The invention provides a lead screw mechanism 160, for example for a vehicle access ramp. The mechanism comprises a support frame 110, a lead screw 150 and an internally-threaded mounting 160. The lead screw 150 has an external thread and is rotatably mounted to the support frame 110. The mounting 160 is mounted on the lead screw such that, when the lead screw rotates, the mounting travels along the lead screw. The mounting comprises a housing 161, an internally-threaded insert 170 rotatably mounted in the housing and engaging with the lead screw, and a lock 164. The lock 164 has a locked position in which the insert 170 is rotatably fixed relative to the housing 161 and a released position in which the insert 170 is able to rotate in the housing 161. The invention provides a mechanism to manually release or disconnect the mounting from the lead screw, which provides a manual means of operation of the vehicle access ramp in the event of a power failure. With the lock 164 in the released position, the insert is free to rotate relative to the housing, so the platform 130 can be moved in or out by hand, moving the housing along the axial direction of the lead screw 150 while the insert rotates around the lead screw thread relative to the housing.
