Depends. Used as in a prop shaft it is probably better than 99%. Used in a PTO drive as in agricultural products, better thna 95% but I don't know.
I think the way to work it out is to figure out the load in the bearings, and then for your joint articulation angle and speed you can work out a surface speed for each bearing, and then using a coefficient of friction of 0.1-0.3, you can work out the power lost. I've never done that.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.