A disengagable coupling comprising a first part (1) affording a bore (3) and a second part (2) comprising a sleeve (5) and a drive flange (6). The sleeve (5) fits in the bore (3) and contains an insert (8) with collars (9,10) between which is defined a chamber (4) for the reception of oil under pressure to expand the sleeve into frictional engagement with the wall of the bore (3) for torque transmission. There may be a pressure release valve response to relative movement between the first and second coupling parts to release the coupling in case of overload.
