An air relay valve in a truck's air brake system uses a signal from the brake pedal to quickly supply air to the brake chambers, improving braking response time. Here's a breakdown of the process:
Function of the Air Relay Valve
The primary purpose of an air relay valve is to decrease the time it takes for air pressure to reach the brake chambers, especially in longer vehicles like tractor-trailers. Without relay valves, the air would have to travel a longer distance from the foot valve (treadle valve) at the front of the vehicle to the rear brakes, resulting in a noticeable delay.
Working Mechanism
Here's how an air relay valve operates:
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Signal Reception: When the driver presses the brake pedal (treadle valve), a control signal (air pressure) is sent to the air relay valve. This signal is not the main source of air for the brakes; it's simply a trigger.
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Valve Activation: The control signal activates the relay valve. The valve typically contains a diaphragm. The air pressure from the control signal pushes against this diaphragm.
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Air Supply Activation: When the diaphragm is pushed, it opens an internal valve within the relay valve. This valve connects the brake chamber to a local air reservoir.
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Air Delivery: The now-open valve allows compressed air from the local reservoir to flow directly into the brake chamber. Because the reservoir is located closer to the brake chamber than the main air tanks at the front of the vehicle, the air reaches the brakes much faster. This immediate pressure increase forces the brake shoes against the drum, initiating braking. This process will push the diaphragm then the rod, then the slack adjuster. The slack adjuster twists to turn the brake camshaft.
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Brake Release: When the driver releases the brake pedal, the control signal to the relay valve is removed. The internal valve closes, cutting off the air supply from the reservoir to the brake chamber. The air in the brake chamber is then exhausted, releasing the brakes.
Benefits of Using Air Relay Valves
- Faster Brake Application: Significantly reduces brake lag, enhancing safety.
- Improved Braking Performance: More consistent and responsive braking across the entire vehicle.
- Reduced Air Consumption: While a small amount of air is used for the signal, the rapid application reduces the need for sustained high pressure from the main tanks.
Summary
Essentially, the air relay valve acts as a "booster" for the air brake system. It receives a signal from the brake pedal and uses that signal to quickly supply air to the brakes from a local reservoir, making braking more efficient and responsive, especially in large vehicles.
