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Curran919

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Gents,

I got into an argument with one of my design engineers and need a sanity check on my assumptions.

Bearings generally specify three load limits:

  • static: constant (DC) load capacity of the bearing while not rotating
  • dynamic: constant (DC) load capacity of the bearing while rotating. I was surprised to see that SKF has some bearings that have a higher dynamic load capacity than static. This limit has nothing to do with dynamic (oscillating/AC) load.
  • fatigue: generally 2-10% of the dynamic load capacity, loads below this point will lead to an 'antispalling' application. This is related to constant load, not oscillating load, as the raceway contact points are already exposed to an oscillating pressure, even if the total bearing load has no oscillating component. Designing a bearing to meet the fatigue criteria is rare in rotating equipment and generally a huge overdesign.
The terms 'dynamic' and 'fatigue' make people think that the compare those values to oscillating (AC)components of the bearing load. I understand though that you simply add the expected 0-pk amplitude of the AC component to the DC component, thereby taking the max instantaneous load. Obviously the AC component can be difficult to quantify, but thats not really integral to the theory.

Have I been misunderstanding something?
electricpete

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I agree with your interpretation.  

The dynamic load capacity is the one used for life calculatioins. 
The dynamic load for a typical machine it would be based on the steady unidirectional load on the shaft (for example wieght / 2 per bearing accounts for weight load). 
The life calculation provides a statistical prediction of failure under fatigue due to this dynamic load. 
example 10% of failures will failure by L10 = (C/P)^3 * 10^6 revolutions

Static load rating is not used to my knowledge.  It may be based on something like a specified deformation (one 10,000th of ball diameter). It is not correlated to any failure prediction that I know of. 




John from PA

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Quote:
Originally Posted by electricpete
Static load rating is not used to my knowledge.  It may be based on something like a specified deformation (one 10,000th of ball diameter). It is not correlated to any failure prediction that I know of.


Static load rating is involved in some specialized applications like antenna positioning gearboxes and wind turbines.  In the antenna positioning applications (think the Goldstone project) where the antenna was 210 feet (65 meters) in diameter there was an azimuth drive and an elevation drive.  Both were essentially static (very low RPM) but had to withstand the buffeting from winds etc.  The same is true of large wind turbines that may be locked into position but still may suffer from the buffeting of the wind.  In the press recently is a 12 MW machine with and it has a 720 foot rotor (220 meters).
tomcd3

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Static load rating (NOT capacity) is used to ensure shock loading in rotating rolling element bearings (REB) doesn't cause RE's to dent the raceway. Same is true for barely or slowly rotating REB.

SKF REB catalog (BU/P1 17000 EN) tells the full story.

BR TMcD @ SKF
801-978-3117
Curran919

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When is the fatigue limit used in design?
tomcd3

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Pu is used in the latest SKF modified life adopted by ISO as L10xyz. It is hidden deep in the calculation as a ratio to the applied load to determine a adjustment factor by way of a chart.


electricpete

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I wanted to ask about the term "fatigue limit"
Is that referring to a loading limit which will prevent fatigue even at infinite cycles (similar to endurance limit for materials). 
tomcd3

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correct Pete.
Curran919

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Quote:
Originally Posted by tomcd3
Pu is used in the latest SKF modified life adopted by ISO as L10xyz. It is hidden deep in the calculation as a ratio to the applied load to determine a adjustment factor by way of a chart.


I can't find much on Pu (fatigue load limit) or anything on this L10xyz factor.

Should I understand that Pu should not be (or is usually not) used directly in bearing specification (design of rotating equipment)?
tomcd3

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As I stated before, download the SKF bearing catalog 17000 EN or purchase ISO spec 281
electricpete

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Quote:
...download the SKF bearing catalog 17000 EN or...

I found the link to that SKF catalogue:
https://www.skf.com/binary/21-121486/Rolling-bearings---17000-EN.pdf
electricpete

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Page 104 describes the fatigue load limit and the static load rating. 

The static load rating is the load that results in a deformation of the heaviest loaded rolling element of 0.0001 times ball diameter.

I didn't find Lxyz, but there is an a_skf factor described on page 94 which is a life modification factor that SKF uses to expand the concept of L10 to include additional factors.
Curran919

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Yes, I read the SKF information on P_u already. The 3 lines the SKF doc has on P_u is not very comprehensive. I understand that is is used as an intermediate quantity to estimate a_SKF and therefore L_nm (can't find anything on L_xyz or L_10xyz).

My question is not whether P_u is useless, it obviously has a purpose in estimating L_nm. However, are there any industries that use it in a direct criterion, i.e. design such that P<P_u? Or is that comparison meaningless with the existence of L_nm?

John from PA

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Quote:
Originally Posted by Curran919


I can't find much on Pu (fatigue load limit) or anything on this L10xyz factor.


For L10, see the article at https://www.stle.org/images/pdf/STLE_ORG/BOK/LS/Bearings/ISO%20281_2007%20Bearing-Life%20Standard_And%20the%20Answer%20Is_tlt%20article_July10.pdf

I
SO 281:2007, which is the standard discussed, is considered current.
tomcd3

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Reply with quote  #15 
As you said Curran, it is typically used to calculate life. L10xyz is the ISO form of the SKF L10mh. See ISO 281 for a non SKF source of info.
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