A frictional drive unit, suitable for effecting linear sliding movement of a structural member into a cantilevered position, includes a frame, at least two sprocket assemblies mounted in bearings in the frame and spaced apart with their rotational axes parallel to each other, and an endless track. This track comprises a plurality of articulated friction pads mounted upon the sprocket assemblies and drivingly engageable with the structural member. Reversible motor means is carried by the frame and is drivingly connected to at least one of the sprocket assemblies.