Speed changing device for hydraulic driving apparatus and speed change control method thereof

In a hydraulic driving apparatus for a vehicle, said apparatus 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 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, and
• a 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 mode selection switch for selecting between a traveling mode position for traveling said vehicle and an operation mode position for operating a machine on said vehicle,
• a mode detection sensor for detecting which one of the traveling mode position and the operation mode position has been selected by said mode selection switch;
• 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 said staged back pressure valve, said staged back pressure valve being adapted to control a return pressure 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;
• and
• a control device;
• wherein, when the detected motor rotational speed lies between a minimum desirable motor rotational speed and a maximum desirable motor rotational speed, said control device, based on the selected position of said mode detection switch, outputs:
• a signal for allowing said hydraulic motor to have a predetermined minimum discharge capacity,
• a signal for reducing an opening degree of said directional control valve, and
• a signal for allowing said staged back pressure valve to have a predetermined high back pressure so as to feed the return oil from a discharge side of said hydraulic motor to a supply side of said hydraulic motor through one of said check valves, and
• wherein, when a pressure applied on said hydraulic motor is larger than a threshold value of braking pressure, said control device further outputs a signal for providing the hydraulic motor with a predetermined motor capacity corresponding to the detected selected position of the mode detection switch.

In a hydraulic driving apparatus for a vehicle having tires, the apparatus 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, and
• a 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:
• an auxiliary transmission connected between an output shaft of said hydraulic motor and the tires of the vehicle, said auxiliary transmission having a first clutch and a second clutch for interrupting torque and for driving the tires at a variable speed of rotation;
• a shifter selected position sensor for detecting a change of the selected position of said 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;
• and
• a control device;
• wherein said control device outputs a signal for engaging one of said first and second clutches in order to measure inlet/outlet pressure of said hydraulic motor while said hydraulic motor is being rotated by a coasting of the vehicle when (a) a change in the selected position of said shift lever is detected and (b) said detected motor rotational speed lies between a minimum desirable motor rotational speed and a maximum desirable motor rotational speed.

In a method of operating a hydraulic driving apparatus, wherein pressurized oil from a hydraulic pump is supplied to a hydraulic motor through a directional control valve, and wherein the directional control valve is actuated responsive to a change in a position of a shift lever having a forward operation position and a reverse operation position, so as to switch between a forward operation of the hydraulic motor and a reverse operation of the hydraulic motor,

• the improvement comprising the steps of:
• switching said directional control valve to a neutral position when there is a change in the position of said shift lever;
• after an elapse of a fixed period of time from the switching of the directional control valve to the neutral position, measuring an inlet pressure of said hydraulic motor and an outlet pressure of said hydraulic motor;
• comparing the thus measured inlet pressure and the thus measured outlet pressure to determine whether the hydraulic motor is operating in the forward direction or in the reverse direction;
• detecting the position of the shift lever;
• and
• reducing a rotational speed of said hydraulic motor when (a) either of the measured inlet pressure and the measured outlet pressure is larger than a brake threshold value, and (b) the direction selected by said shift lever does not match the direction determined by the step of comparing the measured inlet pressure and the measured outlet pressure.

In a method of operating a hydraulic driving apparatus, wherein pressurized oil from a hydraulic pump is supplied to a hydraulic motor through a directional control valve, and wherein the directional control valve is actuated responsive to a change in a position of a shift lever, having a forward operation position and a reverse operation position, so as to switch between a forward operation of the hydraulic motor and a reverse operation of the hydraulic motor,

• the improvement comprising the steps of:
• detecting a change in the position selected by the shift lever;
• determining a rotational speed of the rotational motor;
• when the thus determined rotational speed is between a minimum desirable motor rotational speed and a maximum desirable motor rotational speed, reducing rotational speed of said hydraulic motor upon a detection of a change in the position selected by the shift lever;
• preventing occurrence of cavitation, during the step of reducing rotational speed of said hydraulic motor, by:
• reducing a discharge capacity of said hydraulic motor,
• reducing an opening degree of said directional control valve to generate braking force of said hydraulic motor, and
• feeding a portion of return oil from a discharge side of said hydraulic motor to a supply side of said hydraulic motor in order to increase supply of oil to said hydraulic motor.