A ball screw assembly comprising: a screw shaft along which is formed a first helical groove; a nut along which is formed a second helical groove; a nut cartridge in which the nut is housed, and wherein the nut cartridge is provided with means for attachment to a load to be moved by the assembly; the first helical groove and the second helical groove cooperating to define a track (3, 3'); a plurality of balls arranged in the track and configured to move along the track from a first end to a second end of the track in response to relative motion between the screw shaft and the nut such that rotational motion of the screw is translated to linear motion of the nut via the balls and vice versa, and wherein the balls return from the second end of the track to the first end via a ball return channel in the nut cartridge; and characterised by further comprising: a bypass shoe arranged between the nut housing and the nut and spaced from the track by a predetermined small clearance selected as being a clearance overcome by a preload X equal to the maximum normal operating dynamic load for the ball screw plus a predetermined error margin, wherein when a load applied to the nut does not exceed the preload X, torque is transmitted from the screw shaft to the nut via the balls, and wherein when a load is applied to the nut that exceeds the predetermined preload, the bypass shoe closes the small clearance and engages with the track such that motion of the screw is transferred to motion of the nut via the shoe and bypasses the balls. a screw shaft along which is formed a first helical groove; a nut along which is formed a second helical groove; a nut cartridge in which the nut is housed, and wherein the nut cartridge is provided with means for attachment to a load to be moved by the assembly; the first helical groove and the second helical groove cooperating to define a track (3, 3'); a plurality of balls arranged in the track and configured to move along the track from a first end to a second end of the track in response to relative motion between the screw shaft and the nut such that rotational motion of the screw is translated to linear motion of the nut via the balls and vice versa, and wherein the balls return from the second end of the track to the first end via a ball return channel in the nut cartridge; and characterised by further comprising: a bypass shoe arranged between the nut housing and the nut and spaced from the track by a predetermined small clearance selected as being a clearance overcome by a preload X equal to the maximum normal operating dynamic load for the ball screw plus a predetermined error margin, wherein when a load applied to the nut does not exceed the preload X, torque is transmitted from the screw shaft to the nut via the balls, and wherein when a load is applied to the nut that exceeds the predetermined preload, the bypass shoe closes the small clearance and engages with the track such that motion of the screw is transferred to motion of the nut via the shoe and bypasses the balls. a bypass shoe arranged between the nut housing and the nut and spaced from the track by a predetermined small clearance selected as being a clearance overcome by a preload X equal to the maximum normal operating dynamic load for the ball screw plus a predetermined error margin, wherein when a load applied to the nut does not exceed the preload X, torque is transmitted from the screw shaft to the nut via the balls, and wherein when a load is applied to the nut that exceeds the predetermined preload, the bypass shoe closes the small clearance and engages with the track such that motion of the screw is transferred to motion of the nut via the shoe and bypasses the balls.