estimating time to fill grease cavity.... does grease shrink ? 5

[electricpete]

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 ?


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[JJPellin]

It is my experience that grease will tend to shrink as it looses mass and volume as the oil is depleted. I don't know of any good way to estimate my much shrinkage to expect.

Johnny Pellin

 

[dinjin]

I think manufacturers will ask that you not simply replace the depleted grease but to purge all of the grease out periodically.
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.

 

[Tmoose]

hi Pete,

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.

http://mountingmanager.schaeffler.c....wl81.115.e.pdf;jsessionid=aHD9KSZvDgs_LUxz_R

 

[JJPellin]

I respectfully disagree with both of the last two replies. A grease cavity should never be full. The preferred bearing arrangement for an electric motor is single shield with the shield toward the grease. If the cavity becomes full, there is great risk of collapsing the shield into the balls. We have seen this many times. A motor bearing is much more likely to fail from over-greasing than from under-greasing.

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

 

[electricpete]

Thanks.

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:

http://maintenanceforums.com/eve/forums/a/tpc/f/319103451/m/51520575663

We never see grease come out of bearing drains or releifs after lubrication. I ask myself why is there a disconnect between our experience (grease never comes out) and some OEM recommendations and design (which provides a grease drain and sometimes relief under apparent assumption that grease will come out). We tend to follow the EPRI model where grease intervals tend to be around a year (depends on many factors) rather than the bearing OEM model where grease intervals tend to be around a month. Needless to say if you are greasing more often and reach cavity full stage earlier before grease has had a chance to age, you have a prayer of getting grease to come out a drain. But I think if you pack your cavity half full and then regrease at some interval like yearly, by the time the cavity gets full, you will have dry grease. Something like wire or tie wrap might give you a tiny chance, but I'm not optimistic. You have to ask yourself why the grease would ever flow out of the drain, there are only 2 reasons I can think of: 1 - the cavity is completely full and you are developing a hydrostatic pressure by pumping grease in (in that case I would be worried about the shield.... and if it is a high D*N bearing when the cavity is full I think it's time ti change the bearing/grease) 2- the grease is liquifying due to extreme high temperature (in that case I think the bearing is seeing non-ideal conditions of excess temperature... probably it would tend to separate and you would preferentially lose oil rather than thickener... grease is going to age much faster if oil separates).

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

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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[electricpete]

Attached are more details (2-slides) regarding lack of an “inner bearing cap” on the outboard end of this motor.

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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[electricpete]

Some typo's are corrected attached

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[diskullman]

I agree about that the shield being toward the inboard side, it's a shield, not a seal. The best way is follow whatever the motor manufacturer does.

Russell Giuliano

 

[JJPellin]

I would suggest that your motor has a major manufacturing error. There should be an inner bearing cap on the outboard end. I would drill and tap holes just like the ones on the inboard end and add a cap. It makes no sense to me to have that bearing open the windings. The inner cap on the outboard end will need to be different. You can only have one fixed bearing. If the inboard bearing is fixed, then you need to leave axial room on both sides of the outboard bearing so it can float. If it is impossible to add the inner cap, then I would install a single sealed bearing with a rubber seal toward the windings.

Johnny Pellin

 

[electricpete]

All good comments. My two cents would be again that there are varying opinions for the correct location of the shield. Additionally, I certainly agree that inner bearing cap is desirable, but I am quite sure it was intentional design by the OEM, not some kind of manufacturing accident.

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]

Ref 1 - Mobil

http://hollandindustrial.com/PDF Files/Guide to Electric Motor Lubrication Exxon.pdf

p6/60: “Single shielded bearings, Fig 4, allow a compact arrangement as required in electric motors and can be relubricated as shown.”

P7/60: “Figure 4” [ shows bearing arrangement with single SHIELD TOWARD WINDING, no inner bearing cap, there is an inner shaft slinger present.]

P49/60: “Double shielded bearings protect against contamination but do not allow easy relubrication. Common practice is to pop out the OUTBOARD shield. Allows relubrication, bur risks contamination” [from grease cavity/outboard side]

Ref 2 - TVA

http://reliabilityweb.com/index.php/articles/a_proven_electric_motor_regreasing_program/

“Single-Shielded Bearing - This bearing has a metallic shield on one side only, and is USUALLY INSTALLED WITH THE SHIELD FACING THE MOTOR WINDING. There is a small air gap between the shield and the inner race which will allow grease transfer to the inner bearing cap”

Ref 3 - Heinz Bloch:

http://books.google.com/books?id=aE...&resnum=2&ved=0CBwQ6AEwAQ#v=onepage&q&f=false

Single-Shield Bearings
A large petrochemical complex in the U.S. Gulf Coast area considers the regular single-shield bearing
with the SHIELD FACING THE GREASE SUPPLY (Figure 13-2) to be the best arrangement. Their experience indicates this simple arrangement will extend bearing life. It will also permit an extremely simple lubrication and relubrication technique if so installed. This technique makes it unnecessary to know the volume of grease already in the bearing cartridge. The shield serves as a baffle against agitation. The shield-to-inner-race annulus serves as a metering device to control grease flow. These features prevent premature ball bearing failures caused by contaminated grease and heat buildup due to excess
grease.

Ref 4 - NEMA MG1 – Standard Motors
[Does not mention bearing caps]

Ref 5 - IEEE 841 Severe Duty Motors For Petrochem
“Inner bearing caps shall be provided so that entry of oil or grease into the motor interior is minimized.”
[WHY BOTHER TO SPECIFY IF THIS IS A UNIVERSAL STANDARD FEATURE FOR ALL MOTORS?]

Ref 6 - EASA (Electrical Apparatus Service Association) Mechanical Repair Fundamentals [for motors]

Bearing enclosures fall into the following categories:
• Open bearings
• Shielded bearings
• Sealed bearings
Open bearings are the best choice when it is necessary to replenish the grease in the actual bearing cavity. They are good for applications where it is difficult to maintain the proper amount of grease due to thermal aging and oxidation. Note that the use of an OPEN BEARING REQUIRES THAT AN INNER BEARING CAP BE USED to contain the grease in the bearing. [IMPLIES THAT SHIELDED BEARING DOES NOT REQUIRE BEARING CAP]
Shielded bearings are preferred where the quantity of grease is maintained over long periods of time. The shield is an effective way to reduce the penetration of foreign material into the critical area of the bearing; however, the bearing cavity is exposed to potentially-damaging moisture. Sealed bearings are the best choice where the life and quality of the grease would be satisfactory over a long period of time, were it not for foreign material contaminating the grease in a very short period of time. On motors 50 hp
and larger, sealed bearings (with contact seals) are not a good choice because of the heat generated by friction.


….. Sealed bearings can be a practical solution for a vertically-mounted horizontal motor or a motor that is subject to occassional wash downs. In those cases, modification of the shaft openings may be unnecessary. FOR A MOTOR NOT FITTED WITH INNER BEARING CAPS, where the user requires greasable bearings, the addition of bearing caps may be required. In those cases, motor end play must be considered. See checking end play in Section 8. [ASSUMES NOT ALL MOTORS ARE FITTED WITH INNER BARING CAPS]

Ref 7 - SKF Bearing Installation and Maint Handbook
Using double-shielded bearings not only helps prevent over-greasing but also protects the bearing from dust and foreign matter at assembly, or whenever the bearing is exposed. Using the double-SHIELDED BEARING DOES NOT PERMIT THE ELIMINATION OF THE USUAL HOUSING ENCLOSURES

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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