Patent ID: 7775384

Claim:
A hoist system including an upper hoist assembly supported on an overhead rail for traversing movement along said rail, a hoist chain extending downwardly from said upper hoist assembly which includes a chain drive driving a chain up or down to raise or lower a connected load, said upper hoist assembly supported on a trolley supported for rolling movement along said overhead rail; said upper hoist assembly supported on said trolley so as to be freely movable relative to said trolley in directions parallel to said rail; a tractor carriage also supported for rolling movement along said overhead rail and connected to said trolley for movement therewith; said trolley and upper hoist assembly interconnected with a load sensor which prevents said movement of said upper hoist assembly in directions parallel to said rail and generating signals in response to pulling on said hoist chain with a component force parallel to said rail; a tractor carriage drive normally holding said tractor carriage and connected trolley and activated in response to said load sensor signals correspondingly moving said trolley and said connected upper hoist assembly together along said overhead rail in a direction tending to null said signals.