Hydraulic drive system having a plurality of prime movers

A hydraulic drive system including at least one hydraulic actuator, a plurality of variable displacement hydraulic pumps, each of said hydraulic pumps being connected to said hydraulic actuator in a manner to form a closed hydraulic circuit therewith and said hydraulic pumps being connected in parallel with each other, and a plurality of prime movers each connected to one of said plurality of hydraulic pumps to drive the same, characterized by:

• valve means each associated with one of said hydraulic pumps to fluidly and selectively shut off the hydraulic pump associated therewith from said hydraulic actuator and the other hydraulic pump, said valve means each comprise a two position directional control valve formed with two input ports communicating with one of the hydraulic pumps and two output ports communicating with the actuator, said two position directional control valve being movable between a first position in which the two input ports are brought into communication with each other end the two output ports are shut off from each other and a second position in which the two input ports and the two output ports are connected to each other, wherein said directional control valve comprises a pilot-operated valve normally disposed in the first position;
• and
• control means each associated with one of said prime movers to control said valve means in accordance with an operating condition of the prime mover in such a manner that when the prime mover is not in a normal operating condition, the hydraulic pump connected to the prime mover not normally operating is fluidly shut off from said hydraulic actuator and the other hydraulic pump, said control means for said valve means each comprise additional means operative to control said valve means in response to the flow rate of pressure fluid delivered by the hydraulic pump, to thereby fluidly shut off the hydraulic pump from said hydraulic actuator and the other hydraulic pump when the flow rate of pressure fluid delivered by the hydraulic pump is essentially zero, and wherein said control means each comprise a hydraulic pilot pump driven by said prime mover, a shuttle valve communicating with opposite two pressure chambers of a control cylinder which is provided to drive displacement volume adjusting means for the hydraulic pump, first conduit means supplying an output of said shuttle valve to the pilot operated directional control valve as pilot pressure, the pilot operated two-position change-over valve mounted in said first conduit means and movable between a first position in which the directional control valve has a pilot pressure released therefrom and a second position in which said shuttle valve is brought into communication with the directional control valve, and a second conduit means for supplying pressure fluid delivered by said pilot pump to said change-over valve as a pilot pressure.