1. Tool wear rules

When the shield machine advances forward, the propulsion cylinder must provide sufficient thrust to ensure that the shield machine moves forward. The thrust of the propulsion cylinder acts on the formation through the cutterhead. When the cutterhead rotates, a large friction force will be generated between the part of the cutter facing the formation and the formation. Abrasive wear is the main form of wear of the part of the cutter facing the formation.

The wear of tools generally increases gradually from the inside to the outside. Since the angular speed of the revolution of the tools on the cutterhead is the same, but the radius of each tool is different, the linear speed of each tool is different. In the same time, the external tool travels more, the cutting task is heavier, and the wear speed increases accordingly. But when it comes to the edge knife, due to the closer distance between the knife tips, the rock-breaking workload of each knife is reduced accordingly, and the wear rate is also reduced accordingly.

The difference in linear speed between the center point and the edge position when the cutterhead rotates. When the cutterhead rotates once, the tool installed at the position 3m from the center point of the cutterhead is the distance traveled by the tool installed at the position 1m from the center point. 3 times, theoretically under the same working conditions, the life of the tool at the 1m position should be 3 times at the 3m position.

2. Common failure forms of cutting tools

1.Normal tool wear

During excavation with disc cutters, the penetration rate is related to cutting-edge width. As cutter ring wear increases, the cutting edge becomes wider and, beyond a certain range, affects the excavation rate. Because cutter ring manufacturing processes impose limitations and newly installed cutter rings have a height difference relative to the toothed cutters (the specified height difference varies by cutterhead design), normal wear for face and center disc cutters reaches its maximum when the specified height difference relative to the toothed cutters is reached. Gauge disc cutters, however, generally have a wear limit of 15 mm because of tunnel diameter differences and the requirements of their position.

Normal wear is the main form of tool failure. Its wear shape is relatively uniform within the width of the cutting edge, without eccentric wear or string wear.

2. The knife ring is broken

When the hob is constructed in uneven sandy and pebbled strata, it is difficult for the hob to achieve pure rolling, causing the hob to rotate sometimes but not to rotate. It is easy to cause the cutter ring to break due to the impact of the cutter ring. During the excavation process, due to the sudden hardening of the ground or the falling off of some parts of the cutter head or other iron parts getting stuck between the cutter edge and the formation, local overloading of the cutter ring will cause the cutter ring to concentrate stress and break. At the same time, the interference between the cutter ring and the cutter body does not meet the requirements, which will also cause the cutter ring to break.

3.Bearing damaged

During construction, a cutter ring may sometimes be worn into one or more chord-shaped flats. This is known as uneven wear. It is mainly caused by damaged cutter bearings that prevent free rotation. Failure to detect uneven cutter wear promptly during tunneling is very dangerous. When one cutter fails from uneven wear, the two adjacent cutters can fail from overload. The damage can then spread rapidly outward until every cutter on the cutterhead has failed. Excessive disc-cutter starting torque can also cause intermittent rotation, resulting in uneven cutter-ring wear. Excavated spoil should therefore be observed promptly during tunneling. If long, wide rock chips appear, stop the machine immediately for inspection.

The main reason for bearing damage is that the bearing is overheated and locked due to overload of the bearing or poor cutter head cooling system, or the cutter seal is damaged and oil leakage is not discovered in time. The damage caused by poor bearing lubrication will also cause the cutter ring to wear eccentrically and fail.

4.Tool leakage

The bearings of the vast majority of tools with oil leakage have failed due to the inability to lubricate. Most of the outer rings of the bearings have been pitted or turned blue. At the same time, the floating seals of the tools have failed. A very few tools have leaked oil from the oil filling holes. The main causes of oil leakage are as follows:

(1) Due to rapid changes in rock conditions (mostly relatively developed broken rock formations) or unreasonable tool changes (the wear amounts of adjacent tools vary greatly), individual tools are overloaded, causing pitting and peeling of the load-bearing surface of the tool bearings, and the peeling metal fragments and particles cause seal failure.

(2) Due to the poor cooling system of the cutter head, the temperature of the bearing and lubricating oil is too high, and the floating sealing surface is burned, causing the seal to fail.

(3) Large rocks with high strength or broken front tooth cutters and edge scraper teeth are squeezed into the sealing area of the cutter end cover, causing them to be extruded and deformed, causing the sliding seal to fail.

(4) The life of the tool bearing and floating seal has reached its limit.

(5) During the assembly or repair process of the tool, debris mixed in the tool caused the floating sealing surface to be scratched and failed.

According to hard rock (TBM) construction experience, tool damage caused by tool seal failure accounts for approximately 70%. The damage to the tool seal is related to the assembly of the tool and the pressure and high temperature resistance of the tool seal. Generally, the maximum temperature that the disc hob seal can withstand is 80℃, and the maximum water pressure and air pressure that it can withstand can reach 0.8MPa.

5. Knife ring peeling off

Because the cutter ring is under great force during the process of cutting the rock, whole pieces of the cutter ring fall off the surface of the cutter ring, but the entire cutter ring does not break, which is called "knife ring peeling". The peeling off of the cutter ring is caused by fatigue cracks on the surface of the cutter ring, which gradually expand to cause microscopic fractures and peeling off due to wear. If the peeling piece is small, it generally does not affect the normal operation of the tool.

6, the tool mounting bolts are loose or broken, or the retaining ring is broken or fallen off.

Since the force of breaking rock during excavation is greater, especially when excavation in hard rock, the vibration is also relatively severe, which often causes the tool connecting bolts to loosen and break, causing the tool to be damaged and making excavation impossible. Loose or broken tool mounting bolts may cause the tool to fall off or break. The retaining ring is used to prevent the parallel displacement of the cutter ring along the axis direction. If the retaining ring breaks or falls off, it will cause the cutter ring to shift and cause serious damage.

3. Preventive measures for abnormal tool wear

1. Reasonably select the tunneling thrust. The maximum bearing capacity of the cutterhead can be calculated based on the number of hobs and the maximum radial force that each cutter can bear. Adding the soil pressure reaction force, friction force, rear supporting drag force and tail brush resistance, the maximum thrust acting on the cylinder can be calculated. Therefore, the maximum thrust can be controlled to prevent tool overload.

2. Reasonable control of penetration. According to the shield machine penetration calculation method Pe=V/n, the greater the penetration, the greater the force on the tool. When digging into rock formations above 100Mpa, the penetration should not be greater than 4mm. When excavation in soft rock, it is recommended that the penetration should not be greater than 25mm.

3. Replace protective tools and damaged tools promptly. Damage to individual hobs may cause associated damage to surrounding tools, so check and replace damaged hobs in a timely manner if conditions permit. When installing transition knives, the height difference between adjacent knives should not be greater than 5mm. At the same time, cutting tools should be replaced in time.

4. Ensures the improvement effect of ballast soil, ensures the effect of adding additives, and ensures the fluidity of ballast soil, which can effectively prevent the production of mud cake and reduce the risk of the tool being caught in the mud cake.

(Source: Shield TBM Expert Exchange Network)