KD5ZXG said:
So, my suggestion of roller bearings in place of Maxon's brushes might not have been completely daft. Not sure how much current Bosch intends to push through these? Perhaps this illustration was just about bleeding some unwanted voltage that accumulates.
bosch.jpg
The latter: What that is actually about is charge buildup (static, etc) across bearings that could potentially cause them to be "eaten" over time by effects that can look similar to galvanic corrosion (which is different metals in contact with each other), if arcing can occur between bearings and races (if there is any gap or insulation between them, like grease). If the lubricant itself is also sufficiently conductive, this shoudln't happen anymore.
You would nto want to pass significant current thru the bearings (although technically you could).
At one time I used my frame on Crazybike2 as system ground, particularly for the "12v" lighting. This eventually caused problems with the headset bearings because of the significant currents flowing for things like the car halogen headlight. At some point, for other reasons, I ran actual ground wires for everything, and replaced the bearings (with other used bearings), and stopped having this problem. At higher voltages, I expect the problems would have cropped up quicker and/or been worse.
If you did want to use axle bearings as brushes, you would have to electrically isolate the bearings from the axle (and the metal of the side or end cover, or frame), or you could not use more than one brush, and wouldn't have a current path. Once isolaed from the axle, you could then use one bearing on each end of an axle as a pair of brushes, but if you need more than two you'd need separate bearings for each "brush".
Since running significant current thru them would be problematic in them performing their job as bearings over time, youd' also want to put a pair of "real" bearings that provided the actual load support. Those would make a barrier to putting wiring to or from the brush bearings, so making the system more difficult to engineer.
It's much simpler and cheaper to use regular brushes, for any significant currents.