AFBMA 9 û Ball Brg Load Ratings/Fatigue life û typo? 2

[electricpete]

AFBMA 9 defines dynamic/rotating quantities (no subscript), and static/stationary quantities distinguished by a subscript o.

In particular:
dynamic quantities: Pr=XFr + Yfa and rating C
static quantities: Por = XoFr + YoFa, and rating Co

Paragraph 5.3.2 of AFBMA9 (1990) states: "The life formula [based on dynamic load and dynamic rating] gives satisfactory results for a broad range of bearing loads. However, extra-heavy loads may cause detrimental plastic deformations at the rolling element/raceway contact. The user should therefore consult the bearing manufacturer to establish the applicability of the life formula in cases where Pr exceeds Cor or 0.5Cr, whichever is smaller."

* My question: shouldn't Pr be Por in the above quote?

If not, is there any purpose to calculating Por=XoFr + YoFa…. I don't see any of these quantities used anywhere else in the guide.

 

[jmoritz]

Dynamic loading and static loading often differ by either a little or a lot depending on the equipment and speed and temperature and preload etc., etc..

Consider that in operation, load is added to the bearing as clearance is lost due to temperature differences between the inner and outer rings for example (temperature loading)...inadvertant cross loading....friction loads from free bearings locking in a housing...centrifugal loading.

So when the dynamic load Pr grows to be so high that it exceeds the static capacity Co, the risk of damaging the raceway through plastic deformation exists and threatens the service life of the bearing.

The purpose of calculating the static load is to ensure that the bearing will not deform under this load. But you should check that it won't happen once dynamic loading is considered too. As it says.

Clear as mud.

 

[electricpete]

Thanks for the detailed reply.

"The purpose of calculating the static load is to ensure that the bearing will not deform under this load"

Makes sense, BUT...I don't see any discussion of what to do with Por anywhere in the standard. The logical thing would be to compare it to Cor, but the only statement anywhere close to this is the one I quoted above which refers to Pr, not Por.

 

[Hippo41]

I think you may have found a mis-print. The new SKF General Catalogue explains this in some detail.

You do calculate Po=XoFr+YoFa and compare to Co.

Then you can determine some form of static safety factor.



Lester Milton
Telford, Shropshire, UK

 

[electricpete]

Thanks Lester. When faced with the apparent inconsistency, I just wanted to make sure I wasn't grossly misunderstanding something.

Your response makes me feel better.

 

[jmoritz]

Don't think it's a misprint....consider this:

Static equivalent loads are calculated and compared to the static capacity when the application is considered static, let's say less than 15 rpm. Dynamic life calculations are essentially meaningless in these cases.

Dynamic equivalent loads are calculated and compared to the dynamic capacity when the application is considered dynamic and then the life equations are "valid".

They are separate evaluations for different application types.

You must keep in mind that the total dynamic load cannot surpass the static capacity or you'll get plastic deformation and the fatigue life will never be "achieved".

I tried my best but realise that this is a hard one to explain. Vibrating applications are often cause for concern: they are dynamic but subject to severe shock loading, road graders, rock crushers, things like that. The dynamic loading is unpredictable. Anything with adjusted taper roller bearings -- our favourite topic to avoid. High temperature stuff....super high speed stuff...weird aerospace applications.

It's rare, I'll give you that.

 

[jmoritz]

Okay, I think I thought of a good practical explanation:

I get a lot of bearings sent to me after failure for review. Some fail due to overloading. Could be a pump, a motor, a gearbox, any dynamic application.

However, they failed due to plastic deformation of the material. Inappropriately located bearings that crossloaded so much that the dynamic loading exceeded the static capacity and the rolling elements are literally squashing or squeezing the raceway material around. For example.

I hope that explains it.

It's just something to keep in mind.

 

[electricpete]

It seems plausible that we need to compare dynamic load to static limit. However, if that is the intent of this statement, then this standard does absolutely nothing with the quantity Por=XoFr + YoFa.

Why do we define these parameters, include tables for X0 and Y0, if we never intend to do anything with Por?

 

[jmoritz]

The primary intent of ABMA Standard 9 is to define load ratings and fatigue life. In my copy, which admitedly is not the newest version, it clearly states its purpose and mentions that "the bearing's ability to withstand static loading is also considered".

You're absolutely right, they didn't provide the equation So = Co/Po but they do tell you that Co is the permissible static load and that you shouldn't surpass it. They also tell you how to calculate Co and Po. It's implied that you compare Po to Co.

I can't offer any other explanation as to their decision not to print the static review of a bearing.