Introduction of Semi Trailer Fifth Wheels

A fifth wheel/5th wheel is installed on a tractor, and is connected and disconnected from a semi-trailer through a semi-trailer king pin and a skid plate, and transmits horizontal and vertical forces at the same time.

The fifth wheel plays the role of pulling the semi-trailer and bearing the load of the semi-trailer, and at the same time, it is also subjected to the impact of the tractor's starting, acceleration, deceleration, steering, braking and other working conditions. Therefore, the strength and reliability of the fifth wheel will directly affect the driving safety of a semi-trailer. As shown in Figure 1 below.

(Figure 1. A fifth wheel on a tractor)

A fifth wheel is one of the important parts on a tractor, the followings are introductions about fifth wheels:

1. Technical standards of fifth wheels

The relevant technical requirements and technical standards of fifth wheels are as follows:

1) The general technical conditions of fifth wheels comply with the China standard GB/T31879-2015.

2) The interchangeability requirements of fifth wheels comply with the China standard GB/T13880-2007, and refer to the international standard ISO3842:2001.

3) The strength test of fifth wheels complies with the China standard GB/T20069.

2. The composition of a fifth wheel

As shown in Figure 2 below. A fifth wheel is mainly composed of four parts: a seat plate, a locking device, a connecting-disconnecting mechanism and 2pedestals.

(Figure 2. The composition of a fifth wheel)

3. Classification of fifth wheels

(1) Classification according to the specifications of the king pin matched with the fifth wheel

1) 50# (2.0") fifth wheel.

2) 90# (3.5") fifth wheel.

(2) Classification according to the number of swing axis of afifth wheel

1) Single axis fifth wheels

As shown in Figure 3 below. Single-axis fifth wheel refers to a fifth wheel whose seat plate can swing longitudinally around the transverse axis of the fifth wheel.

(Figure 3. A single-axis fifth wheel)

Because a single-axis fifth wheel has high driving stability, it is suitable for semi-trailers that run at high speeds on better roads, have lighter loads, and have a higher cargo center of gravity. Tractors using this kind of fifth wheel are generally used in conjunction with container semi-trailers, warehouse grid semi-trailers and bulk freight semi-trailers, etc.

2) Two-axis fifth wheels

As shown in Figure 4 below. A two-axis fifth wheel refers to a fifth wheel whose seat plate can simultaneously swing longitudinally around the transverse axis of the fifth wheel and laterally swing around the longitudinal axis of the fifth wheel.

(Figure 4. A two-axis fifth wheel)

Due to the two-axis swing design, the height of the fifth wheel is higher than that of the single-axis fifth wheel, and the lateral stability of the vehicle is poor during high-speed driving and turning. Therefore, this style of fifth wheel is not used in general highway transportation.

This fifth wheel is suitable for low-speed transportation of large and heavy special equipment. Due to its two-axis four-way swing design, it can protect the tractor beams to a certain extent if it is transported under heavy loads at different road conditions.

Classification according to the material of the fifth wheel cover plate:

1) Welding fifth wheels

This fifth wheel has two kinds of seat plates: the pressing forming seat plate and the CNC cutting seat plate.

a. As shown in Figure 5 below. The pressing forming seat plate is generally made of 8mm-12mm thick steel plate after blanking, forming, and stretching, and the panel is welded with various ribs. This kind of seat plate is widely used and has been tested by markets for a long time.

(Figure 5. A pressing forming fifth wheel)

b. As shown in Figure 6 below. The CNC cutting seat plate is generally made of 16mm-32mm thick steel plate directly through plasma cutting, and then welded with various ribs. This kind of seat plate is generally used under heavy-load and over-heavy-load.

(Figure 6. A CNC cutting fifth wheel)

4. Selection of fifth wheels

The selection of fifth wheels mainly considers the following factors:

1) D value of a fifth wheel

The D value of a fifth wheel must meet the design requirements to ensure the safety of a vehicle. The calculation method of D value is shown in Figure 8 below:

(Figure 8. Calculation formula of the fifth wheel D value)

2) Model of the king pin on the semi-trailer

The corresponding fifth wheel has two styles, 50# (2") and 90# (3.5").

3) Rated vertical load of fifth wheels

A fifth wheel is usually selected according to the design load of the tractor and the design load of the semi-trailer. Generally, the vertical load (T) range of a fifth wheel is 20 tons≤T≤75 tons.

4) Installation height of a fifth wheel

It is determined according to the design requirements of the tractor and the height of the matched semi-trailer. Generally, the range of the height (H) of a fifth wheel is 150mm≤H≤250mm.

5. Installation of fifth wheels

As shown in Figure 9 below. A fifth wheel is connected with the chassis of a tractor through the corrugated mounting plate using special mounting bolts.

(Figure 9. Installation of a fifth wheelⅠ)

When installing, the 2 bottoms of the 2Pedestals must be on the same plane and close to the corrugated mounting board. The bottom of the 2pedestals should not be suspended or there must be no gaps. As shown in Figure 10 below.

(Figure 10. Installation of a fifth wheelⅡ)

6. The use and maintenance of a fifth wheel

A fifth wheel is an important safety component that affects the driving of the vehicle and requires regular maintenance.

(1) The maintenance cycle description of a fifth wheel is shown in Table 1 below:

(Remark: Due to the different road conditions and fifth wheel usage requirements in each country or region. The maintenance and maintenance cycles are for reference only.)

Inspection Item	Inspection Contents	Aintenance Cycle
		Initial driving	Every   500 KM	Every 2500 KM	Every 5000 KM
Pedestal	Check for cracks or other damage				●
	Check whether the fastening bolts of the Pedestal   are loose			●
Seat Plate	Check for cracks and severe wear			●
	Clean and replenish grease	●	○	●
Lock jaw, Locking Bar, Wearing   Ring	Check for severe wear			●
	Clean and replenish grease	●	○	●
shock-absorbing sleeve	Check for damage				●
Safety Lock	Check for proper operation	●			●
Operating handle	Whether it is flexible or stuck	●		●
Grease nipple/Grease tube	Check and clean the tube to make it   unblocked			●
	replenish grease	●		●

(Table 1.Maintenance table of a fifth wheel)

● Indicates maintenance

○ Indicates that it can be maintained in advance

(2) Replace a wearing ring and lock jaw:

The wear limit of a wear ring and lock jaw in a fifth wheel is shown in Figure 11 below. If the wear exceeds the limit value, it must be replaced immediately.

(Figure 11. Wear limit of a wear ring and lock jaw)

(3) Lubrication of fifth wheels

The lubrication of fifth wheels is very important in maintenance and service. The following points need to be paid attention to:

a. When a vehicle is used for the first time, grease should be added to all parts that need to be lubricated.

b. When the vehicle is often unhooked, the lock jaws, bolts and wear ring should be maintained. Remove the sludge (the sludge contains a lot of sand will increase the wear of the parts), and replenish new grease.

c. Clean the fifth wheel panel (or wear plates) and the trailer skid plate regularly, and apply grease on the surface after cleaning.

d. Fill grease regularly through the grease nipple until grease overflows from the oil outlet. If the oil pipe is blocked, the oil pipe must be cleared first, and then grease is replenished.

7. Fifth wheel troubleshooting table (problem, cause and correction)

As shown in Table 2 below.

Problem	Cause	Correction
A fifth wheel   cannot be connected with a semi trailer	(1) The position of the fifth wheel   is too high or too low (2) Uneven skid plate of the trailer (3) Deformation of the lock jaw (4) Problems with the tension spring (5) Bending deformation of the   operating handle	(1) The trailer skid plate and the fifth   wheel seat plate should be kept on the same level or 50mm lower than the seat   plane of the fifth wheel (2) Replace the skid plate (3) Replace the lock jaw (4) Replace the tension spring (5) Replace or correct the operating   handle
A fifth wheel   cannot be disconnected from a semi trailer	(1) The king pin is stuck on the fifth   wheel (2) The lock jaw or lock bolt is   damaged, or the adjusting screw is incorrect	(1) Check whether the thickness of   the connecting plate meets the requirements (2) Replace the lock jaw or lock bolt   and re-adjust the screw
The fifth wheel   is not working	(1)   Deformation of the lock jaw (2)   The tension spring is damaged	(1) Replace the lock jaw (2) Replace the tension spring
There is relative motion between the fifth   wheel and the semi trailer	1) The shock-absorbing sleeve is worn   and the space is too large (2) King pin is worn (3) The gap of the locking devices   and the king pin is too large	(1) Replace the shock-absorbing   sleeve (2) Replace the king pin (3) Adjust or replace the locking devices   or/and the king pin

(Table 2. Fifth wheel troubleshooting table (problem, cause and correction)

From: WONDEE Autoparts