Warning - This trick does not work well for high speeds. Removing
and replacing the rubber seals results in an imperfect seal. At high speed, centrifugal
force causes the conductive grease to fly out of the bearing. For high speeds,
encasing the bearing in a hard material such as epoxy might help.
I needed bearings that conducted electricity. My tests showed that
if a bearing was not spinning then it had a low resistance. If I
spun it, however, it had about 20Mohm resistance which is basically
infinite. Pictures are shown below of how I tested this.
To make bearings conduct I took ordinary bearings, removed the
rubber seals from either side, removed as much of the grease that came
with it as I could and put conductive grease in its place. The resistance
went from 20Mohm with the original grease to about 200ohm to 500ohm
across two bearings with the conductive grease. So for one bearing the
resistance during rotation was about 100 to 250ohm. See below for more
about how the measurements were made.
Note that the first time I did it the results weren't so good (about
250kohm). This was probably because the first time I did it (October 14)
I just used my xacto knife to pick out the original oil so undoubtably there was
some left. This time
(October 19) I removed all oil by removing the seals and putting the now
opened bearings
in a container of degreasing fluid available from automotive supply stores
(I used MotoMaster Dunk Degreaser). I left them in it for about 10 minutes
and then ran high pressure hot water from the kitchen sink through them
and then repeated the
whole procedure a second time. In the end there was no sign of oil
(in fact one was a bit squeaky when turned by hand). I then put in
the conductive grease. This time the results were in the 100ohm to 250ohm
range for a single bearing.
In the middle of the picture is the bearing (bottom)
and the two rubber seals (top). They were carefully removed using the
xacto knife blade. Be careful to damage the rubber as little as possible.
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Testing the conductive bearing
I wanted to minimize any eddy currents and triboelectric effects from
interfering with my measurements so I measured across a spinning
metal shaft using two bearings. I held the outside of the bearings
against the shaft so that it would spin freely with the shaft. The inside
of the bearing did not spin at all. The result was that no parts were scraping
against each other. I tested before changing the grease and then
again after.
A close-up of a bearing assembly. The inner diameter
of the bearing is 1/4" so I'm sandwiching it between the head of a 1/4" bolt
and a nut. The outer part of the bearing is not touching anything and is
free to spin. The wire is attached to the assembly by wedging it between
two nuts.
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The wires go to my meter probes. This is a picture
of the position of things during the measurement. The red handled pliers
are holding one bearing assembly in place such that the bearing is rotating
with the shaft. The other bearing assembly is being held in place
on the other side of the shaft.
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This shows the shaft without the bearing assemblies
and pliers to help you pick them out from the clutter in the picture on
the left.
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Measurement being made. The meter is showing
436.27ohms. In this case the bearings are directly on opposite sides
of the shaft.
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In this case the bearing on the right is being held
about 1 inch higher than the bearing on the left (not visible) though they
are still on opposite sides of the shaft.
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The reading on the meters fluctuated a lot probably due to the
fact that the bearings were being held in place by hand and that the
shaft was wobbling during rotation. Note that this measurement was
made across both bearings so divide by two for a single bearing.
Resistance without rotation was in the single digit range so it is
lost in the variation in resistance observed during rotation.
Using the conductive bearing for grounding with high voltage
I later used this conductive bearing trick to ground a part of a
capacitor that was rotating at very high speed, up to 8400 RPM.
The other side of the capacitor was stationary wand was the high
voltage side. The whole capacitor had around 14kV across it.
Details of
this rotating charged cylinder, non-conventional propulsion
experiment can be found here.
The experiment. The inner of the two cylinders the one that's
grounded inside using the conductive bearing and is rotating.
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The grounded, conductive bearing rolling against the upper
surface of the rotating bowl/cylinder.
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