Abstract: In a hydraulically expandable friction coupling for connecting a shaft to a hub there are two concentrically arranged cylindrical sleeves between which a pressure medium is enclosed. The pressure of said pressure medium against the sleeves is adjustable by adjusting means in order to transmit a predetermined torque between the shaft and the hub. In order to ensure slipping between the outer sleeve and the hub when said predetermined torque is exceeded i.e. when the coupling is overloaded, the outer sleeve is formed with a greater wall thickness than the inner sleeve or at least one additional sleeve is fixed on the outside of the outer sleeve. The wall thickness of said additional sleeve is such that the combined wall thickness of the outer sleeve and the additional sleeve exceeds the wall thickness of the inner sleeve.