P.J. van Dijk
Safety systems against snag loads in cranes.
Report 2007.TEL.7204, Transport Engineering and Logistics.
Sometimes when a crane hoists a load, the load clasps; that is called a snag
situation. This could happen for example with a maritime container due to
irregularities of the ship cells.
At the moment a snag situation occurs the hoisting motors have to be braked
from full power to zero in a very short time. Due to inertia, enormous forces
are acting at the equipment. These forces could be three to four times higher
in comparison with normal hoisting conditions. Therefore an anti-snag device
needs to be installed to prevent damage of the crane.
From the late 1970's several safety mechanisms have been invented for snag
situations. They can be divided into two groups: hydraulic systems and devices
that make use of torque limiters.
The first hydraulic safety systems, invented in the 1980's, continually analyse
the pressure in the boom of the crane. When a predetermined limit is
exceeded, valves are opened, so that the fluid can flow into a reservoir.
In principle are the hydraulic motors connected to a reservoir, so that they
can not build pressure any further.
With the introduction of the modern container cranes in the 1990's the
configuration of the hydraulic equipment changed. In this case are the hoisting
ropes around pulley, which are connected to hydraulic cylinders. Under normal
hoisting conditions these cylinders do nothing; they are only pressurized by
valves in an outward position.
During a snag load forces in the hoisting ropes exceed a limit, which causes the
valves to open. At the same time the hoisting motors are switched off and the
hydraulic cylinders take up the inertia forces by making an inward movement.
After that the container could be trimmed, skewed or listed by separate
supervision of the cylinders, so that it does not snag any more.
Half way the 1990's the first anti-snag methods are introduced, which make use
of mechanical torque limiters. This device functions as a clutch under
normal conditions. When in a snag situation the needed torque exceeds a
limit, load and drive become separated. After that will trimming, skewing or
listing be done by auxiliary motors, to get the load free from snagging.
Before torque limiters were not an option as safety systems for cranes,
because conventional types are sensitive for wear, which causes maintenance
costs. But due to the development of more advanced mechanical torque
limiters wear is reduced and lifetime increased.
Because of this there is a tendency that torque limiters become more and
more a serious replacement for the hydraulics. Most recent inventions are
namely based on torque limiters.
Another advantage of torque limiters is that they react faster than hydraulic
system, because only mechanical parts are involved. This fast reaction time
will lower forces in the crane structure.
Further is it more simple to install a torque limiter in a crane than an
hydraulic system. This is especially an advantage when the crane already
Finally, these devices are very compact and do not cause environmental
pollution. This is in contrast to the much bigger hydraulic cylinders, which
cause environmental impact due to oil leakages.
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