Tag Archive for: TBM slewing bearing

What Is a Slewing Bearing?

A slewing bearing is a large rotating component that supports heavy loads while enabling rotation between two structures. These bearings handle axial loads, radial loads, and tilting moments simultaneously. They are essential in cranes, excavators, wind turbines, and tunnel boring machines.

Unlike standard bearings that operate at high speeds, slewing bearings in tunnel boring machines work under extreme slow-speed, heavy-load conditions. They must withstand enormous forces while maintaining precise rotational control. The bearing serves as the critical connection between the stationary shield structure and the rotating cutter head.

Why Tunnel Boring Machines Need Slewing Bearings

Tunnel boring machines (TBMs) operate under some of the most demanding conditions in heavy engineering. These massive machines excavate tunnels through rock, soil, and mixed ground conditions. The cutter head must rotate continuously while supporting the entire thrust load of the machine.

Extreme loads and slow rotation

The TBM main bearing is a typical large, low-speed, and heavy-load slewing bearing. It supports the cutter head’s weight, the thrust force from the machine’s hydraulic system, and the tilting moments created when the cutter head encounters uneven rock conditions. The bearing must maintain rotational accuracy while carrying loads measured in thousands of tonnes.

Unique design requirements

Since TBM are designed specifically for geological conditions, bore diameter, and tunnel type, the slewing bearings used must also be designed and manufactured individually for each machine. Each bearing design must be based on individual service life requirements and specifications. This is not a standard product—it is a custom-engineered component for each project.

Critical reliability

The dismounting and repair of a defective main bearing is expensive and involves massive effort, or may not be an option at all. For major projects like the Gotthard Base Tunnel, the reliability of the bearings and smooth operation of the machines is crucial. Four rothe erde slewing bearings were used in that project alone.

How Slewing Bearings Work in Tunnel Boring Machines

Tunnel boring machines use two primary types of slewing bearings: main shield bearings and erector bearings. Each serves a distinct function.

Main shield bearings: connecting cutter head to thrust system

Main bearings connect the cutter head with the thrust system and carry all the cutter head loads for secure and continuous operation. They ensure that the cutter head rotates reliably, allowing it to fight through rock and withstand the loads acting on it.

The main bearing must handle three load types simultaneously:

• Axial load: Thrust force from the machine’s hydraulic cylinders pushing the cutter head forward
• Radial load: Forces perpendicular to the tunnel axis from steering and ground conditions
• Tilting moment: Overturning forces when the cutter head encounters uneven resistance

Erector bearings: positioning tunnel segments

Erector bearings allow for complete rotation of the erector, which positions pre-manufactured concrete segments against the tunnel wall, providing it with stable support. The concrete elements primarily secure the tunnel, and in the second step, these elements support the TBM when moving forward.

These bearings are generally used as single-row four-point bearings or crossed roller bearings. Each type offers application-specific advantages: four-point bearings are extremely resistant to deformations of the adjacent construction, while crossed roller bearings provide low torque resistance for low consumption values and longer service life.

Types of Slewing Bearings for TBM Applications

Different TBM applications require different bearing configurations. The choice depends on load requirements, precision needs, and structural constraints.

Three-row cylindrical roller bearings (main bearings)

The most common configuration for TBM main bearings is the three-row cylindrical roller bearing. This design has three rows of cylindrical rollers arranged axially: two rows of thrust rollers and one row of radial rollers between them. The axial and radial loads are mainly borne by the main thrust rollers, radial rollers, and corresponding raceways, while the tilting moment is comprehensively supported by all three rows.

The challenge with this design is that under load, the inner ring’s axis tilts while the outer ring remains normal. This creates localized contact between the inner ring and radial rollers, causing stress concentration that accelerates wear and shortens service life. Researchers have addressed this by designing radial roller raceways with arc-shaped cross-sections and matching curved rollers, which improves contact posture and reduces stress concentration.

Double-row tapered roller bearings (main bearings)

Alternatively, double-row tapered-roller bearings may be used as main bearings. These bearings handle combined radial and axial loads with high rigidity. The tapered design allows for precise clearance adjustment and good load distribution.

Single-row four-point contact ball bearings (erector bearings)

Four-point contact bearings are specifically engineered for erector applications. Each ball contacts the raceway at four points, enabling the bearing to handle bidirectional axial loads and tilting moments. These bearings are extremely resistant to deformations of the adjacent construction, making them suitable for the variable conditions encountered during tunnel construction.

Crossed roller bearings (erector bearings)

Crossed roller bearings use cylindrical rollers arranged at 90-degree angles to each other. This design provides line contact between rollers and raceways, resulting in higher rigidity than ball bearings. Thanks to their low torque resistance, crossed roller bearings make for low consumption values and longer service life in erector applications.

How Tunnel Boring Machine Slewing Bearings Are Manufactured

Manufacturing TBM slewing bearings requires specialized capabilities. These are not off-the-shelf products—they are custom-engineered for each machine.

Size and manufacturing capabilities

Slewing bearings can be manufactured with a diameter of up to 9.6 meters as a single piece. Segmented slewing bearings are produced with a diameter of up to 18 meters. The main bearing diameter can reach 8.61 meters. China has achieved full product spectrum coverage from 3 meters to 8.61 meters in diameter.

Material and heat treatment

TBM main bearings require high-quality forged alloy steel with induction-hardened raceways. The bearing must achieve 55-62 HRC surface hardness with adequate hardened layer depth to withstand the extreme contact stresses. Industry standards for TBM main bearing steel are being developed, with YB/T 6360-2025 published in 2025.

Lubrication challenges

The main bearing of a TBM is a typical large, low-speed, and heavy-load slewing bearing. Lubrication and cooling usually occur via an oil bath and circulation to ensure good lubrication and heat dissipation. The immersion height of the oil bath is usually 1/2 to 2/3 of the height of the main bearing, and an oil supply system achieves oil circulation. Understanding the lubrication flow field is critical to optimizing lubrication and cooling methods.

Common Challenges and Failure Modes

TBM main bearings face unique challenges due to their large size, complex structure, and harsh operating conditions.

Uneven load distribution

The complex working environment makes the main bearing subject to huge and uneven loads. This causes abnormal peeling and wear of the raceway and rollers, affecting service life. The TBM main bearing often contends with uneven loads due to complex geological conditions.

Thrust side defects

Existing research predominantly concentrates on raceway radial side defects or roller defects. However, defects of main bearings occur mainly on the thrust side. The derivation of defect frequency for thrust side defects remains an area requiring further study.

Lubrication flow field complexity

The main bearings of TBMs are often larger than normal bearings in both size and number of components. They are subject to demanding working conditions and complex behavior in their internal lubrication flow field. The oil-air two-phase distribution in the main bearing is directly related to the lubrication condition and heat dissipation performance.

How LDB Bearing Supports Tunnel Boring Machine Applications

LDB Bearing designs and manufactures high-quality slewing bearings for heavy machinery applications, including tunnel boring machines. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC and gear teeth hardened to 50–60 HRC.

LDB’s TBM capabilities:

• Custom design: Application-specific bearings engineered for individual TBM specifications
• Quality manufacturing: ISO 9001-certified manufacturing with documented inspection reports and full material traceability
• Precision engineering: Capabilities to meet the tight tolerances required for TBM main and erector bearings
• Global reach: Serving 73 countries with over 500,000 units in service

Serving the heavy machinery industry for over two decades, LDB understands the critical nature of TBM bearings. When a tunnel boring machine stops, the project stops. LDB offers the technical expertise and quality assurance that tunnel construction projects demand.

Contact LDB Bearing today to discuss your tunnel boring machine slewing bearing requirements.

FAQs

1. What is the difference between a TBM main bearing and an erector bearing?
The main bearing connects the cutter head to the thrust system and carries all cutter head loads. The erector bearing positions precast concrete segments against the tunnel wall during construction.

2. What type of slewing bearing is used as a TBM main bearing?
Three-row cylindrical roller bearings are the most common configuration, with two thrust roller rows and one radial roller row. Double-row tapered roller bearings are used as an alternative.

3. How large can TBM slewing bearings be?
Single-piece slewing bearings can be manufactured up to 9.6 meters in diameter. Segmented designs can reach 18 meters. Main bearing diameters can reach 8.61 meters.

4. Why is lubrication challenging for TBM main bearings?
TBM main bearings are large, low-speed, and heavy-load components. The oil-air two-phase distribution directly affects lubrication and heat dissipation. The immersion height is typically 1/2 to 2/3 of the bearing height.

5. What causes TBM main bearing failures?
Uneven load distribution from complex geological conditions causes abnormal peeling and wear. Thrust side defects are particularly common but have received less research attention than radial side defects.