Hydraulic drive system for hydraulic work vehicle

• US 6,220,028 B1
• Filed: 12/03/1998
• Issued: 04/24/2001
• Est. Priority Date: 12/04/1997
• Status: Expired due to Term

• Current Assignee: Hitachi Construction Machinery Company Limited (Hitachi, Ltd.)
• Original Assignee: Hitachi Construction Machinery Company Limited (Hitachi, Ltd.)
• Inventors: Hirata, Toichi, Gotou, Yasuharu, Ishikawa, Kouji, Nakamura, Tsuyoshi, Sugiyama, Genroku, Toyooka, Tsukasa
• Primary Examiner(s): Look, Edward K.
• Assistant Examiner(s): Lazo, Thomas E.
• Application Number:
  US09/204,068
• Time in Patent Office: 873 Days
• Field of Search: 604/31
• US Class Current: 60/431
• CPC Class Codes: B60W 2710/0644 Engine speed
• CPC Class Codes: E02F 9/2235 including an electronic controller
• CPC Class Codes: E02F 9/2246 Control of prime movers, e.g. depending on the hydraulic load of work tools
• CPC Class Codes: E02F 9/2296 Systems with a variable displacement pump
• CPC Class Codes: E02F 9/26 Indicating devices E02F5/145 takes precedence

Abstract

A hydraulic drive system is provided with a signal guide system for guiding a delivery pressure of a pump as a command pressure via a line and a port, another signal guide system for guiding a pilot pressure of a pilot pump as another command pressure via a line and a port, a tilting control unit for controlling the displacement of the pump by the command pressures guided to the ports, a controller having a command current-setting unit for setting a relationship between a target engine speed and a command current, a target engine speed-indicating unit for an engine, and a solenoid-operated proportional valve for allowing the pilot pressure of the line to pass as is while no drive signal is outputted from the controller but for being reduced in opening to change the pilot pressure to a smaller value when a drive signal is outputted from the controller.

Claims

1. A hydraulic drive system for a hydraulic work vehicle, said system being provided with:

a motor,

a variable displacement hydraulic pump operatively driven by said motor,

a first tilting control signal guide system for guiding, as a first tilting control signal, a delivery pressure of said variable displacement hydraulic pump,

a second tilting control signal guide system for guiding a second tilting control signal adapted to control torque to be absorbed in said variable displacement hydraulic pump,

a unit for generating said second tilting control signal, and

a tilting control unit for controlling a displacement of said variable displacement hydraulic pump so that a delivery flow rate of said variable displacement hydraulic pump decreases as a value of said first tilting control signal increases and said torque to be absorbed in said variable displacement hydraulic pump decreases as a value of said second tilting control signal increases, characterized in that said hydraulic drive system comprises;

a controller;

a unit for outputting to said controller an upper limit signal which determines an upper limit of said torque to be absorbed in said variable displacement hydraulic pump; and

a control device arranged in said second tilting control signal guide system and operatively controlled by a drive signal outputted from said controller to perform control so that, when said drive signal is not outputted from said controller, said second tilting control signal is maintained at a predetermined large value and, when said drive signal is outputted from said controller in correspondence to said upper limit signal outputted from said upper limit signal output unit and determining said upper limit of said torque to be absorbed in said pump, said value of said second tilting control signal is changed from said predetermined large value to a small value.

2. The hydraulic drive system according to claim 1, wherein:

said second tilting control signal generating unit is a pilot pump operatively driven by said motor, said second tilting control signal is a pilot pressure delivered from said pilot pump, and said second tilting control signal guide system comprises a tilting control signal line for guiding said pilot pressure; and

said control device is a solenoid-operated proportional valve arranged in said tilting control signal line.

3. The hydraulic drive system according to claim 1 or 2, wherein said upper limit signal output unit is a target rotational speed indicating unit for indicating a target rotational speed of said motor.

4. The hydraulic drive system according to any one of claims 1-2, wherein said upper limit signal output unit is a mode selector switch arranged in association with modes of work by said hydraulic work vehicle.

5. The hydraulic drive system according to any one of claims 1-2, wherein:

said hydraulic drive system further comprises an actuator, which is operative by pressure oil delivered from said variable displacement hydraulic pump, and a directional control valve arranged in a line, which communicates said variable displacement hydraulic pump and said actuator with each other, and provided with a center bypass passage such that, when said directional control valve is in a center valve position, pressure oil delivered from said variable displacement hydraulic pump is guided to a reservoir but, when said directional control valve is in a valve position other than said center valve position, the pressure oil delivered from said variable displacement pump controls said actuator; and

said tilting control unit comprises a unit for controlling a displacement of said variable displacement hydraulic pump so that said displacement increases as a flow rate of pressure oil, which is flowing through said center bypass passage, decreases.

6. The hydraulic drive system according to any one of claims 1-2, wherein said upper limit signal output unit is a rotational speed detector for detecting an actual rotational speed of said motor.

7. The hydraulic drive system according to any one of claims 1-2, wherein said hydraulic work vehicle is a hydraulic excavator.