Abstract: Disclosed are methods and apparatus for controlling the flow of hydraulic fluid to and from a work cylinder. The disclosed system includes a reversible variable displacement pump interconnected between a work cylinder and a selector valve which may connect the pump with any of a plurality of fluid reservoirs. The fluid reservoirs contain hydraulic fluid at variable pressures so that the pressure at the work cylinder may be raised to pressures well above the pressure differential capacity of the pump at maximum flow rate by selectively increasing the pressure on the reservoir side of the pump. The system includes means for controlling the pressure at each port of the pump so that the pressue on the output port is always greater than the pressure on the input port regardless of the direction of flow through the pump and regardless of pressures at other points in the system.

Abstract: Disclosed is a crane employing a fluid cylinder and piston assembly secured to the crane boom and operatively interconnected with the cable between the boom sheave and the load hook to vary the vertical distance between the boom sheave and the load hook in response to relative vertical movement between the crane boom and the platform to or from which a load is to be transferred. Sensors which generate a signal in response to vertical movement are associated with each platform which is movable with respect to a fixed horizontal position. Signals from the sensors are used to control the position of the piston in the cylinder and thereby vary the vertical separation between the hook and the boom in relation to relative vertical movement between the boom and the landing platform.