Cable stretch asymmetries in multi-cable spreader suspension systems undergoing combined translations and rotations

The work presented in this paper proposes that for widely spaced cable reeving configurations in container-handling cranes there is an interesting, and unexpectedly useful, phenomenon defined henceforth as geometrical extension which gives rise to unevenly distributed cable stretch and, hence, uneven tensions across the system. Geometrical extension is entirely caused by the reluctance of the trolley, spreader and cable system to conform, as a whole, to certain overall shapes. Such shapes are developed because of the various translational and rotational motions which can be experienced at the trolley (that is at the attachment point of the cables running down to the spreader). Other cables stretch effects, such as those due to static and dynamic (inertia) loads, are additional to the geometrical extension (which can, in some circumstances, manifest as a compression instead). The net effect of cable stretch asymmetries is to stiffen up the suspension for certain cases of translation and rotation. This has a direct, and useful, influence for anti-sway control.