Specifications

Service Training
Page 4.2
Section 20
09.04
4.2.2 OPERATION OF THE HYDROSTATIC TRANSMISSION
The oil flow (20 L/min at n
min
1, 60 L/min at n
max
) generated by boost pressure pump G goes through filter
O to control valve block N entering it at port E1 and leaving at port E, from where it goes to port E of the
travel control unit P.
The oil flows through the restrictor (45) and leaves the travel control unit at port F to enter control valve block
N at port F, and then the feed valves (25) and the boost pressure valve (26), which stabilizes the boost
pressure at 17.5 bar.
4.2.2.1 TRAVEL CONTROL UNIT P
The way valve (47) which is pressurized to 12 bar and open at first is arranged in parallel to the restrictor
(45). After the boost pressure increases to 17.5 bar at F the valve is pushed to the closed position. The
pressure-relief valve (44), which is also arranged in parallel to the restrictor (45), ensures a constant
differential pressure of 11 bar between E and F. The feed and control pressure of 17.5 bar goes through
the way valve (48), travel direction, and the pressure reducing valve (43) to ports Y and Z and from there
to the servo piston (24) to which equal pressure is applied on each end. The pressure of 17.5 bar is applied
via way valve (46), the brake valve and port BR to the disc brakes as brake release pressure. This pressure
is also applied to the way valve (49), speed control valve as boost pressure for control of the engine. The
higher pressure with a differential pressure of 11 bar created by restrictor (45) and valve (44) is applied at
the pressure reducing valve (43) as boost pressure.
When an accelerator pedal is depressed, way valve (48) determines the direction of travel while valve (49)
and pressure reducing valve (43) establish the pilot pressures controlling engine speed and pump output.
When the stroke of the accelerator pedal creates a differential pressure of 4 bar between Y and Z, the pump
begins delivery and the truck starts to move. At the same time a pressure of 7 bar goes from valve (49)
through port VF to the speed control piston, increasing the engine speed to approx. 1200 rpm (jump in
speed). As the accelerator pedal is depressed further, the differential pressure between Y and Z rises to
approx. 10 bar, whereas the pressure at VF remains constant. The Q
max
of the pump (19) and Q
min
of the
hydraulic motors (52) is reached at a differential pressure of 10 bar (primary/secondary control) without
increasing the engine speed, however. Depressing the pedal still further modulates valve (49) and
increases the pressure in proportion to the pedal stroke to approx. 17.5 bar (end of pedal stroke). The engine
is brought to maximum RPM and the truck achieves maximum speed.
Depressing the brake pedal fully when an accelerator pedal is operated opens valve (46) so that the
pressure goes from F (feed pressure) to valve (43) as boost pressure. This reduces the differential pressure
between E and F; consequently the differential pressure between Y and Z drops to under 4 bar, causing
the pump to downstroke to zero output. At the same time port BR is connected via T2 to tank, reducing the
brake release pressure and so applying the brakes. A reduction in pressure to under 12 bar at port F (leak
in the closed circuit) opens valve (47). As a result restrictor (45) is bypassed and no differential pressure
can build between E and F, preventing an upstroking of the pump when the brake is applied.