Ball bearing life span calculation

[oramone]

Hello people

I have tried to calculate the life span of a bearing I want to use. Here is
the formula I used:

L10 Life = (Rating8)3 x 1,000,000 Hours
(LoadE)3 60 x n

http://www.google.no/url?sa=t&rct=j...=hNKUU7FeIET1F0dleocnVQ&bvm=bv.47008514,d.bGE

where:
L10 Life Number of hours that 90% of a group of bearings should attain or exceed
prior to onset of fatigue failure
Rating8 Basic dynamic load rating for a given bearing
LoadE Equivalent radial load impressed on a bearing including radial and axial
loads.
n RPM

6309 bearing spec:

Max dynamic load: 35,1kN
Max static load: 19kN

And the load my bearing will exposed for is 0,58kN.

Here is my calculation:

L10 Life = (35kN)3 x 1,000,000 Hours
(0,58kN)3 60 x 600rpm

L10 Life = 6 081 250 Hours!!

This is more than 711 years!! This isn`t possible, have I made some wrong calculations??

 

[electricpete]

I think your calculation is right if your load is right (I assume you accounted for both radial and axial load).

There sometimes extra factors added to account for things like lubrication.

It is a consequence of the x^3 behavior that life changes dramatically with load.
At very low loads, predicted life increases dramatically, similar to behavior of a fatigue limit. ie. fatigue failure is basically not predicted at these load levels. There are still a lot of non-ideal factors that can kill your bearing: improper installation, lubricant contamination, etc.

=====================================
(2B)+(2B)' ?

 

[electricpete]

Clarification in bold:

At very low loads, predicted life increases dramatically, similar to behavior of a fatigue endurance limit

=====================================
(2B)+(2B)' ?

 

[Tmoose]

Like E-pete said, "equivalent load" must be calculated.

SKF's published "fatigue load limit" = 1.34 kN page 11 here -

http://www.skf.com/files/876653.pdf

So your bearing life as limited by surface fatigue/spalling should be infinity X 2.

Now "all" you have to do is evaluate the lubricant for a nice fat kappa at operating rpm and temp, get the machining of shaft and housing shoulders and seats done as SKF, FAG etc request (demand), assure the bearings are assembled in a non damaging way, provide means of introducing nice clean lube is introduced well before the lube's L10 life is exceeded, protect the bearing from shaft currents, etc.
Nothing to it.

 

[oramone]

Thanks allot for the respond people!

Good to hear that my calculations are correct. Of course there will be more factors that will determine the life span, like temperature, lubrication, environment etc. But the calculation give me a good "pin point".

I looked at the SKF brochure you posted, thanks! But I`m having problem to exactly understand what the "fatigue load limit" means. Is this a minimum load that the bearing need to prevent fatigue wear?

I used the "Minimum Load" formula in the SKF brochure. And after a calculated, I found out that the minimum load for my bearing would be 0,07kN (7,14kg). My load is well above that limit, so I guess everything is just fine?

 

[caoimhin1]

Everything is fine, but you could make it even better by choosing a 6209 or 6009 and thereby saving yourself a few pennies.

 

[Tmoose]

fatigue load limit is similar in concept (and probably based on) to steel's "endurance limit."

http://www.skf.com/skf/support/html/dictionary/dictionary.jsp?dictPage=f

Minimum load is the load below which the balls or rollers may skid, rather than roll. Skidding can cause surface damage, invalidating life calculations. Preload can keep the balls tracking and rolling when the equivalent load is less than the "minimum."