DAF LF45, LF55 Series. Manual — part 447
©
200436
2-9
Description of components
OPERATION OF BRAKE COMPONENTS
ΛΦ45/55 series
6
3
Empty/load relay valve
In rest position, relay piston 4 is in its upper
position and connecting point 2 (brake cylinders
on front axle) is bled via connecting point 3.
When the foot brake is applied, the relay piston is
forced downwards via connecting point 41, thus
opening valve 5. At connecting point 2 pressure is
built up until a set value is reached. Relay piston
4 is then once again forced upwards until there is
a state of equilibrium.
Air has also entered simultaneously via
connection point 42 (load sensing valve). This will
force piston 6 to the left. Through a bore in piston
6 the pressure now also reaches the central
surface of the relay piston (4). This pressure will
depend on the loading of the rear axle. As a
consequence, the output pressure of this valve is
in part dependent on the braking pressure of the
rear axle.
The input pressure at connecting point 41 is also
applied to the left-hand side of piston 6, via two
openings. If no pressure enters via connecting
point 42, due to a fault, piston 6 will be forced to
the right. The pressure at connecting point 41 will
now also reach the central surface of relay piston
4. In this situation, the valve simply operates as a
relay valve, and will no longer reduce.
When the foot brake is released, the pressure at
connecting points 41 and 42 will disappear. Relay
piston 4 will be forced upwards by the pressure
beneath it, thus opening the bleed system.
1
2
3
42
41
R600493
4
6
5
1
2
3
42
41
R600494
OPERATION OF BRAKE COMPONENTS
2-10
©
200436
Description of components
3
ΛΦ45/55 series
6
2.7 LOAD SENSING VALVE, AIR SUSPENSION
Purpose
Automatic control of the brake pressure is
dependent on the pressure in the bellows and
therefore on the load condition of the vehicle.
Thanks to the integrated relay valve, the brake
cylinders are aerated and bled quickly.
Operation
The control valve is activated by the pressure of
the left and right bellows via connecting points 41
and 42. The actuated piston (i) that moves
against the pressure of the spring (j), brings the
tappet (g) to a position that corresponds to the
load condition. The calculated average of the
bellows pressure on the left and right is the
determining factor in this.
The compressed air provided by the foot brake
valve flows via connecting point 4 into space A,
pushing piston b to the left. Outlet "d" is closed
and inlet "m" is opened, causing compressed air
to enter space C to the left of diaphragm "e".
Relay piston "f" is operated via duct F and
chamber G.
43
41
42
3
2
1
4
E
C
C
F
D
A
G
B
a
n
o
m
d
g
i
j
f
e
l
k
h
c b
R600455
r
s
©
200436
2-11
Description of components
OPERATION OF BRAKE COMPONENTS
ΛΦ45/55 series
6
3
At the same time, compressed air flows through
the open valve (a) and duct E into space D to the
right of diaphragm "e". Due to this control, the
output pressure at partial load and low control
pressures is increased to max. 1.4 bar). If the
control pressure increases further, piston "n" is
moved to the left against the pressure of spring
"o" and valve "a" closes.
As pressure builds up in space G, relay piston "f"
is pressed downwards. Outlet "h" closes and inlet
"k" opens. The air at connecting point 1 now flows
to the brake cylinders via connecting point 2.
Now pressure will start to build up in space B
under relay piston "f". As soon as this pressure is
somewhat higher than that in space G, the piston
is pushed upwards and closes inlet "k".
When piston "b" is moved to the left, the vanes (l)
attached to it will gradually loosen the diaphragm
(e) from the fixed vanes in the valve housing. As
a result, the effective diaphragm surface will
gradually increase. As soon as the force of the air
to the left of the diaphragm exceeds that to the
right, piston "b" will move to the right. The inlet
(m) will be closed and a set position is reached.
43
41
42
3
2
1
4
E
C
C
F
D
A
G
B
a
n
o
m
d
g
i
j
f
e
l
k
h
c b
R600455
r
s
OPERATION OF BRAKE COMPONENTS
2-12
©
200436
Description of components
3
ΛΦ45/55 series
6
The position of tappet "g", which depends on the
position of piston "i", is indicative of the effective
diaphragm surface and therefore of the output
brake pressure.
The position of tappet "g" determines to what
extent piston "b" must be moved with the vane
disc (l) to allow the valve to build up pressure.
Due to this movement, the effective surface of the
diaphragm will alter.
In full-load position, this surface and that of piston
"b" are equally large. The control pressure at
connecting point 4 is therefore let through (ratio
1:1) to spaces C and G. The output pressure at 2
will now be equal to the control pressure at
connecting point 4.
If the pressure decreases at connecting point 4,
piston "b" will be pushed to the right by the
pressure in space C. Bleed vent "d" will open and
the pressure in spaces C and G will fall. The relay
piston will be pushed up due to the pressure still
present in space B, causing bleed vent "h" to
open. The pressure at connecting point 2 will now
fall via bleed vent 3.
A stop bolt in front of tappet "g" ensures that this
valve can always provide the minimum brake
pressure if the bellows pressure delivered falls
below the minimum effective pressure due to a
fault. The factory setting of this bolt must not be
changed.
The simulation connection (43) is for controlling
the valve. By connecting an air hose to it, the
bellows will be pneumatically closed, allowing the
valve to be operated with a random test pressure.
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