German coupling and system specialist Malmedie has launched the ISC-HOIST system at the TOC Europe conference in Amsterdam.
The system aims to address the problems experienced on ship-to-shore (STS) standard main and boom hoists with regard to emergency-stops or sudden power cut-offs. In these circumstances, the rotational kinetic energy (inertia) of the motors cannot be immediately removed – and this energy generates peak overloads to the drive train and specifically to the gearbox against the closed brakes.
Additionally, due to the “traditional” dual brake circuits (high speed service brake and low speed emergency brake), damaged bearings, gears and other components occur due to the resulting tooth flank change of the gearing.
According to a statement from Malmedie: “High speed service brakes are not installed for safety reasons – this role has been taken already long time ago by secondary emergency brakes at the rope drums because this brake circuit covers also gearbox failures, etc. The service brakes are still in place for a simple technical reason: during normal operation, the motors can accelerate and decelerate the load (motors are acting like brakes) – but when the motor speed is zero, it cannot develop torque in order to hold the load. This is done by setting the service brakes.”
However, Malmedie continued, on emergency stops: “Both brake circuits are set because both are ‘failsafe’ and that leads to problems. “Synchronising” both brake circuits is a rather theoretical approach and practically almost impossible – additionally, it´s not eliminating the root cause (inertia) at all. In a typical main hoist arrangement and as per a sample case study (elasto-kinematic body calculation), the max. occurring torque on the reducer and drive train is about 6.7 times the static load torque (E-Stop down with full speed and empty spreader).
Malmedie’s new Inertia Stress Compensating Hoist (ISC-HOIST) aims to solve these problems by taking two steps: eliminating the motor inertia on emergency stops down and substituting the service brakes, which will also eliminate the inertia of high speed brake discs.”
Upon emergency stops or power cuts while lowering the load, the motors can “idle” through the incorporated freewheels in the motor coupling, thus removing any peak overloads due to their inertia on the drive train.
In normal operation, the freewheels are kept closed due to the permanent existing load direction (gravity) – and they automatically open when this load direction is “cut” when the brakes are set.
This “first step” solution was introduced by Malmedie already at TOC Europe 2016 in Hamburg. As a layout option, when the freewheel is an integral part of the Gearbox, the motors can be directly connected to the Gearbox, reducing the quantity of components.
To substitute the service brakes, so called (active) “Motor-Locks”, electromagnetic Gear-Couplings, face geared and with a positive fit are used. The Motor-Locks are normally open (spring released during normal operation), enabling the motors to idle on emergency stops down.
Deceleration is done by ramping down the motors itself, the motor acts as a brake, and once the motor speed is zero, the Motor-Lock is electromagnetically set to hold the load. Electromagnetic Gear-Couplings are available from various manufacturers, e.g. from “Heid Antriebstechnik” in Austria. They are small, simple and lubrication or any other maintenance is not required. They can be installed at the backend of the motors or even integrated in the motors.
Rope drum calliper brakes are still in place to stop the load in case of emergency stops or power cuts or as a result of any malfunction of the system (controlled by speed encoders). Instead of single “big” callipers, dual “small” callipers can be used and operated in two independent circuits alternatively.
However, there are also alternative system layouts possible. External freewheels in combination with motor couplings, Motor-Locks at the back end of the motor or at the high speed shaft of the gearbox and/or single calliper brakes.