Independently actuated pressure relief system

Claim 1

A large industrial machine having a primary hydraulic system for actuating operative members, the primary hydraulic system is provided with a primary hydraulic pump, a primary hydraulic motor, a directional control valve, a primary hydraulic fluid reservoir, and at least two primary hydraulic lines; one of the primary lines couples the primary pump to the primary motor through the directional control valve and the other primary line couples the primary reservoir with the primary motor through the directional control valve; the primary pump forming a primary source of pressurized hydraulic fluid; a pilot hydraulic control system is used to control the position of the directional control valve, the pilot hydraulic control system is provided with a secondary hydraulic pump forming a secondary source of pressurized fluid and a secondary hydraulic fluid reservoir; the primary hydraulic system is further provided with a pressure relief system comprising at least one pressure relief valve which is in fluidic communication with one of the two primary lines between the primary motor and the directional control valve, and the primary reservoir; the pressure relief valve is provided with a biasing means having a biasing force for normally biasing the pressure relief valve into a closed position, the improvement comprising: coupling the pilot hydraulic control system to the pressure relief valve to form the biasing means so that the pressure relief valve is biased into a closed position by the pressure of the secondary hydraulic system, the pilot hydraulic control system is provided with a solenoid actuated, two-position, three-way valve for coupling and decoupling the pressure relief valve to and from the secondary source of hydraulic pressure and the secondary reservoir, thereby regulating the biasing force directed to the first pressure relief valve.