Understand the principle of pneumatic braking and solve 80% of the problems of maintenance

Cardholders know that the performance of the truck is closely related to its braking system, and the most problem with the motor transmission is that the vehicle cannot drive, but the failure of the braking system often brings irreversible consequences when driving.

For truckers, it is important to have a certain understanding of how the braking system works in their car. This brief description of the principle of the truck gas circuit is summarized hereby, hoping to help cardholders in the maintenance and overhaul of the gas circuit in the future. If there is an inappropriate introduction, please correct and discuss it, learn and make progress together.

The working principle of the pneumatic assisted braking system of common trucks is shown in the figure. The fresh air filtered by the air filter is first compressed and pressurized by the air compressor (air pump), and the gas output by the air compressor is a high-pressure gas with a certain temperature and humidity, which will cause damage to the pump valve if used directly.

Therefore, it is necessary to cool down and filter the water through condenser tubes and dryers. After that, the dry gas is distributed to each air reservoir by the four-circuit protection valve, and the common ones are front axle brake air reservoir, rear axle brake air reservoir, trailer brake air reservoir and auxiliary air reservoir.
 

How the pneumatic assist braking system works in common trucks

When the vehicle is braking, the driver steps on the brake pedal to directly operate the brake valve (brake master cylinder), and the brake valve enters the air pressure signal to the relay valve of each bridge through the connecting pipeline to control the relay valve to connect to supply air to each brake chamber (brake cylinder), and the brake cylinder moves after ventilation and rotates the shoe friction brake drum through the mechanical structure such as the adjustment arm and camshaft to achieve vehicle deceleration and braking.

The air valve parts of the models produced by each manufacturer are different, but the main valve parts of the braking system are the same. The main air pump valves of common trucks are: four-circuit protection valve, foot brake valve, relay valve, quick release valve, differential valve, load sensitive valve, handbrake valve, etc.

▎1. Four-circuit protection valve

In order to ensure the reliability of the vehicle braking system, multi-circuit common braking is generally adopted. When a circuit pipeline breaks and fails and leaks, the four-circuit protection valve can ensure that the air pressure of other circuits is not affected, and the braking system can still work without failure.

During the operation, the compressed air from the air compressor is supplied to the air reservoir of each circuit after the four-circuit protection valve, and the whole vehicle gas circuit is divided into four interconnected and independent circuits.

Under normal circumstances, the four-loop protection valve is actually a five-way joint, which only plays a protective role when a circuit is broken or leaked. The air outlet of the four-circuit protection valve is generally connected to the front axle brake air reservoir, the rear axle brake air reservoir, and the auxiliary air reservoir.

▎2. Foot brake valve

The foot brake valve is the brake master cylinder that card friends often say, and it is the main control device in the air brake system.

When the vehicle is braking, the driver controls the brake pedal and is directly connected with the brake master cylinder, and the brake pedal stroke is used as the input signal, and the air pressure is controlled as the output signal to control the gas flow of the brake cylinder and then determine the size of the braking force. One of the most important characteristics of a foot brake valve is the linear relationship between the input and output.

In the design, some brake master cylinders have a smaller ratio of output and input in the front section and a larger proportion in the rear section, so the driver will reflect that the effect of light braking is not obvious, but heavy pedaling is prone to locking. Of course, the characteristics of air brake are determined by the characteristics of the overall system, and the subsequent relay valves and sub-pumps are also related.

▎3. Relay valve and quick release valve

In the air brake system, the air reservoir, the brake master cylinder and the brake cylinder are all far apart, and the roundabout inflation and exhaust through the pipeline will lead to too long lag time between braking and disbraking, and too slow braking response.

The actuator valve is usually installed on the gas pipeline between the brake master cylinder and the brake cylinder and close to the brake cylinder, and the quick-release valve is installed behind the actuator valve. The relay valve can ensure that the cylinder is quickly inflated during braking, and the quick-release valve can ensure that the cylinder is quickly vented when the brake is released.

▎4. Differential valve

The main function of the differential valve is to prevent the service brake and the parking brake from being used at the same time, and to avoid the overlapping of the forces of the brake chamber and the spring chamber in the double chamber compound brake cylinder, so as to avoid the overload of the mechanical components.

When the handbrake is pulled, if the driver steps on the downward brake at the same time, the differential valve will work, and the compressed air will enter the spring air chamber of the brake cylinder through the differential valve, so as to release the parking brake in time to prevent the mechanical structure from being overloaded. In order to reduce the number of valves in the pneumatic system, the differential valve and the relay valve are usually designed as a combination valve, i.e., a differential brake valve.

5. Load valve

The load sensor valve is essentially a pressure regulator that can change the pressure of the rear axle braking circuit according to the load of the vehicle, which is installed on the frame, and the elastic arm of the load sensor valve is connected with the axle, and the vehicle load is sensed by the bending degree of the axle steel plate.

For freight vehicles, the load of the rear axle is very different when empty and fully loaded, if the rear axle brake air pressure is the same in both cases, it will cause the vehicle rear axle no-load brake locking or full load braking weakness.

When the car is unloaded, the load sensor valve can automatically reduce the braking force of the rear wheels, and when the car is fully loaded, the load valve can automatically increase the braking force of the rear wheels. In order to ensure that the braking force of the front and rear wheels meets the appropriate ratio when the vehicle brakes, and ensure that the rear wheels do not lock up when the car brakes urgently.

▎Afterword

Through the above introduction, cardholders can realize that vehicle air brake is a relatively complex system, and the quality of braking performance is the overall effect of the combination of all valves and mechanical components in the system.

After understanding the working principle of the braking system, the braking system must be maintained reasonably and timely when using the vehicle, and frequent inspection and maintenance is an important means to ensure driving safety and avoid unnecessary losses.