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2025
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I only learned today how the dual-chamber brake wheel cylinder works.
In China's medium and heavy truck market, there is a wide variety of brake types. Whether it is the air disc brakes popular in Europe and strongly promoted by domestic regulations, the air wedge drum brakes used by manufacturers like FAW Jiefang J7, or the air camshaft drum brakes that have been popular in the domestic truck market for decades, they all share one key feature: air brakes. Compared to the hydraulic brakes commonly used in passenger cars, although air brakes require the installation of an air compressor, air tanks, and various air valves, making the overall structure extremely complex, bulky, costly, and noisy, they provide a much larger braking torque (with an average braking torque per axle above 30,000 Nm and peak braking torque reaching up to 50,000 Nm). This alone ensures the absolute dominance of air brakes in the heavy truck market.
▎What is a brake chamber?
The key component that converts the energy of compressed air into mechanical energy for braking is the brake chamber, also known in the industry as a brake actuator. Our national standard GB7258 strictly requires that every axle must have a braking system.
For conventional 4×2 front-engine, rear-drive two-axle trucks with a total vehicle weight of ≤18 tons, the front steering axle is generally equipped with a single-chamber brake chamber that only has braking function, while the rear drive axle is equipped with a dual-chamber brake chamber that has both braking and parking functions.
According to the national standard GB/T5620 'Road Vehicles — Terms and Definitions for Brakes of Motor Vehicles and Trailers,' a single-chamber brake actuator installed on the front steering axle is called a service brake actuator, which refers to a device in the vehicle's service brake system that provides driving force to the brake using compressed air.
There is another type called a parking brake actuator, which refers to a device in the vehicle brake system that uses mechanical methods such as stored-energy springs to provide driving force to the brake, keeping the parked vehicle stationary.
A dual-chamber brake actuator installed on the rear axle is officially named a spring brake actuator, defined as 'a combined unit of a service brake actuator and a parking brake actuator.'
The dual-chamber brake chamber, also known as the spring brake chamber, can achieve both service braking and parking braking, and is widely used on drive axles and trailer axles. Its structure is slightly more complex compared to a single-chamber service brake chamber. Based on the cross-sectional diagram of the spring brake chamber, we provide a brief analysis of its working principle:
The compressed air ports of the spring brake chamber are clearly marked as '11' and '12' to help us identify the type of port. The '11' port is the service brake port, connected to the service chamber of the brake chamber, while the '12' port is the parking brake port, connected to the parking chamber of the brake chamber.
We need to clarify one concept: service braking is 'air-applied braking,' while parking braking is 'spring-applied braking.' For clarity, we first isolate the functions of the '11' and '12' ports, temporarily separating the operations of the two brake chambers to make it easier to understand.
Service Brake Chamber Activation: Compressed air is input through the “11” port, eventually reaching the service brake chamber. The diaphragm (rubber cup) at the front end of the service brake chamber is acted upon by the air pressure, pushing the push plate and push rod forward, producing braking force. This ultimately causes the push plate/push rod to extend, completing the service brake activation.
Service Brake Chamber Release: Compressed air is cut off from the “11” port. Under the influence of the return spring, the diaphragm (rubber cup) and push plate at the front end of the service brake chamber return to their original positions, releasing the service brake.
Parking Brake Chamber Activation: Compressed air is cut off from the “12” port, allowing the energy storage spring to fully release. The energy storage spring sequentially pushes the energy piston, parking pin shaft, and diaphragm (rubber cup) forward to generate braking force, ultimately causing the push plate/push rod to extend and completing the parking brake activation.
Parking Brake Chamber Release: Compressed air is input through the “12” port, filling the parking brake chamber. The air pressure pushes the energy piston backward, and the parking braking force generated by the air pressure is greater than the spring force provided by the energy storage spring. The energy storage spring is then compressed again to the end of the brake air chamber.
At this point, the parking pin shaft, diaphragm (rubber cup), push plate/push rod, and other transmission components return to their original positions under the force of the return spring, completing the release of the parking brake.
Let's assume a scenario and organize the working states of the brake chamber in the order of 'park → drive → park,' that is: release parking brake → apply service brake → release brake → engage parking brake. The following diagram is drawn to help everyone understand.
In order to avoid the possibility of dual braking caused by the service brake chamber being pressurized while the parking brake chamber is depressurized (the braking torque becomes extremely high when the two types of braking are combined, which can lead to premature overload failure of the brake chamber and all downstream brake components), a differential relay valve is designed in the compressed air pipeline to prevent the simultaneous overlap of the two types of braking.
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