Bearing Life Calculation

[mickb22]

Has anybody any advise on calculating bearing life combining static load (crushing) and dynamic load (wear). The static load is high relative to the dynamic loads 6 x 1.

 

[electricpete]

I think I am still a little confused on the subject of static vs dynamic loading of bearings. Here was a thread with some good comments from another member thread821-75531

My feeble understanding when you say dynamic loading is much less than static loading... that means that the load is lower when the unit is rotating then when it is stationary?

If such were the case I would think life calculations should be based solely on the dynamic loading and dynamic rating. If the static rating is ever exceeded (while stationary or rotating) then damage may occur but it is right away, not a cuumulative fatigue lifetime issue.

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[electricpete]

If those loads you are calling static are present when the machine is rotating (for instance rotor weight), they should be added into the dynamic loads.

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[MASSEY]

I agree with electricpete, when we speak of "life" calculations, they always refer to rpm or rotating speed.

Certainly static rating is vitally important and cannot be exceeded, preferably not even approached.

An important note comparing static vs. dynamic however is that when more than one brg is used, say two brgs. for example: the staic rating for the brg. combination is double that for a single brg. But dyanimic rating is 2^7/9.

If you are able to incorporate three brgs. then the static rating is triple that for a single brg. while dynamic is 3^7/9 etc.

Again, the best approach is to contact a reputable brg. manufacturer, tell them what you are dealing with, and let them help you reach a succussful solution.

 

[EnglishMuffin]

Massey: Whilst it is true that static rating is important, it is not true to say that it cannot be exceeded. The degree to which this can occur depends on the amount of rotation present during the load application.

 

[MASSEY]

EnglishMuffin:

It is my understanding that Dynamic loading should never exceed the Static load rating. If rotation is occurring the load would probably be considered dynamic unless it is rotating very slowly.

I would be interested in seeing some reference from manufacturers stating that the Static load rating may be exceeded whether Static or Dynamic.

Thanks.

 

[MASSEY]

That is to say that if the load is rotating it is no longer static, but if the load is static then the static load rating should not be exceeded.

And although the dynamic load may be larger than the static load rating. The load must continue to be dynamic, and if it stops rotating then the actual load that used to be dynamic must not exceed the static load rating.

Clear as mud.

 

[EnglishMuffin]

Massey : See the following thread: thread821-81044. I merely stated that the static load rating can be exceeded - I did not say under what circumstances. The rating is somewhat arbitrary - a small amount of permanent deformation occurs even below the static rating, so for some applications you should use a lower rating . If the bearing is rotating while the load is being applied, more deformation is considered permissible because it will produce a groove rather than a dent, and so a higher rating can be used.

 

[MASSEY]

EnglishMuffin:

Thanks, I read the thread listed above. It sure sounds like you and the other posters know what you are talking about.

Thanks for the insight.

 

[diamondjim]

It is rather strange that the static
load limitation is based on rolling
element diameter squared, plus many
other factors and the dynamic load
limitation is based on the diameter
cubed, plus many other factors.

Have always wondered about this.
For slow moving slewing bearings
is .01 rpm, the static capacity is
often used. Even this limit might
have a safety factor or 2 or more
before the bearing explodes or
ruptures. If the loads are dynamic
and fast rpm is involvled the limits
are much lower. Dynamic life might
be defined as .0001 x the rolling diameter
as being the allowable deformation of the
raceways. This is an arbitrary value
and the bearing might see another
20 percent more life before it siezes
up. Static bearing loads may result in
.0003 times the rolling diameter in the
raceways and still not fail.
It really is dependent on the application
whether static or dynamic values should
apply and what defines the useful life
of a particular bearing application.
time
raceway

 

[Tmoose]

>>IF<< the operating conditions are clean, >>AND<< bearing speed and lube viscosity is high enough to provide a decent "kappa" (over 2.0) >>THEN<< ElastoHydroDynamic lubrication will be achieved, and bearing surfaces will be separated by oil, and the dynamic load/life calculations are based on fatigue, not wear. If the equiv. loading keeps the stress below the ball/race fatigue limit, then bearing (not lube) life is infinite. SKF puts this fatigue limit right in the catalog. For many types of bearings FAG suggests if the equivalent load is less than 1/8 the basic dynamic load the life can be infinite. However, If the equivalent load induces stresses greater than the endurance limit, then a number of revolutions or cycles can be calculated representing theoretical life to reach some modest level of surface damage. Higher rpm simply completes the number of cycles sooner.

If the lube is dirty or thin, or the rpm are low, then the balls and races will touch and scuff, and life will be more like wear, and the L10 calculation would probably apply.

As others said, the static load is limited by deformation.
Precision bearings (machine tool spindles) are usually limited to about 1/3 the "static load," as even slightly dented races will create crummy finishes (and probably even initiate fatigue failures at much lower stress levels when the spindle revs up later).

A low speed bearing would really benefit from keeping loads well below the static capacity, and using lube with oil thick enough for a good kappa at a few rpm. Oil that thick oil will probably reduce the upper speed limit.