TUNNEL BORING
Bearing up
under pressure
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
Atunnel 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
signi cant 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. As
Paul Dysiewicz, engineering manager at
SKF, explains: “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,” states Dysiewicz. So,
what exactly are the key challenges
associated with the design, operation
and maintenance of these components,
and how can these be addressed to get
the best outcomes?
CUTTING-DISC BEARINGS
A large TBM uses several 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 also 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,” he adds.
“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 nite element analysis
tools to optimise key bearing design and
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