A worm gear drive has a self locking gear train. The gear train comprises a worm 34 fitted to a motor shaft 32 and in mesh with a worm wheel 35 which drives the output 26. The motor shaft 32 extends between two thrust bearings 29, 60 having faces 54 which contact respective axial ends of the motor shaft 32. The contact between at least one of the thrust bearings 29 and the shaft 32 is adapted to provide a high frictional force.