Some double universal joints use a centering joint to ensure that the angle of the input and output are equal to maintain a constant velocity through the joint. From what I can tell, this centering joint consists of a ball joint that can slide through a slot. It would seem to me that this arrangement would not give an exactly equal angle for the input and output at all possible angles, so is an approximately equal angle deemed acceptable? Or am I missing something about this arrangement that forces the angles to be exactly equal?
