1. Shield Machine Main Bearings: The “Choke Point“ of National Heavy Equipment

Shield machines and full-face tunnel boring machines (TBMs) are the “national heavy equipment“ for underground space development, widely used in subway, railway, highway, water conservancy and other tunnel engineering projects. The main bearing is known as the “heart“ of the shield machine, responsible for transmitting cutterhead torque and withstanding boring loads.

 

However, this “heart“ has long been firmly held in the hands of foreign brands. Previously, 90% of domestic super-large shield machine main bearings relied on imports, with German ThyssenKrupp, Swedish SKF and other brands occupying 85% of the market share. Single unit procurement costs exceeded 20 million yuan, with lead times up to 12 months – severely constraining China‘s tunnel engineering construction schedules. Imported main bearings have been used in over 5,000 projects globally, while domestic brands account for only over 30 sets. The main bearing is listed as one of the 35 “choke point“ key technologies restricting China‘s industrial development.

 

The consequences of main bearing failure are extremely severe:

 

Extreme impact loads causing raceway spalling – Shield machines face multiple complex operating conditions during boring, including start-stop, turning, correction and pitching, enduring strong impact loads with coupled multi-degree-of-freedom bending, torsion, axial and oscillating forces. Rolling contact fatigue is the primary failure mode of main bearings, with raceway surfaces developing fatigue cracks under repeated alternating stress that progressively grow into spalling. Research shows that raceway damage risk exceeds roller damage risk, and main thrust raceway damage risk exceeds the other two raceways. Raceway fatigue, internal gear fatigue and corrosion are the primary failure modes of shield machine main bearings.

 

Mud intrusion and seal failure – When shield machines bore underground, the muck, mud and groundwater excavated by the cutterhead constantly threaten the main bearing interior. If lip seals are poorly installed or grease injection is insufficient, mud and sand can easily enter the seal cavity, scoring and wearing the seal lips. Once the sealing system fails, sand and gravel particles carried by mud enter the bearing, mixing with grease to form abrasive paste that accelerates raceway and rolling element wear – the bearing rapidly fails.

 

Grease degradation and loss – Shield machine main bearings operate under low-speed heavy-load conditions, with grease subjected to extremely high contact stress. After moisture and chemicals from mud intrude, grease is diluted and emulsified, losing load-carrying capacity, leading to raceway scoring, wear and local deformation.

 

Extremely difficult maintenance – Main bearings can exceed 8 meters in diameter with single unit weights over 40 tons. Once failure occurs inside the tunnel, repair is extremely difficult. Traditional solid bearing replacement requires lifting the entire shield machine to the surface, with project delays measured in months and economic losses in the tens of millions of yuan.

 

2. NMT Localized Main Bearings: From “Choke Point“ to “World Record“

With support from national key projects, NMT collaborated with research institutes and enterprises to overcome key technologies in material formulation, heat treatment processes and precision control, fully establishing the main bearing“s complete chain of “design – materials – manufacturing – testing.“

 

In 2025, NMT successfully developed the world‘s largest integral shield machine main bearing, setting three world records:

 

8.61 meters in diameter – world‘s largest

 

62 tons in weight – world‘s heaviest

 

100,000 kN axial load capacity – world‘s highest

 

This main bearing is applied to 18-meter-class shield machines and can continuously bore over 10 kilometers. An expert panel led by Academician Qian Qihu evaluated that all indicators reached “international advanced level for similar products,“ with some indicators reaching “international leading level.“

 

From the 3-meter class to the 12-meter class, NMT has achieved full series coverage of shield machine main bearings. The first domestic 11-meter-class large-diameter shield main bearing has been successfully manufactured, and the 14.57-meter super-large diameter earth pressure balance shield “Jiaoping No.1“ equipped with indigenous main bearings has officially rolled off the production line. Main bearing prices are only 1/3 of imported alternatives, with significantly shorter delivery cycles.

 

In terms of materials, NMT main bearings are developed using domestic specialty steel, employing 20Cr2Ni4A carburizing steel with isothermal transformation control for gradient structure. Using low-oxygen rare earth steel key technology, rare earth bearing steel shows over 40 times improvement in tensile-compressive fatigue life and 40% improvement in rolling contact fatigue life. Raceway roundness is controlled to ≤2μm, roller cylindricity to ≤1μm, with errors controlled within 0.001mm.

 

3. Three-Row Cylindrical Roller Construction – Precision Design for Ultra-Heavy Loads

TBM main bearings primarily use three-row cylindrical roller bearings, consisting of inner ring, outer ring, main thrust rollers, auxiliary thrust rollers and radial rollers. Each row has its own role – main and auxiliary thrust rollers bear the enormous axial forces, while radial rollers bear radial loads and tilting moments. This configuration enables a single bearing to simultaneously withstand axial, radial and tilting moment compound loads, achieving maximum load capacity within extremely limited space. The most commonly used three-row three-column cylindrical roller combination bearings are available in internal gear and external gear configurations.

 

During shield machine boring, the main bearing endures complex concentrated loads including enormous axial forces and tilting moments. NMT optimizes internal bearing structure design using nonlinear spring equivalent cylindrical roller modeling methods for finite element analysis to reduce maximum contact loads. Under eccentric loads, the safety factors of the main thrust raceway and main thrust rollers are relatively small – NMT‘s precision crowning effectively reduces edge stress concentration that causes early spalling.

 

The cage is one of the key components of the main bearing. NMT shield machine slewing bearing cages use integral construction with main material of alloy steel forgings. Aluminum bronze or lead brass is used as anti-friction material at the cage guide surface and contact areas with raceways, balancing strength and anti-friction performance.

 

4. Triple Seals + Centralized Lubrication – Blocking Mud and Groundwater

When shield machines bore underground, the muck, mud and groundwater excavated by the cutterhead constantly threaten the main bearing interior. The sealing system is a complete set of sealing devices installed between rotating and stationary parts to prevent muck from entering the main bearing. NMT shield machine main bearings feature a multi-seal and centralized lubrication system specifically designed for tunnel boring conditions:

 

▸ First layer: Outer ring labyrinth seal – Multiple tortuous channels use centrifugal force from rotation to expel mud, muck particles and moisture, forming the first non-contact protection barrier.

 

▸ Second layer: HBW grease cavity (Chamber A) – A grease pump supplies HBW grease, filling the labyrinth cavity with high-viscosity HBW. HBW grease is pressurized to slightly higher than the earth pressure, ensuring grease flows toward the earth chamber to prevent mud water backflow. The shield machine system uses a control program to set HBW grease pressure P1 = P0 + ΔP1, linking the first seal cavity pressure with the earth pressure.

 

▸ Third layer: EP2 grease cavity (Chamber B) + FKM secondary seal – Chamber B supplies constant EP2 grease through a multi-point grease pump, with injection pressure slightly higher than Chamber A pressure. EP2 grease is squeezed out from the first seal lip to lubricate the lip and support the seal. The shield machine system sets EP2 grease pressure P2 = P1 – ΔP2, ensuring a relatively stable pressure difference between the first and second seal cavities.

 

▸ Chamber D detection cavity – Chamber D is empty – if gear oil or grease appears, the presence of corresponding media in the cavity indicates the condition of the sealing system, enabling real-time seal condition monitoring.

 

5. Product Series – Full-Series Shield Machine Main Bearings

NMT offers a complete main bearing product system covering multiple shield machine and TBM applications:

 

▸ 8-Meter-Class Ultra-Large Main Bearings

 

8.61 meters in diameter, 62 tons in weight, 100,000 kN axial load capacity – setting three world records. Applied to 18-meter-class shield machines capable of continuous boring over 10 kilometers. Three-row cylindrical roller construction, deep-case carburizing, 20Cr2Ni4A carburizing steel. Compatible with external gear, internal gear and flangeless configurations.

 

▸ 11-Meter-Class Large-Diameter Main Bearings

 

The first domestic 11-meter-class large-diameter shield main bearing further promotes localization of shield machine core components and breaks through large-diameter main bearing technical barriers. Expert panel leader, Academician Yang Huayong of the Chinese Academy of Engineering, commented: “This will further promote the localization of shield machine core components, break through large-diameter main bearing technical barriers, and is of great significance for promoting the complete localization of shield machines.“

 

▸ 3-Meter to 12-Meter Full-Series Main Bearings

 

Full series coverage from 3-meter class to 12-meter class. Reliability and other indicators reach international levels, with prices only 1/3 of imported alternatives. The 14.57-meter super-large diameter earth pressure balance shield “Jiaoping No.1“ equipped with indigenous main bearings has officially rolled off the production line.

 

▸ Small and Medium Diameter Main Bearings

 

Demonstration applications of small and medium diameter main bearings have been completed. Small-diameter shield machine main bearings typically use four-point contact ball and crossed cylindrical roller configurations, with some using double-row tapered roller configurations.

 

6. Quality Control – Full-Chain Assurance from Materials to Finished Products

NMT implements a strict quality control system for shield machine main bearings:

 

Self-reliant materials – Developed using domestic specialty steel, establishing self-reliant material specifications for key components of large main bearings, overcoming key technologies in purification preparation, homogenization control, low-stress regulation, large-depth hardening and composite material processing.

 

Precision machining – High-precision machining ensures raceway roundness ≤2μm, roller cylindricity ≤1μm, with errors controlled within 0.001mm.

 

Full-chain technical system – Established a full-chain technical system for large main bearings including structural design, material preparation, processing assembly and testing evaluation.

 

Reliability verification – Products pass reliability load testing verification, meeting host machine usage requirements and installation conditions. Conducted force analysis, structural optimization and life evaluation of main bearings, establishing domestic main bearing application verification technical solutions.

 

7. Application Cases – From Laboratory to Tunnel Construction Sites

▸ 8-Meter-Class Domestic Main Bearings Pass Installation Application Demonstration

 

An expert panel led by Academician Qian Qihu evaluated that all indicators of the domestic 8-meter-class large-diameter main bearing reached the advanced level of similar imported products, with some indicators reaching the international leading level.

 

▸ 11-Meter-Class Main Bearing Successfully Manufactured

 

The first domestic 11-meter-class large-diameter shield main bearing was successfully manufactured at Luoyang Bearing. Expert panel leader, Academician Yang Huayong of the Chinese Academy of Engineering, gave high praise.

 

▸ “Jiaoping No.1“ 14.57m Shield with Indigenous Main Bearing

 

The 14.57-meter super-large diameter earth pressure balance shield “Jiaoping No.1“ equipped with indigenous main bearings officially rolled off the production line in Changsha, achieving self-reliance and control of shield key components.

 

▸ Full-Series Main Bearings in Volume Production

 

Full-series main bearings from 3-meter to 12-meter classes are arranged in sequence. By overcoming key technologies in material formulation, heat treatment processes and precision control, the “final link“ of full industrial chain self-reliance has been completely opened. Domestic main bearing installation rates continue to rise, with long-term partnerships established with CRCHI, China Railway Construction and other enterprises.

 

8. Lifecycle Cost Optimization – From “Controlled by Others“ to “Self-Reliant“

The breakthrough in shield machine main bearing localization brings not only technological self-reliance but also huge optimization of total lifecycle costs:

 

Hidden costs of imported main bearings:

 

Single unit procurement cost exceeds 20 million yuan

 

Lead times up to 12 months

 

Import dependency 90%

 

Continuous supply cut-off risk

 

Slow after-sales response, long spare parts lead times

 

Core advantages of NMT domestic main bearings:

 

Procurement cost reduced by 67% – Price only 1/3 of imported alternatives

 

Delivery cycles significantly shortened – From 12 months to weeks

 

Full series coverage – 3-meter to 12-meter+ classes, meeting different diameter shield machine requirements

 

Technical indicators internationally leading – Some indicators reaching international leading level

 

Self-reliant and controllable – Completely eliminates import dependence and supply cut-off risks

 

Full-chain technical support – Design, materials, manufacturing, testing – all self-reliant

 

Extended life of equipment – Longer overall equipment overhaul intervals

 

For shield construction companies, tunnel engineering firms and underground space development units, the total cost of ownership with NMT domestic shield machine main bearings is significantly lower than the procurement and maintenance costs of imported main bearings – what you save is not just tens of millions in procurement price difference, but the enormous project delay risk and the operational certainty of self-reliance and control in tunnel engineering.