Geometry and kinematics of multicable spreader lifting gear

This paper presents a novel theoretical study of the geometry and derived kinematics of a typical multicable suspension system as encountered on a rubber tyred gantry (RTG) crane. Machines of this sort are used extensively in the international container handling business but, to date, little has been known about the precise motions of cable suspended spreaders other than general intuitions about fore-aft, lateral and rotational oscillations. Such motions are initiated by driver-controlled motion of the gantry itself (by torques applied to the driving wheels) and by across-the-vehicle motions of the trolley from which the spreader and container payload are suspended. The work reported here shows the complete derivation for spreader coordinates, relative to the trolley, for any translational and/or rotational displacement. Conventional geometrical and trigonometrical principles are used throughout the development. This research forms an integral part of a larger programme of work to propose strategies for accurate spreader motion control based on nonlinear dynamic models.