The most complete maintenance manual for semi-trailers, truckers, be sure to save it

During transportation, safety is always the top priority for truck drivers! If renting a trailer is for convenience, then trailer maintenance is for survival. Everything must be checked and in order before hitting the road.

Based on feedback from previous semi-trailer drivers and the extensive practical experience of professional personnel at Dongfang Post-Station's aftermarket services, we have summarized a comprehensive guide on semi-trailer usage, maintenance, and upkeep knowledge here to provide reference for truck drivers:

▎1. Introduction to the Main Assemblies of Semi-Trailers

● Suspension system (rigid): Transmits loads and absorbs vibrations.
 

● Axle and Hub System: The suspension system is equipped with single, double, or triple axle assemblies of the same structure. The tire and rim assembly is fastened to the hub with wheel bolts, which support the weight of the entire vehicle.
 

● Landing Gear Lifting Mechanism: To ensure that the semi-trailer can be reliably parked after being detached from the tractor, two landing legs, either linked or single-action, are installed at the front end of the chassis. The mechanism consists of a screw transmission, bevel gear transmission, two-speed reduction gearbox, drive rod, and operating handle.

When the handle is pushed in, turning it allows high-speed lifting and lowering. When the handle is pulled out, turning it allows low-speed lifting and lowering. When the landing legs touch the ground, they can only be lifted or lowered at low speed.
 

● Towing Pin and Guide Plate/Bracket: At the lower front end of the chassis, there is a towing pin and a towing pin connecting plate that connect to the tractor saddle. The towing pin is mainly used to transmit the towing force from the tractor and the braking force during braking.
 

▎2. Semi-Trailer Electrical Circuit

A semi-trailer transmits the lighting signals of the tractor to the trailer by connecting the spiral cable plug of the tractor to the electrical socket at the front end of the trailer.

Therefore, the vehicle is equipped with an internationally compatible electrical system that matches the tractor. The electrical system generally consists of a seven-pin socket, multi-core cables, junction boxes, waterproof connectors, and various types of lighting components.

● Seven-core wire circuit connection diagram:

▎3. Semi-Trailer Air Lines

The three main components: emergency relay valve, ABS valve, and brake chamber. Most faults come from these three components.

● Emergency relay valve: The pneumatic components of the vehicle's braking system achieve emergency braking when the trailer air supply line is damaged or air intake is cut off, reducing brake lag time and allowing for quick brake release. It replaces the brake valve, proportioning valve, quick-acting valve, and relay valve in the original trailer braking system.
 

● ABS (Anti-lock Brake System): An electronic control system that monitors and controls vehicle speed during braking. It consists of components such as ABS solenoid valves, ABS wiring harness, ABS sensors, and toothed rings. Working together with the relay valve to perform braking functions, it prevents wheel lockup. The sensors detect the wheel speed, and when the speed drops to zero, a signal is sent to the solenoid valve, which then adjusts the air pressure in the brake lines to reduce braking force, preventing wheel lockup and enhancing driving safety.
 

● Brake Chamber
 

▎4. Semi-Trailer Brake System

It consists of the tractor's air pipe connectors, inflation pipeline, control pipeline, air reservoirs, brake chambers, relay valves, ABS valves, ABS sensors, and wheel brakes.

The inflation pipeline connects to the tractor's air reservoir via quick couplings; the control pipeline connects to the tractor's brake valve via quick couplings. When the semi-trailer brakes, all brake chambers operate simultaneously.
 

● Driving Brake: During driving, the driving brake (foot brake) is generally used, which facilitates deceleration and stopping while moving forward. 

● Parking Brake: The parking brake is the brake used when the vehicle is stationary, also known as the "handbrake." Its main function is to prevent the vehicle from rolling forward or backward.
 

● Emergency Air Brake (Emergency Braking): This is the automatic emergency braking action performed by the trailer's braking system when the air supply pipe loses pressure or leaks severely. It will not activate if the control pipe is broken, but the 'foot brake' will fail. 

Importance: When the truck's air pump is damaged, the air supply pipe is disconnected, or the air reservoir leaks severely, the vehicle's 'foot brake' fails or performs poorly. The emergency air brake is an important safety measure for the vehicle and personnel! 

▎ Normal Braking: Working Brake Status:

When the brake pedal is pressed, the control line ① sends air to the emergency relay valve ⑤, which controls the air supply from air reservoir ⑥ to the brake chamber ④ to generate braking force; when the brake pedal is released, the emergency relay valve quickly releases the compressed air from each brake chamber, rapidly disengaging the braking state.

▎ Feedback braking status during the inflation process:

When a trailer is parked for a long time and there is no air pressure in the brake system, the parking brake system (spring brake) comes into play. When the tractor supplies air to the trailer's air tanks through the supply line, the emergency relay valve is in a safe braking state. 

In the safe state, the emergency relay valve first sends compressed air to each brake chamber. Once the trailer's air tank pressure rises to 4.5 bar, the safe state is released. This function is referred to as the feedback braking function, where the brake chambers generate braking force while being charged, serving as a safety protection function.

▎Automatic braking state during emergency air loss:

When the air supply line ② is accidentally damaged while driving, the compressed air in the trailer ⑥ air tank enters the brake relay valve ⑤ and then the brake chamber ④, automatically performing braking measures to ensure the vehicle's safety in emergency situations. This allows the vehicle to drive a short distance to a repair location, but it cannot last for long. When the tractor stops and needs to disconnect the trailer ② air supply line, the trailer brake chamber can also be seen operating, based on the same principle. 

▎5. Semi-Trailer Inspection and Maintenance

The vehicle requires basic inspection and maintenance. Proper inspection and maintenance work can prevent unnecessary safety hazards and reduce operating costs.
 

● Semi-trailer Inspection

● Hub Maintenance
Work Content: Disassemble and inspect the hub system, clean the axle ends and replace failed lubricating grease in the inner and outer bearings with qualified new grease, check the bearing wear to see if it affects usage, and inspect the working performance of components such as brake pads, brake drums, and brake shoes. Perform maintenance or replacement according to the inspection results.
 

Cycle:

① BPW axle maintenance frequency: 120,000–150,000 km.

② VALX axle maintenance frequency: 2 years or 400,000 km (after maintenance-free period: 1 year or 200,000 km).

③ BPW 400K/600K: 2 years or 400,000 km / 3 years or 600,000 km.

④ Fuwa axle maintenance frequency: 50,000–70,000 km.

● Greasing

Content: Apply grease or lubricate parts such as the semi-trailer landing gear, control arms, camshaft, balance beam pins, and leaf springs.
Cycle: For frequently used vehicles, grease should be applied once every half to one month, or after heavy rain. Infrequently used vehicles need to be greased before being put back into operation. Apply sufficient grease to ensure full lubrication, reducing component wear and damage caused by mechanical friction.

▎6. Container and Tire Maintenance

● Container Maintenance
Introduction to the terminology of container structures:
 

Container Repair Methods:

(1) Patch Repair: For holes, cracks, etc.

(2) Part Replacement: For extensive damage or severe deformation.

(3) Putty and Paint Touch-Up: For minor deformations or scratches without damage.

(4) Adhesive Repair: For cracks, commonly used on composite panel containers.

(5) Liner Plate Welding: For cracks or weld failures.

(6) Sheet Metal Correction: For deformations or bends, such as hinges, locks, etc.

(7) Sealant Application: For water leaks on the roof or side panels.

(8) Steel Reinforcement: For overall deformation of composite panels or bloating.
 

● Tire Maintenance
 

The effect of tire pressure:
 

Experts from the Dongfang Post Station aftermarket service team will periodically share knowledge about trailers and maintenance tips with truck drivers, hoping to help everyone, with safety as the top priority.