The unit comprises an electric motor and a gear assembly which may be installed within a building wall and operates to open or close a window or door. The motor drives a pair of gears, the second of which frictionally drives a shaft which moves axially to provide a clutch action. An electric motor (5) drives a pinion (7) and a gear wheel (8) freely rotating on an axially sliding shaft (9) on which are fixed a bevel pinion (13), a bush (14) and a friction wheel (10) which bears on one side of the gear wheel (8). The bevel pinion drives a bevel gear (17) and a pinion (18) which provides an output through a gear (19). When the motor is energised and the load is light the bevel gears drive the friction wheel against the free gear wheel (8) providing a rotational output. When resistance is encountered the bevel gears cause the friction wheel to separate from the free gear and the drive to cease. Further, the load dependency may be arranged to operate in both directions.
