Civil Engineering Reference
In-Depth Information
Fig. 10.6
3D-impression of the
cutting wheel
too great. In addition, very large forces would operate on this closed steel
part of the cutting wheel.
In order to overcome this problem, the design of theTBM provided for a sec-
ond machine with a small diameter in the centre of the large cutting wheel.
This so-called active centre cutter was fitted with its own coil circuit and had
the task of drilling out and removing the soil in the centre of the excavation
front to prevent clumping.The active centre cutter worked independently of
the large cutting wheel and could revolve much faster but also slower, going
in the same or the opposite direction to the large cutting wheel. Fur-
thermore, it was possible to move the active centre cutter in the axial direc-
tion with respect to the cutting wheel.
The cutting wheel was driven by the driving mechanism which could be
tilted in its entirety by means of a ball joint and sliding sleeves and could
be moved in the axial direction supported by bearings in the steel structure
between the submerged wall and the pressure wall.This made it possible to
tilt the cutting wheel and to move it forwards.The axial force was transferred
to the main frame through three groups of hydraulic steering cylinders.
Main bearing seal
The seal of the main bearing between the rotating part of the driving mech-
anism, to which the cutting wheel is attached, and the stationary part of
the unit is an essential (and vulnerable) part of a tunnel boring machine.The
bearing not only makes it possible for the cutting wheel to rotate smoothly,
it must also be watertight and must be able to stem and reduce the prevail-
ing soil and water pressures at the excavation front. The main bearing seal
of the boring machines used for the WesterscheldeTunnel had a robust con-
struction and consisted of a number of 'small chambers' which were sepa-
rated from each other by neoprene lip seals. In the first chamber, the
so-called labyrinth, grease was constantly pumped under a higher pressure
than the one prevailing in the working chamber, so that there was a constant
loss of a certain amount of grease. This was not only important to lubricate
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