A load-bearing plate is mounted by a plurality of pivotal links above a base plate, and solely for vertical movement toward and away from the base plate. An intermediate frame including two, spaced, parallel side bars is supported by the links between the two plates for swinging movement on the base plate about a pair of spaced, parallel axes. Driving means, such as a hydraulic screw jack is mounted between the two side bars for swinging movement therewith, and has a reciprocable piston rod attached to one of two, parallel shafts mounted at opposite ends in slots in the side bars to reciprocate longitudinally of the bars during the raising and lowering of the load-bearing plate. The reciprocable movement of the two shafts oscillates the links which in turn cause the load-bearing plate to reciprocate only vertically relative to the base plate.