Introduction service brake:
The service brake is used to reduce the speed of the vehicle and bring it to a standstill. The service brake is part of the “air‑consuming” section, which is colored yellow in the image below. On the page: basic air brake system, the different parts are explained that are indicated below with colors (blue, yellow and green).
The service brake comprises the following components: the foot brake valve, the load‑sensing control valve (ALR), the relay valve and the diaphragm cylinders on the front brake calipers and rear brake calipers or brake drums.
1. Air compressor
2. Dryer with wet reservoir
3. Four‑circuit protection valve
4. Air reservoir circuit 3
5. Air reservoir circuit 1
6. Air reservoir circuit 2
7. Foot brake valve
8. Load‑sensing valve (ALR)
9. Diaphragm cylinders (front)
10. Diaphragm cylinders (rear)
11. Spring brake cylinders (rear)
12. Check valve
13. Parking brake valve
14. Relay valve
- Blue: air supply
- Yellow: air consumption
- Green: parking brake / emergency brake
In the four‑circuit protection valve (3) the brake circuits are separated from each other. In the image below the brake circuits are shown in colors:
- circuit 1 (red) is the brake circuit of the rear wheels;
- circuit 2 (blue) is the brake circuit of the front wheels;
- circuit 3 (green) is the brake circuit of the auxiliary brake or parking brake;
- circuit 4 is the brake circuit of the trailer or semi‑trailer (not shown).
From the four‑circuit protection valve (3) the service brake circuits of the front and rear brakes are separated from each other. The air reservoirs (5 and 6) serve as storage to allow repeated braking in quick succession. The service brake is operated with the foot brake valve (7). The foot brake valve allows graduated (proportional) braking.
From the foot brake valve, the brake pressure is directed to the ALR (load‑sensing brake valve, number 8). Here, the delivered air pressure is adjusted to the load of the vehicle. The ALR sends the air pressure to the relay valve (9), in which the air pressure from the ALR determines how much air pressure from reservoir (6) is admitted to the diaphragm cylinders (11) of the rear brakes.
Foot brake valve:
With the foot brake valve, the driver operates two service brake circuits. These circuits are separated independently of each other and can be controlled in small steps (graduated). After braking, the diaphragm cylinders are vented via the foot brake valve.
The symbol shows the inlets and outlets of the foot brake valve:
- 11 and 12: inlet 1 and inlet 2;
- 21 and 22: outlet 1 and outlet 2.
The image below shows the components in the foot brake valve. The explanation follows underneath.
Legend
Components:
1. foot brake pedal
2. operating piston
3. spring plate
4. upper piston sleeve
5. chamber A
6. middle piston sleeve
7. chamber B
8. control piston
9. chamber C
10. lower piston sleeve
Connections:
- 11: inlet 1
- 12: inlet 2
- 21: outlet 1
- 22: outlet 2
The following text refers to figure 1 (above) and to figures 2 and 3 (below the text). The red color indicates the supplied air pressure from the reservoirs, and blue the atmospheric pressure.
Figure 1:
In the rest position, the middle and lower piston sleeves (6 and 10) seal the inlets of the foot brake valve. No flow of air pressure takes place from the inlet to the outlet. The springs (the cross‑sections of the springs are shown as white circles) push piston sleeves 4, 6 and 10 into their rest position.
Figure 2:
The driver operates the brake pedal (1) with the foot. The operating piston (2) is thereby actuated mechanically and moves downward. The spring plate (3) and the piston sleeves (4 and 6) are also moved downward. A passage is created between inlet 11 and chamber 5. Because the piston sleeves are round, the air flows around them to outlet 21. In this way, pressure is initially built up in brake circuit 1. The reason that the pressure to the rear axle rises a few tenths of a second faster than the pressure to the front axle is that there are usually more valves in the circuit to the rear axle. By actuating these valves a little earlier, the brakes on the front and rear axle will still start braking at the same time.
Figure 3:
The air pressure in circuit 1 of the rear axle acts in chamber B (number 7 in the figure above), which pneumatically pushes the lower control piston (8) downward. This means the air flow to circuit 2 is not controlled mechanically but pneumatically. The incoming air pressure from the reservoir enters via connection 12 and is passed on to connection 12.
Circuit 2 is operated pneumatically when sufficient pressure has built up in circuit 1. If there is a leak in circuit 1, no compressed air flows into chamber B. The driver will have to press the brake pedal deeper so that the middle piston sleeve (6) contacts the control piston (8), so that the braking movement is still transmitted mechanically to circuit 2.
After the brake pedal is released, the piston sleeves are pushed back up by their spring force. Circuits 1 and 2 are closed again. The remaining air pressure in chambers A, B and C flows to the outside via connection 3 of the foot brake valve. This prevents a residual pressure from still causing an unwanted pressure in the diaphragm cylinders, which would result in dragging brakes.
Automatic load‑sensing control valve (ALR):
The brakes on the rear axle are not supplied directly from the foot brake valve.
The weight on the rear axle determines the maximum braking force, so the braking force must be matched to the load. High brake pressure on an unladen rear axle carries a high risk of wheel lock, and low brake pressure on a laden axle barely slows the vehicle down.
On the pneumatic ALR we see the following connections:
- 1 and 2: inlet and outlet
- 41 and 42: control (connections air bellows)
The ALR (number 8, the automatic load‑sensing brake pressure regulator in the overview image) measures the load in one of two ways:
- mechanically: this type of ALR is mainly used on older vehicles or vehicles with leaf springs. By means of a lever that is connected to the chassis with a cable or rod, the height of the vehicle relative to the wheel suspension is measured. On a laden axle the leaf springs deflect and the lever moves downward;
- pneumatically: the air pressure in the air springs is measured by the pneumatic ALR. On a loaded vehicle the load, and thus the bellow pressure, increases.
The ALR sends the air pressure, adjusted to the load, to the diaphragm cylinders of the rear brakes.
Diaphragm cylinder:
A diaphragm cylinder is a type of brake cylinder. The energy supplied by the air pressure is converted into motion in the diaphragm cylinder. By means of a lever this motion is converted into a force that presses the brake lining against the brake disc or brake drum.
The force with which the brake lining is pressed on depends, among other things, on the lever ratio and the air pressure in the diaphragm cylinder.
In the image alongside we see two diaphragm cylinders:
- left: diaphragm cylinder of the front axle;
- right: diaphragm cylinder with the spring brake cylinder of the rear axle mounted underneath.
The spring brake cylinder is discussed on the page: auxiliary and parking brake.
Next to the images of the diaphragm cylinders we see the symbol.
In the next image we see a cross-section of a diaphragm cylinder. The compressed air from the foot brake valve or relay valve enters at number 1 into the chamber to the left of diaphragm (2). The air pressure pushes the diaphragm to the right against the spring force of spring (3). The push rod follows this movement and tilts lever (5) around its pivot. At that moment braking occurs.
When the brake is released, the air pressure to the left of the diaphragm drops and the spring pushes the diaphragm and the push rod back into the rest position.
A diaphragm cylinder on the rear axle works according to the same principle, but in the housing there is also a spring brake cylinder which provides the parking brake.
Legend:
1. air connection
2. diaphragm
3. spring for rest position
4. push rod
5. lever
The animations below show the outward stroke of the diaphragm cylinder that operates the lever, and an S-cam that, when rotated, presses the brake shoes against the brake drum. Note: both animations do not run in sync. In reality, during the outward stroke of the diaphragm cylinder, the S-cam will move the brake linings against the drum.
Related page: