Patent ID: 6182448
Filing Date: 2001-02-06
Classification: F16H

Abstract:
In a hydraulic driving apparatus for a machine including:a hydraulic pump driven by a driving source,a variable displacement hydraulic motor for receiving pressurized oil from the hydraulic pump to output a driving force,a shift lever for selecting between forward operation of the hydraulic motor in a forward direction and reverse operation of the hydraulic motor in a reverse direction,an oil tank,a closed center directional control valve provided between the hydraulic pump and the hydraulic motor for switching said hydraulic motor between forward operation and reverse operation,a first pipe for passing pressurized oil from the directional control valve to the hydraulic motor when the hydraulic motor is to be operated in the forward direction and for passing return oil from the hydraulic motor to the directional control valve when the hydraulic motor is to be operated in the reverse direction, anda second pipe for passing pressurized oil from the directional control valve to the hydraulic motor when the hydraulic motor is to be operated in the reverse direction and for passing return oil from the hydraulic motor to the directional control valve when the hydraulic motor is to be operated in the forward direction;the improvement comprising:a shifter selected position sensor for detecting a change in a selection by the shift lever;a motor rotational speed sensor, for detecting a rotational speed of said hydraulic motor;a staged back pressure valve having an outlet connected to the oil tank;a return circuit formed between said directional control valve and the staged back pressure valve, said staged back pressure valve being adapted to control a return pressure of return oil in the return circuit at any one of at least two different pressures;first and second check valves, each of said check valves having a first side and a second side, the first side of said first check valve being connected to the first pipe between said directional control valve and said hydraulic motor, the first side of said second check valve being connected to the second pipe between said directional control valve and said hydraulic motor, and the second side of each of said check valves being connected between said directional control valve and said staged back pressure valve by said return circuit, so that when the first check valve is opened during switching to a forward operation, return oil can flow from the second pipe through the first check valve and the first pipe to the hydraulic motor, and when the second check valve is opened during switching to a reverse operation, return oil can flow from the first pipe through the second check valve and the second pipe to the hydraulic motor; anda control device;wherein said control device, upon detection of a change in the selection of said shift lever to a forward operation or to a reverse operation while the detected motor rotational speed is lower than a minimum desirable motor rotational speed, drives said machine with the driving force of said hydraulic motor, andwherein said control device, upon detection of a change in the selection of said shift lever to a forward operation or to a reverse operation while the detected motor rotational speed lies between said minimum desirable motor rotational speed and a maximum desirable motor rotational speed, outputs:a signal for reducing an opening degree of said directional control valve,a signal for allowing said staged back pressure valve to have a predetermined high back pressure so as to feed return oil from a discharge side of said hydraulic motor to a supply side of said hydraulic motor through one of said check valves, anda signal for allowing said hydraulic motor to have a predetermined minimum discharge capacity to reduce the rotational speed of said hydraulic motor.