Faster, green snag load protection



Lawrence E. Chizmar, Jr, Pintsch Bubenzer, Flemington, NJ, USA


Snag loads on container cranes happen fast. Your snag load protection must be faster to prevent damage; easy to reset to save time and as green as possible to avoid spills.

Don’t get too hung-up with snag events

Periodic snag loads are an unfortunate occurrence during container handling operations particularly with STS (ship to shore) container cranes. A snag event is defined as a sudden stopping of the lifting spreader, particularly during unloaded, high-speed hoisting when the spreader hangs up in a vessel cell guide. The main hoist drive must be stopped and the rotating kinetic energy of the drive absorbed or dissipated as heat as quickly as possible, to prevent damage to the spreader, hoist ropes and other hoist system elements. Although there is significant stretch in long hoist wire ropes, once a snag occurs it is essential that the hoisting motion be stopped as quickly as possible, to avoid damage. Once the snag event is over, it is essential that the crane be placed back into service as quickly as possible.

Conventional hydraulic snag load protection systems in use on many STS Container Cranes utilize long-stroke hydraulic cylinders to relieve the sudden strain on main hoist ropes in a controlled manner. As the snag load hydraulic system is signaled by a snag event to release the hoist wire rope, hydraulic oil flows through metered orifices, heating the oil and thereby dissipating some of the hoist drive kinetic energy as heat. The reaction time of such a system includes time delays associated with the drive control system and the time required to move the hydraulic cylinders to force the oil through the orifices.

Once the snag event has ended, the hydraulic cylinders must be reset, the system checked for leaks, and any damage assessed.

Given that snag events are quick, typically lasting 0.5 seconds or less, it is possible that the reaction time of such a hydraulic snag system can exceed the actual snag event time, thereby not completely eliminating damage. In addition, a significant amount of on-going maintenance is required to change oil, filters, seals and the cylinders which adds to the overall cost of a hydraulic snag load protection system, and includes the possibility of hydraulic oil spills.

The new green non-hydraulic snag load system

A recently developed, non-hydraulic snag load protection system is now available with reaction times more suitable for preventing damage, which can be caused by snag load events. The key elements of this new approach to snag load protection are mechanical safety couplings in the high-speed main hoist drive train, which react in 0.005 seconds to a snag event by decoupling the main hoist motors from the drive. (Figure 1 shows an installed safety coupling).

Quick-setting main hoist emergency brakes are then triggered to stop the main hoist drums, with partial brake torque achieved in 0.030 seconds, and full brake torque applied in 0.100 seconds. The graph in Figure 2 is an actual chart recording, indicating point B, 0.030 seconds, and point C, 0.100 seconds. This recording shows actual times from a test on one of the new operational snag load systems. This new system is controlled by a separate PLC with special sensors, quick-setting main hoist emergency brakes, individual main hoist rope load measuring pins and high-quality main hoist high-speed brakes with lowinertia aluminum composition brake discs. The safety couplings in the high-speed main hoist drive and/or the individual hoist rope load measuring pins sense the snag in the hoist ropes instantaneously. The safety couplings separate, taking the inertia of the main hoist motors out of the drive. The snag system PLC coordinates with the crane drive PLC to shut off power to the main hoist motors, and trigger the fast-setting main hoist emergency brakes to stop the main hoist drums. All of this is accomplished in 0.1 second, minimizing the possibility of spreader, hoist rope or hoist system damage.

Putting the crane back into service is quick and simple

Following a snag event, separation of the safety couplings and stopping of the main hoist drive, the crane operator can lower, not hoist, the spreader at an unpowered slow lowering speed generated by the snag system PLC slightly opening the high speed brakes so that the spreader can be freed from the vessel cell guide and checked for damage.

Once checks have been made, and no damage noted, the safety couplings are manually reset by aligning the couplings and gently hammering the coupling safety elements that separated on each coupling back into place. This takes no more than a few minutes. The snag system PLC control is then reset with a key switch, coordinated with the crane control and the crane can be placed back into service.

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