The Working Principle of TBM Cutter Disc Used in Tunnel Engineering

Full-section rock tunnel boring machine (TBM) is a special construction machine for tunnels, which is widely used in subway, railway, highway, municipal, hydropower tunnel projects.

The cutter head is the key component of TBM, which has the functions of excavation, stabilizing the tunnel face, stirring muck and so on. TBM disc roller cutter is widely used in all kinds of tunnel boring machines, mainly used in full section rock tunnel boring machines, shield TBM and pipe jacking machine. 

Ⅰ. Force and rock breaking mechanism of TBM disc cutter

Each TBM disc cutter cuts rock with a three-direction interaction between the cutter ring blade and the rock. 

1. The normal pushing force points to the excavation surface and is provided by the thrust of the cutter head.

2. The tangential rolling cutting force points to the tangential direction of the disc cutter, which is provided by the torque of the cutter head;

3. The lateral force on the edge of the disc cutter is generated by the squeezing force of the disc cutter on the rock and the centrifugal force of the rotation of the TBM roller cutter head, and it points to the center of the cutter head. It points to the center of the cutter head and has a small value. It does not belong to the same order of magnitude with the other two forces and is generally not considered. 

When the cutterhead is working, the disc cutter first contacts the excavation surface, and is pressed against the rock surface under the action of thrust. With the rotation of the cutterhead, the TBM disc roller cutter revolves around the central axis of the cutterhead on the one hand, and revolves around its own axis at the same time.

TBM disk cutter cuts a series of concentric grooves on the palm surface under the joint action of the thrust and torque of the cutter head. When the cutter rotates and presses into the rock, the TBM disc cutter will produce comprehensive effects such as extrusion, shear and tensile cracking on the rock.  The slag thus formed is composed of crushed body, crushed body and rock fragments, and the proportion of each part depends on the rock properties, cutter ring geometry, pushing pressure and cutter spacing. 

Ⅱ. The arrangement of the TBM disc cutter on the cutter head

1. Analysis of the layout of TBM cutter

The arrangement of the TBM disc cutter on the cutter head should satisfy certain mechanical and geometric laws. Generally, the general layout should meet the following requirements. 

As far as possible to make the disc cutters and cutter plate getting the uniform force, so that the radial load on the large bearing is zero.

The front cutters should provide the rock breaking surface for the rear cutters, so as to form the front and back cutters for successive rock breaking. 

Therefore, TBM disc cutter are generally arranged in single or double spiral mode on the cutter head, and adjacent cutters are arranged in a certain phase difference. 

The TBM cutter disc usually has three types of single disc, double disc and triple disc. The arrangement of the TBM roller cutter on the cutter head should facilitate the formation of sequential rock breaking.

Since the double disc cutters and the triple disc cutters cannot well meet the requirements of all TBM roller cutters for rock breaking in sequence, and are prone to uneven wear, resulting in deterioration of cutter force and cutter waste, so the face cutter and gauge cutter should choose single disc cutter as far as possible. 

However, in order to save the space of the cutter head, both the shield machine and the tunnel boring machine are mostly equipped with double disc cutters or triple disc cutters in the center of the cutter head.

2. The principle and method of determining the disc cutter spacing

No matter which method is used to arrange the tools, the cutter spacing is one of the first technical issues to be considered. There are two ways to consider the cutter spacing.

(1) On the same machine, the cutter spacing is unchanged, so as to change the thrust of the tbm cutter head to adapt to the change of rock strength.

(2)Increase or decrease the number of cutters on the same cutter head or change the number of cutter rings on each disc cutter to change the cutter spacing to adapt to the rock strength. 

The first consideration method is simple and practical, and has many applications.

Considering the cutter head space, when excavating in mixed strata, the selection principle of cutter spacing is generally as follows.

(1) When there are few center cutters, they are arranged according to the optimal cutter spacing. They are often arranged with relatively small cutter spacing to compensate for the difficult cutting conditions that the TBM roller cutter may encounter during the tunneling process.

(2)The distance between the front cutters is between 50 and 120mm. For soft rock layers, take a large value, and for hard rock layers, take a small value.

(3)When the tunnel is mainly composed of soft rock and has a small amount of hard rock, the cutter spacing is selected according to the soft rock, and when the hard rock section is dug, the tunneling can be slow. 

(4) When the tunnel is dominated by hard rock and has medium hard rock, the cutter spacing shall be selected according to the principle of taking both into consideration.

(5) The gauge cutters are located on the outer edge of the cutter head, with high speed and serious wear, and are rarely arranged according to the optimal cutter spacing.

With the comprehensive utilization of high strength, wear resistance and other technologies of the disc cutter, the depth of cut of it has been greatly improved, but at this time in order to achieve the required critical thrust, it is necessary to expand the distance between the cutters accordingly, otherwise it will cause the TBM roller cutter is working in the former optimal zone.

In tunnel construction, the cutter head is one of the most important core components, commonly known as the head of the tunnel boring machine. The management of the cutter head directly determines the success or failure of the entire project, the investment of the project cost and the guarantee of the project duration.