My robot has wheels, so it has no knees. But it does
have a shoulder, elbow and wrist. For each joint, I
use a pair of sealed ball bearings pressed onto a
section of stainless steel shaft. Cheap, easy and
reliable. I wouldn't even think about making my own
bearing races. The shaft for the shoulder is 3/8", for
the elbow is 1/4", for the wrist is 1/8".
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