What is the working principle of the air brake system used in freight cars?

The parking brake system of trucks and buses is usually called "air brake". After parking, the driver only needs to pull a valve on the dashboard, and there is a sound of deflation under the car, and then the car stops steadily. Before driving, the car should also "bomb the air pressure", and the car can only start after the air pressure exceeds a certain pressure. So how does this air brake work? Today, we will talk about the structure and working principle of the air brake for freight cars in detail, and how to make the car run without air pressure.

First of all, let's take a look at the working schematic diagram of the common pneumatic braking system. Pneumatic braking system is a dynamic braking system with compressed air as power source, which is often used in trucks and large trucks. The braking force is greater than that of hydraulic braking system, but the system structure is complex and there are many parts. Its working principle is actually very simple. Compressed air is generated by an air pump and stored in an air storage tank. When the car needs braking, the driver controls the brake pedal and opens the control valve of the brake master cylinder, so that the compressed air in the air reservoir enters the brake master cylinder and pushes the brake master cylinder to open outwards, thus pushing the wheel brakes to act and generating braking force to slow down the car until it stops. Nowadays, cars are basically controlled by two pipes with independent front and rear wheels. When one pipeline loses its braking function, the other pipeline can still work normally, so as to prevent the automobile from completely losing its braking force.

In order to make the car more safe and reliable, there are generally four sets of braking devices on the car, namely, the service braking system, the parking braking system, the emergency braking system and the auxiliary braking system. Service brake system and parking brake system? * * * Use a set of wheel brakes, but their control systems are completely separated. When one system fails, the other system can also make the car stop reliably; The emergency braking system is generally the same set of devices as the parking braking system, but there are also some devices connected with it; The auxiliary braking system includes exhaust braking, hydraulic retarder and eddy current retarder. They can enhance the braking force and reduce the load of the braking system to a certain extent, but they can't completely stop the car.

At present, large trucks and buses that use pneumatic braking usually use strong spring parking braking, which is also commonly known as "pneumatic braking". The following figure is the working principle diagram of the common strong spring parking brake system. Let's use this diagram to analyze its working principle. The system has three main components, namely hand brake valve, relay valve and strong spring parking brake. First, let's look at the structure and working principle of these components.

1, hand brake valve

The hand brake valve is essentially an on-off valve, and its interior is actually a two-position three-way valve rod. When the car is running normally, the hand brake valve is in the open state, and the compressed air in the air reservoir leads to the relay valve, so that the relay valve is opened, the compressed air enters the strong spring brake, the strong spring is compressed, and the parking brake is released; When the car stops, the hand brake valve is disconnected, cutting off the pipeline between the gas storage tank and the relay valve. The relay valve is closed, the compressed air is exhausted, and the spring of the powerful spring parking brake pops up, so that the wheels are braked, thus making the car stop reliably.

The hand brake valve is also an emergency brake valve on the car. When the car's service braking system loses its function, operating the hand brake valve can not only stop the car, but also control the braking force of the car at any time. The hand brake valve has the following functions: different handle angles correspond to different relay valve openings, thus controlling the braking force of wheels. When it is completely closed, the maximum parking braking force is generated. Generally speaking, the maximum parking braking force of a car is more than 60% of the maximum driving braking force. Therefore, the parking brake of trucks is very powerful, and it is generally required to park reliably on a slope of 30% under the condition of full load.

2. Relay valve

The essence of the relay valve is an amplifier valve, which only plays the role of controlling the air volume with a small amount of air, but it will not change the braking performance of the car. In addition, due to the long wheelbase of the freight car, the distance between the brake master cylinder and the brake master cylinder is far away, and the air capacity of the rear axle brake master cylinder is relatively large. If the brake master cylinder is directly controlled, the braking of the car will be reversed. The response time will be too long and the braking distance will be too long. Therefore, setting the relay valve closest to the rear brake cylinder, directly supplying air from the air reservoir through the rough air pipe, and controlling the opening and closing of the relay valve through the thin air pipe on the brake master cylinder can greatly shorten the braking response. In view of time, it has played the role of "fast charge and fast release". Its working principle is shown in the following figure. The number "1" on the interface represents the air inlet, "2" represents the air outlet, "3" represents the air outlet, and "4" represents the control air inlet.

3. Strong spring parking brake

Powerful spring parking brake is the core and most complicated part of the system. If the hand brake valve and relay valve are control elements, then it is an actuating element. It is a complex with dual functions of service braking and parking braking. The service brake air chamber and the parking brake air chamber are separated by partitions and controlled by the service brake control pipeline and the parking brake control pipeline respectively. They operate independently, but use a set of driving mechanisms.

The specific structure and working principle of the powerful spring parking brake are as follows. Generally speaking, it is divided into two parts. The front part is the service brake air chamber, and the brake piston retracts under the action of the spring under the normal driving state, and no braking force is generated; When the brake pedal is depressed, compressed air enters the rear of the service brake piston in the front air chamber, pushing the push rod to extend outward, and then pushing the wheel brake to act, generating braking force. The second half is the parking brake, which has a strong spring (elasticity is about 5000N). When the car stops, the powerful spring stretches out to push the service brake piston outward and push the wheel brake to generate braking force, which is about 60% of the maximum braking force of the car. When the car runs normally and the air pressure is sufficient, the hand brake valve is opened, and compressed air enters the parking brake air chamber in front of the strong spring through the relay valve, which compresses the strong spring to the limit position, thus releasing the parking brake.

The working process of the whole system is as follows: start the car, the air pressure of the braking system reaches above 0.65MPa, release the hand brake valve, and compressed air enters above the piston of the relay valve to make the piston go down and open the relay valve; Waiting for the compressed air at the air inlet of the relay valve to instantly pass through the relay valve, enter the parking brake air chamber of the powerful spring parking brake from the air outlet, and compress the powerful spring to the limit position, thereby releasing the parking brake; When parking, the driver pulls the hand brake valve, and the compressed air above the piston in the relay valve is discharged from the hand brake valve. Then the relay valve is closed under the action of the spring and the exhaust port is opened. The compressed air in the parking brake air chamber is discharged from the exhaust port of the relay valve, and the strong spring pops up to push the wheel brake to brake.

If the service braking system fails during driving, the parking braking system will act as an emergency braking system. The driver pulls the handle of the manual brake valve to control the pressure of the piston entering the relay valve, and then control the opening of the relay valve, so that the pressure of compressed air entering the parking brake air chamber is not enough to completely compress the powerful spring. At this time, the powerful spring partially extends to push the wheel brake to generate partial braking force, thus slowing down the car. When the hand brake valve is completely closed, the strong spring is completely extended and the emergency braking force reaches the maximum. In addition, if the brake system of the automobile has pressure or no pressure at all due to pipeline leakage, the parking brake spring is completely pushed out at this time, so that the automobile can be reliably parked in the same place, so this kind of "gas cutting brake" is also called the safest brake system.

Then, if the automobile engine fails, or there is no pressure in the braking system for some reason, at this time, the strong spring is always in the stretching position, and the wheel maintains the maximum braking force. At this time, if you need to start or trailer, how to release the parking brake? In this case, we only need to unscrew a screw behind the parking brake air chamber. The front end of the screw is connected with the parking brake piston. When turning outwards, you can compress the strong spring, and then release the thrust of the strong spring on the wheel brake to loosen the brake. It should be noted that before unscrewing the screw, the wheels must be covered to prevent personal injury caused by the movement of the car at the moment when the parking brake is released.

The most common fault of strong spring parking brake system is air leakage. It should be noted that in many cases, the leaked parts are not necessarily faulty parts. We should analyze which part is out of order according to the working principle of this system. For example, the air leakage at the exhaust port of the hand brake valve is often not the hand brake valve is broken, but the internal seal of the relay valve is damaged, and the air leakage flows into the hand brake valve and is discharged from the exhaust port. In this case, replacing the hand brake valve cannot eliminate the fault, and the relay valve needs to be replaced. In addition, when the hand brake valve is released, air will leak from the exhaust port of the master brake cylinder, but it will not leak when the hand brake valve is pulled. This situation is not the fault of the master brake cylinder, but the internal air leakage of one of the master brake cylinders. When the hand brake valve is released, we can put the screwdriver on the brake cylinder and hear the sound of air flowing in the brake cylinder, which means that the brake cylinder is leaking and can be replaced.

In addition, in the northern winter, if the water in the air reservoir is not released in time, the water content in the brake system will be too high, which will also lead to the freezing of the brake system pipeline, which will lead to the failure of parking brake release. This kind of fault is very troublesome, because there are many fault points, each pipe joint may freeze, and it is impossible for us to open all the pipes for inspection, so in this case, we usually drive the car to a warm warehouse to thaw. I hope the ice has melted. If there is no warm warehouse, it will be a great luck. Remove the pipe joint step by step until you find the frozen part. Therefore, before the car enters the winter, it is necessary to replace the drying tank, clean the water in the gas storage tank, and insist on draining water every day during use to prevent the brake pipeline from freezing.