DAF LF45, LF55 Series. Manual - part 448

©

 200436

2-13

Description of components

OPERATION OF BRAKE COMPONENTS

ΛΦ45/55 series

6

3

2.8 LOAD SENSING VALVE, LEAF SUSPENSION

Purpose
Automatic control of the brake force depends on 

the deflection of the springs and therefore on the 

loading condition of the vehicle. Thanks to the 

integrated relay valve, the brake cylinders are 

aerated and bled quickly.

Operation
The control valve is attached to the chassis and 

connected to the rear axle by means of a rod.

With unladen vehicles, the distance between the 

regulator and the axle is largest and the lever (j) 

points fully downwards. When the vehicle is 

loaded, this distance decreases and the lever 

moves upwards, towards full load position.

Pin i rotates at the same time as the lever and as 

a result thereof moves to the right via the control 

groove in bearing cover p. Rod "q" brings the 

tappet (g) in a position that corresponds with the 

loading condition.

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.

3

2

1

4

E

C

C

F

D

A

G

B

a

n

o

m

d

g

q

j

i

f

e

l

k

p

h

c b

R600456

OPERATION OF BRAKE COMPONENTS

2-14

©

 200436

Description of components

3

ΛΦ45/55 series

6

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 fan 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.

The position of the tappet (g), which is dependent 

on the position of lever "j", is indicative of the 

effective diaphragm surface and therefore of the 

output brake pressure.

3

2

1

4

E

C

C

F

D

A

G

B

a

n

o

m

d

g

q

j

i

f

e

l

k

p

h

c b

R600456

©

 200436

2-15

Description of components

OPERATION OF BRAKE COMPONENTS

ΛΦ45/55 series

6

3

The position of the 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 the tappet (g) ensures that 

this valve can always provide the minimum brake 

pressure if lever "j" is in too low a position due to 

a fault. The factory setting of this bolt must not be 

changed.

OPERATION OF BRAKE COMPONENTS

2-16

©

 200436

Description of components

3

ΛΦ45/55 series

6

2.9 ABS VALVE

The ABS valve must keep the pressure constant 

in the brake chamber during an ABS control, or 

decrease the pressure in the brake chamber 

regardless of the pressure leaving the foot brake 

valve.

If the ABS valve is not operative, it has no 

function and the input pressure at connecting 

point 1 is the same as the output pressure at 

connecting point 2 to the brake chamber.

Increasing pressure at connecting point 2
Input pressure at connecting point 1 coming from 

the foot brake valve will lift diaphragm 5 from 

seat 7, causing the brake pressure to be guided 

to the brake chamber via connecting point 2.

The input pressure will also be guided through a 

bore past the magnet coil (10) in space 19 under 

diaphragm 6, causing diaphragm 6 to form a seal 

on seat 8. Connecting point 2 is thus sealed off 

from the bleed vent.

1

2

R600264

6

8

19

10

7

5

2

1

R600629

P  (bar)

t (sec)

2