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Join Date: Sep 2010
Posts: 22

Pump Bearing Life

09/26/2010 4:08 AM

Hi, I need some help regarding client's pump specification:

"Ball or roller bearings shall be sealed for life. They shall be rated to give a minimum life of 100,000 hours at maximum load without replacement. Bearings shall be protected by water throwers and lip seals."

Now, my humble understanding is that if bearings are sealed for life, they cannot be re-greased, and since grease has limited shelf life (say 2 years), it is impossible for such bearings to last for 10 years. So the specs statement is contradictory. Bearing life of 100,000 hrs applies only to those types with re-lubrication facility.

Please advise whether my understanding is correct, and can I challenge the consultant on this point? Thanks!

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Guru
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#1

Re: Pump bearing life

09/26/2010 5:41 AM

I can't believe grease only has a 2 year shelf life, and anyway it's service life you are interested in.
(At risk of being pedantic) You may well be right that the spec' is "unacheivable", but a don't think it's "contradictory".
If you can show from bearing manufacturers data that it's unacheivable they you will doubtless have just cause to negotiate the spec'.
Del

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#2
In reply to #1

Re: Pump bearing life

09/26/2010 6:00 AM

Thanks Del. Allow me further clarifications as I'm a rookie when it comes to pump mechanical details...

The pump manufacturer says that their bearing lifespan is only 20,000 hrs. Reason given is that bearings are wear-and-tear parts so need to be replaced.

In any case, grease cannot last for 10 years without replacement, right?

For motor bearings, I looked up the Siemens motor technical data. It says under average operating conditions, a nominal bearing lifetime of 100,000 hrs can be achieved. But further down the page it says that "the bearing lifetime of motors with horizontal type of construction is at least 40,000 hrs if there is no additional axial loading at the coupling output and at least 20,000 hrs with the admissable permitted loads." The pump manufacturer interpreted this as: no-load life is 100,000hrs, and 20,000 hrs under normal working conditions.

Is he correct or not?

Many thanks for your invaluable advice :)

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Guru
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#3
In reply to #2

Re: Pump bearing life

09/26/2010 7:06 AM

In view of ...The pump manufacturer says that their bearing lifespan is only 20,000 hrs. Reason given is that bearings are wear-and-tear parts so need to be replaced. It sounds to me that your customer's spec' is unrealistic.

In any case, grease cannot last for 10 years without replacement, right?
I have no idea as it's not my field, but I have no reason believe that under light loads grease couldn't last longer than 10 years. I'm sure many front wheel drive cars have rear wheel bearings which last langer than 10 years (unfortunately not the case in my Nissan Micra)

Like I said, not really my field, but I'd make sure you don't agree or sign up to any spec' which you think is unacheivable. On the other hand many manufacturers quote ridiculously short lifetimes for things to avoid claims, E.G Claimed brush life on small DC motors is ludicrously short, yet I've never seen a worn out brush on any of the peristaltic pumps we make (and I've been at this job for 12 years).
Del

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#4
In reply to #2

Re: Pump bearing life

09/26/2010 7:26 AM

1. if you check SKF site you will find that the grease life under designed condition of 1,00,000 Hrs is common.

2. bearing L10 like of 20,000 Hrs is quite low. Yes they are wear and tear parts but they may be designed for the higher life (if properly selected).

3. Average running condition means under designed load. No axial loading of course is self explanatory- the driven must isolate its axial thrust from the mortor. And admissible permitted load are the loads that are worse than the designed load, but are permissible, hence pump manufacturer's interpretation is not correct.

No one would buy a product that works for even infinite time without load and fails quickly as it is loaded. You can contest his interpretation.

UD15

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#5

Re: Pump bearing life

09/26/2010 8:28 AM

Thanks Del and Guest. I will communicate with the pump manufacturer again to get the corrected data.

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#6

Re: Pump Bearing Life

09/26/2010 12:15 PM

I've changed more pumps than I care to remember. Nearly all have been due to seal failures. OK most of the pumps were for nasty dirty horrible stuff. Do the consultants want 11½ years constant running out of the seals? The fact water throwers are mentioned would suggest they expect some leakage. If there is leakage you can almost guarantee some will find it's way to the bearings.

To be honest it sounds like they live in cloud cuckoo land!

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#7

Re: Pump Bearing Life

09/27/2010 1:22 AM

To extend the bearing life, there is no reason why a grease cavity cannot be built in (or recommended) forward of the motor bearing, to give protection to the bearing. This set-up is common in marine applications - even the small inspection class ROVs use this.

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#8

Re: Pump Bearing Life

09/27/2010 2:39 AM

piquek -- I'm a bit late into this discussion. .....

I've experienced firsthand the discussion of pump manufacturers' life rating data for antifriction bearings. The design engineers tell the marketing guys what liquid ends will at shutoff and maximum impeller diameter, speed and suction pressure will be used on which bearing frames or motor frame sizes. At Worthington in the '70's we used L10 life of 16,000 hours as the criteria. Now understand that the L10 life is a statistical thing. If the marketing people think that their experience tells them that raising the rating by some selected limits on liquid ends and applications will get them into the 20,000 hour range and give them a marketing advantage over slower moving competition then they might well do it.

But understand that in a volute pump operation at maximum efficiency you have zero continuous radial load if the pump is designed perfectly or just a few percent of shutoff load if the design is sloppy. The other predictable load is thrust load from suction pressure. That is usually a small percentage of maximum radial load. If it isn't then the engineering guys usually get involved and the selection involves not only a review of bearing loads but all the work involved in special mechanical seals.

There are also unpredictable loads such as from cavitation, coupling misalignment, operation conditions away from maximum efficiency and a host of other causes. Since such conditions are by their nature unpredictable they usually don't become part of the application criteria without some business decisions getting into the picture.

Also note that there are a whole class of centrifugal pumps that use vane diffusers. These have minimal radial load at any operating point by their nature. Example barrel pumps and deep well and sump pumps where bearings are far away from the impellers and the main function of bearings is to carry the axial or vertical loads. I also seem to recall double volutes to get balance in pumps, The deeply cored casings were a real foundry man's nightmare. Also common practice in 2 stage volute pumps, both vertical and horizontal split cases is to have the volutes for each impeller spaced 180 degrees apart to partially balance each other leaving a smaller force producing moment.

In today's age of computer tools a sophisticated pump company would provide the sales force with a computer based application analysis that among other things would provide the necessary bearing calculations and statistical probability of bearing failure base on application predictions.

And, BTW, a pump manufacturer, as an OEM, should be able to obtain special bearings assembled with a modern long life synthetic oil base grease that can survive running temperatures without oxidation or heat deterioration for more years life as long as contaminants are excluded. In this situation I'd defer to the bearing manufacturers and what they would advise. And I would indeed go there if the 12 year life was a realistic requirement.

Ed Weldon

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#9

Re: Pump Bearing Life

09/27/2010 6:53 AM

I have also gone through similar situations while going through thick booklets of tender specifications prepared by the consultants. I understand that consultants are paid on per page basis so more pages more money. They specify painting,packing specs running in pages. Also they ask for technical details of the products, almost asking for design details.

Coming to your problem I would like to know if they have specified the brand names in specs?. If yes then you should ask the mfrs. to confirm the specs. If you are in hurry to submit the offer then just point out as deviation and latter on sort out with the consultant.

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#10

Re: Pump Bearing Life

09/27/2010 9:00 AM

"Ball or roller bearings shall be sealed for life. They shall be rated to give a minimum life of 100,000 hours at maximum load without replacement. Bearings shall be protected by water throwers and lip seals."

This is not the kind of statement that I would accept from a consultant. It is bs contradictory, almost impossible to achieve and according to no standard known to me.

API 610, which demands a pretty high standard, only specifies a continuous run of 3 years as a design basis. They also demand an L10 life of 25,000 hrs at normal running conditions and at least 16,000 hrs at maximum conditions. This is a far cry from the demanded 100,000 hrs. In fact API 610 stipulates that grease nipples do not need to be fitted, only if the grease life exceeds 25,000 hrs. This is usually not achievable in anything other than very small lightly loaded pumps. (I do not use sealed bearings in pumps).

If you do the bearing life calculations and you can achieve 100,000 hours at maximum load, (pump OEM thinks not) the next step is to do the re-greasing frequency calculations (SKF, FAG et al all have these calculations on line). You will find that the re-greasing interval is substantially shorter than the bearing life. A "greased for life" bearing will last for marginally longer than the re-greasing interval of a re-greasable bearing (assuming grease relief facility).

If you assume that the "sealed for life" specification is real, then why on earth would you use "water throwers" and lip seals. Lip seals are pretty much useless things for pump bearings anyway, (effective life is measured in months) and they most certainly will not be effective for nearly 12 years continuous running. Hence the move by API which states that "lip type seals SHALL not be used" and instaed stipulates the use of "replaceable labyrinth or magnetic type end seals" in order to protect bearings. (One of many manufacturers of these seals can be found here).

In my opinion sealed bearings are useful only for very small "machines" or domestic use where re-greasing is not really practical - washing machines, swimming pool pumps etc.

Lip seals and water throwers are a waste of money and should never be specified.

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#12
In reply to #10

Re: Pump Bearing Life

09/27/2010 11:10 AM

GA, Prof. Lots of good up-to-date info in your post.

Ed Weldon

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#11

Re: Pump Bearing Life

09/27/2010 10:09 AM

The bearings used as wheel bearings at both ends of many modern cars are "sealed for life" (i.e the bearings have integral seals) and typically last 10 years or 200,000 miles. Assuming an average speed of 35 mph, that is only 6000 operating hours or so.

Any additional lip seal will wear out quickly, because the lip is not lubricated, unless the fluid being pumped is providing the lubrication.

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#13

Re: Pump Bearing Life

09/27/2010 9:07 PM

Thanks Prof and everyone for your advices. It certainly confirms my suspicion of the unreasonable specification :p And I have learnt a lot from all of you.

Will keep posted of the outcome...

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#14

Re: Pump Bearing Life

09/27/2010 10:48 PM

In my research attempts I have found an SKF article which I find useful regarding sealed-for-life bearings:

http://findarticles.com/p/articles/mi_qa5322/is_200412/ai_n21360835/

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Anonymous Poster (1); Del the cat (2); Ed Weldon (2); GM1964 (1); MoronicBumble (1); pjquek (4); suresh sharma (1); The Prof (1); TonyS (1)

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