=========LITERATURE (Mixed recommendations): ==========
1. Many motor OEM's have provided grease fittings with doulbe-shielded bearings and recommended regreasing in the associated literature (without specific mention of the type of bearing).
2. EPRI NP 7502 (Electric Motor Maintenance) page 2-23 indicates that double-shielded bearings can and should be periodically relubricated, possibly with a lower quantity and less frequent interval..
3. EPRI Lube Notes June 1998 indicates that double-shielded bearings should not be relubricated.
4. NRC IN 94-51 implies that greasing of double-shielded bearings may cause failure.
5. Previous plant evaluation of NRC IN 94-51 indicates our plant does not consider greasing of double-shielded bearings inappropriate. No changes to plant lubrication procedures were made as a result of the information notice.
6. The EPRI Large Electric Motor User's Group (LEMUG) has endorsed the EPRI NP7502 approach to lubricate double-shielded bearings. Additionally, the EPRI LEMUG has recommended that both the relubrication quantities and the relubrication intervals for double-shielded bearings may be adjusted by a factor of 2 compared to the EPRI NP7502 recommendations for open bearings, in order to decrease the possibility of filling the cavity and damaging the shield.
7. EPRI Lube Notes Dec 2004 - do not lube double-shielded.
8. Heinz Bloch - "Basic Lubrication Knowledge Challenged" - Pump Zone / Pumps and Systems March 2002. - "The knowledge that grease consists of 85% oil, the rest being soap and other additives, is decades old. Moreover, there is a body of literature that shows that the oil portion of the grease bleeds or seeps through the gap between the shields and the earing inner ring, and that it is capillary action that causes the oil the leave the soad and wet the rolling elements. .... The original design intent for shielded bearings was to keep the user from overgreasing the bearing and to depend on capilalry action to bleed primarily oil, not soap, through this gap. It was also the original intent to use shielded bearings for light to medium loads and moderate speeds, and to periodically regrease shielded bearings in these applications. ... In a followup conversation, one bearing manufacturer's application engineer neertheless claimed that double-shielded bearings are no longer considered regreasable because of recently reduced clearances between shield bore and bearing inner ring diameter. The engineer stated tha unlike older shielded bearings that had a radial gap of 0.016", the newer bearings have gaps of only 0.010". Most competent reliability professionals, mechanics and maintenance technicians would find it difficult to understand why oil would manage to creep across a 0016" gap, but not a gap of 0.010".... Regreasable shielded bearings are considered the best choise for light to medium loads in the dn range from 108,000 to 300,000, and one can go with either sealed (non-greaseable) or shielded (regreasable) in the greay range from 80,000 to 108,000"

==========EXPERIENCE ============
We have lubricated most doulbe-shielded bearings since our plant started operation 25 years ago. I have seen like Ed reported in another thread at least two cases recently of double-shielded bearings that we have greased and after 25 years we find them rough when rotated by hand or strange vib pattern... time waveform with erratically-spaced spikes. Opened them up and found grease was very black.... seems clear regreasing as not effective. Both were 2-pole TEFC motors. As I remember it looked a little worse on the inboard end where the bearing runs hotter in TEFC motors.

=========What Now?============
In those cases where it is a critical motor and we have experienced problems, we do a design change to replace the bearings with single-shielded bearing (shield toward winding). Another option would be periodic bearing replacement of the double-shielded bearings (20 years?) but we haven't embraced that idea.

So we still have a very large population of double-shielded bearings which we have no intention to change bearings unless we have to. So the questions underlying a decision whether or not to grease these bearings are #1 - does it do any good to regrease them, #2 - does it do any harm to regrease them #3 - is it worth the effort

#1 - Regarding Benefit - As stated above I agree wholeheartedly that relubrication of double-shielded bearings is not as effective as relub of open bearings based on what we have seen. What is still an unknown in my mind is whether there is some relubrication taking place (more than if we ignored the bearing). From intuition of seeing how oil behaves (spreads all over everything... climbs up walls!), it certainly makes sense that oil from the grease wants to migrate everywhere it can and I don't see why some would avoid migrating into the bearing.
#2 - Regarding harm - I don't think there is much more probability of overlubricating a double-shielded bearing than a standard bearing. Sure there is a possibility fo denting the shield not present for an open bearing... shouldn't happen if you run with the grease plug open after relubing and also be stingy with the quantities.. Possible extra grease trapped between the shields? I don't know but I doubt it.
#3 - Regarding effort - We already have a greasing program in-place for these motors. Efforts for greasing even on a continuing basis are far far less than would be efforts to replace the bearings.

Years ago we had a presentation from a SKF representative for our maintenance crew.
About re-greasing sealed 2RS & 2Z bearings he was very clear.
1) Do not take out the shields
2) Do not wash the bearings (solvent)
3) Do not repack the bearings.

He told us why (contamination) and the quantity and quality entered by SKF would be very difficult to match. There are other design shielded bearings with lubrication holes on the innner or outer ring. Typically we encountered them on shafts of drilling equipment, or other non-standard machinery.
Electro-motors are a different kettle of fish, and normally are somewhere stationary.

Good info. To clarify, we don't remove seals to "relubricate". We just add fresh grease to the cavity periodically.

Is there anyone else in the world who greases double-shielded bearings?

quote:

Originally posted by electricpete:
We just add fresh grease to the cavity periodically.

Pete,

I assume that when you say "cavity" you mean free bearing housing space next to the bearing.

1. Do you rely on SKF formulas/plots when calculating the amount and frequency? If so, are you using bearing geometry only or housing cavity size?.

The latter question is valid not only for double shielded bearings but also for single shielded bearings since when re-greasing, the fresh grease does not go directly to the bearing as that at initial librication. Instead, it fills up the housing cavity and then portion of the base oil gradually gets dispensed (migrate) to the bearing internals.

It looks like DD/ZZ bearing use different phenomenon for re-greasing: copillary effect for double shielded vs. a different dispensing effect ( as portion of grease is being stored in the housing cavity) on single ones.

2. Do you have any luck using ultrasound guns instead of formulas for determining the right amount of grease?

4. In my opinion, there is a general flaw with motor bearings as user has no information nor has he control over the amount of grease being accumulated over the time in the housing cavity as a result of re-greasing. Initial lubrication requirements done by OEM are stringent and call for about 25% in the bearing internals and about same in the housing cavity.

Regards,
David

"I assume that when you say "cavity" you mean free bearing housing space next to the bearing."
Yes.

"1. Do you rely on SKF formulas/plots when calculating the amount and frequency? If so, are you using bearing geometry only or housing cavity size?. "
We actually use EPRI intervals and quantities which in general add a little more grease a little less often. We rely only on bearing geometry...don't have info on the cavity size.

"The latter question is valid not only for double shielded bearings but also for single shielded bearings since when re-greasing, the fresh grease does not go directly to the bearing as that at initial librication. Instead, it fills up the housing cavity and then portion of the base oil gradually gets dispensed (migrate) to the bearing internals. It looks like DD/ZZ bearing use different phenomenon for re-greasing: copillary effect for double shielded vs. a different dispensing effect ( as portion of grease is being stored in the housing cavity) on single ones."
I think you are assuming single-shielded installed with shield toward the winding. I agree it is different lubrication effect and probably not as effective with shield between the cavity and the bearing.

"2. Do you have any luck using ultrasound guns instead of formulas for determining the right amount of grease?"
Haven't tried it. Haven't been too impressed with that idea yet, but that may just be my ignorance. But I do think that condition monitoring should play a key role in the program to identify when grease conditions are getting bad. I think normal vib analysis can do this without ultrasound. As I mentioned before what we have seen is randomly-spaced peaks in time waveform. If I get a chance I will post them.

"4. In my opinion, there is a general flaw with motor bearings as user has no information nor has he control over the amount of grease being accumulated over the time in the housing cavity as a result of re-greasing. Initial lubrication requirements done by OEM are stringent and call for about 25% in the bearing internals and about same in the housing cavity."
I agree there is some uncertainty about how much grease accumulates in the cavity and when it will be full, but we have to play the cards we are dealt. I used to think that if I knew the initial cavity size and amounts added since day 1, I could calculate the cuumulative effect of the additions over the life of the bearing, but interesting that old Heinz says that 85% of the grease is oil? So perhaps only 15% of the grease volume added remains over time? Seems like a tough nut to calculate even if we knew all the other stuff. If we see specific evidence that a cavity is full or running hot, we generally make note of it and make plans to address it but we haven't had this happen too often.

Yes Pete it can work. 1st time seen it done some 25 years ago fan in exhaust duct of concentrator dryer. Bearings would not make a month. Had to be sealed heat and dust etc but 1/2 shot of grease added once a week and bearings lasted about four years.Forgot to grease. No grease new one died fast again.Ended argument it got greased if not died. Seen done since in tight spots always worked. Later motor crashes ran hot 2 pole. Got one just before crash could not turn by hand. Dr bearing welded seized so soaked in solvent overnight. Spun no problem. Cut open like new.