Hi John,

     There is really no problem with ball pivots, only with some people's 
understanding of them! If you have a ball on the end of the arm rotating up 
against a flat, the centre of rotation is on the axis at a point equal to the 
radius of the ball beneath the surface. If you have a flat on the end of the 
arm rotating up against a ball, the center of rotation is at the centre of 
the ball. I can send you drawings if it would help. 

     Happy thanksgiving!

     Chris Chapman

Hi John,



     There is really no problem with ball pivots, only with some people's understanding of them! If you have a ball on the end of the arm rotating up against a flat, the centre of rotation is on the axis at a point equal to the radius of the ball beneath the surface. If you have a flat on the end of the arm rotating up against a ball, the center of rotation is at the centre of the ball. I can send you drawings if it would help. 



     Happy thanksgiving!



     Chris Chapman