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Hydraulic drive apparatus

  • US 6,192,681 B1
  • Filed: 06/01/1998
  • Issued: 02/27/2001
  • Est. Priority Date: 11/21/1996
  • Status: Expired due to Term
First Claim
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1. A hydraulic drive system comprising an engine, a variable displacement hydraulic pump driven by said engine, a plurality of actuators driven by a hydraulic fluid delivered from said hydraulic pump, a plurality of flow control valves for controlling flow rates of the hydraulic fluid supplied from said hydraulic pump to a plurality of actuators, and pump displacement control means for controlling the displacement of said hydraulic pump so that a differential pressure Δ

  • PLS between a delivery pressure Ps of said hydraulic pump and a maximum load pressure PLS among said plurality of actuators is maintained at a first setting value Δ

    PLSref, said pump displacement control means including first setting modifying means for modifying the first setting value Δ

    PLSref of said pump displacement control means depending on a rotational speed of said engine, wherein said hydraulic drive system further comprises;

    an unloading valve for controlling the delivery pressure Ps of said hydraulic pump so that the differential pressure Δ

    PLS between the delivery pressure of said hydraulic pump and the maximum load pressure PLS among said plurality of actuators is maintained at a second setting value Δ

    Pun higher than said first setting value Δ

    PLSref, and second setting modifying means for modifying the second setting value Δ

    Pun of said unloading valve depending on the rotational speed of said engine (1) in match with change in the first setting value Δ

    PLSref modified by said first setting modifying means in such a manner that the second setting value Δ

    pun does not become smaller than the first setting value Δ

    PLSref.

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