DAF 95XF. Manual - part 450

HYDRAULIC GEARBOX CONTROL (HGS)

General

2-2

Engaging gears
To engage the gears, move the gear lever
forwards or backwards.
In the neutral position, the front and rear
plungers in the gear lever unit (3) are in the
same position (balanced), while the gear
engaging cylinder (1) on the gearbox is also in
the central position.
To engage a gear, move the gear lever from the
neutral position forwards or backwards.
The front and rear plungers in the gear lever unit
(3) then move in opposite directions. This
creates a fluid flow in the circuit of the gear
engaging cylinder (1).

This fluid flow moves the piston shaft in or out of
the gear engaging cylinder, causing the selector
shaft to rotate and engaging the correct gear.

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Description of components

HYDRAULIC GEARBOX CONTROL (HGS)

3-1

3. DESCRIPTION OF COMPONENTS

3.1 OPERATION OF GEAR LEVER UNIT

General
Inside the gear lever unit you will find four
plungers with different diameters to compensate
for the varying diameters on the left- and
right-hand sides of the plunger in the gate
selection and gear engaging cylinders.
As a result of this compensation, the displaced
volumes at the gear lever unit and the gate
selection and gear engaging cylinders are
identical on both sides.
In the central section of the unit, an operating
reservoir (F) connected to the cylinders is
located. The connector holes (A) have a
diameter of 0.6 mm. This diameter has been
kept as small as possible in order to reduce
wear on the sealing cups (B) of the pistons.
These connection holes have a twin function.
When a piston moves upwards, any absent
liquid is passed on to the piston via a duct (C)
and a valve (D).
The other purpose of the holes is to permit air to
escape during the filling/bleeding of the system.
In the gate selection cylinders, an extra hole (G)
has been bored. Via this second hole, fluid can
always be topped up, even if the lower hole
remains closed by the sealing cup (B). The
stroke for gate selection is extremely small
(approx. 4 mm). For the gear engaging cylinder
operation cylinders, a single hole is enough,
because these pistons make a stroke large
enough to always free the lower hole.
During the compression stroke, the delivery (C)
is blocked by the closure of the duct, by the
compression valve (D). If the piston is still
located above the lower hole (A), fluid will flow
back into the operating reservoir (F). Once the
hole has been passed by the rubber sealing
cup, pressure will be built up, and the gear
engaging or gate selection cylinder will be
operated via the pneumatic pipes.

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D

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HYDRAULIC GEARBOX CONTROL (HGS)

Description of components

3-2

Pressure-relief valve
Inside the gear lever unit, there is a
pressure-relief valve (E) for each dual-operation
circuit.
These pressure-relief valves prevent an
undesirable build-up of pressure within the
system as a result of temperature increases.
The pressure-relief valve is located between the
two plungers which belong to a single
dual-operation circuit.
The connection between the right-hand cylinder
and the internal reservoir is closed by a ball.
This ball (2) is pressed into its seat by a
spring (3).

The ball will be pressed from its seat if the
pressure within the system builds up equally on
both sides and exceeds a particular limit.
The piston (1) affected by this pressure will
move to the right and press the ball from its
seat.

As a result, the fluid can only flow from the
right-hand plunger towards the internal reservoir.
This will cause the pressure under the
right-hand plunger to drop.
The left-hand plunger will move up and the
right-hand plunger down, until the pressures
have stabilised.
As a result, the gear lever will be displaced
slightly. The pressure-relief valve has been
placed in such a position that the gear lever
always moves against the weight of the gear
lever:
-

towards the rear in the gear engaging
direction;

towards the front in the gate selection
direction.

The displacement of the lever as a result of the
operation of the pressure-relief valves will not be
noticed in practice, because the system is
self-correcting during gear engaging operations.

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Description of components

HYDRAULIC GEARBOX CONTROL (HGS)

3-3

Bottom springs
Underneath the plungers in the gear lever unit,
there are coil springs (L) which ensure that the
displacement of the gear lever is restricted in
the event of internal/external leakage and failure
of the internal stops.
The self-correction action of the system will
reset the neutral position after a number of gear
engaging actions.

Internal stops
An internal stop (1) has been fitted on top of the
gear lever unit.
This stop ensures that the gear lever remains in
the desired position in the event of minor
internal/external leaks in the system.
This is achieved by means of two plastic blocks
with recesses.
These two plastic blocks are separated by a
spring-loaded ratchet.

If the internal stop should fail, the movement of
the gear lever will be restricted by the bottom
springs inside the gear lever unit.

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