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.