Abstract: Combined with the actual project, the application characteristics of crawler cranes are elaborated, focusing on the crawler crane-based metro shield hoisting construction process, for the heaviest pieces (front shield) and the longest pieces (connecting bridge) down the well hoisting, which can provide reference for similar shield hoisting construction applications.

1 Project example

The shield construction in a metro project is mainly composed of front shield, middle shield, tail shield, cutter plate and other components, the largest of which is mainly the front shield or middle shield, with a height of 3.0-3.4m and a maximum weight of 85-110t.

The parameters of the main components to be lifted in this metro project are shown in Table 1. The 350t truck crane and the 120t truck crane were used for this purpose. In order to reduce the number of transport vehicles used and to save construction costs, crawler cranes were used for the lifting of the shield structure.

Name	Diameter /mm×mm×mm	Quality /t	Tip
Front shield connecting bridge	Φ6500×4000      12700×1900×1500	11014	Heaviest component Longest component

2 Crawler crane structural features

In engineering applications, the tonnage of the equipment parts used is often larger, in the process of consignment, installation, commissioning, etc. will bring many technical problems, but also bring certain safety issues, for this stage of engineering construction is often used to crawler cranes. Crawler crane is a jib structure lifting equipment, in the same area can achieve automatic rotation operations, at the same time with a small ground pressure, flexible operation, lifting capacity, low mass centre, climbing capacity, traction coefficient, etc., its structural composition as shown in Figure 1.

It is widely used in wind power, municipal, petroleum, nuclear power, chemical and other large construction projects, which can effectively reduce the labour intensity of construction personnel, improve the construction process and help to ensure construction quality.

2.1 Metal structures

The main components of the metal structure are steel sections and plates, which are made of metal in the fabrication plant and can effectively bear a certain load strength. The metal structure is mainly The metal structure itself should have good stability, strength and stiffness as it provides the crawler crane with various load-bearing functions. The metal structure is the main skeleton of the crawler crane and is responsible for carrying the crane’s own weight as well as the external loads.

2.2 Working mechanism

2.2.1 Lifting mechanism

The hoisting mechanism is mainly used to lift and lower the heavy load, as shown in Figure 2. The lifting structure is mainly based on a drive shaft or coupling that connects the prime mover to the high-speed shaft, and after a reduction gear makes the reel rotate. The reel receives the rotating power and achieves a forward and reverse winding and releasing motion, which in turn enables the lifting and lowering of the load and the vertical up and down movement of the pick-up device.

2.2.2 Slewing mechanism

The slewing motion of a crawler crane refers to the rotational movement of the car and the lower workshop. The structural equipment for completing the slewing motion is called the slewing mechanism, and the crawler crane operates under the entire slewing mechanism to achieve slewing operations, enabling the crawler crane to carry out construction work in a linear range. The crawler crane slewing mechanism is shown in Figure 3.

1. main boom 2. driver’s cab 3. frame 4. body weight 5. track 6. turntable

7. overhead counterweight 8. fixed counterweight 9. herringbone 10. overhead mast

2.2.3 Variable amplitude mechanism

The amplitude of a crawler crane refers to the distance between the centre of the hook and the centre axis of rotation in the machine. The luffing mechanism in a crawler crane serves the purpose of improving the efficiency of the equipment by changing the elevation angle of the boom to achieve the lifting and lowering of heavy loads. Crawler cranes increase their lifting capacity by means of luffing movements to extend the operating range to the vertical plane.

2.2.4 Travelling mechanism

The travelling mechanism of a crawler crane is mainly the crawler chassis, which includes the frame, drive wheels, guide wheels, crawler frame, drag chain wheels, crawler plates, etc., as shown in Figure 4. The travelling mechanism is a combination of drive wheels, guide wheels and support wheels of the crawler beam supporting the entire crawler vehicle, transferring the entire load to the lower branch of the crawler and the upper branch supporting the towing wheel. When the drive wheels are in a rotary motion, the combination of the engagement of the tracks with the drive wheels and the adhesion force keeps the tracks from sliding, thus enabling the machine to move forward.

2.3 Power unit and control system

The power source of the crawler crane is the main core component that determines the structural characteristics and lifting performance of the crawler crane. The power unit of the crawler crane provides the driving force for the crawler crane’s own operation and lifting action, allowing the structure of the components to move in an orderly manner. The control system is used for braking and stopping the various mechanisms in order to change the crane’s movement characteristics and to complete various operational actions. The control system is equipped with operating valves (mainly consisting of stops, brakes, clutches, etc.), special safety devices and speed control devices.

1.boom luffing rope 2. suspension boom rope 3. luffing winch 4. hoisting winch rope

3 Shield crane hoisting process

3.1 Lifting of the heaviest part of the front shield down the shaft

The 250t crawler crane components were transported to the site in advance, in accordance with the 20~30m

The jib will be connected and commissioned in the form of a 20-30m main boom, a 9-12m jib, a 160t main hook and an 80-100t jib. In exceptional cases, the boom can be assembled at another site and then transported to the safe and optimal working site.

Once the shield components have been transported to the site, the main hook spreader is attached to the main lifting point of the shield and the secondary hook spreader is attached to the secondary lifting point. Tighten the main and secondary hooks and raise the main and secondary hooks alternately, keeping the front shield diameter level.

Pull the transport vehicle out of the way. Lower the main and secondary hooks alternately, keeping the front shield 0.5~1.0m from the ground and ensuring that the radius of operation of the main hook is 10~12m from the machine pit and that the machine is well positioned.

Use alternating main and secondary hooks to lift the shield evenly and balance it, slowly raising the main crawler hook when it reaches a distance of 0.5m from the ground and slowly lowering the secondary hook at the bottom of the shield to keep the main crawler hook fully lifted. The crawler crane is lifted down to the shield in position and the main and secondary hooks are withdrawn, thus completing the descent of the front shield.

3.2 Lifting of the longest part of the connecting bridge down the shaft

The transport vehicle for the bridge components is brought to the construction site in advance and the main hook spreader is attached to the two front lifting points of the bridge, while the secondary hook spreader is attached to the two rear lifting points of the bridge. Tighten the main and secondary hooks and raise the main and secondary hooks alternately so that the bridge is 300-500 mm from the truck.

Pull the transport vehicle away. Lower the main and secondary hooks alternately, keeping the connecting bridge 0.5~1.0m from the ground level, ensuring that the main hook is 10~12m from the pit and that the machine is well positioned.

The crawler crane will slowly lift the top of the shield by the main hook, while the secondary hook at the bottom of the shield will slowly descend, keeping the bridge at a 15° incline, and then slowly lower the bridge into the shaft.

1. rotary table 2. rotary support inner ring 3. rotary support inner ring 4. pinion

5. rotary reducer 6. rotary motor   

4 Conclusion

This paper focuses on the application of crawler cranes in shield hoisting construction. By elaborating on the structural features of crawler cranes, it focuses on the construction process of metro shield hoisting based on crawler cranes, with a focus on the hoisting of the heaviest part (front shield) and the hoisting of the longest part (connecting bridge) down the well, confirming the feasibility and practicality of crawler crane shield hoisting construction technology.

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