An Introduction to Semi-Trailer Brake System Design and Emergency Fault Handling

With the rapid development of road transportation, semi-trailer tractors, with their high load capacity, economy, and efficiency, have attracted industry attention and gradually become the main vehicle type for long-distance logistics. Many drivers who are new to semi-trailers lack understanding of the structure, design, and maintenance of these vehicles, and often find themselves helpless when brake system failures occur, sometimes even causing serious accidents. As the saying goes, 'A journey of a thousand miles begins with a single step,' it is actually very necessary for us to study carefully and understand the vehicles we drive.

Liaoning Yingkou CIMC Single-Axle Container Trailer

● The semi-trailer braking system meets the basic requirements

ABS Brake Performance Test

The braking system design of semi-trailers has mandatory requirements. Due to the special articulation characteristics of the tractor-semi-trailer combination, while meeting the basic functions of vehicle deceleration and parking, the following requirements must also be met:

1. The interconnection between the trailer and tractor brake systems, ensuring that the braking actions work in coordination.

2. The sequential coordination between the trailer and tractor brake systems, with braking occurring in the following order: front wheels of the tractor, trailer wheels, and rear wheels of the tractor, to ensure vehicle stability.

3. When the trailer is detached from the tractor, the brake line is cut off; or in emergency cases where the trailer brake line is broken or damaged, the trailer brake system should apply automatically to ensure safety. (Note: this does not refer to the simple concept of spring parking brake activation)

4. When the vehicle combination is fully loaded and in the towed state, it must be able to park safely and reliably on a 16% gradient; the trailer must have a separate parking brake system to ensure braking capability when parked independently from the tractor.

● Diagram of semi-trailer brake air circuit layout

 Currently, domestically manufactured semi-trailers use a dual-pipeline design. In the diagram, ① is the control line, ② is the supply line, and they are connected to the tractor via ③ control air couplings. In practice, the commonly seen air supply line on vehicles is the red spiral hose, while control lines are generally yellow or blue spiral hoses.

1. Air Supply Line

The tractor supplies air through the ② supply line, which passes through ⑤ emergency relay valve to supply air to ⑥ air reservoir, and at the same time, air is supplied to the parking brake secondary cylinder (parking brake air chamber portion of the brake secondary cylinder) through ⑧ manual valve. The parking brake is released when the air in the reservoir exceeds 4 bar.

2. Control Line

When the brake pedal is pressed, air passes through the ① control line, actuating ⑤ emergency relay valve. At this point, the ⑥ air reservoir begins to supply air to the ④ brake secondary cylinders on each axle, and the vehicle starts braking, entering the braking state.

● The important role of the emergency relay valve

Emergency Relay Valve

The emergency relay valve is a core component of the entire trailer braking system. It integrates multiple functions, including those of a relay valve, distribution valve, check valve, and quick-release valve. Trailers equipped with an emergency relay valve significantly reduce braking response time. In fact, the emergency brake valve has more functions than these; it has three main functions:

1. Working brake state during normal braking

Emergency Relay Valve Working Brake Diagram

This is the function we feel most directly. When the brake pedal is pressed, control line ① sends air, and the emergency relay valve ⑤ controls ⑥ air reservoir to supply air to ④ brake wheel cylinders to generate braking force; when the brake pedal is released, the emergency relay valve quickly releases compressed air from each wheel cylinder, rapidly releasing the braking state.

2. Feedback brake status during the air filling process

When the trailer is parked for a long time and there is no air pressure in the brake system, the trailer parking brake system (air-loss brake) comes into effect. When the tractor charges the trailer air tank through the air supply line, the emergency relay valve is in a safety braking state.

In the safety state, the emergency relay valve first sends compressed air to each brake chamber. When the air pressure in the trailer air tank rises to 4.5 bar, the safety state is released. This function is referred to as the feedback braking function. The brake chambers generate braking force while being pressurized, which is a safety protection feature.

3. Automatic braking state in the event of an emergency air-loss failure.

   When the trailer's air supply line ② is accidentally damaged, the compressed air in the trailer's air tank ⑥ enters the brake chamber ④ through the emergency relay valve ⑤, automatically performing braking measures to ensure vehicle safety in emergency situations. When the tractor stops and it is necessary to disconnect the trailer's air supply line ②, the trailer brake chamber can also be seen operating, based on the same principle. ● ABS anti-lock braking system needs to be installed. 

The semi-trailer braking system is divided into two parts: the tractor and the trailer. Traditional braking systems use various purely mechanical components to control the entire braking system and cannot sense the driving conditions of each axle or tire in contact with the ground. In emergency situations, the braking system controls the vehicle using a fixed braking mode, which can be insufficient even with good driving skills.

ABS Anti-lock Braking System Main Control Unit (WABCO)

Happiness comes from safety. Simply put, the ABS system collects complex signals through sensors, processes them with a central processor, precisely controls the ABS hydraulic control valves, and adjusts the braking force distribution of each wheel, constantly sensing the vehicle's braking status. It's like the driver has many assistants helping to control the vehicle. Although it is the same action of pressing the brake pedal, its effect is completely different.

● Precautions for using the automatic brake adjustment arm

Automatic Adjusting Arm 

The automatic adjusting arm was initially widely used in the braking systems of large buses. With the technological upgrades of high-end truck products, products equipped with automatic adjusting arms have gradually come into our view. The automatic adjusting arm eliminates the labor-intensive task of manually adjusting the brake clearance (the gap between the brake drum and brake pads), maintaining consistent braking performance.

   It should be noted that if the brake pads or brake drums are excessively worn, the brake camshaft acting on the brake shoes may not return to its original position, causing 'dragging' or even 'locking,' which can create safety hazards. Therefore, vehicles equipped with an automatic adjuster arm must be strictly maintained and inspected according to the prescribed maintenance standards, and components should be serviced or replaced as needed.

● Requirements of the ABS system for brake drums and brake pads

BPW Asbestos-Free Brake Pads

    The ABS system sensor is installed on a fixed bracket of the axle housing and requires a relatively dry and clean environment to function properly. When choosing replacement brake pads, high-quality products without asbestos should be selected. Some so-called 'semi-metallic' or 'fully metallic' asbestos brake pads produce wear dust that adheres to the surfaces of the brake hub components, leading to failure of the ABS sensor. Brake pads that are too hard can also cause irreparable damage to the brake hub, and the braking system is prone to overheating.

Brake Drum

If the brake drum is out of round or the inner surface is rough and uneven, it can cause severe and uneven tire wear, and the ABS system may not function effectively in rainy or snowy weather. ● How to respond in case of a brake system failure

Internal and External Threaded Pipe Plugs

When a braking system fails and the vehicle cannot move to a safe area, it is very dangerous. Knowing basic emergency procedures can reduce damage and prevent accidents. Vehicles should always carry commonly worn parts, such as brake air hoses, raw rubber tape, internal and external threaded plugs matching the brake pipe fittings, quick-connect air hose joints, etc. Most brake system failures occur in the brake air hoses, fittings, or when the diaphragm of the brake chamber ruptures or leaks.

A few common five-minute emergency methods:

Raw tape

The main PVC pipes of the brake system are worn and leaking. Cut off the leaking parts and connect the two ends using quick connectors.

Stainless Steel Quick Connector

If the brake system wheel cylinder connection hose is damaged or the wheel cylinder air chamber diaphragm is leaking, and it is difficult to find a suitable hose or there is no special tool to remove the diaphragm, making on-site replacement difficult and rendering the vehicle unable to move, what should be done?

① Emergency methods for damaged parking brake lines or air chambers

Schematic diagram of the parking brake pipeline distribution valve interface

The red arrow indicates the air inlet port of the parking brake pipeline. If the parking brake (spring brake) pipeline is damaged, unscrew the air inlet fitting of the damaged pipeline and tighten a plug into the air inlet port. At this point, the pipeline is disconnected, and the air inlet port is temporarily sealed so no air pressure passes through, ensuring the normal operation of the other pipelines in the entire braking system.

驻车制动解除螺栓位置示意图

Parking brake release bolt is loosened

At the same time, unscrew the parking release rod of the brake caliper or adjust the brake adjustment arm to release the brake. At this point, the emergency parking brake release is complete, and the vehicle can continue to drive after the damaged line is disconnected.

Diagram of the connection pin between the brake wheel cylinder and the control arm

However, it should be noted that for vehicles equipped with an automatic control arm, the method of loosening the control arm is ineffective. The control arm wheel cylinder connection pin must be removed to release the parking brake.

② Emergency measures for damaged brake lines or air chambers

Brake line distribution interface equipped with ABS anti-lock system

The damage to the brake line is relatively simple, and the joint of the intake end (relay valve or ABS solenoid valve) of the damaged pipeline is also unscrewed, and the air intake port can be tightened with a plug. The vehicle can be driven in an emergency, but it should be noted that the tire on the side of the damaged pipeline does not brake at this time, and safety should be paid attention to when driving in an emergency.

Regardless of the method, the vehicle should repair the damaged parts as soon as possible after driving to a safe area to ensure safety first.
 
● Editor's postscript

All braking states of the trailer brake system completely maintain the smooth flow of the air supply pipeline in terms of safety design. The foot brake and parking brake measures of the tractor are effective by supplying air to the control pipeline. Friends who are new to semi-trailers should not be confused with the braking system of ordinary vehicles.

Critical brakes, which are not advisable to easily change the core components, will affect the operation of the entire brake system. The author has to emphasize again the Achilles heel of the king of private brakes, the loss of control of the excess air makes the adjustment arm, brake hub, and brake pads unable to withstand the pressure, resulting in collapse and failure.

Go out happily and go home safely! Remember to take care of your vehicle properly and don't let happiness slip under your feet.