normal sliding pattern in a ball bearing 2

[electricpete]

On the first page of this document

http://www.mrcbearingservices.com/docs/lubrication.pdf

there is a description of the typical sliding pattern in a ball bearing.

It shows sliding in one direction in the center, sliding in the other direction in two bands outside the center. Two stripes separating the bands where no slippage occurs.

I have heard this before and I'm sure it's correct but I just can't picture why it would be this way.

My two questions:
1 - can anyone explain to me the reason for this pattern?
2 - If I have a ball bearing with constant contact angle and disassemble after many years of service, shouldn'tI expect to see distint wear zones... particularly those two strips with noe wear? (I've never seen it).

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[Skogsgurra]

This is a tough one. I understand it like this:

A The unloaded ball always has a smaller radius than the curvature of the outer race.

B The ball is compressed when there is radial load on the shaft.

C Compressing the ball means wider contact area.

D This means that there are several radii involved in the "rolling". Shorter radii away from the outer race centerline and longer radii close to the centerline.

E Constant angular velocity (or the ball would turn into a "screw") demands that all radii rotate at the same speed.

From this, one can only draw one conclusion; the surfaces in the center of the race and the surfaces away from the race have to slide with regard to each other, and in opposite directions. There are only two "tracks" where proper rolling occurs, and that is on either side, about half-way out from the raceway center. As shown in the figure you are referring to.

The closest static equivalent to this is the neutral fiber that you have when bending a material, like steel or a batten. The outer side is stretched and the inner side is compressed while the center (the neutral fiber) is neither stressed nor compressed.

Gunnar Englund

http://www.gke.org

 

[Gerry45]

I've always known such markings as 'Heathcote slip bands'. Unfortunately I've no idea who Heathcote was but I have seen them them many times.

They tend to show up best when lubrication has been marginal (i.e. allowing some ball~raceway surface interaction) and loading heavyish (i.e. wide contact ellipse with large differences in rolling radius) and constant.

Stop a brg that has been operating in these conditions before too much damage is done, and by using a 'Talysurf' or similar sensitive equipment, it is quite easy to identify the worn and unworn portions in the ball running-path. The depth of the worn portons are typically less than a micron, (in fact often the difference is just a surface-finish modification), so macro-damage from contaminated lubrication etc. will easily mask the formation of the Heathcote bands.
Gerry

 

[volpegrigia]

Sorry about getting on this late. The issue is well covered in Ted Harris bokk on bearings.

All angular contact bearings show a wear band, even the perfectly lubricated ones. It may just show up as a dull band where the width is a function of the elements in play (ball and raceways). You may not be able to distinguish the two lines, but all bearings wear. The cage of such bearings is typically darker than the original color since it will have accumulated the small wear particles (oxidized) generated during breaking-in of the part. This is particularly true for bearings with steel balls and it is not present with bearings that have ceramic balls. The slippage (both negative and positive) forces at the ball raceway interface are taken by the lubricant thus the wear only occurs at start-up and shut-down when the speed is insufficient to provide the separation of the surfaces. Try a simple experiment. Take a small rubber ball and press it on a plate of glass with some talcul owder on it. You will be able to see the effect of the plastic deformation on slippage.
The intriguing part is that there is no accepted method to rate the wear life of bearings and the only accepted method to assess durability is fatigue. The intersting fact is that few bearinsg fail due to fatigue (I saw charts showing less than 10%) while the rest fail due to other causes!