There is an inner boss (1) and a radially spaced coaxial ring forming rigid coupling components. the rings are radially toothed (5,8) with alternate spacing on their facing sides so the converging teeth when torqued define a chamber (17) for respective cylindrical rollers (4.1,4.2) in positive contact between the boss and ring casing surfaces. Teeth flanks (6,10) bounding the chamber (17) are concave. they conform to the periphery of the roller and the casing surfaces of the boss. They extend over a chord (11) extending between one end of the chamber and the radiused foot of the boss (1) tooth (8) at the far end of the chamber.
