A coupling for coupling together two mutually coaxial, rotatable parts, comprising a substantially cylindrical coupling member which is intended to be mounted between the parts to be coupled together and which is expandable both radially outwards and radially inwards so as to exert on the parts a coupling pressure through the intermediary of the coupling member. The coupling member incorporates a pressure chamber which is intended to be supplied with pressure fluid for expansion of the coupling member and which is provided with a pressure relief device which is triggered upon relative rotation of the aforementioned parts. The coupling member is configured to exert a greater coupling force on a first of the aforementioned parts than on the other of these parts, and the pressure relief device is intended to be activated upon relative rotation between the first part and the coupling member, but to permit rotation between the other of the parts and the coupling member.
