Control device for hydraulically propelled work vehicle

A control device for a hydraulically propelled work vehicle, comprising:

• a variable displacement hydraulic pump which is disposed in an upper rotating body portion and which is driven by an engine;
• an engine revolution speed control means which controls a revolution speed of said engine;
• a hydraulic motor for vehicle propulsion which is disposed in a lower moving body portion and which is driven by hydraulic fluid ejected from said hydraulic pump;
• a hydraulic actuator for performing work which is driven by hydraulic fluid ejected from said hydraulic pump;
• a first and a second control valve means, respectively provided between said hydraulic pump and said hydraulic motor for vehicle propulsion, and between said hydraulic pump and the hydraulic actuator for performing work, and controlling the flow amounts of pressurized hydraulic fluid which respectively is supplied to said hydraulic motor for vehicle propulsion and to said hydraulic actuator for performing work;
• a displacement variation means which varies the displacement of said hydraulic pump;
• and,
• a displacement control means which, at least when a pump pressure results in a surplus torque, in order to limit an input torque, determines a target displacement for input torque limitation which becomes smaller as the pump pressure is higher and controls said displacement variation means accordingly;
• wherein,
• said hydraulically propelled work vehicle is equipped with at least a center joint provided in a flow conduit between said hydraulic pump and the hydraulic motor for vehicle propulsion;
• characterized in that:
• said control device further comprises a condition discrimination means which discriminates whether the current operational condition is a vehicle propulsion condition or a performing work condition wherein,
• said engine revolution speed control means limits an engine maximum revolution speed to a first revolution speed when it is discriminated that the current operational condition is the vehicle propulsion condition, and limits the engine maximum revolution speed to a second revolution speed not greater than said first revolution speed when it is discriminated that the current operational condition is the performing work condition;
• said displacement control means limits a maximum displacement to a first displacement value when it is discriminated that the current operational condition is the vehicle propulsion condition, and limits the maximum displacement to a second displacement value higher than said first displacement value when it is discriminated that the current operational condition is the performing work condition;
• and,
• said first and said second revolution speeds and said first and said second displacement values are so determined that said first revolution speed multiplied by said first displacement value at least as relating to the pressure for vehicle propulsion on level ground is less than said second revolution speed multiplied by said second displacement value as relating to pressure below the range of torque limitation control in order to reduce a pressure loss of said center point.