Indication means for indicating suitable conditions for the transfer of loads between two stations movable relative to each other in a vertical plane

Abstract From the Canadian patent

For a crane, carried by one of two stations movable relative to each other in a vertical plane and, adapted to permit a hook assembly when attached to a load on the other station to follow relative motion prior to lifting of 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 negative, and relative acceleration acting away from the direction of approach indicates conditions suitable for initiating lifting.