A ratarder for a power transmission shaft comprises a liquid immersed coupling between the shaft and a drag member, the drag member being normally entrained to rotate with the shaft by viscous drag in the coupling. A dry brake mechanism, having a frictional drag greater than that of the coupling, is provided to restrain the drag member for relative movement to take place in the coupling. In the embodiments shown, the coupling comprises a friction disc pack alternately connected to the shaft and the drag member, the brake mechanism firstly restraining the drag member and subsequently shifting it axially to engage the friction disc pack.The coupling is suitable for use in heavy motor vehicle transmission systems.