Abstract: A safety shaft coupling member for coupling together two coaxial shafts and ncorporating an overload safety device, which causes complete interruption of the transmission of torque when an acceptable maximum torque is exceeded, comprises a hub sleeve which is adapted to be fixed to a driven shaft, and a cup-shaped assembly in which the hub sleeve is freely rotatably mounted and is capable of limited axial displacement. The hub sleeve and the cup-shaped assembly have concentric toothed end faces situated in a common plane extending normal to the axis of rotation of the coupling member, and a first annular piston has two annular sets of teeth arranged to mesh with the toothed end faces.