"To Grease or Not to Grease" - That is the Question!
Attending a pump symposium in Houston has always been an interesting experience for me and I particularly enjoy being a discussion group leader, sometimes for being able to help with an answer, and in other instances, learning from other folks. Our Discussion Group #3 ("Bearings and Lubrication") had gotten into a heated discussion about grease lubrication and we could not stop debating way after the session had officially ended.
What's the big deal about grease lubrication? That's what many of us thought. But, as the debate continued, we realized we all knew less then we thought we did. It is no secret that the "refinery" folk look down upon grease lubrication. To them, it is like going back to the Stone Age, and the only acceptable solution is oil. Power generation goes even further: forced lubrication, sophisticated oil skids, controls, bells and whistles but visit your neighboring water or wastewater plant, and you will find that grease lubrication is still the main method of lubrication, even for large pumps and high power levels; Why is that?
Is there a criterion that delineates grease applications from oil baths? I looked up the ANSI spec: "Oil lubrication is standard. Bearing housing shall be tapped for a constant level oil feed regulator or level indicator. Other methods of lubrication may be specified." As such, it said, "may be specified", but when?
Next, I looked up the API-610, also another "unless otherwise specified" standard (126.96.36.199, 8th edition, and 188.8.131.52, 9th edition). It does have a NDm = 500,000 (N is RPM, and bearing mean diameter dm is in millimeters), as a limit of rolling element bearings, or if energy density of 5.4 million (HPxRPM). So, a 1.5x1-6 pump (1.38-in. shaft) running at 3,600 RPM, with dm ~ 2.11-in. = 54 mm, would have Ndm = 193,000 i.e. fine for grease lubricants. A larger size, with a 2.5 ft shaft (dm ~ 4 in. = 102 mm), would have Ndm = 366,000, also fine. However, API prohibits shielded or sealed bearings, but does not rule out greased bearings and stated "provision shall be made for re-greasing the bearings in service." (5.11.4.c 9th Ed.)
I then looked at the SKF catalog, which said "the speed rating for grease lubrications are 15 to 25% lower (depending on the bearing type) than the speed rating for oil lubrications, which apply to oil bath lubrication". (p. 25) On page 127, SKF said, "grease has the advantage over oil as it is more easily retained in the bearing arrangement, particularly where shafts are inclined or vertical and it also contributes to sealing the arrangement against contaminants, moisture or water". Can we use this information as a foundation to say "yes" to grease? With a conservative 25% reduction on Ndm criteria: 500,000 x 75% = 375,000? If so, the examples above still qualify the grease for both small and large pumps. The defense rests. Oil proponents, you may approach the bench for cross-examination.
So, if we agree that grease is still alive and well (as evident by successful usage by those who do not know what they should or should not do), the next question is how to grease.
Should bearings be open to grease everywhere? If so, should lubrication be forced across the balls or flow on through?
What happens to the grease that goes over the inside of the bearing housing of the pump? Does it stay there building up more and more after each re-grease? What about the effect on motor windings?
Perhaps a single shield with a controlled clearance between it and an inner race would work better. Are you laughing yet? Well a large petrochemical company in West Virginia isn't and has expressed satisfaction with such an arrangement:
There is another reported arrangement where the shield is on the other side allowing grease to freely access the bearing while controlling its admission past it to the inside of the pump or the motor. I have heard of this setup before, but cannot tell if it works as well in practice as it does in theory.
Sometimes a controlled grease release is recommended by a pump manufacturer providing a special grease retainer and a double shielded bearing arrangement (note that the bearing shields have some clearance between the inner race and we are not talking about bearing seals used for greased-for-life arrangements or for small power applications.
After our discussion group meeting was over, I went to the exhibit hall to speak to some pump manufacturers about specific types of grease/bearing arrangements that they supply. Interestingly enough, few were familiar with the specifics, and we made little progress beyond generalities and in the end, the consensus was: "if you find out, let me know!"
What do you use at your plant and if you are a manufacturer? What are your standard offerings and options?