1. Core Failure Pain Points of Shield Machine & TBM Bearings

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. A main bearing with an 8-meter diameter bears maximum axial forces of 105 kN, radial forces of 104 kN, and tilting moments of 105 kN·m during operation. Once the main bearing fails, the consequences are severe.

 

However, main bearings for shield machines in China still face many challenges. Domestic main bearings suffer from frequent raceway spalling failures (average life only 60% of design value), high seal system leakage rates (over 25% failure rate within 300 hours), and rapid grease degradation (50% life reduction at high temperatures) , resulting in domestic shield machines achieving less than 70% of the single-pass boring distance of foreign counterparts. High-end core components such as main bearings still rely on imports, with import dependency reaching 90%, and German ThyssenKrupp, Swedish SKF and other brands occupying 85% of the market share.

 

Common failure modes:

 

Extreme impact loads causing raceway spalling and collapse – 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 cracks initiating at the working surface or subsurface. Under severe service conditions and complex load combinations, main bearings frequently develop abnormal damage. Raceway surfaces develop fatigue cracks under repeated alternating stress, progressively growing into spalling. 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. 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. 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. Seal failure rates exceed 25% within 300 hours.

 

Grease degradation and loss – Shield machine main bearings operate under low-speed heavy-load conditions, with grease subjected to extremely high contact stress. Grease life is reduced by 50% at high temperatures. After moisture and chemicals from mud intrude, grease is diluted and emulsified, losing load-carrying capacity. Once main bearing lubrication fails, it leads to raceway scoring, wear and excessive local deformation.

 

Abnormal roller wear and fracture – Main bearing rollers in shield machines are prone to abnormal wear, indentations, spalling and even cracking during operation. Once main bearing rollers are damaged, in-tunnel repair is extremely difficult – the main bearing must be excavated and repaired above ground, causing major impacts on the engineering project.

 

Extremely difficult installation and maintenance – Shield machine main bearings are massive and heavy (diameters can exceed 8 meters, single weight 41 tons), and they operate mainly underground. Once failure occurs inside the tunnel, especially for the main drive and cutterhead, 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. Replacing a TBM main bearing during boring requires extremely complex civil engineering schemes and changeover operations.

 

2. Large-Diameter Three-Row Cylindrical Roller Construction + Deep-Case Carburizing – Core Assurance for Ultra-Heavy Loads

✅ Three-row cylindrical roller construction – TBM main bearings mainly use three-row cylindrical roller bearings. The main bearing consists 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.

 

✅ Deep-case carburizing heat treatment – NMT applies deep-case carburizing heat treatment to shield machine main bearing raceways and rolling elements, forming a high-hardness fatigue-resistant surface layer (HRC60-62) while the core maintains sufficient toughness (HRC40-45) to absorb boring impact loads. This “hard outside, tough inside“ material property enables the bearing to maintain structural integrity under extreme conditions of 105kN-level axial forces and 104kN-level radial forces. Using 20Cr2Ni4A carburizing steel with isothermal transformation control for gradient structure, contact fatigue life (10⁷ cycles) can be increased to 3200MPa. Raceway surfaces are superfinished to mirror finish, significantly reducing friction coefficient and heat generation.

 

✅ Ultra-pure rare earth bearing steel – NMT uses ultra-pure bearing steel produced with low-oxygen rare earth steel key technology. The research team at the Institute of Metals, Chinese Academy of Sciences, spent over a decade discovering that the root cause of performance fluctuations in rare earth steel is oxygen content, and developed “low-oxygen rare earth steel“ technology by controlling oxygen levels. Rare earth bearing steel shows over 40 times improvement in tensile-compressive fatigue life and 40% improvement in rolling contact fatigue life. Improved material purity directly suppresses fatigue crack initiation and extends rolling contact fatigue life.

 

✅ Optimized roller crowning and contact geometry – NMT applies precision logarithmic crowning to main thrust rollers and radial rollers for more uniform contact stress distribution, effectively reducing peak contact stress. 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.

 

3. Position-Specific Bearing Solutions – Precision Fit for Four Core Shield Machine Systems

▸ Main Drive Bearings (Three-Row Cylindrical Roller Bearings)

 

The main drive bearing is the most critical bearing in the shield machine, connecting the cutterhead to the drive system and withstanding all boring loads from the cutterhead. NMT offers three-row cylindrical roller bearings with main thrust roller row, auxiliary thrust roller row and radial roller row. Outer diameters can exceed 8 meters, with single unit weights exceeding 40 tons. Deep-case carburizing and precision grinding processes ensure high reliability under extreme conditions of 105kN-level axial forces and 104kN-level radial forces. Compatible with external gear, internal gear and flangeless configurations. Widely used in earth pressure balance shield machines, slurry shield machines and hard rock TBMs. Design life: 10,000 hours of fault-free operation (or cumulative boring of 10 km+), operating speed 1-3 rpm.

 

▸ Cutterhead Drive and Gearbox Bearings

 

The cutterhead drive system is mounted on the shield body through high-strength connection bolts, driven by the main bearing to rotate the drive plate, providing torque to the cutterhead. Gearbox bearings endure high-frequency alternating loads and impacts. NMT offers tapered roller bearings and cylindrical roller bearings with high load capacity and impact resistance to handle the complex forces during cutterhead boring. Due to the non-uniformity of geological structures, the magnitude and direction of shield machine boring resistance can change dramatically – NMT bearings are designed to adapt to extreme conditions such as impact and eccentric loads.

 

▸ Screw Conveyor Bearings

 

The screw conveyor is responsible for discharging excavated muck from the shield body – its bearings are in long-term contact with highly abrasive muck and mud. NMT offers heavy-duty sealed spherical roller bearings and split cylindrical roller bearings, with multi-seal structures blocking muck and mud intrusion, and wear-resistant raceways resisting continuous scouring from abrasive materials. The grease pressure in the seal cavity should be set slightly higher than the excavation face balance pressure.

 

▸ Segment Erector and Auxiliary System Bearings

 

The segment erector requires precise rotation and positioning – its slewing ring and spherical plain bearings endure alternating loads. NMT offers four-point contact ball slewing rings and spherical plain bearings with high precision ensuring accurate segment positioning. Conveyor systems, belt conveyors and other auxiliary equipment bearings also feature heavy-duty sealed designs, adapting to the high-dust, high-humidity tunnel environment.

 

4. Multi-Seal System – Blocking Mud, Muck 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 bearings feature a multi-seal 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: Polyurethane main seal (HBW grease cavity / Chamber A) – High-wear-resistance polyurethane material with lip tightly contacting the rotating surface, effectively blocking high-pressure mud and fine particle intrusion. Chamber A uses a grease pump to supply HBW grease, filling the labyrinth cavity with high-viscosity HBW, pressurized slightly higher than the earth pressure to ensure grease flows toward the earth chamber. If grease pressure is too low, external mud and sand can enter the lip seal and damage it.

 

▸ Third layer: FKM secondary seal + EP2 grease cavity (Chamber B) – FKM (fluoroelastomer) secondary seal resists mud and chemical corrosion, paired with a stainless steel dust shield to form the final defense line. Chamber B supplies constant EP2 grease through a multi-point grease pump, with injection pressure slightly higher than Chamber A – EP2 grease is squeezed out from the first seal lip to lubricate the lip and support the seal.

 

▸ Centralized grease lubrication system interface (Chamber D detection cavity) – NMT bearings feature centralized lubrication ports compatible with shield machine centralized grease systems. 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. Shield machine cutterhead bearings, bearing seals, gearboxes, screw conveyors and other components all require centralized grease lubrication for sealing and lubrication.

 

5. Specialized Lubrication Solutions – Resisting Mud Dilution and Extreme Loads

Shield machine main bearings operate under extreme low-speed heavy-load, mud-splash conditions, with ordinary grease life reduced by 50% at high temperatures. NMT offers graded lubrication solutions:

 

▸ Main Drive Bearings: Extreme-Pressure Water-Resistant Heavy-Duty Grease

 

Suitable for the ultra-heavy load conditions of main bearings with 105kN-level axial forces and 104kN-level radial forces. Formulated with refined mineral oil and specialty additives, featuring excellent extreme-pressure anti-wear performance, high adhesion, good chemical stability, and excellent anti-rust and anti-corrosion properties. Excellent water washout resistance and mud dilution resistance – prevents grease emulsification after mud moisture intrusion. Should have a stable colloidal structure to ensure good lubrication and sealing performance under complex shield machine operating conditions.

 

▸ Cutterhead & Gearbox Bearings: Impact-Resistant Extreme-Pressure Grease

 

Suitable for cutterhead drive and gearbox bearings subject to frequent impact loads. Formulated with solid lubricant additives (molybdenum disulfide or graphite) for additional protection under boundary lubrication conditions. Excellent anti-fretting wear performance for alternating loads during cutterhead boring.

 

▸ Screw Conveyor Bearings: High-Viscosity Anti-Wear Grease

 

Suitable for screw conveyor bearings in contact with highly abrasive muck. High-viscosity base oil with extreme-pressure additives maintains stable oil film thickness under low-speed heavy-load impact, resisting abrasive wear.

 

▸ Main Bearing Seal Grease (HBW/EP2) – Compatible with shield machine main drive sealing systems. HBW grease fills the first seal cavity, EP2 grease fills the second seal cavity. Good pumpability and shear stability, maintaining stable sealing performance in long-distance delivery lines.

 

6. Stock Availability & Customization – Emergency Support for Tunnel Engineering

Common shield machine bearing models (three-row cylindrical roller bearings, tapered roller bearings, spherical roller bearings) kept in stock – Covering over 80% of bearing replacement needs for earth pressure balance shields, slurry shields and hard rock TBMs. Emergency tunnel engineering failures receive rapid response – minimizing project delays.

 

Custom solutions available – Special bore/OD/width (standard range up to 8 meters+), custom three-row cylindrical roller configurations, deep-case carburizing (20Cr2Ni4A carburizing steel), special seal materials (polyurethane/FKM), centralized lubrication system interfaces, custom clearance, split construction (for in-tunnel installation).

 

Rapid customization from samples or drawings – Provide old bearing samples or equipment drawings – NMT completes surveying and custom production quickly. Imported bearing single unit procurement costs exceed 20 million yuan, with lead times reaching 12 months – NMT custom lead times are significantly shorter, with reduced procurement costs. Domestic 3-meter-class main bearings have already been successfully applied in Shenyang Metro projects, operating smoothly in complex geological conditions with all performance indicators meeting design requirements. Cost-effective replacement for imported brands (SKF, Timken, FAG, NSK, NTN) with significant savings.

 

7. Factory Simulation Tests – Stable Tunnel Boring Operation

All products pass four specialized tests before shipment:

 

Ultra-heavy load fatigue life test – Simulating shield machine boring compound loads (105kN-level axial force, 104kN-level radial force, 105kN·m-level tilting moment) to verify fatigue and spalling resistance

 

Mud environment seal test – Simulating high-pressure mud and muck environments during underground boring to verify multi-seal system effectiveness

 

Low-speed heavy-load lubrication performance test – Simulating shield machine low-speed (1-3 rpm), heavy-load conditions to verify grease oil film stability under extreme contact stress

 

Dimensional precision and rotational torque inspection – 100% inspection of bore, OD, width, radial clearance and starting torque for consistency. Raceway roundness ≤2μm, roller cylindricity ≤1μm

 

Defective units scrapped. Consistent batch-to-batch dimensions, clearance and load performance simplify spare parts management and planned replacement scheduling for shield construction companies.

 

8. Lifecycle Cost Optimization – Reducing Downtime and Project Delay Losses

One day of unplanned shield machine downtime loses the entire tunnel construction schedule and project performance. As the “heart“ of the shield machine, once the main bearing fails, the repair cycle is measured in months, with economic losses in the tens of millions of yuan. Replacing a TBM main bearing during boring requires extremely complex civil engineering schemes and changeover operations, significantly impacting project schedules. Frequent bearing replacements increase direct procurement costs and cause project delays and contract default risks.

 

Hidden costs of cheap heavy-duty solid bearings:

 

Raceway spalling reduces main bearing life to only 60% of design value – frequent failures impact project schedules

 

Seal system failure rate exceeds 25% within 300 hours – mud intrusion rapidly destroys bearings

 

Grease life reduced by 50% at high temperatures – lubrication failure causes raceway scoring and local deformation

 

In-tunnel repair of damaged main bearing rollers is extremely difficult – the main bearing must be excavated and repaired above ground

 

Imported bearing single unit procurement costs exceed 20 million yuan, with lead times reaching 12 months

 

Replacing a TBM main bearing during boring requires extremely complex civil engineering schemes and changeover operations

 

NMT shield machine bearings deliver stable service life of 10,000-15,000 hours in main drives, cutterhead drives, screw conveyors and segment erectors (domestic main bearings have already achieved 10,000 hours of fault-free operation):

 

Replacement frequency cut by over 50% – Fewer annual unplanned shutdowns – tunnel project schedules assured

 

Three-row cylindrical roller construction bears ultra-heavy loads – Maintains structural integrity under 105kN-level axial and 104kN-level radial forces

 

Deep-case carburizing resists raceway spalling – High surface hardness, tough core – fatigue life greatly improved

 

Multi-seal system blocks mud and muck – Polyurethane main seal + FKM secondary seal + labyrinth seal – 300-hour failure rate reduced to below 5%

 

Specialty grease resists mud dilution – Water-resistant, mud-resistant, non-emulsifying – grease service life extended

 

Extended life of equipment (cutterheads, gearboxes, screw conveyors) – Fewer disassembly cycles – longer overall equipment overhaul intervals

 

For shield construction companies, tunnel engineering firms and underground space development units, the total cost of ownership with NMT shield machine bearings is significantly lower than the repeated replacement cost of cheap bearings and the long wait times for imported bearings – what you save is not bearing price difference, but the enormous project delay risk and operational certainty of “replacing one bearing, shutting down an entire tunnel line“