INTERNATIONAL AND SPECIALIZED TRANSPORT
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SEPTEMBER 2013
38
the radius would be increased by 5 m as
the boom from where the lift takes place
sits on the ring. This creates the illusion
that the crane is stronger than it is. In
addition, a larger ring diameter (with the
rest of the crane components remaining
equal) would result in a larger radius and
could wrongly lead someone to believe that
the crane has increased in capacity.
But even when the radius is measured
from the edge of the ring, how do we
handle twin-ring cranes? Mammoet,
Sarens, Bigge and ALE (to name a few)
have all designed and built cranes that
have an inner and an outer ring. It requires
knowledge of those cranes to compare and
select the right one for the job.
Here is where the load moment makes
its introduction. The load moment does
away with centre of rotation and twin-ring
confusion. (The load moment is expressed
in tonne-metres). It is determined by
multiplying the maximum capacity by the
distance from the boom butt hinge point
to the hook block (or strand jack anchor),
where that maximum is measured.
The load moment also does not
discriminate between a ring-shaped
track or a straight track, a full ring or a
partial ring (see figure) and whether the
counterweight sits on the opposite site of
the ring or is positioned in the middle of
the ring or maybe even anchored to the
ground (see figure).
Balancing forces
Counterweight is used to keep the crane
in balance during a lift. Smart use of
counterweight can increase the capacity
of a crane in certain cases. The application
of super lift counterweight increases the
capacity of a crane without having to
mobilise or invest in a larger machine. The
application of super lift counterweight,
however, does not necessarily mean that a
500 tonne crane can now lift 700 tonnes. It
is mostly an increase in capacity at larger
radii. The boom of a crane is exposed to
the highest compression load in a near
vertical position. It is calculated and
engineered to take a maximum load in that
configuration. As the radius increases, the
angle of the boom with vertical increases
as well and the compression forces in the
boom gradually decrease. By adding super
lift counterweight, heavier loads can be
lifted at larger radii, herewith increasing
the compression force in the boom to a
value closer to what it was engineered for.
This can make a 500 tonne crane behave
like a 700 tonne crane without actually
having to lift 700 tonnes.
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THE KNOWLEDGE
ABOUT THE AUTHOR
Marco van Daal has
been in the heavy
lift and transport
industry since
1993. He started at
Mammoet Transport
from the Netherlands
and later with Fagioli
PSC from Italy. His
experience resulted in a book The Art of
Heavy Transport, available at:
com/books/the-art-of-heavy-transport/
ALE AL.SK190
