DAF 95XF. Manual - part 125

Description of components

DESCRIPTION OF BRAKE COMPONENTS

1-37

1.17 PARKING-BRAKE VALVE

PARKING-BRAKE VALVE WITH
(SEMI-)TRAILER CONNECTION

Purpose
The parking-brake valve enables simultaneous,
controlled operation of both the parking-brake
system of the tractor and the (semi-)trailer
brakes.

Operation
The parking-brake valve has 3 positions:
A:

driving position

parking position

test position

Driving
With the handle in the driving position, there is a
through connection for the reservoir pressure
(port 1) to the connections for the spring-brake
cylinders (21) and the (semi-)trailer (22). The
exhaust is closed now.
The output pressure at ports (21) and (22) is
now approx. 8 bar (see graph).

Emergency brake
When the handle is moved backwards against
the spring pressure, stem (3) will be forced
downwards by eccentric (2). Chamber (a) can
now be vented and as a result the pressure at
port (22) will drop. Via the bore in valve (10) the
pressure at port (22) will also drop. Spring (4)
forces piston (5) down until valve (6) comes into
contact with the seal collar of stem (3). A state
of balance is now achieved.

When the handle is moved against stop (7), the
exhaust will remain open, so that the spring
brakes and the (semi-)trailer brakes will be
applied to their maximum (max.
emergency-brake position).

R600089

2
3

R600092

ǹ 0006

DESCRIPTION OF BRAKE COMPONENTS

Description of components

1-38

Parking brake
When the handle is pulled past stop (7), it is
locked in position.
Ports (21) and (22) will remain pressureless, so
that the spring brakes and the (semi-)trailer
brakes are still applied to their maximum.

Test position
When the handle is moved beyond the parking
position, cam (8) will move stem (9) downwards,
causing the bore in valve (10) to be closed and
the valve to be raised from its seat.
The reservoir pressure can now be passed to
port (22) via a bore in piston (5). As a result, the
(semi-)trailer brakes will be released. Port (21)
remains vented, so that the spring brakes keep
the brake shoes applied.
The combination is now braked only by the force
exerted by the spring-brake cylinders on the
towing vehicle. This will enable the driver to test
whether the combination can be held when the
trailer brakes are not applied. When the handle
is released, it will automatically return to the
parking position.

Releasing the brakes
When the handle is once again moved fully
forwards, stem (3) will move upwards, against
valve (6), and will push the valve from its seat in
piston (5). As a result, the reservoir pressure
can reach ports (21) and (22). The pressure in
chamber (a) returns to approx. 8 bar.

R600093

P1 = 8 bar

6.5

+0.7

° 85°

P(bar)

R 600099

ǹ 0006

Description of components

DESCRIPTION OF BRAKE COMPONENTS

1-39

1.18 FOUR-CIRCUIT SAFETY VALVE

KNORR design without circuit 3
reverse flow function

Purpose
The purpose of the four-circuit safety valve is to
split up the brake system into four parallel
circuits, and at a failure of one circuit, to protect
the remaining circuits against full venting.

Operation
The opening pressure is the pressure applied at
port (1) for opening valves (8), (9), (10) and (11)
when the brake system is not pressurised.
The opening pressure is determined by
diaphragm surface (a) and by the force of the
spring acting on the diaphragm.

The static closing pressure is the pressure in the
intact circuits whereby the valves in these
circuits are forced onto their seats, if an air
leakage occurs in a faulty circuit, and the
compressor fails to refill the system.
The static closing pressure is determined by
diaphragm surface (a) and (b) and by the force
of the spring operating on the diaphragm.

R600043

ǹ 0006

DESCRIPTION OF BRAKE COMPONENTS

Description of components

1-40

Four circuits are connected to the valve, i.e.
circuits 1, 2, 3 and 4.
Circuits 1 and 2 (ports 21 and 22) supply the
service brake at the rear and front axle. Circuit 3
(port 23) supplies the parking brake and
(semi-)trailer brake. Circuit 4 (port 24) supplies
the other compressed-air consumers.
Compressed air enters the valve via port (1) and
travels via the three small by-pass valves (5),
(6) and (7) into the system. At the same time
pressure builds up under valves (8), (9), (10)
and (11). When a predetermined pressure (the
opening pressure) has been reached, these
valves will open so that the diaphragms are
lifted, against the pressure of the adjustable
springs. The compressed air can now flow into
the four circuits unobstructed.
It should be remembered that circuits 1, 2 and 4
have a lower opening pressure than circuit 3.
See main group “Technical data”.
If, for example due to a leakage or line fracture,
a circuit fails, the pressure in the other circuits
will first drop until the dynamic closing pressure
of the faulty circuit is reached. The closing
pressure cannot be precisely indicated, because
it depends on the speed at which the pressure
drops. That it why it is called “dynamic” closing
pressure.
Subsequently, the intact circuits are refilled to
the opening pressure of the faulty circuit.
The purpose of the three by-pass valves in
circuits 1, 2 and 4 is to refill a pressureless
brake system up to the opening pressure of the
faulty circuit after the circuit with the lowest
opening pressure (in view of tolerances) has
failed. This is achieved as follows.
Via port (1), compressed air from the
compressor is applied to the underside of valves
(8), (9), (10) and (11), which remain closed for
the time being by the springs. These circuits
receive a (limited) supply of compressed air
through by-pass valves (5), (6) and (7). As a
result a slight pressure builds up in the intact
circuits, and therefore under the diaphragms.
This will cause the opening pressures of the
intact circuits with a by-pass valve to drop to a
value below the pressure of the faulty circuit. As
a result, the intact circuits will first be refilled up
to the opening pressure of the faulty circuit.

R600044

ǹ 0006

Content .. 123 124 125 126 ..