Stiga Park 2WD Workshop Manual page 44

The charge pump (1) and the main pump (2) are integrated into one unit, the hydraulic
pump (C) which is located separat in front of the engine.
The engine drives the the hydraulic pump (C) with a constant speed. The charge pump (1)
is feeding the main pump (2) with 35-45 bar. The oil, fed into the main pump during
operation, is a replacement for the leak oil from the front and rear axle drives (A and B) and
the main pump (2).
The charge pump sucs its oil from the oil container (4) and through the filter (5).
The oil flow and the flow direction through the main pump (2) - front hydraulic motor (9) -
rear hydraulic motor (7) is controlled by the speed pedal, mechanical connected to the
main pump (2). The main pump pressure is depending on the power requirements at the
wheel axles and is limited by the engine power.
The hydraulic motors and the hydraulic pump have a small oil leakage (1-4%), which
increases with increased power requirement (increased pressure). The leakage oil is
collected inside the unit housings an forwarded back to the oil tank through the leak flow
lines.
The motors are connected in serie with the rear hydraulic motor (7) first, when driving
forwards. This means, due to the leakage in the rear motor, that the machine under normal
conditions drives on the rear wheels only. When the rear wheels begin to slip (rotate with
1-4% higher speed than the front wheels) also the front wheels start to drive and the
slipping is avoided. This fact is not noticed by the operator, since the machine is driven with
its four wheels after demand.
By-pass valve
The main pump (2) is equipped with a by-
pass valve (8), connected to its clutch
release lever. When the by-pass valve is
open, the main pump is disconnected from
the oil circuit by an open passage out into
the housing.
The pressure drop over the pump is such
neutralized and the oil can flush free in the
system. See the figure.
The by-pass valve is intended to make it
possible to push the machine without heavy
resistance from the main pump.
WORKSHOP MANUAL
PARK
Chapter
4 - Hydraulic system
8
EDITION
2018
8
2
Page
13