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Principle and Function of Heavy Truck Follow-up Valve
The relay valve is a part of the automotive air brake system. The air inlet of the relay valve is connected to the air reservoir, and the outlet is connected to the brake chamber. When the brake pedal is pressed, the output air pressure of the brake valve serves as the control pressure input for the relay valve. Under the action of the control pressure, the inlet valve is pushed open, allowing compressed air to flow directly from the air reservoir through the inlet into the brake chamber, without passing through the brake valve. This greatly shortens the air supply line to the brake chamber and accelerates the charging process of the air chamber. Therefore, the relay valve is also called an accelerator valve. The relay valve is used at the end of long pipeline routes to allow the compressed air from the air reservoir to quickly fill the brake chamber.
Application of Relay Valve
For example, in the brake system of trailers or semi-trailers. In the brake system of heavy-duty trucks, the relay valve helps shorten the reaction time and the pressure build-up time. Generally, a differential relay valve is used. It prevents the simultaneous operation of the service brake and parking brake systems, avoiding the overlapping of forces in the combination spring brake cylinder and spring brake chamber, thereby preventing overloading of mechanical transmission components and allowing the spring brake cylinder to quickly charge and exhaust air.
Generally, a differential relay valve is used. It prevents the simultaneous operation of the service brake and parking brake systems, avoiding the overlapping of forces in the combination spring brake cylinder and spring brake chamber, thereby preventing overloading of mechanical transmission components and allowing the spring brake cylinder to quickly charge and exhaust air.
Differential Relay Valve
1. Purpose: To prevent the overlapping of forces in the combination spring brake cylinder and brake chamber (spring brake chamber) when the service brake and parking brake systems operate simultaneously, thereby avoiding mechanical transmission component overload and allowing the spring brake cylinder to quickly charge and exhaust air.
2. Working principle: In the driving state, under driving conditions, the hand brake valve continuously supplies air to chamber A through port 42. Pistons (a) and (b) are pressed downward, closing the exhaust valve (e), and pushing the valve stem (c) downward, opening the intake valve (d). Compressed air from the air reservoir enters through port 1 and exits through port 2, providing compressed air to the spring brake chamber connected to port 2, thus releasing the spring brake.
When the service brake system operates independently, controlling the main brake causes compressed air to enter chamber B through port 41, pushing piston b downward. Due to the reactive forces from chambers A and C, the pressure reaching chamber B has no effect on the operation of the differential relay valve. The compressed air continues to flow to the spring brake chamber's spring brake section, thereby releasing the brake. At the same time, the compressed air directly from the tractor brake valve acts on the diaphragm portion again.
When the parking brake system operates independently, controlling the hand brake valve causes chamber A to be completely vented. Piston (a) is not subjected to pressure, and piston (b), exposed to the air pressure in the C chamber reservoir, is pushed upward, opening the exhaust valve (e). At the same time, the valve stem (c) rises, closing the intake valve (d). In this way, according to the position of the hand brake lever, the spring brake cylinder releases gas through port 2, valve stem (c), and exhaust valve 3, thus applying the spring brake.
During partial braking, the exhaust valve (e) closes after exhaust, and the air pressure in chambers A and C balances, keeping the differential relay valve in a balanced position. However, during full braking, the air inlet (d) continues to open. When both the service brake and the spring brake operate simultaneously: A. When the service brake exhausts—that is, when the spring brake cylinder operates—compressed air enters chamber B through port 41, acting on piston (b). Since chamber C is exhausted, piston (b) moves downward, closing the exhaust valve (e) through the valve stem (c) while opening the inlet valve (d). The compressed air from port 1 passes through chamber C to reach port 2 and enters the spring brake chamber.
The spring brake is released according to the increase in service brake pressure, thereby avoiding the overlapping effect of the two brakes. When the pressure at port B rises above the pressure in chamber B, the pressure in chamber C pushes piston (b) upward, closing the inlet valve (d) and bringing the differential relay valve to a balanced state.
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