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Introduction to Common Brake Valves Basic Knowledge of Emergency Relay Valves
The performance and quality of the valve body on a truck directly affect the vehicle's braking performance, with different types of valves serving their respective functions. However, many people may not be very familiar with the roles of the various valves on a truck. The following is some information about the emergency relay valve, compiled and shared for everyone.
The emergency relay valve, also known as the tri-axle synchronizing valve, is installed on semi-trailers. Currently, on many vehicles, this is the only valve in the trailer's air system.
Currently, there are two mainstream relay valves: WABCO and Haldex. As for the specific interfaces, WABCO specifies that port 1 is the air source port (connected to the red spiral hose), port 2 is connected to the ABS valve, with no air pressure, port 4 is the control port (connected to the yellow spiral hose), also with no air pressure, ports 1–2 are connected to the trailer air tank and normally have 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 driving brake. If the air supply circuit breaks, the operation of the emergency relay valve will cause the trailer to automatically brake. While driving: port 1 usually has air, port 4 has no air; when the brake is pressed: air flows into port 4, the piston moves downward, and the air pressure from 1-2 passes through port 2 to the ABS valve, causing the brake wheel cylinders to work. When parked: air flows into port 4, the piston moves downward, and the air pressure from 1-2 passes through port 2 to the ABS valve, causing the brake wheel cylinders to work, which is the same as when the brake is pressed. When the air pressure in the air reservoir leaks to a certain extent, the spring in the spring chamber automatically pushes out, functioning as a parking brake. When the spiral air hose is accidentally broken: ports 1 and 4 have no air pressure, but the air pressure from 1-2 is directed through port 2 to automatically apply the brakes.
The national standard (GB12767—1999) also has provisions regarding the emergency relay valve. If one of the air supply lines (or other possible connection methods) is broken or leaks, the driver must be able to use the driving brake, emergency brake, or parking brake control device to activate the trailer brakes fully or partially.
The above is the air pressure curve during vehicle braking, where the output air pressure from outlet 2 increases linearly with the pressure at control port 4.
The above diagram shows the air pressure curve when one unit (red spiral tube disconnected) is leaking. When the pressure at intake port 1 (red spiral tube) drops to around 2.5 kg, outlet 2 begins to fill to match the pressure of the air reservoir, which means that when the red spiral tube breaks, the trailer will automatically brake.
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