COMPONENT OPERATION / HST Pump

Cutoff Valve

The cutoff valve regulates the maximum circuit pressure and protects the circuit on the high pressure side.

When the high-pressure side in the travel circuit reaches the specified circuit pressure, the control pressure acting on the displacement angle control cylinder through the forward/reverse selection solenoid valve is reduced.

When the control pressure of the displacement angle control cylinder is reduced, the pump displacement angle is controlled in order to deliver only the pressure oil flow rate corresponding to the specified pressure. The heat generation can be minimized as the circuit pressure on the high-pressure side is regulated by the pump flow rate control.

The cutoff valve regulates circuit pressure on the high-pressure side. Therefore, the cutoff valve pressure is usually set at 2 to 3 MPa (20 to 31 kgf/cm2, 290 to 435 psi) lower than that of the high pressure relief valve.

T3-4-10

COMPONENT OPERATION / HST Pump

HIGH-PRESSURE RELIEF VALVE

The high-pressure relief valve regulates the maximum pressure of the travel circuit and protects the circuit on the high-pressure side. The high-pressure relief valve has the check valve function for HST charge. When the pressure oil flow rate to the inlet circuit is insufficient, the HST charge pressure oil is routed to the inlet circuit.

LOW-PRESSURE RELIEF VALVE

The low-pressure relief valve regulates the HST charge pressure.

The low-pressure relief valve always operates during machine operation. The relieved oil returns to the hydraulic oil tank through the pump housing for lubrication and cooling.

Spring Spring Seat Piston Adjusting Screw Plug

NOTE: The illustration shows the low-pressure relief valve.

T3-4-11

COMPONENT OPERATION / HST Pump

Hst Charging Pump

The HST charging pump is coaxially connected to the shaft of the HST pump. Therefore, when the engine runs, the HST charging pump rotates with the HST pump together. The HST charging pump is a trochoid type pump. The inner rotor and the outer rotor rotates together by engaging so that hydraulic oil is drawn to chamber A and delivered from chamber B.

T3-4-12

Outline

COMPONENT OPERATION / HST Motor

HST motor is a bent-axis type variable displacement axial plunger motor and consists of the regulator, the shaft, the plunger, the rotor, and the valve plate.

HST Motor:

The EP motor solenoid valve is provided in the regulator of the HST motor.

T3-5-1

COMPONENT OPERATION / HST Motor

(Blank)

COMPONENT OPERATION / HST Motor

Operation

COMPONENT OPERATION / HST Motor

The output shaft of the rotary group is supported to the motor casing by two bearings. The center pin connects the output shaft to the rotor and is located in the rotor center hole. The valve plate is provided in the rotor end.

The spring applies the appropriate force to the rotor and brings the rotor into contact with the valve plate.

There are seven plungers in the rotor. Rotation direction of the HST motor is determined by entering pressure oil into either port A or port B. When traveling forward, pressure oil from the pump flows to the rotor through port B of the valve plate. When the plunger is pushed by pressure oil, component force (FV) of pushing force (F) rotates the output shaft.

As the output shaft rotates the rotor, the plungers reach port A in sequence. When a plunger reaches port A, pressure oil in the rotor is pushed by the plunger. Therefore, pressure oil is returned to the HST circuit.

The valve plate is connected to the regulator through the control rod. The valve plate position is changed by the operation of the regulator. Thus, the HST motor speed is changed.

T3-5-4

COMPONENT OPERATION / HST Motor

Regulator

COMPONENT OPERATION / HST Motor

The regulator consists of the servo piston, the control rod, the balance spring, the spring, and the spool.

The EP motor solenoid valve is provided in the regulator of HST motor.

The servo piston of the regulator is connected to the motor valve plate via the control rod. When operating the servo piston, the valve plate position is changed. Therefore, the motor displacement angle is changed and the motor speed is controlled.

The servo piston operation is controlled by the EP motor solenoid valve.

Ep Motor Solenoid Valve

The EP motor solenoid valve is a proportional solenoid valve and controlled by the command signal from the HST control unit. The command signal is a current value. The motor displacement angle is the maximum at the maximum current value and is the minimum at the minimum current value. The spool position is changed by changing a current value so that the servo piston operation is controlled.

T3-5-6