Stiga Park Series Workshop Manual page 50

Edition Manual
2014-09-20 Workshop Manual, GGP Park 4 Hydraulic system
The engine drives the charge pump (1) and the main pump (2) with a constant speed. The
charge pump is feeding the main pump. 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).
The charge pump sucs its oil from the rear axle drive volume and through the filter (5). The
rear axle drive is supplied with oil from the oil container (4).
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) in the rear axle drive (A). The main pump pressure is depending on the
power requirements at the wheel axles and is limited by the engine power.
The hydraulic motors have a small oil leakage (1-4%), which increases with increased
power requirement (increased pressure). The leakage oil is collected inside the axle drive
housings an forwarded back to the oil tank through the leak flow lines.
The motors are connected in serie with the front motor (9) first, when driving forwards. This
means, due to the leakage in the front motor, that the machine under normal conditions
drives on the front wheels only. When the front wheels begin to slip (rotate with 1-4% higher
speed than the rear wheels) also the rear 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 valves
The axle drives are equipped with by-pass
valves. Each by-pass valve is connected to
their clutch release lever. When the by-pass
valve (10) is open, it allows oil to flow into
the motor housing and the pressure drop
over the motor is such neutralized. The by-
pass valve is intended to make it possible to
push the machine without heavy resistance
from the axle drive.
The front by-pass valve is equipped with a
mechanic interlock which always resets the
valve, if previously opened, at driving
attempts forwards
An attempted to drive the machine forwards with the rear
by-pass valve (8) closed and the front by-pass valve (10)
open will result in an powerful oil flow into the front axle
drive housing. Since the leak flow line (11) not are
dimensioned for this flow and the main flow line (13) is
blocked, it will result in a hazardous pressure rise in the
front axle drive housing. This pressure rise forces the oil to
presses out through the sealings and can cause damages.
Chapter
10
8. Rear clutch release lever,
connected to the rear by-
pass valve.
10. Front clutch release lever,
connected to the front by-
pass valve.
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7
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