Abstract: A disconnectable transmission assembly comprising a coupling member within which is rotatably mounted in bearings a viscous coupling having a housing member and a hub which respectively carry interleaved plates. The coupling member and the housing member are connectable and disconnectable by means of a deformable sleeve which closes a chamber formed in the housing member and which can be supplied by fluid under pressure from a cylinder by movement of a piston. Deformation of the sleeve, which is connected to the housing member, engages the bore of the coupling member thus connecting the housing member and the coupling member for rotation. When the chamber is de-pressurized there is a running clearance between the sleeve and the coupling member.