The fifth wheel is not just a piece of iron

The saddle is a key component connecting the tractor and the semi-trailer. It plays an important role in transmitting the longitudinal and braking forces between the semi-trailer saddle and the trailer. In addition, the saddle must also bear the impact of various movements of the tractor such as starting, turning, braking, etc. Its structure directly affects the operating safety of the entire system. 

The tractor saddle is a swingable horseshoe-shaped wear-resistant steel plate. The trailer pressure acts on this plate. There is a semicircular hole with a notch in the center, and the hole wall is made of impact-resistant forged steel.

1. Composition of the saddle

1. Saddle body, the main component of the saddle.

It is composed of a shell (also called a seat cover), a front frame plate, a rear frame plate, an M-shaped frame plate, an inner frame ear, an outer frame ear, a U-shaped plate, and a tail slide cover.

2. Brackets, used to fix the saddle.

The two brackets are connected to the vehicle frame through the saddle bottom plate, thereby fixing the saddle and playing a supporting role.

3. Pull rod, used to control the relative movement of the lock hook and the inclined iron.

The pull rod separates the lock hook from the saddle by driving the inclined iron to move horizontally. When the semi-trailer is operating normally, the pull rod is fixed by the locking boss, thereby locking the lock hook and the saddle to ensure the connection between the tractor and the semi-trailer.

4. Inclined iron, used to control the separation and locking of the lock hook

The inclined iron moves horizontally through the pull rod, assisting the lock hook to rotate a certain angle to achieve the locking and separation of the traction pin.

5. Lock hook, used to control the locking and separation of the traction pin

When the inclined iron moves to the appropriate position, the U-shaped lock hook will be pulled by the spring and rotate around the pin seat to a certain angle, realizing the locking and separation of the traction pin, and at the same time realizing the connection and separation of the two vehicles. 

2. Working principle of saddle

1. Disengagement: Release the safety and manually pull the pull rod outward. The movement of the pull rod causes the inclined iron to move horizontally. At the same time, the spring generates tension due to deformation, driving the lock hook to rotate around the pin seat. At the same time, the opening of the lock hook turns to the outside, realizing the uncoupling of the traction pin and completing the separation of the two vehicles.

2. Hooking and connecting: Repeat the above steps to turn the opening of the lock hook to the outside. When the tractor makes the opening of the lock hook face the traction pin, move backward until a collision occurs. The lock hook rotates to the starting position under the action of the spring rebound. At this time, release the pull rod and the inclined iron automatically returns to its original position. At the same time, the safety pin also returns to the initial position, realizing the self-locking of the lock hook and achieving smooth connection. 

3. Classification of saddles

According to the size of the traction pin, they are divided into No. 50 and No. 90, which are commonly referred to as No. 50 saddles and No. 90 saddles. A trailer used for ordinary road transportation is usually equipped with a No. 50 saddle, while a No. 90 saddle is more commonly used on engineering machinery transport trailers and heavy-load trailers.

In addition to the different corresponding trailer towing pins, the heights of the two are also different when installed on the same tractor chassis. The height of the No. 50 saddle from the ground is smaller than that of the No. 90 saddle. The height of the trailer after coupling is also lower, and the vehicle stability is better.

The 90-ton saddle has a stronger towing capacity and can tow a semi-trailer with a total mass of less than 100 tons, while the 50-ton saddle can only tow a semi-trailer with a total mass of less than 50 tons. At the same time, in order to maintain better interchangeability, most tractors use the 90-ton saddle.

4. Saddle Maintenance

The life of the saddle depends on whether grease is applied to the saddle panel, the matching surface of the traction pin and the lock hook, the lock hook, the pin block, and the horseshoe mouth when it is used for the first time. Please use qualified heavy-duty grease. It is recommended to use heavy-duty grease containing graphite or molybdenum disulfide. Its good extreme pressure and anti-wear properties will ensure that the saddle surface will not produce abnormal wear under normal working conditions.

After the first use of grease, clean the above lubrication parts every 5000km and re-apply grease. Before applying grease each time, make sure that the traction pin is free of impurities and burrs.