Abstract: A coupling which serves to transmit torque from a shaft to the end portion of a rotary mandrel in a web winding or unwinding machine has a socket which is defined by a sleeve-like insert in the end portion of the shaft. The socket has a lateral inlet which can be closed by a pivotable gate, and the gate can be held in closed position by a wheel which is pivotable on the end portion of the shaft between operative and inoperative positions. The sleeve and the gate extend beyond one axial end of the wheel to define a relatively long socket for the end portion of the mandrel so that the latter can stand pronounced bending and other stresses.