Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering
Transport Technology

P.J. van Dijk Safety systems against snag loads in cranes.
Literature survey, 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.

Hydraulic systems
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.

Torque limiters
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 exists.
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.

Reports on Transport Engineering and Logistics (in Dutch)
Modified: 2007.12.04; , TU Delft / 3mE / TT / LT.