Measurement and characterization of precision spherical joints

Spherical joints based on a ball and socket configuration have been developed for parallel kinematic machines. Four prototypes are implemented using point, rolling, sliding and aerostatic contact mechanisms; each with a magnetic preload between the ball and the socket. A spherical kinematic test mechanism has been constructed to automate the measurement of joint accuracy, revealing a mean positional error of 3–5 μ m and a maximum positional error of 12–22 μ m for the joint prototypes. The flow properties of the aerostatic contact joint are modeled using a Navier–Stokes flat-plate approximation and found to be accurate to within 10% for a 20 μ m air gap. The tensile load capacity is 50–100 N rendering these joints suitable for high accuracy non-contact applications, such as inline measurement and inspection.