Abstract: A freewheel coupling for drivingly connecting co-axial driving and driven shafts includes preventer means adapted to automatically prevent engagement except when the speed of rotation of the driving and driven shafts is substantially synchronized. The preventer means comprises a ball bearing having a ball race located in circumferential trackways in inner and outer races attached respectively to first and second coupling parts, and having a plurality of slots extending laterally from one of the trackways so as to permit relative axial movement of the coupling parts by movement of the balls of the ball race in the slots.