Why don't ball bearings conduct electricity?

[mmagliaro]

I have been experimenting with some of the tiny ball bearing sold by places like Boca Bearings.

Things like this:


I want to use them in my next steam engine project on the axles. 

What I'm shocked to find is that there is very spotty electrical connectivity between the inner race and
the outer one.  That is, if you press a steel or brass axle into the center, and connect an ohmmeter between
that axle and the outside edge of the bearing, instead of having zero ohms resistance, it
is very intermittent, and never better than about 20-30 ohms.

I would have thought it impossible for at least one of the steel balls inside the races to be touching both
the inner and outer rings of the bearing at the same time, and even if it were a completely broken
circuit, I would have expected to see an open circuit (infinite ohms), not some wonky
low-but-not-low-enough value like 20, 30, 40 ohms.

Can anybody enlighten me as to what's going on here?
(and I did try soaking the bearing in alcohol and then acetone just to make sure there was no oil or
grease causing this).

[peteski]

As a non-chemist, take my opinions with a grain of sodium chloride. 

Couple of reasons:

1. steel might have a thin layer of oxide on the surface which is non conductive.
2. A film on non-conductive lubricant on the contacting surfaces isolates them electrically.

If you have some silver-bearing grease , try using it to lubricate the bearing and see if it improves the contact.

[up1950s]

Stainless Steel ? ... Look at this chart http://www.tibtech.com/conductivity.php , then add lube and 2 points of rolling contact per ball .

[C855B]

Yes, more than likely they're stainless steel. Stainless steel is not a good conductor - about 40X more resistance than copper - but I don't think that's the problem. Stainless is "stainless" because of the chromium content, which forms a protective chromium oxide film on the surface. I wasn't able to quickly find conductivity data on CrO, but think that's gotta be the culprit.

[nkalanaga]

Mike:  I looked too, and nearly everything I could find on Google was either useless or paywalled.  Since a lot of metal oxides don't conduct well, I suspect you're right. 

Besides all of the other problems, I wouldn't be surprised if ball bearings are designed NOT to conduct.  Arcing between the balls and the races would cause pitting, which would soon ruin the bearing.

[mmagliaro]

Two probes touching the outer race = 0 ohms.  Two probes touching the inner race = 0 ohms.
I suppose I need to test two probes touching only one of the little balls in the center.  It won't be easy,
but I think I can do it.

And yes, I believe all these little 1.5mm (or so) bearings use stainless balls.  But maybe the inner and outer races are not stainless steel?   Doesn't make sense.  I'd think the whole thing is stainless.

[peteski]

Oh man, there are so many good ballz and li'l ballz references in the last few threads - must resist...

Here is a serious thought:  Wash off any existing lubricant (acetone, paint thinner on naphtha would do that well) and lube it with Neolube, then check conductivity.

[rschaffter]

I think there is simply not enough load on the bearing to have the balls make good contact; it's basically point contact on the bottom ball or two... 

[Doug G.]

I think Rod's got it. There just isn't enough of a tight connection anywhere to allow efficient conductivity. It's kind of like laying a wire on top of another and measuring resistance between the two ends of the two wires.

Doug

[C855B]

I just ran across an interesting little tidbit in researching this subject - it is possible the balls could be ceramic. Not unheard of in precision micro bearings.

[mmagliaro]

These are definitely not ceramic.   They are steel.  I ordered them from Boca that way, and I can clearly see them.
As for the pressure on the actual steel balls, it seems to me that with a shaft pushed through the metal, if
I hold the outer race and press the shaft very hard toward one side, it certainly has to be pressing on those
things pretty hard.

This is a bummer.  If these things cannot be counted on to conduct well, I will have to bail on my idea of using ball bearings and just use conventional bronze friction bearing blocks.  They will work fine, but I was pushing to see if I could make something super-fantastic-ultra-free-rolling.

[peteski]

I guess you won't even try a conductive lubricant.    And I thought you were an experimenter.   

[sizemore]

These are definitely not ceramic.   They are steel.  I ordered them from Boca that way, and I can clearly see them.
As for the pressure on the actual steel balls, it seems to me that with a shaft pushed through the metal, if
I hold the outer race and press the shaft very hard toward one side, it certainly has to be pressing on those
things pretty hard.

This is a bummer.  If these things cannot be counted on to conduct well, I will have to bail on my idea of using ball bearings and just use conventional bronze friction bearing blocks.  They will work fine, but I was pushing to see if I could make something super-fantastic-ultra-free-rolling.

If BB's were the preferred for performance why not use a wiper on the back of the wheels like Kato Mikes?

Humbly Submitted,
The S.

[JMaurer1]

You have an outer race, an inner race and the balls (and ball race to keep the balls from falling out). The whole point of using balls is to minimize the contact area on the inner and outer race. The balls just barely touch the races. Then everything is covered in lube. The balls usually are completely covered in lube which then insulates them from the races. Clean the bearings well with alcohol or some other cleaner/degreaser and conductivity will increase (but most likely still occasionally be intermittent). Bearings are not expected to be conductive so no effort has been made to make them so.

[PROPULDUDE]

I'll bet caged / sealed ball bearings conduct  far better. Test one, I no longer work with them, nor have any.