Bearing Design Standard 2

[corus]

Is there a recognized design standard for bearings that provides a factor on a static load for normal service?
Alternatively is there a recommended factor that should be used with a reliable reference?
Efunda gives factors but the source is not given.

corus

 

[EnglishMuffin]

According to "Ball & Roller Bearings, Theory Design & Application" Brandlein et Al, (basically FAG), the index of static stressing is defined to be:

fs = Co/Po

where Co = Basic Static Load Rating
Po = Equivalent Static Bearing Load

The value of Co is computed based on the total rolling element and raceway deformation equaling .01% of the rolling element diameter on the most heavily loaded rolling element. This is a somewhat arbitrary criterion, and as will be seen below, it may often be exceeded without disastrous consequences.

The above reference quotes the following acceptable values for fs, the logic of which I do not profess to fully comprehend :

Occasional rotation : fs = 1
Less deformation acceptable (50% of standard) : fs = 1.5
Larger slewing, no shock : fs = 0.6 balls, 0.7 rollers
Shock loads or small oscillation angles : fs = 0.7 balls, 0.75 rollers
Spherical roller thrusts : fs = 4 (because of unfavorable lip loading)

I don't know if this is what you are after. I have frequently used safety factors of 2 or 3, because the real loading conditions are often unknown. It all depends on how accurately you can predict the loading. Of course, bear in mind that with static load ratings, if you only exceed the rating one time - you will permanently brinell the bearing some definite amount. On the other hand, nothing noticeable will necessarily occur immediately – possibly just some extra noise.

 

[vanstoja]

FMC Corp. Book 3251, 1970, "Bearing Technical Journal", Chapt. 3 on Rating and Life states that "The Anti-Friction Bearing Manufacturers Association (AFBMA)has adopted standards developed by the member companies. The AFBMA standards on ratings were approved as USA Standard B 3.11-1959 by the organization now known as American National Standards Institute (ANSI)." Loading analysis is covered in Chapt. 4 of FMC's 50 page book. Possibly the ANSI standard number or issue year has changed since 1970.

 

[Gerry45]

EM, I'm pleased to see you saying that static ratings can be exceeded in appropriate conditions. So many others believe that the Co figs indicate a 'never never exceed' value for the brg.

Just a couple of points come to mind:-

Firstly I think that in the latest ISO76 they have slightly changed the way they define the rating. It was 0.01% indent depth, now they say the Co correspondes to a max contact stress at the most heavily loaded element of approximately 4000 N/mm2(depending on brg). I've no idea why they made this change, I just cant see that it really helps anyone (other than perhaps makes the standards people feel useful!)

Secondly, regarding the brinelling and noise creation, I think the concern here applies only really to situations where the brg is stationary, or virtually so, when an extreme load is applied to it. The loads, in this situation, will create indents that will be detectable by vibration/noise measurement when the brg is subsequently run at speed under a much lighter 'normal' load.

However, if the duration of the shockload and movement of the brg is sufficient, the indents/brinels will tend to elongate and overlap. Since the rolling elements wont then bump in and out of 'ruts', vibration/noise doesn't become an issue at subsequent light-load running.


Gerry'

 

[EnglishMuffin]

Yes, now that you mention it, that newer definition rings a bell. 0.01% deformation is about the same as 4000 N/mm^2 for roller bearings of 52100 steel. I wonder if the reason for the change has to do with the greater prevalence of silicon nitride bearings ? I have no idea what the acceptable static stress level for these is without looking it up, but permanent deformation in the case of a bearing made completely of SiN would probably not be a useful concept.

 

[corus]

Is the 4000 MPa referred to a limiting stress at room temperature, and if so will this reduce with higher temperatures?

corus

 

[EnglishMuffin]

Since the size of the indentation is still the bottom line, as far as I'm aware, then at higher temperatures I assume you would want to go with a lower stress. Maybe Gerry45 can help - he seems to have the standard.

 

[ehsan1364]

hi,please tell me what is the bearing design

 

[diamondjim]

This has always been a problem. The
dynamic capacities are almost always
lower than the static capacities.
Earthquake loads imposed severe limits
on many bearings. Catastophic failure
can be as high as 3 to 5 times the static
capacity depending on the shape and size
of the bearing and its support structure.
I would be surprised if this value does
not also correlate to some Rockwell C
value for the steel surfaces if they have
adopted it as a standard. Subsurface
shear stresses are still limiting the
capacities and these have been based on
material hardness below the Rockwell C case
for bearings and their rolling elements.
I will have to review the standard and see
what else is new. Thanks for the posting.