several grades? : how many grades and where are stated ?@rauliruegas If memory serves the bearings in the SME 5 are the hardest commercially available. The bearings in the Triplanar are not commercially available; Triplanar has a security clearance to get the bearings they use. Only one supplier exists worldwide that makes them.
Specifically as it was explained to me by Herb Papier, the problem he ran into was that over a period of years he noticed that many arms needed to be readjusted. He found that the reason was that the bearings would fail over time in the field- due to use and minor abuse the bearing points would blunt. Since he wanted to reduce friction, he used the smallest bearings he could find which made the problem worse. So brute force was applied (IMO); he found and installed the hardest metal bearings made anywhere, which solved the problem.
SME deals with the reliability issue by having physically larger bearings. So their arms tend to have more friction.

