Smallest possible rolling resistance

Question

So, question is how to achieve the lowest possible friction in a scenario where there is a steel bar rotating AGAINST the circular frame at high rpm. Now, when it comes to sliding friction lowest coefficient of friction is hard steel against greased hard steel at it's around 0.029.

However for rolling friction coeefficient is typically around 0.001 or ~30 times less. In ideal case rolling friction is 0 since since surfaces never slide relative to each other. Of course in practice it is not so so we have some small finite friction.

Logically, putting ball bearings on the ends of the bar eliminates (ideally) sliding friction. I have tried this with hand roling hybrid ceramic ball bearing against thick plastic frame, friction is indeed very low, even with applied pressure.

So, i am going with ball bearings on the ends of the bar to take advantage of the rolling friction.

Question is which is the best combination of materials.

I guess best is hard steel bearing against hard stainless steel frame, greased of course.

In wiki article hardened steel ball bearings on steel COF is given as 0.0010 to 0.0015.

https://en.wikipedia.org/wiki/Rolling_resistance

But possible downside is this may lead to more sliding, in contrast with ball bearing with rubber or plastic covering vs plastic frame where there would be hardly any sliding but probably more friction overall due to soft materials, more microscopic deformations etc.

Some amount of sliding definitelly happens in ordinary bearings and there are some new (not so new) concepts where sliding is supposedly eliminated.

But sliding is supposedly not large in the bearings.

So, steel bearing against greased steel frame seems like best comb, there is surely gonna be certain amount of sliding and it's hard to predict exactly how much.

Also, hardened stainless steel seems to be most slippery, from personal experience. And as they say as well.

Also, good grease of appropriate viscosity is also important, but question is purely about materials.

I am also considering a nano teflon powder to further reduce friction.

Anyone with experience in this area, your opinion please.

Answer 1

What causes the rolling friction is the deformation of the material Figure source: electronicsteacher.com

So if you need to have reduced friction you would need surfaces that are not easily deformable. Usually that amounts to hardness (and increased bearing strength).

In order to achieve that you'd need some type of heat treatment (depends on the material). E.g. Gas Nitriding, Carbo-Nitriding etc.

Answer 2

You never want the outer race or the inner race to slip against anything except for the balls. Technically you don't want it balls to slip either but if slip has to happen anywhere, you want it to be between the ball and the races because they are designed to handle it. You want slip nowhere else which means maximum friction everywhere else. This is why you grease the balls of a bearing but not the inside of the inner race or the outside of the outer race.

For example, on a car: We grease the ball bearings that the wheel axle sits in, but we choose a high friction material like rubber for the tires. We don't grease the tires and the road.

That means maximum friction between the outer race and whatever it is connected to, and maximum friction between the inner race and whatever it is connected to. There is never a reason to grease the outside of the outer race or the inside of the inner race (well technically there is one, and that is to prevent galling when press-fitting but that's a different issue).