COMPONENT

OPERATION ／ HST Pump

・Pilot pressure creating mechanism（YUKEN ／ Serial No. 1001 ～ 4499）

1.The relationship between pump speed and pilot pressure (ΔPc) is as follows.

2.This shows the spool position when the engine is stopped. The spool is pushed to the left because of the spring force and the port C which creates pilot pressure is closed.

3.The spool begins to move when the engine speed has gone beyond low idle speed. The opening of the control section is then increased and pilot pressure is created (point X). The point X is decieded by setting load of the spring ③ -1. The spring load setting is adjusted by the shims.

COMPONENT OPERATION ／ HST Pump

4.When the engine speed is increased, a differential pressure before and after the orifice ④ , the spool ① is moved to the right and the opening of the control section is increased, so that the pilot pressure is increased, as shown in the figure. The spring ③ -2 determines the pilot pressure gradient. Adjusting the smaller spring set load creates the pilot pressure characteristics to gain horsepower and yield optimum characteristics.

COMPONENT OPERATION ／ HST Pump

・Low-pressure relief valve（YUKEN ／ Serial No. 1001 ～ 4499）

The oil sent from the inching valve assembly acts on the port (A), pushes out the valve stem (1) at a pressure of 1.57MPa or over and flows into the casing through the port (B) to hold the pressure in the low-pressure circuit.

・High-pressure relief valve

（YUKEN ／ Serial No. 1001 ～ 4499）

Relieving

1.The oil from the high-pressure port (A) acts on the difference in area between the two parts having the diameters D and d. (D>d).

2.When the oil pressure in the high-pressure port (A) reaches the high-pressure setting, the valve body (1) opens and the oil in the high-pressure port (A) flows to the low-pressure port (B).

Sucking

When the pressure in the high-pressure port (A) is lower than the pressure in the low-pressure port (B), the valve body (2) opens and the oil in the low-pressure port (B) flows to the highpressure port (A) to prevent cavitation.

COMPONENT OPERATION ／ HST Pump

・HST Pump（UCHIDA ／ Serial No. 4501 ～）

The HST pump is the swash plate type axial piston pump with automotive control (DA control). The cylinder block with nine pistons in total is connected to the drive shaft using a spline; and the main pump is connected to it. The charging pump is built-in in the HST pump and it connected to the drive shaft to replenish the oil in the closed travel circuit and for piloting the automotive control.

COMPONENT OPERATION ／ HST Pump

・High-pressure relief valve

This valve determines the maximum pressures in the closed circuit and protects the high-pressure circuit. It also has an anti-void unit, which refills the oil to low-pressure side from the drain circuit directly to prevent the cavitation.

1-Rod

2-O-ring

3-Spring

4-CAP screw

5-O-ring

6-Adjusting Spring

7-Poppet

< When relieved>

The force of high pressure circuit [A] is larger than the spring force due to the spring (6), thus press down the poppet (7) to the position shown in the figure and the oil in the high pressure circuit [A] will be drawn into the charging circuit [B].

8-Valve sheet

Ａ-HST HP (LP) circuit

Ｂ-Charging circuit