Bearing and seal design considerations for tunnel boring machines

The safe, reliable operation of tunnel boring equipment relies on the integrity of critical bearings and seals

Once construction starts, there’s no going back. A tunnel boring machine (TBM) is not equipped with a reverse gear, so it is imperative that the machine is able to complete its job without suffering any significant mechanical failure on route. Where problems do occur, accessing the machine for repair can be a complex, costly and time-consuming project, especially with the main slewing bearing installed inside the TBM’s gear box. The main slewing bearing is the heart of the machine and in case of failure, it cannot be replaced easily, potentially causing months of delays and cost overruns that could easily add up to millions of dollars.  

For design engineers, the challenge of delivering high levels of reliability and availability are compounded by the extremely tough working conditions associated with tunnelling works. TBMs combine several highly undesirable operating conditions for key components, including slow rotating speeds, high static and shock loads, and a wet, dirty environment. Together, these characteristics are a recipe for accelerated wear and component damage. 

For decades, SKF has collaborated with major TBM manufacturers to develop robust bearing, sealing and lubrication solutions that can meet the requirements of the most demanding tunnelling projects in the world. This article describes some of the key challenges associated with the design, operation and maintenance of these components, and the solutions available to address them. 

Cutting disc bearings
A large TBM uses a large number of cutting discs, each rotating on a pair of taper roller bearings (TRB). In operation, these bearings are subject to high transient loads, which rise dramatically as the disc is forced into the rock and are suddenly released as the material fractures. The rotating speed of the disc depends on its position on the cutter head, but it is typically in the range of 10 to 20 rpm. The use of ever-larger TBMs has led to a commensurate increase in cutter disc diameter, which has grown from a standard of 13 inches some years ago to 17, or even 19 inches, on the largest modern machines. 

The location of cutting disc TRBs makes it very likely that bearing surfaces will become contaminated during operation. This, combined with the presence of extreme shock loads means bearing manufacturers must design the bearings to cope with uneven loading and highly localised forces on rollers and raceway surfaces. At the same time, consideration must be given to the nature of a premature bearing failure should it occur. The priority here is to avoid complete fracture of a raceway, which would lead to blocking of a disc leading to damage of multiple discs. 

Building a bearing that can handle this environment requires careful attention to geometry, material selection and surface treatment approach. Design engineers need to make use of proprietary finite element analysis tools to optimise key bearing design and manufacturing parameters, to deliver the best combination of characteristics. Small adjustments to raceway geometry can have a big impact on the bearing’s ability to cope with irregular loads, for example, while the use of precisely controlled case-hardening of raceways provides rolling surfaces of sufficient hardness while retaining a tough core to prevent bursting in failure conditions. 

Testing cutting disc bearing presents another challenge for manufacturers. There is no test rig available that can adequately reproduce the rigours of real-world operation, and operating conditions can vary significantly from project to project. As a result, bearing design engineers must rely on a combination of experience and computer simulation when evaluating a new design. 

The condition of bearings removed from worn cutting discs can also provide useful information about their operating conditions. Expert analysis of wear and damage to bearing surfaces can reveal opportunities to modify operation and maintenance procedures to maximise bearing life. In cases where projects have faced unexpectedly challenging ground conditions, SKF engineering teams have even been able to use insights gained from damaged bearings to produce customised solutions designed to offer improved performance. 

Since disc replacement operations are conducted underground, they must be as straightforward as possible, requiring minimal use of time, manpower and specialist tools. The reliable operation of the taper roller bearings inside the discs, however, relies on an appropriate level of pre-load, which must be set by the maintenance team when the bearings are mounted. 

In practice, operators have limited ability to assess bearing preload in the field and must rely on the torque measured on the arrangement during assembly. To assist operators in this critical assembly step, the TRBs used in cutting discs are manufactured to tight dimensional tolerances, thereby ensuring the closest possible correlation between mounting torque and preload. Other factors come in to play as well, however, including the condition and lubrication of mounting hardware, so careful inspection of components and adherence to standard mounting protocols are essential. Bearing suppliers can also provide support to operators in the development of appropriate disc bearing replacement procedures and the training of maintenance personnel.