess grease must be able to find its way out of the bearing - ususally their is a cavity where the grease can flow to and that cavity usually has a drain plug that should be opened before regreasing and closed again when the bearing has obtained a stable temperature. initially after regreasing the bearing temperature might go up a little due to the churning effect of the "extra" grease. after say 5 minutes of running the temperature should go down again to the normally observed temperature. when the temperature stays high, you may have added to much grease.

the amount of grease to be used when regreasing is based on the assumption that some grease will leave the bearing when running over time - thus 5 grams should be regarded together with the regreasing frequency. "5 gram grease" though is not a very accurate indication of the amount needed - the volume added varies with the density of the grease. for most greases that does not make much difference, but for some greases (based of a PFPE fluid) it would in fact mean only half the volume, since the density of a PFPE fluid is about twice that of a standard grease.

the primary purpose of regreasing is to stabilize the amount of grease in the bearing over time - and thus it is better to grease more frequently smaller portions then one "big shot" every 4000 hrs or so. a added advantage of regreasing more frequently is that the average quality of the grease will stay on average somewhat higher over time.

there are various formulae that may be used to calculate the optimum regreasing interval based on bearing type, type and magnitude of the load, whether mounted horizontal or vertical etc. all formulae assume the use of a standard lithium grease with a operating temperature of 70 degrees C - higher operating temperatures need more frequent regreasing, lower temperatures less frequent regreasing. when using a higher quality grease (Li-complex) regreasing intervals may be extended.

 

[Aptx4869]

Dear romke
Thank you for the reply

"when regreasing open bearings you need to make sure that excess grease must be able to find its way out of the bearing - ususally their is a cavity where the grease can flow to and that cavity usually has a drain plug that should be opened before regreasing and closed again when the bearing has obtained a stable temperature. initially after regreasing the bearing temperature might go up a little due to the churning effect of the "extra" grease. after say 5 minutes of running the temperature should go down again to the normally observed temperature. when the temperature stays high, you may have added to much grease."

If the grease comes from the drain plug, doesn't this mean that the bearing cavity is full of grease? Isn't this considered overgeasing? Or you mean another cavity different from bearing cavity

Again if the grease will flow from bearing cavity to another cavity, that means the bearing cavity is already full, is this good?

 

[romke]

when you regrease a bearing you push a certain amount of grease through a bore that ends near the bearing. thus there is some extra grease added to the bearing, assuming that is was not filled completely. the idea is to introduce the grease to be added when the bearing turns, so the added grease is spread over all the cavities between the rolling elements evenly, while at the same time more or less pushing out the same amount of "used" grease. to push some used and excess grease out, will take some time and as long as it is not pushed out sufficiently the temperature of the bearing will be temporarily higher. the excess grease must go somewhere and therefore there is usual a cavity with a drain plug. leaving the drainplug open during regreasing makes it easier for the excess grease to leave the bearing, by pushing through the open bore with a small screwdriver you can also check whether old hardened grease may prevent the bearing shedding excess grease. of course after that the drainplug should be reinstalled to prevent dust or moisture entering the bearing.

if there is a drainplug that is never opened and the bearing is regreased at frequent intervals overgreasing will occur and you might reach the stage where the "excess grease cavity" will be completely filled. when you carry on regreasing eventually the cavities in the bearing itself will also be completely filled which will lead to overheating, oxidation of the grease and finally the complete destruction of the bearing....

 

[Aptx4869]

Dear romke

I have seen some bearings with the same design you mentioned. There is a bearing cavity that only has a supply port and no drain port. The drain port is in the excess grease cavity. Are you saying that bearing cavity overgreasing doesn't happen on this type as long as you keep the excess grease cavity clean ?

 

[romke]

No, you can still overgrease a bearing when you put in more grease then recommended. But the bearing will be capable to work out some of the grease when there is room where it can go to.

See here for more info on grease lubrication of bearings:

https://www.amrri.com/PDF/LubeCoach-volume-and-frequency-recommendations.pdf

 

[electricpete]

One small question, based on your experience does over-greasing causes the single shielded bearings to have their shield "caved-in" and contact the balls ? or just this happens on double shielded bearing

If the shield is facing the cavity, then it certainly can happen on single-shielded bearing. Which direction to put the shield on a motor is not always straightforward. If there is no inner cap, then clearly the shield goes toward winding rather than toward cavity. If there is an inner cap, you have a choice. Bearing OEM’s like SKF will suggest to put the shield toward the cavity to “meter” the grease into the bearing. It prevents the scenario where too much grease finds its way into bearing itself during greasing and the bearing runs hot for a few hours after greasing while the rolling elements push the grease out of the ball path.

(although I read it is not recommended to re-grease a double shielded one) ?

Another area of non-universal agreement. Many would say no. I would say relubrication still provides fresh grease in the cavity, and the oil from the fresh grease can migrate through the clearance. SKF states that double shielded bearings (when not lubricated) have a life of only twice the relubrication interval. For a high Dm*N machine, that’s not very long. In reality I’ve seen them last, but I still think modest relubrication can help for high D*N double shielded bearings, especially in continuous-run applications (my opinion, not everyone agrees). On the other hand, if we have a low D*N double-shielded bearing that only runs intermittently I definitely wouldn’t bother attempting to add grease.


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