estimating time to fill grease cavity.... does grease shrink ?
estimating time to fill grease cavity.... does grease shrink ?
(OP)
I am trying to estimate the time that it will take to fill up a bearing grease cavity through periodic regreasing over a period which may be 5, 10, or more years.
My question: Can I expect that grease added retains it's original volume, or does it tend to shrink as the oil bleeds out ?
My question: Can I expect that grease added retains it's original volume, or does it tend to shrink as the oil bleeds out ?
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(2B)+(2B)' ?





RE: estimating time to fill grease cavity.... does grease shrink ?
Johnny Pellin
RE: estimating time to fill grease cavity.... does grease shrink ?
As it heats under load, it liquifies, and many of the extreme pressure additives run off. You obviously want to keep the grease cavities full to prevent the ingress of water.
RE: estimating time to fill grease cavity.... does grease shrink ?
Does your application lack purge valves, plugs etc? Or is the management of the purged grease poor, so motor windings are put at risk?
This FAG publication has a few sections on grease lubrication and relubrication. Sounds like generally it is important to add more than "just enough" new grease, so the old grease adjacent to the bearing is pushed away too.
ht
RE: estimating time to fill grease cavity.... does grease shrink ?
I would never recommend purging a grease cavity on a motor. By definition, this requires pumping the grease cavity full. The old grease is depleted of oil, but it is not harmful. It is like an empty sponge. You need to replenish the oil and refill the sponge. Adding just the right amount of new grease is the best method to accomplish this.
We have more than 5000 grease lubricated electric motors. Most of them run at 3600 rpm. Our method was based on a motor greasing conference that we conducted with Exxon-Mobil Lubricants, Siemens Motors, SKF Bearings and rotating machinery experts from two of our refineries and our corporate central engineering group.
Johnny Pellin
RE: estimating time to fill grease cavity.... does grease shrink ?
If any further thoughts on the original question, I'd be interested to hear.
It seems any discussion of greasing practices is bound to be controversial.
fwiw, my observations and opinions seem similar to Johnny's. Here was a similar discussion:
h
Interesting comment shields toward the grease (outboard) rather than toward winding (inboard). I have heard SKF say that.. the sheild will protect the bearing from getting too much grease, but still allow oil and tiny amounts of grease thru.
We tend to put shields toward the windings, mostly because we have always done it that way and it makes more sense to our maintenance staff (I have never imposed a strong preference).
I have a little bit of a case study posted here (in particular summarized in my attachment posted 6 Apr 11 11:1):
thread237-295585: Possible causes of TEFC NDE brg spinning on shaft
Two things to note:
1 - this particular motor doesn't have inner bearing cap on outboard end.... so if you put the outboard shield outboard (toward endbell cavity / away from motor), the inboard side of the bearing would be wide open and completely unprotected from motor internal cooling air... definitely not a good situation as someone mentioned in the thread. Because of that I am almost positive that shield toward winding is best for this particular motor, but perhaps not for others.
2 - We went to investigate a vibration problem. That in itself is an interesting thing you can see in the powerpoint (inner ring spun on shaft). We saw the cavity full of grese. I did some measurements of the bearing cavity of this 6313 bearing (toward the end of the powerpoint) and estimated the grease cavity is only 13 cubic inches or so. If it is initially half full and then we add 1.2 ounces (2.1 cubic inch) every year or so, we have only 3 relubrications before cavity would be full (absent shrinkage... unknown). I was a little surprised at that. I have a suspicion this cavity may be small for the bearing, but I don't have much other data handy at the moment.
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(2B)+(2B)' ?
RE: estimating time to fill grease cavity.... does grease shrink ?
I assume everyone would agree that the shield absolutely needs to go toward the winding, for the outboard bearing of this motor. Otherwise the bearing is wide open to contamination from the motor internal cooling air.
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(2B)+(2B)' ?
RE: estimating time to fill grease cavity.... does grease shrink ?
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(2B)+(2B)' ?
RE: estimating time to fill grease cavity.... does grease shrink ?
Russell Giuliano
RE: estimating time to fill grease cavity.... does grease shrink ?
Johnny Pellin
RE: estimating time to fill grease cavity.... does grease shrink ?
I have collected below some related references on various "sides" of these issues. In some cases I used caps to emphasise quoted words and added my own comments in [square brackets]
I know there was an SKF reference somewhere that suggested shield away from winding, but I can't find it.
Regarding suitability of motor with shield and without bearing cap, I have two thoughts:
1 – Clearance in a shield is not too much different than clearance in an inner bearing cap (maybe 10 mils). The path through the inner bearing cap clearance would be a straight line of about 1/2 ". The path through shield clearance would be much shorter distance, but also involve 2 or more right angles. Once it penetrates the shield, dirt would be in the bearing... whereas once it penetrates the inner bearing cap, dirt would be in the cavity. I can see based on all this that bearing cap may provide better sealing, but it doesn't seem like a huge difference to me.
2 – This particular motor survived 3600 rpm operation for 8 years with shield and no inner bearing cap. Upon disassembly, races and balls were fine... the final failure had nothing to do with lack of inner bearing cap. This doesn't prove it's a good design, but just a datapoint.
Again, there is no doubt inner bearing cap is better. But I don't think lack of inner bearing cap in a motor purchased 25 years ago should be viewed as an unthinkable design, such that manufacturing error is the only explanation. I have never paid attention to this feature before, but I am pretty sure that the next opportunity I get to look into a sister motor (same OEM/model number), I will find the same configuration.
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(2B)+(2B)' ?