Material specifications for bearings 1

[BevanSmith]

I need the material specs like ultimate tensile strength and poisson's ratio for the ball and race of typical ball bearings.

What can I assume the ball and race are made of? I have been looking in SKF catalogue and on the web but no clear guidelines as to which material standard bearings are made of.

Thanks for the assistance if any

 

[swall]

SAE 52100 steel through hardened to Rc 58 min. is typical.

 

[dinjin]

The balls are as earlier stated. The races could be made out of many different steels depending on size and cross sections. Often these are case hardened to 58Rc and in larger bearings may be induction hardened. The steels could be normalized and or quench and tempered. UTS above 100,000psi and up to 150,000psi depending on the steel.

 

[Tmoose]

I think UTS of 150,000 psi at RCH 58 is a bit low for any steel.

Some values for 52100 here -

http://www.surfrainbow.com/DLpdf/General-Bearing-Balls.pdf

Bearing races are relatively easily broken by impact ( snapped ) obeying to the harder=more brittle chestnut

 

[dinjin]

TMoose,
You are right about the 52100 Steel if it is fully
or thru hardened but the raceways may not be fully
hardened and simply case hardened. We generally
use 1050 4140 or 4150 for raceways. I see your reference
uses 1020 which really is a low carbon steel and its
UTS is just above 50000psi. Sub surface shear stresses
often appear below the case and the lower UTS must often be considered or checked. Thanks for the references.

 

[flash3780]

52100 is typical, but high speed turbine bearings use M50, M50-NIL, or Pyrowear 675.
An interesting side-note on bearings: manufacturers tend to thermodynamically induce compressive stresses in bearing raceways. The intent is to force microcracks, etc closed and keep them from propagating.

 

[tbuelna]

BevanSmith,

There is no such thing as a "typical" ball bearing. As flash3780 noted, a common thru-hardening steel alloy is E52100. But since steel cleanliness can have a huge effect on bearing life, and because there is significant cost pressure on production ball bearings, each bearing manufacturer carefully selects and customizes the exact steel alloy they use for any particular bearing. It is a compromise between material cost and required fatigue life.

Very high grade double vacuum melt E52100 CEVM (AMS 6444) bearing steel will give the best fatigue life, but is very costly. M50 (thru hardening) or M50-NiL (case hardening) will give good performance at temperatures up to about 600degF, versus about 350degF for 52100. 440C is a thru hardening stainless alloy. Sometimes 9310 (carburizing) steel alloy is used where a race needs to be part of a structure, like a gear shaft. And other steel alloys such as 4150 are used on large bearings where the race surface is locally induction hardened.

Tensile strength specifically, is generally not a concern with rolling element bearings. The most common failure mode in a properly lubed and maintained rolling element bearing is a sub-surface initiated race spall. These spalls are due to fractures that propagate up from sub-surface shear failures, so technically shear strength is what matters. The nucleation point of these shear fractures are usually microscopic inclusions (such as non-metallic oxides) within the steel. Vacuum melt steels are mostly free from these inclusions, which is why they tend to perform so much better in fatigue than air melt steels.

Retained austenite can also result in soft spots in the race structure with thru hardening bearing steels. So their heat treat usually includes a sub-zero treatment immediately after the initial quench.

Depending upon how your bearing is loaded (ie. which race is fixed with regards to load) and due to differences in relative contact geometries, one race will always have less fatigue life than the other. In most cases the inner race will be the limit. So having contact geometries and materials that equalize the elastic strains between the balls and races is very important.

Sorry for the long-winded post, but the answer to your question is that with regards to bearing fatigue life, properties like tensile strength and poisson's ratio are relatively meaningless. So maybe you might want to ask a different question.

Hope that helps.
Terry

ps. flash3780- could you elaborate on how manufacturers "thermodynamically" induce compressive stress in a race surface. Is it something similar to what nitriding does? And maybe give an explanation, if possible, of how this surface compression is not relieved by the repeated mechanical strains resulting from the rolling element contacts? It's something I've not heard of before, but sounds very interesting.

 

[flash3780]

In surface hardened bearings, bearing manufacturers often induce compressive stresses in the bearing raceway during heat treatment and by subsequent grinding which has been shown to improve the bearing's fatigue life.

From Harris, "Advanced Concepts in Bearing Technology":

"Heat treatment used for hardening rolling bearing components can exert very significant influence over the state of residual stress. Depending on the steel composition, austenizing temperature, quenching severity, component geometry, section thickness, and so forth, heat treatment can provide either residual compressive or residual tensile stress in the surface of the hardened component. Temperature gradients are established from the surface to the center of the part during quenching and after heating. The differential thermal contraction associated with these gradients provides for nonuniform plastic deformation, giving rise to residual stresses. Additionally, volumetric changes associated with the phase transformation occurring during heat treatment of steel occur at different times during quenching at the part surface and interior due to the thermal gradients established. These sequential volumetric changes, combined with differential thermal contractions, are responsible for the residual stress state in a hardened steel component. The sequence and relative magnitudes of these contributing factors determine the stress magnitude and whether the surface is in residual compression or tension..."

 

[tbuelna]

flash3780,

Thanks for the Harris reference. I read through it.

Harris indeed says that residual surface compressive stress is beneficial for fatigue life. And that it can result intentionally from carefully controlled finish grinding pressures or from more traditional processes like shot peening. He also states that a controlled run-in procedure can produce a beneficial work hardening effect on the race surface.

He also states that unintentional(?) residual stress, both tensile and compressive, can result from uneven heat transfer conditions present in the race section during quenching. But he did not elaborate on how this can be reliably controlled. I have seen through hardened bearing races with small soft spots that resulted from contact with the quenching dies, so I agree that it can happen. But how would you reliably control such a process? Would you use some sort of selective surface masking like copper plating?

I'd like to know more if possible. Thanks.
Terry