COMPONENT OPERATION ／

HST Pump

<Idling>

The pressure difference between fore-and-aft the orifice (6) is small since the output volume of the charging pump is small, thus the differential piston (5) will be located at the position shown in the figure. Hence, low control pressure due to reduction in section between the charging circuit [C] and the control circuit [B] will be drawn into the control circuit [B].

The pressure difference between fore-and-aft the orifice (6) becomes larger since the output volume of the charging pump is large, thus the differential piston (5) will be pressed up to the position as shown in the figure. Hence, high control pressure will be drawn into the control circuit [B].since the section area of the passage between the charging circuit [C] and the control circuit [B] will becomes large.

COMPONENT OPERATION ／ HST Pump

（３）DA control system

1-Low pressure relief valve

2-O-ring

3-DA valve

4-Adjusting screw

5-Adjusting Spring

6-O-ring

7-Differential piston

8-Orifice

9-Solenoid valve [b]

10-Solenoid valve [a]

11-O-ring

12-O-ring

13-Control piston

14-Tilt piston

15-Guide ring

16-Turcon glide ring

17-O-ring

18-Swash plate

19-Slide ring

Ａ-DA valve, primary (from charging pump)

Ｂ-DA valve, secondary (to control circuit)

Ｃ-DA valve, secondary (to charging circuit)

Ｄ-DA valve drain

Ｅ-Tilt piston (x1)

Ｆ-Tilt piston (x2)

COMPONENT OPERATION ／ HST Pump

Neutral position

COMPONENT OPERATION ／ HST Pump

<Forward driving>

When the shift lever is set at [F], the solenoid valve [a] (10) is energized, which press the control piston (13) connected to the solenoid valve upward to the position shown in the figure, and the oil in the control circuit generated by the DA valve (3) will be drawn into the tilt piston right chamber [F]. As a result, a hydraulic force directly proportional to the pressure of the control circuit [B] is applied to the tilt piston (14) and the tilt piston (14) will move downward to the position shown in the figure. The swash plate (18) is connected to the tilt piston (14) through the slide ring (19), and the HST pump discharges the oil to the forward drive circuit.

COMPONENT OPERATION ／ HST Pump

<Backward drive>

When the shift lever is set at [R], the solenoid valve [b] (9) is energized, which press the control piston (13) connected to the solenoid valve downward to the position shown in the figure, and the oil in the control circuit [B] generated by the DA valve (3) will be drawn into the tilt piston left chamber [E]. As a result, a hydraulic force directly proportional to the pressure of the control circuit [B] is applied to the tilt piston (14) and the tilt piston (14) will move upward to the position shown in the figure. The swash plate (18) is connected to the tilt piston (14) through the slide ring (19), and the HST pump discharges the oil to the backward drive circuit by rotating counterclockwise shown in the figure.

COMPONENT OPERATION ／ HST Pump

(4)Tilting mechanism of the swash plate

The tilt piston is connected to the pin on the swash plate (tile plate). The springs are built into the tilt piston and the spring force works to return the tilt piston to neutral (zero discharge). The control pressure is led to the tilt piston decides the tilt piston position and pump discharge amount. The relation of the solenoid and the pressure discharge port and travel direction

[The relation of the solenoid and the pressure discharge port and travel direction]

Acting solenoidDischarge portPilot pressure measuring ports

Solenoid (a)Ａ (Forward)Ｘ２

Solenoid (b)Ｂ (Reverse)Ｘ１

COMPONENT OPERATION ／ HST Pump

COMPONENT OPERATION ／ HST Pump

・Recovering force from tilt and spark-advance（UCHIDA ／ Serial No. 4501

Hydraulic power works on the swash plate through the piston. The control plate's high and low changing point is set so that the angle is advanced into the rotation direction: therefore the application point on the high-pressure section, worked on the piston, slightly deviates from the supporting point of the swash plate. Because of the deviation between the supporting and application points a recovering power from the tilt is generated. (referred as Recovering Power from the tilt or Self-Recovering Power)

The eccentric pin at the adjusting screw end is connected with the control plate groove. The spark advance of the control plate has been adjusted (by turning the adjusting screw) by HST manufacturer in the bench test before the delivery.

COMPONENT OPERATION ／ HST Pump

・Inching function（UCHIDA ／ Serial No. 4501 ～）

The inching valve is separated from the HST pump to which it is connected by the brake link and rod.When the brake pedal is depressed, the linked lever is turned and it turns the spool so that the oil flow in the spool is connected to the oil flow between the P-port and T-port. Because the DA pilot pressure is linked to the P port, when the oil is drained in the oil tank the pilot pressure is lowered so as to reduce the pump tilt.

This function is used when the machine is to be controlled at a low speed while the engine speed is increased and the speed of working equipment is increased.

COMPONENT OPERATION ／ HST Pump

・Automatic speed change function（UCHIDA ／ Serial No. 4501 ～） q

The pilot pressure in relation to the engine speed is linked from the DA valve to the tilt piston on the side selected by the solenoid valve. When the engine is running in low idling the pilot pressure is lower than the spring force in the tilt piston so that the pump is set to neutral and the machine is stopped. When the engine speed increases the control pressure also increases and the spring is under pressure, which inclines the pump. It means that the discharged quantity varies with the engine speed so that speed control is continuously available by use of the accelerator.

：Displacement by the HST pump

P3

N

V

：Pilot pressure

：Engine speed

：Travel speed

・Torque control function（UCHIDA ／ Serial No. 4501 ～）

When the pressure increases, the recovery of the swash plate tilt is increased and the swash plate tilt angle is reduced. This is an approximate torque fixing control. As the torque set is based on the pilot pressure, and the balance between the spring force of the tilt piston and the recovery force from the swash plate tilting, the set torque varies with the engine speed.

If the engine is loaded beyond its torque, then the engine speed tries to reduce from N1 to N2. When the engine speed is reduced, the HST pump input torque also reduces and the HST pump tilt angle is balanced at a point appropriate to the engine's torque. It means that when the load increases, the engine speed slightly decreases and the HST pump automatically performs the tilt angle control without stopping the engine effectively adjusting to the engine torque.

COMPONENT OPERATION ／ HST Pump

・Middle speed fixing solenoid valve（UCHIDA ／ Serial No. 4501 ～）

When the [AUTO] mode is selected, the DA pilot pressure in the HST pump is connected to the X port on the HST motor. When the [LOW] mode is selected, the decompressed charging pressure from the G port is supplied to the X port at a fixed rate. Because of that the HST motor tilt angle is fixed at the middle and the maximum speed is reduced.

COMPONENT OPERATION ／ HST Motor

・ HST Motor（YUKEN ／ Serial No. 1001 ～ 4499）

This HST motor is a bent axis type axial piston motor and the variable-displacement type in which the displacement can be changed in two steps. The bent axis type features that the cylinder block tilts with respect to the shaft. By sending the pressurized oil from charging pump to the chamber A or B of the piston ⑥ , the maximum, middle or minimum cylinder block ⑩ tilting angle can be obtained.

1-Nut

2-Shaft

3-Push rod

4-Piston assembly

5-Port plate (Valve plate)

6-Piston

7-Pin

8-Set screw

9-Bush

10-Cylinder block

11-Body

12-Piston (for middle-speed control)

13-System pressure control valve

14-Electromagnetic selector valve