Abstract: For use with apparatus for transferring a load between two stations movable relative to each other in a vertical plane by means of a crane carried by one of the two stations having a hook assembly for attachment to the load on the other station for lifting the load, a lift-off indicator measures either the relative velocity or relative vertical accelerations of the two stations and from signals representing relative velocity of approach showing values between a maximum positive and a negative, and relative acceleration acting away from the direction of approach indicates conditions suitable for initiating lifting.

Abstract: Control of vertical motion of a suspended crane hook assembly to accommodate vertical motion between a station carrying the crane and a station carrying a load is achieved by a transducer on the hook assembly and a control cord depending onto the load station. Tension above a set value in the cord produced by a crew member on the station causes the hook assembly to descend and below the set value causes it to rise. Movement of the stations causes a continuous adjustment of tension and the hook assembly follows the motion, extra tension being applied to guide the hook assembly down to the load and vice versa.