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Introduction to Conventional Brake Valves Basic Knowledge of Emergency Relay Valves
The performance and quality of the valve body on a truck have a direct impact on the vehicle's braking effect, and different types of valves play their respective roles. However, many people may not yet fully understand the functions of the various valves on a truck.
The emergency relay valve, also known as the three-axle synchronization valve, is installed on semi-trailers. Currently, on many vehicles, this is the only valve in the trailer's air circuit.
Currently, there are two mainstream relay valves: WABCO and Haldex. Regarding specific connections, WABCO specifies that port 1 is the air supply port, connected to the red coil hose; port 2 connects to the ABS valve, with no air pressure; port 4 is the control port, connected to the yellow coil hose, with no air pressure; ports 1-2 connect to the trailer air tank, normally with air; and port 3 is the exhaust valve. For Haldex, port 1 is labeled ENERAY, port 2 is labeled DELIVERY, port 4 is labeled SERVICE, and ports 1-2 are labeled RESERVIOR.
The emergency relay valve is used to control the trailer service brakes. If the air supply circuit breaks, the action of the emergency relay valve will automatically apply the trailer brakes. While driving: port 1 normally has air, port 4 has no air; when the brake is pressed: port 4 is filled with air, the piston moves down, and the air pressure from 1-2 passes through port 2 to the ABS valve, causing the brake calipers to work. When parking: port 4 is filled with air, the piston moves down, the air pressure from 1-2 passes through port 2 to the ABS valve, causing the brake calipers to work, just like the braking state. When the pressure in the air reservoir leaks to a certain level, the spring in the spring chamber automatically extends, functioning as a parking brake. When the spiral air pipe accidentally breaks: ports 1 and 4 have no air pressure, and the air pressure from port 1-2 is automatically applied to the brakes through port 2.
National Standard (GB12767—1999) also includes provisions regarding the emergency relay valve. When one of the air supply pipes is disconnected (or other possible connection methods are used) or leaks, the driver must be able to control all or part of the trailer brakes using the service brakes, emergency brakes, or parking brakes.
The above is the air pressure curve during vehicle braking, and the output air pressure of outlet 2 increases linearly with the pressure at control port 4.
The above image shows the air pressure curve when a single port (red spiral tube disconnected) leaks. When the pressure at intake port 1 (red spiral tube) drops to around 2.5 kg, outlet port 2 begins to charge until it matches the pressure of the air reservoir, meaning that when the red spiral tube breaks, the trailer will automatically brake.
● Editor's Note:
Thanks to the introduction by forum truckers, I believe everyone now has some understanding of the emergency relay valve and its working characteristics. In your daily truck life, you not only need to know how to drive but also need to learn more about truck-related knowledge. I also hope there will be more good articles like this for everyone to study and improve.
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