Introduction of Semi-trailer Landing Gears(Landing Legs)

A Landing gear is usually installed in the front of a semi-trailer, which consists of the left and right leg. Its function is to support the front end of a semi-trailer after the semi-trailer is disconnected from the tractor. As shown in Figure 1 below.

Figure 1. Installation position of landing gear

A landing gear is an indispensable part of a semi-trailer. It is mainly introduced from the following aspects:

1. Technical standards of landing gears

(1) The technical standard of a landing gear complies with the China standard GB/T2677-2015

(2) The welding standard of a landing gear complies with the standard of the Ministry of Transport of the People's Republic of China JB/T5000.3-2018

2. The composition of a landing gear

As shown in Figure 2 below. A landing gear is mainly composed of the following five assemblies: a driving leg assembly, a driven leg assembly, a landing gear foot assembly, a crank assembly and connection shaft (linkage landing gears include this part)

Figure 2. Exploded diagram of a landing gear

3. Classification of landing gears

According to the structures of landing gear, it is usually divided into the following two types:

(1) Single-acting landing gear(Two-side operation landing gear)

As shown in Figure 3 below. Both legs of this landing gear are equipped with a crank handle and a gearbox, The left and right legs can be raised and lowered independently by the crank. This landing gear is highly adaptable and can be used on both flat and uneven roads.

Figure 3. Single-acting landing gear

(2) Linkage landing gear (One-side operation landing gear)

As shown in Figure 4 below. This landing gear has a gearbox and a crank on only one of the two legs. The leg with the gearbox and the crank is called the driving leg, the other is called the driven leg. The two legs are connected by a middle connection shaft. Rotating the crank clockwise or counterclockwise will raise or lower the 2 legs at the same speed. Compared with the single-acting landing gear, the lifting efficiency of this landing gear is doubled. This landing gear is only suitable for flat roads, otherwise it must be reliably leveled with pads before it can be used normally.

Figure 4. Linkage landing gear

4. Selection of landing gears

The following aspects need to be considered when selecting the landing gear:

(1) Styles of landing gears

Landing gears have usually two styles:

1) Single-acting landing gear

2) Linkage landing gear

(2) Static load and lifting load

The static load and lifting load of landing gears are as follows, as shown in Table 1 below:

S/N	Static load   （T）	Lifting load（T）
1	50	24
2	60	25
3	60	28
4	80	28
5	90	32

Table 1. Static load and lifting load of a landing gear

 

(3) Lifting travel

The common lifting travel range of landing gears are: 350～480mm (14”～19”).

(4) Speed ratio of the high gear and low gear

The common speed ratios of high gear and low gear are as follows, as shown in Table 2 below.

S/N	Speed ratio of the low gear（mm/r）	Speed ratio of the high gear（mm/r）
1	0.71	5.40
2	0.95	8.50
3	1.00	7.50
4	1.00	11.00
5	1.00	24.60

Table 2. Speed ratio of the high gear and low gear

5. Installation of landing gears

Before mounting, it is necessary to confirm that the landing gear parameters meet the design and use requirements of the semi-trailer. The installation process of landing gears needs to meet the following requirements (taking a linkage landing gear as an example)

(1) The left leg and right leg are perpendicular to the upper plane of the semi trailer frame.

(2) The output shaft of the driving leg and the input shaft of the driven leg must be on the same axis and perpendicular to the longitudinal axis of the semi-trailer.

(3) The landing gear must be equipped with horizontal pull rods, diagonal pull rods, and longitudinal pull rods to ensure the support strength of the landing gears, as shown in Figure 5 below.

Figure 5. Installation of a landing gear

(4) 2 limiting blocks needs to be firmly welded on the trailer at the upper end of the installation bottom plate of the landing gear, as shown in Figure 6 below.

Figure 6. Installation of landing gear

(5) Adjust the lifting height of the left leg and right leg to make the difference between A1 and A2 less than 5mm, as shown in Figure 5 above.

(6) Assemble the intermediate connection shaft (round shaft or square shaft, no need to do so for the single-acting landing gear). A proper axial movement clearance shall be ensured, as shown in Figure 7 below.

Figure 7. Installation of landing gear

(7) Tighten the bolts with a torque wrench (torque value: 182～245Nm).

After the landing gear is installed, check whether the landing gear works normally through test. Requirements: the raising and lowering of the landing gear at high or low gears are flexible, the expansion and contraction of two legs are synchronous, and the fast and slow gears can be switched smoothly. Otherwise, it shall be adjusted and corrected.

6. Use of landing gears

(1) Operation of landing gears

As shown in Figure 8 below:

1) Landing: lower the landing gear in high gear until it touches the ground. Shift to low gear and raise the landing gear to the required height.

2) Lifting: firstly use the low gear to keep the landing gear off the ground, and then use the high gear to fully retract the landing gear.

3) Shifting operation: gently shake and pull out the crank to the limit position, shift to low-speed gear. On the contrary, gently shake and push the crank to the extreme position, shift to high-speed gear.

 

Figure 8. Operation of landing gear

(2) Precautions for use of landing gears

1) The semi-trailer must be parked on a flat road with sufficient support (such as a cement road). It is not allowed to use landing gears to support the semi-trailer on slopes or soft roads. Otherwise, the landing legs are easy to be bent.

2) Landing gears that match the height of the semi-trailer should be selected. The lifting height must not exceed the limit position. That is: when the red area (limit position) of the inner leg of the landing gear appear, please stop lifting immediately, and retract the landing gear, till the red area be hided, as shown in Figure 9.

Figure 9. Using of landing gear

3) When parking on an unavoidable low-lying road, use a rectangular sleeper to pad the lower leg to the suitable height, as shown in Figure 9 below.

Figure 10. Using of landing gear

4) When unhooking or hooking up, the tractor should not drive the trailer to slide to avoid damage to the landing gear from dragging on the ground.

5) Before unhooking, use the high gear to quickly make the landing gear foot contact the ground, and then use the low gear to lift, so that the load is completely transferred to the landing gear. Then unhook.

6) After the tractor is hooked up, the landing gear must be completely retracted before moving to ensure that the distance between the landing gear and the ground is more than 300mm. After the operation is completed, place the crank in the hook. It is not allowed to place the crank at will, and it is not allowed to take off the crank, otherwise the inner legs will fall due to vibration during driving, causing the landing legs to collide with the ground and be damaged.

7. Lubrication of landing gears

In order to prevent the grease in the landing gear from failing after long-term use, good lubrication of the landing gear cannot be guaranteed, so it is necessary to regularly replenish the grease in various parts. It mainly involves the following parts:

(1) Grease storage box in the inner leg

The grease storage box in the inner leg shall be filled with sufficient grease twice a year or during regular maintenance.

(2) Thrust ball bearing in the inner leg

Thrust ball bearings shall be filled with sufficient grease twice a year or during regular maintenance.

(3) The large and small bevel gears in the legs

Fill the bevel gears with sufficient grease twice a year or during regular maintenance.

(4) Gears in the gearbox

Add sufficient grease to the gears in the gearbox twice a year or during regular maintenance or when the gearbox produces abnormal noise when the crank is shaken.

8. Maintenance cycle of landing gears

The routine maintenance cycle of landing gears is shown as Table 3 below:

Inspection items	Contents	Every time   before departure	Every three   months	Every six months	Every year	Every time for   maintenance
Mounting bolts/nuts	Check the torque			●		●
Intermediate connecting   shaft	Check for   deformation and cracks		●			●
The shaft and split pin on the foot	Check for deformation and   cracks	●				●
Crank and bolt	Check if it is missing	●
Gearbox components	Check for wear and damage					●
	Replenish grease				●	●
Gearbox cover	Check for damage		●			●
Outer square tube	Check the welding condition   of the square tube				●	●
	Check whether it is deformed   or damaged			●		●
Inner leg	Check whether it is deformed   or damaged			●		●
Input and output shafts	Check for bends and cracks			●		●
Big and small bevel gears	Check for wear and damage					●
	Replenish grease				●	●
All bushings	Check for wear				●	●

Table 3. Landing gear maintenance

9. Troubleshooting of landing gears

Troubleshooting table-problems, causes and troubleshooting methods are shown in Table 4 below:

Problems	Causes	Troubleshooting
It is difficult to shake the   crank (1st time to use)	(1) The   intermediate connecting shaft pulls or pushes the output shafts of the left   and right legs too tightly, and there is no axial movement clearance, which hinders the   rotation of the gear. (2) The coaxiality deviation   of output shafts is too large (3) The ground slope where the semi-trailer is   parked is too large	(1)   Increase the axial   movement clearance of the intermediate connecting shaft (2) Reinstall and adjust the landing gear (3) Park on the level ground)
It is difficult to shake the crank	(1)   Bending deformation of gear shaft (2) Local interference of inner and outer legs due to impact deformation (3) Gear damage (4) Exceeding the stroke limit causes deformation and damage   of the screw and nut (5) The screw and nut are deformed and damaged   due to the instantaneous impact during loading or unhooking	(1)   Replace the gear shaft (2) Replace   the deformed leg (3) Replace the gear (4) Replace the inner leg (5) Replace   the inner leg
Works normally under no load and cannot be lifted under heavy load	The key, or keyway, or keyseat of gear shafts in the gearbox is broken	Replace the damaged parts
Only one leg can be lifted-the leg with gearbox can be lifted	Check whether the connection shaft    bolts or the small bevel gear, or the key, or the key seat, or the key   way are damaged	Replace the damaged parts
Only one leg can be lifted-the leg without gearbox can   be lifted	Check whether the small bevel gear, or the key, or the   key seat, or the key way are damaged	Replace the damaged parts
Difficult or unable to shift   gears	The steel ball spring in the   dual gear shaft assembly falls off, or the positioning sleeve is damaged and   jammed	Reinstall the steel ball and spring or the   damaged positioning sleeve

Table 4. Landing gear troubleshooting table-problems, causes and troubleshooting methods

Compiled by  WONDEE Autoparts