A coupling comprises a pair of members: a first member comprising an outer convex spherical periphery centred about a central point and a torsional axis extending through the central point, and a second annular member has an inner concave spherical periphery centred on the central point and complementary to the outer periphery of the inner first member. The spherical surfaces of the inner and outer members co-act to transmit radial loads and loads acting along the common. Axles extend radially of the central point and couples the first and second annular for transmitting torsional load from one to the other. The each of the pair of members is rotatable one relative to the other about the said central point in a direction constrained by the axle arrangement.