Air bearing kinematic couplings

The “kinematic coupling” is a well-known device used to achieve highly repeatable positioning of one machine or instrument element relative to another. Normally, it consists of three precision balls attached to one machine element in a triangular pattern. These balls mate with three V-grooves in the surface of the mating element, creating ball/flat point contacts at six locations on each body. Friction forces at the contacts limit the repeatability and accuracy of the coupling, and can result in wear over time. This paper describes a novel coupling design where we replace the ball/flat contacts with an intermediate element consisting of a double-sided porous carbon air bearing with one planar surface and one concave spherical surface. These air bearings ride on corresponding surfaces on the two bodies to be mated, thus restricting motion along a single line in space while introducing essentially zero friction in the other directions. With six such elements appropriately placed, all 6 d.f. of relative motion between the two bodies can be restrained. Experimental measurement of coupling repeatability shows that the air bearing kinematic coupling (ABKC) outperforms traditional ball/groove couplings both in positioning repeatability and load carrying capacity.