Malmedie is well known for its range of safety couplings, and they are a key part of the Safety Overload System (SOS) developed by Pintsch Bubenzer and Malmedie that is now in use as an alternative to a hydraulic anti-snag system on more than 150 cranes. Malmedie has now developed the ISCC to protect the main hoist gearbox during an emergency-stop situation when the crane’s hoist is travelling in the other direction.

Most terminal operators typically specify oversized brakes in the hoist system, to offer more protection against a falling load and to allow for some loss of braking performance as the brakes and lining material age. During normal operation, this does not cause any problems, as the mechanical brakes are mainly used as holding brakes.

However, in an overspeed or other emergency-stop situation, the brakes come on very quickly, and stopping the main hoist system so suddenly has a major impact on the gearbox.

In a main hoist system, the gearbox (or reducer as they are otherwise known) sits between the hoist motors and hoist rope drum. The hoist motor input shafts are known as the “high speed” side of the gearbox, and the output side to the rope drum is the “low speed” side of the system. The brakes on the hoist motor side are called the service brakes, while there are (normally) separate secondary emergency brakes that act on a flange on the rope drum itself.

Any situation that stops the rope drum suddenly, such as a sudden snag or an emergencystop puts a very high load on the gearbox’s internal components, which have to absorb the inertial forces of the gearbox itself, plus specifically the huge inertial forces coming from the main hoist motors and motor couplings with brake discs as it is braked and brought to a stop. 

In an emergency-stop situation,  the secondary brakes on the main hoist drum normally come on slightly ahead of the service brakes on the high speed side of the gearbox, due to the different type of brake system used in each. “That leads, especially on the e-stop “down” (lowering), to a reversal on the tooth flanks in the gearbox caused by the rotating masses from motors, coupling and the gearbox itself,” explained Malmedie.

The principle of the ISCC is to separate the rotating masses quickly, so as to absorb the energy coming in from the motors, without damaging the main hoist gearbox, which is typically the most expensive single component on a crane. ISCC does this through an integrated freewheel in the motor coupling. 

During normal operation, the coupling is kept closed by the load on cranes rope, which is always present regardless of which direction the motors are turning (one way for hoisting, and the other for lowering). However, during an e-stop, the load on the drive system from the ropes instantly becomes zero when the emergency brakes are set. This triggers the freewheel to open and allows the motors to idle, eliminating the sudden and dangerous torque load in the drive train.

The gearbox still has to absorb the load from its own rotating mass, but Malmedie calculates a freewheel coupling between the hoist motors and gearbox can eliminate 35% of the stress on the gearbox.

The ISCC system is patentpending and is now undergoing operational tests. 

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This complete item is approximately 300 words in length, and appeared in the July 2016 issue of WorldCargo News, on page 24. To access this issue download the PDF here.