A hydraulic frictional coupling for either mounting a hub on a shaft, or for connecting two shafts, (Figs. 5(a), 5(b)) (not shown) comprises an annular chamber (c, 16, 36) housing an annular piston (d, 15, 35), the chamber and the piston having matching tapers. Oil fed (e.g. at h) to one end of the chamber, moves the piston axially to cause radial expansion connecting the shaft and hub or two shafts. Oil fed (e.g. at i) to the other end of the chamber allows the coupling to be disconnected.