Dear Royce in South Brazil, I know a little about metals and the coefficients of friction. I, through extensive tests making worm gearing, found that any steel, the harder the better, suffered relatively little galling when used with SAE 92 bronze. This bronze is common for use as bushings and easily available. It has lead in it. When I used SAE 68 aluminum bronze, a much harder material that contains about 9% aluminum, my gears would wear and the steel worms would develop radial scratches. You would think that the harder bronze would have much better wearing characteristics but that isn't the way it goes. Also, aluminum bronze is difficult to machine.
My experience with Teflon and nylon gears, or machined pieces, is that dimensionally the material is not stable after machining. That stuff can move 1% to 2% over night. Mysteries of life. A one inch SAE 92 bronze bar 6" long is very cheap and easy to find. It machines easy. Arthur---Mexico City
