Spherical Bearing Help 2

[crunchie12268]

I am using a SKF 22207E spherical bearing with a SKF N 207 ECP in a cantilever arrangement. With my setup I am placing approximately 4100 lbf of radial load on the 22207E bearing, when I am using the calculator on SKFs website I get a rolling frictional moment of 2.34lbf. Now I have a moment arm on the shaft that is running through these bearings of 1.2 in so I get a force of 1.95 lbf required to overcome the frictional moment (my customer calls this "tension"). Here is my problem... the specification is calling for a min "tension" of 0.4 lbf and I cannot change anything but the bearings, it has to be the same dimensionally too. I can't change the operational parameters such as the radial load either. I am beginning to think the 0.4lbf is not attainable.. anyone have any suggestions?

 

[desertfox]

Hi crunchie

Reading your post I don't see a problem because the specification calls for a minimum tension of 0.4lbf and not a maximum, therefore the 2.34 is greater than the minimum tension required.
I guess I might be reading your post incorrectly.

 

[crunchie12268]

Sorry I may have been a bit ambiguous.. they would like to be able to rotate the shaft with as little as 0.4 lbf when loaded.

 

[desertfox]

Hi crunchie

If its a frictional force that you are trying to control its impossible, friction cannot be restricted to a single figure but I agree with you I think its unattainable.

 

[GregLocock]

SKF's figures are for a new bearing. That bearing is far too big for that load.

Try running one in for a hundred hours (WAG) at 100 rpm(WAG) and remeasuring the friction.

You use of units is so horrible I'm not going to try and work anything out, but perhaps a smaller diameter bearing would have less friction for a given load, or you could consider flushing whatever lubricant out and replacing it with something else.

Alternatively switch bearing types, I'd guess that in that housing size you could get a self aligning ball bearing which will have less friction.

Most of the above have been used in production in extremis.





Cheers

Greg Locock


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[crunchie12268]

The bearing is too big for the load? Dimensionally too big? The shaft size is as small as I can get with the deflection seen at that load... this is using a 17-PH material also.

 

[Tmoose]

How much axial load do you have?

What rpm?
Are you using some standard grease in your bearing, and the calculation?
Different Greases have different dN (speed) ratings depending on their channeling etc and resulting heat generation behavior. I'd expect frictional moment to be somewhat related.

Some folks have reduced heat generation of oversized bearings by removing alternate rollers. Some may even have received the bearing manufacturer's blessing.

others' Suggestion to investigate other types of self aligning bearings is good.

 

[GregLocock]

4000 lb is 16 kN. That bearing is good for 80 kN crudely. If you are interested in friction then you don't choose bearing size and type by looking at the shaft diameter.



Cheers

Greg Locock


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[tbuelna]

crunchie12268-

The friction moment result you got from the SKF calculator should have been in in-lbf if you selected imperial units. However, the friction moment loss produced in a rolling element bearing can vary with speed. For example, a spherical roller bearing having a fairly heavy roller complement and operating at high Dn can experience large amounts of roller skidding within the sector of the bearing circumference that is not loaded.

You need to be careful in how you interpret the results you get from that calculator, and in what inputs you use'

 

[Tmoose]

RE: bearing loading

The Co ( basic dynamic load) is the starting point for most bearing life calculations.
It often is described as the "load" which the pampered bearing will support for 1,000,000 revolutions before spalling starts to appear ( the end of L10 statistical life ).
A machine running 24/7 at a mere 100 rpm will complete 1,000,000 revolutions in about a week.
A wheel bearing would accumulate 1,000,000 revs in less than 2000 miles.
The life is generally figured related inversely to the load^3. Double the load, the life is reduced to 1/8. Halve the load, life improves 8X.

For a few decades FAG and SKF have subscribed to a theory, based on testing, that bearing life especially at lower loads far exceeds calculated life, and in fact can have "infinite life" if the load is below the "fatigue limit" (and cleanliness and lubrication are excellent.)
In ISO 281:2007 reportedly there is a concept of a fatigue limit within its bearing life calculation method.

Some feel the concept of bearing endurance or fatigue limit is in error.

http://www.stle.org/assets/document/8-10_tlt_commentary_iso_standard_281_2007_part_ii.pdf

I would first aim for an infinite bearing life, using my time instead to design excellent sealing and ensuring lubrication life rather than calculating the somewhat nebulous although frequently obligatory L10 bearing life. The FAG spindle calculating program I enjoyed using for several years red flagged any bearing choices that would result in a "contact stress" that exceeded their prescribed fatigue limit.


As you found, For some machines the shaft sizes are sometimes first dictated by stiffness/deflection requirements. Machine tools are an example.


SKF says the fatigue load limit for the 22207E bearing is 9.3 kN.

http://www.skf.com/binary/12-121486/SKF rolling bearings catalogue_tcm_12-121486.pdf

FAG says generically the fatigue limit is ~ Co/8.
They list a fatigue load limit of 9.4 Kn here -

http://www.schaeffler.com/remotemed...chaeffler_2/tpi/downloads_8/tpi_183_de_en.pdf

4100 lbs ~ 18 kN, well above the "endurance limit."
I guess I would not consider that 22207 bearing grossly oversized.

An FAG 2207 self aligning ball bearing (same envelope as you 22207) has a Co of 35 kN and a fatigue limit of just .56 Kn

 

[tbuelna]

Tmoose-

Nice post. In reality, the fatigue life of a particular rolling element bearing can vary widely from other bearings of the same design used in similar applications. As you noted, factors such as lubrication or contamination can have a huge effect on actual fatigue life. So even the "fatigue load limit" listed by SKF (and others) must be carefully considered with regards to factors such as lubrication, maintenance, operating environment, safety/reliability, etc.

 

[GregLocock]

Great paper on the mysterious presence or absence of the fatigue limit.

Cheers

Greg Locock


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