Introductions of Semi-trailer King Pins

A king pin is installed on a semi-trailer and cooperates with a fifth wheel on a tractor to realize hooking up and dropping the semi-trailer and the tractor, as shown in Figure 1 below.

(Figure 1. Position of semi trailer king pins)

The king pin is one of key parts connecting the tractor and the semi-trailer. The introduction of king pins including the following aspects:

1. Classification of king pins

The semi-trailer king pin has two models: 50# (2”) and 90# (3.5”).

For 50# king pins, needing to correspond China standard GB/T 4606-2006 (or referring to ISO 337:1981).

For 90# king pins, needing to correspond China standard GB/T 4607-2006 (or referring to ISO 4086:2001).

According to different assembly forms, king pins can be divided into the following two styles:

1) Welded type king pins

As shown in Figure 2 below. Welded king pins are available in two models: 50# (2”) and 90# (3.5”). The welded king pin is directly welded to the skid plate of the semi-trailer and cannot be replaced.

The raw material of this kind of king pin is generally 20CrMnTi. It needs surface carburizing and quenching treatment to enhance wear resistance of its surface. The optional skid plate thickness is 8mm, 10mm, 12mm, 14mm and 16mm.

(Figure 2. Welded type semi trailer king pins)

2) Bolted type king pins

As shown in Figure 3 below. The bolted king pin consists of a retention plate, a king pin (interchange head) and self-securing bolts.

(Figure 3. Components of a bolted type semi trailer king pin)

There are also two types of bolted type kingpins: 50# (2”) and 90# (3.5”).

As shown in Figure 4 below. The bolted king pin is to weld the retention plate on the semi-trailer skid plate, and then fix the interchange head on the retention plate with self-securing bolts. When the bolted king pin is worn or accidentally damaged or must be replaced duo to other reasons, remove the eight bolts and replace a new interchange head. It’s easy to maintain.

The raw material of this kind of king pin generally adopts 40Cr and undergoes quenching and tempering treatment. The optional skid plate thickness is 8mm, 10mm, 12mm, 14mm and 16mm.

(Figure 4. Bolted type semi trailer king pins)

2. Model selection of king pins

The king pin is a class A force-bearing component on a semi-trailer. The following two factors should be considered when selecting a king pin:

1) D-value of king pins

Different semi-trailers have different loading designs, and the D-value of the king pin must meet the semi-trailer’s design requirements to ensure safety of the vehicle. The calculation of D value is as follows:

2) The thickness of a skid plate

If the selected king pin does not match the thickness of the skid plate, the king pin cannot be properly matched with the 5th wheel, or cause abnormal wear of the king pin.

For the selection of a king pin and a skid plate, please refer to the following Table1 and Table 2.

(Table 1- Selection table of welded king pins)

(Table 2-Selection table of bolted king pins)

3. Installation of king pins

1) Welding of welded king pins

As shown in Figure 5 below. For welding a king pin, preheat at the welding position before welding, with the preheating temperature within the range of 200 ~ 300℃, and then use CO2 gas shielded welding to weld.

Note: Welding is not allowed within the range of Φ110mm on the upper surface of the king pin.

(Figure 5. welded type semi trailer king pins)

2) Welding of bolted king pins

Before welding, preheat the welding position of the retention plate. The preheating temperature is within the range of 200-300℃. The welding method is the same as that of a welded king pin. After cooling, use self-securing bolts to lock the interchange head. The reference tightening torque is 190NM.

（Note: Self-securing bolts can only be used once!)

4. Use and maintenance of king pins

1) Inspection of king pins

a. Check the king pin for defects such as scars, premature wear, cracks, etc.;

b. Measure the diameter of the king pin and replace it in time if it reaches the wear limit. The wear limit size of a king pin is shown in Figure 6 below:

(Figure 6. King pin wear limit requirements)

c. Check the skid plate for scars, distortion and foreign objects, otherwise it will cause abnormal wear of the king pin;

d. Check whether any self-securing bolt is loose or broken. Otherwise, replace the self-securing bolt and tighten it according to the specified torque.

e. Regularly check the above items according to the instruction manual of each automobile manufacturer.

2) Dealing with the wearing of king pins

During the running of a semi-trailer, the king pin rotates relative to the fifth wheel, causing the king pin to wear. The handling method is as follows:

a. Adjust the position of the lock bolt of the fifth wheel to eliminate the gap between the king pin and the lock hook;

b. The king pin must be replaced when the wear of the king pin reaches the wear limit.

5. Lubrication of king pins

a. During initial use, add an appropriate amount of grease around the end of the king pin;

b. After the vehicle is used for a period of time, due to the frequent drops and hooks of the tractor and semi-trailer, necessary maintenance shall be carried out for the king pin: remove the oil sludge on the king pin (the oil sludge contains a lot of sand, which will aggravate the wear of the king pin) and supplement new grease.

From: WONDEE Autoparts