Hydraulic drive system

A hydraulic drive system comprising a variable displacement hydraulic pump (1), a plurality of actuators (6, 7) driven by a hydraulic fluid delivered from said hydraulic pump (1), a plurality of directional control valves (8A, 8B) of closed center type connected to said hydraulic pump (1) through hydraulic fluid supply lines (22A, 22B) for controlling flows of the hydraulic fluid supplied to said plurality of actuators (6, 7), a plurality of control lever units (30A, 30B) for operating said plurality of directional control valves, and pump control means (2n;

• 2p) for controlling a delivery rate of said hydraulic pump (1) to become a flow rate corresponding to input amounts by which said plurality of control lever units (30A, 30B) are operated, wherein said hydraulic drive system further comprises:
• a plurality of load pressure detecting lines (12A, 12B) for detecting respective load pressures of said plurality of actuators (6, 7), and a maximum load pressure detecting line (13) for detecting maximum one of the load pressures detected by said plurality of load pressure detecting lines (12A, 12B),
• bypass variable throttle means (40) disposed in a bypass line (5) branched from a hydraulic fluid supply line (3) of said hydraulic pump (1) and having a downstream end led to a reservoir, said bypass variable throttle means (40) being operable to reduce an opening area thereof as the input amounts of said plurality of control lever units (30A, 30B) increase, thereby raising a delivery pressure of said hydraulic pump,
• a plurality of first pressure adjusting valves (9A, 9B) disposed respectively downstream of variable throttle portions (8a, 8b) of said plurality of directional control valves (8A, 8B) for controlling outlet pressures of said variable throttle portions (8a, 8b) to be kept substantially equal to the maximum load pressure detected by said maximum load pressure detecting line (13),
• a second pressure adjusting valve (41) disposed downstream of said bypass variable throttle means (40) in said bypass line (5) for controlling an outlet pressure of said bypass variable throttle means (40) to be kept substantially equal to the maximum load pressure detected by said maximum load pressure detecting line (13), and,
• an on/off valve (15) disposed in at least one of said plurality of load pressure detecting lines (12A, 12B) for selectively making the load pressure of the associated actuator (6) detected or not detected.

A hydraulic drive system comprising a variable displacement hydraulic pump (1), a plurality of actuators (6, 7) driven by a hydraulic fluid delivered from said hydraulic pump (1), a plurality of directional control valves (8A, 8B) of closed center type connected to said hydraulic pump (1) through hydraulic fluid supply lines (22A, 22B) for controlling flows of the hydraulic fluid supplied to said plurality of actuators (6, 7), and a plurality of control lever units (30A, 30B) for operating said plurality of directional control valves, wherein said hydraulic drive system further comprises:

• a plurality of load pressure detecting lines (12A, 12B) for detecting respective load pressures of said plurality of actuators (6, 7), and a maximum load pressure detecting line (13) for detecting maximum one of the load pressures detected by said plurality of load pressure detecting lines (12A, 12B),
• bypass variable throttle means (40) disposed in a bypass line (5) branched from a hydraulic fluid supply line (3) of said hydraulic pump (1) and having a downstream end led to a reservoir, said bypass variable throttle means (40) operating to reduce an opening area thereof as input amounts by which said plurality of control lever units (30A, 30B) are operated increase, thereby raising a delivery pressure of said hydraulic pump,
• a plurality of first pressure adjusting valves (9A, 9B) disposed respectively downstream of variable throttle portions (8a, 8b) of said plurality of directional control valves (8A, 8B) for controlling outlet pressures of said variable throttle portions (8a, 8b) to be kept substantially equal to the maximum load pressure detected by said maximum load pressure detecting line (13), and
• a second pressure adjusting valve (41) disposed downstream of said bypass variable throttle means (40) in said bypass line (5) for controlling an outlet pressure of said bypass variable throttle means (40) to be kept substantially equal to the maximum load pressure detected by said maximum load pressure detecting line (13), and
• pump control means (2n) for carrying out negative flow control so that a delivery rate of said hydraulic pump (1) is increased corresponding to a reduction in flow rate downstream of said second pressure adjusting valve (41) in said bypass line (5).

A hydraulic drive system comprising a variable displacement hydraulic pump (1), a plurality of actuators (6, 7) driven by a hydraulic fluid delivered from said hydraulic pump (1), a plurality of directional control valves (8A, 8B) of closed center type connected to said hydraulic pump (1) through hydraulic fluid supply lines (22A, 22B) for controlling flows of the hydraulic fluid supplied to said plurality of actuators (6, 7), and a plurality of control lever units (30A, 30B) for operating said plurality of directional control valves, wherein said hydraulic drive system further comprises:

• a plurality of load pressure detecting lines (12A, 12B) for detecting respective load pressures of said plurality of actuators (6, 7), and a maximum load pressure detecting line (13) for detecting maximum one of the load pressures detected by said plurality of load pressure detecting lines (12A, 12B),
• bypass variable throttle means (40) disposed in a bypass line (5) branched from a hydraulic fluid supply line (3) of said hydraulic pump (1) and having a downstream end led to a reservoir, said bypass variable throttle means (40) operating to reduce an opening area thereof as input amounts by which said plurality of control lever units (30A, 30B) are operated increase, thereby raising a delivery pressure of said hydraulic pump,
• a plurality of first pressure adjusting valves (9A, 9B) disposed respectively downstream of variable throttle portions (8a, 8b) of said plurality of directional control valves (8A, 8B) for controlling outlet pressures of said variable throttle portions (8a, 8b) to be kept substantially equal to the maximum load pressure detected by said maximum load pressure detecting line (13), and
• a second pressure adjusting valve (41) disposed downstream of said bypass variable throttle means (40) in said bypass line (5) for controlling an outlet pressure of said bypass variable throttle means (40) to be kept substantially equal to the maximum load pressure detected by said maximum load pressure detecting line (13), and
• pump control means (2p) for carrying out positive flow control so that a delivery rate of said hydraulic pump (1) is increased corresponding to an increase in command values from said plurality of control lever units (30A, 30B).