A lead screw assembly comprising: a screw shaft along which is formed a first helical groove; a primary nut along which is form a second helical groove; the first helical groove and the second helical groove cooperating to define a track (10,20); means for applying a pre-load of a predetermined value to bias the primary nut into engagement with the screw shaft such that rotary motion of the screw is translated to linear motion of the nut via the track and vice versa; and further comprising: a secondary nut arranged around the primary nut spaced from the first helical groove by the predetermined preload, wherein when a load applied to the assembly exceeds the predetermined preload, the secondary nut is configured to engage with the first helical groove such that motion of the screw is transferred to motion of the secondary nut and bypasses the primary nut; the assembly characterised by further comprising: a torque transfer tube; and a transfer member between the primary nut and the torque transfer tube is configured to transfer the movement of the primary nut to the torque transfer tube, and wherein the secondary nut is provided between the transfer member and the torque transfer tube. a screw shaft along which is formed a first helical groove; a primary nut along which is form a second helical groove; the first helical groove and the second helical groove cooperating to define a track (10,20); means for applying a pre-load of a predetermined value to bias the primary nut into engagement with the screw shaft such that rotary motion of the screw is translated to linear motion of the nut via the track and vice versa; and further comprising: a secondary nut arranged around the primary nut spaced from the first helical groove by the predetermined preload, wherein when a load applied to the assembly exceeds the predetermined preload, the secondary nut is configured to engage with the first helical groove such that motion of the screw is transferred to motion of the secondary nut and bypasses the primary nut; the assembly characterised by further comprising: a torque transfer tube; and a transfer member between the primary nut and the torque transfer tube is configured to transfer the movement of the primary nut to the torque transfer tube, and wherein the secondary nut is provided between the transfer member and the torque transfer tube. a secondary nut arranged around the primary nut spaced from the first helical groove by the predetermined preload, wherein when a load applied to the assembly exceeds the predetermined preload, the secondary nut is configured to engage with the first helical groove such that motion of the screw is transferred to motion of the secondary nut and bypasses the primary nut; the assembly characterised by further comprising: a torque transfer tube; and a transfer member between the primary nut and the torque transfer tube is configured to transfer the movement of the primary nut to the torque transfer tube, and wherein the secondary nut is provided between the transfer member and the torque transfer tube. a torque transfer tube; and a transfer member between the primary nut and the torque transfer tube is configured to transfer the movement of the primary nut to the torque transfer tube, and wherein the secondary nut is provided between the transfer member and the torque transfer tube.