Hydraulic circuit for controlling actuators in a construction vehicle

Claim 1

In a hydraulic circuit system for controlling actuators in a construction vehicle which includes: a prime mover; a fixed displacement pump driven by said prime mover; a variable displacement pump driven by said prime mover and connected at the outlet side thereof to the actuators through their respective directional control valves; a controlling cylinder means hydraulically connected through a first pilot line to an outlet of said fixed displacement pump and mechanically connected to a swash plate of said variable displacement pump for controlling volumetric output of said variable displacement pump; and a drain line with an orifice connected between said first pilot line and a tank for draining hydraulic fluid from said fixed displacement pump, the improvement comprising: a. a bypass line, one end thereof being connected to said first pilot line and the other end thereof being connected to said drain line downstream from the orifice; b. an unloading valve means provided in said bypass line for selectively unloading the output pressure of said fixed displacement pump when the unloading valve means is in an open state; and c. a second pilot line, one end thereof being connected to said output line for said variable displacement pump and the other end thereof being connected to an actuating means of said unloading valve means for selectively operating the actuating means of said unloading valve means in response to the variation of the output pressure of said variable displacement pump, whereby said prime mover is prevented from stalling upon an overloading condition of said actuators and whereby power loss of both pumps is significantly minimized when said directional control valves are respectively in neutral positions so that the actuators are not actuated.