Generalized transmission index and transmission quality for spatial linkages

This paper proposes a generalized transmission index for spatial mechanisms, based on the virtual coefficient between the transmission wrench screw and the output twist screw. This index is applicable to single-loop spatial linkages, with fixed output and single or multiple degrees of freedom. We show that the transmission index introduced by Sutherland and Roth, the pressure angle of higher-pair mechanisms and the transmission angle of linkages are special cases of our index. A method is developed to compute the transmission wrench screw in spatial single-loop linkages. Furthermore, the transmission quality is defined based on the generalized transmission index to evaluate the global performance of a mechanism.