Hydraulic drive system for construction machine

A hydraulic drive system for a construction machine comprising a prime mover, a hydraulic pump of variable displacement type driven by said prime mover, at least one hydraulic actuator driven by a hydraulic fluid delivered from said hydraulic pump, control valve means connected between said hydraulic pump and said actuator for controlling a flow rate of the hydraulic fluid supplied to said actuator dependent on the input amount of operating means, first pump control means for controlling a displacement volume of said hydraulic pump dependent on the input amount of said operating means such that the delivery rate of said hydraulic pump is increased with an increase in said input amount, second pump control means for limiting an available maximum value of the pump displacement volume such that an input torque of the hydraulic pump does not exceed an output torque of said prime mover, and prime mover control means for controlling a revolution speed of said prime mover, said available maximum value of the pump limited by said second pump control means being variable set in accordance with a delivery pressure of the hydraulic pump;

• wherein said prime mover control means includes:
• first control means for monitoring saturation of the delivery rate of said hydraulic pump provided by said second pump control means, and for controlling the revolution speed of said prime mover to be maintained at a predetermined speed before said hydraulic pump reaches a predetermined state at which said saturation occurs while further controlling the revolution speed of said prime mover to increase to avoid said saturation of the pump when said hydraulic pump reaches said predetermined state,
• wherein said first control means includes means for detecting the input amount of said operating means and increases the revolution speed of said prime mover when said hydraulic pump reaches said predetermined state and the input amount of said operating means exceeds a predetermined value.

A hydraulic drive system for a construction machine comprising a prime mover, a hydraulic pump of variable displacement type driven by said prime mover, at least one hydraulic actuator driven by a hydraulic fluid delivered from said hydraulic pump, control valve means connected between said hydraulic pump for said actuator for controlling a flow rate of the hydraulic fluid supplied to said actuator dependent on the input amount of operating means, first pump control means for controlling a displacement volume of said hydraulic pump dependent on the input amount of said operating means such that the delivery rate of said hydraulic pump is increased with an increase in said input amount, second pump control means for limiting an available maximum value of the pump displacement volume such that an input torque of the hydraulic pump does not exceed an output torque of said prime mover, and prime mover control means for controlling a revolution speed of said prime mover, said available maximum value of the pump limited by said second pump control means being variable set in accordance with a delivery pressure of the hydraulic pump;

• wherein said prime mover control means includes:
• first control means for monitoring saturation of the delivery rate of said hydraulic pump provided by said second pump control means, and for controlling the revolution speed of said prime mover to be maintained at a predetermined speed before said hydraulic pump reaches a predetermined state at which said saturation occurs while further controlling the revolution speed of said prime mover to increase to avoid said saturation of the pump when said hydraulic pump reaches said predetermined state,
• wherein said prime mover control means further includes second control means for detecting the input amount of said operating means and increases the revolution speed of said prime mover when said input amount exceeds a predetermined value.

A hydraulic drive system for a construction machine comprising a prime mover, a hydraulic pump of variable displacement type driven by said prime mover, at least one hydraulic actuator driven by a hydraulic fluid delivered from said hydraulic pump, control valve means connected between said hydraulic pump and said actuator for controlling a flow rate of the hydraulic fluid supplied to said actuator dependent on the input amount of operating means, first pump control means for controlling a displacement volume of said hydraulic pump dependent on the input amount of said operating means such that the delivery rate of said hydraulic pump is increased with an increase in said input amount, second pump control means for limiting an available maximum value of the pump displacement volume such that an input torque of the hydraulic pump does not exceed an output torque of said prime mover, and prime mover control means for controlling a revolution speed of said prime mover, said available maximum value of the pump limited by said second pump control means being variable set in accordance with a delivery pressure of the hydraulic pump;

• wherein said prime mover control means includes:
• first control means for monitoring saturation of the delivery rate of said hydraulic pump provided by said second pump control means, and for controlling the revolution speed of said prime mover to be maintained at a predetermined speed before said hydraulic pump reaches a predetermined state at which said saturation occurs while further controlling the revolution speed of said prime mover to increase to avoid said saturation of the pump when said hydraulic pump reaches said predetermined state,
• wherein said first control means includes means for providing a first target revolution speed of said prime mover and means for providing a second target revolution speed of said prime mover that is higher than said first target revolution speed, whereby said first control means outputs said first target revolution speed before said hydraulic pump reaches said predetermined state, and outputs said second target revolution speed when said hydraulic pump reaches said predetermined state.

A hydraulic drive system for a construction machine comprising a prime mover, a hydraulic pump of variable displacement type driven by said prime mover, at least one hydraulic actuator driven by a hydraulic fluid delivered from said hydraulic pump, control valve means connected between said hydraulic pump and said actuator for controlling a flow rate of the hydraulic fluid supplied to said actuator dependent on the input amount of operating means, first pump control means for controlling a displacement volume of said hydraulic pump dependent on the input amount of said operating means such that the delivery rate of said hydraulic pump is increased with an increase in said input amount, second pump control means for limiting an available maximum value of the pump displacement volume such that an input torque of the hydraulic pump does not exceed an output torque of said prime mover, and prime mover control means for controlling a revolution speed of said prime mover, said available maximum value of the pump limited by said second pump control means being variable set in accordance with a delivery pressure of the hydraulic pump;

• wherein said prime mover control means includes:
• first control means for monitoring saturation of the delivery rate of said hydraulic pump provided by said second pump control means, and for controlling the revolution speed of said prime mover to be maintained at a predetermined speed before said hydraulic pump reaches a predetermined state at which said saturation occurs while further controlling the revolution speed of said prime mover to increase to avoid said saturation of the pump when said hydraulic pump reaches said predetermined state, wherein said first control means includes:
• first detection means for detecting a differential pressure between the delivery pressure of said hydraulic pump and the load pressure of said actuator,
• second detection means for detecting the delivery pressure of said hydraulic pump,
• first means for setting a first target revolution speed of said prime mover,
• second means for calculating a first target displacement volume from the differential pressure detected by said first detection means to hold that differential pressure constant,
• third means for calculating a second target displacement volume for the input torque limiting control by the use of at least the pump delivery pressure detected by said second detection means and a predetermined input torque limiting function,
• fourth means for calculating a second target revolution speed of said prime mover higher than said first target revolution speed based on said first and second target displacement volumes,
• fifth means for calculating a difference between said first target displacement volume and said second target displacement volume, and
• sixth means for outputting said first target revolution speed when said difference is less than a predetermined value, said second target revolution speed when said difference becomes larger than the predetermined value.