RELIABILITY AND SPEED
To draw a range of opinion on this topic PS invited the comments of one German and two American specialist engineers.
Reliability and speed are two of the main issues for the layout and design of motor cable reel systems installed on container cranes. Wampfler differentiates two types. On one side the spreader reel on STS cranes for the vertical application; on the other side the reel for the main power supply on STS and RMG cranes.
Spreader applications are always low voltage with lifts between 40 and 80 metres and constantly increasing speeds (up to 240 metres/min). They operate at a very high duty cycle. This means high mechanical stress and tensile load on the cable. Reel bodies of spreader reels are mainly designed for one layer with different types of spooling device for a well-defined winding of the cable and minimization of wear and friction on the cable.
Main power reels are mainly of monospiral type for speeds between 30 and 150 metres/min and travel lengths up to 800 metres with high voltage cables. Actual requests also specify speeds up to 240 metres/min for the main gantry travel. This means very high stress on the cable especially while passing over centre feed points.
Higher speeds for the reel movements result in increasing numbers of moved containers per hour and therefore in increased speed for ship unloading and loading. This means higher productivity at the terminal and shorter standstill of ships in port.
Wampfler mainly uses three different types of drive units. For small to medium sized travel lengths and speed requirements Wampfler recommends the drive unit with standard motor and magnetic clutch.
These permanent magnetic clutches are available for output torque up to 25Nm and can be paralleled by maximum two. As an additional feature there is also a safety system for the cable so that the real tension will never exceed the maximum admissible tensile load of the cable. Magnetic clutches are very efficient drive units, easy to install and to adjust due to the application's requirements.
For applications with high ambient temperature and dusty environments, transfer cars in steel mills for example, Wampfler recommends the use of torque motors as drive units. These motors are stout, heavy and resistant to tough environmental circumstances. The torque motors have a long lifetime but, compared to the drive units with a permanent magnetic clutch torque motors, are more expensive.
For high-speed applications and/or long travel lengths we recommend the use of frequency controlled drive units. The interface towards the cable reel control unit and the crane PLC is realised by different bus systems or by hard wiring contacts (potential free contacts). The actual torque will be adjusted continuously to the required torque during operation (torque mode). The alternative to this is speed mode. Technically, frequency controlled drive units can be used for all applications but this drive unit is the most expensive.
Looking to the different type of cables, we recommend consideration not only of investment costs (purchase price) of the cable but also the operational costs and the appearing costs during service and therefore during standstill times of the system. Highstress operations such as spreader applications require cabling with high maximum admissible tensile loads to cover the application and to warrant a certain lifetime of the cable in the given application. This is due to the high-speed movements in vertical operation.
Power cables and power cables with fibre optics must be optimized with regard to outer diameter and maximum tensile load because of increasing travel lengths and speeds of the main travel.
As to preferences we must differentiate between two types of customers: end users and OEM's. The focus of the OEM is mainly on price, easy installation and commissioning while end users want a reliable, high quality system with a long service life and probably a long warranty. End users also want a cable reel system that is quick to service and easy to change the cable. There is also a tendency to limit the number of suppliers to simplify spare part inventory control.
Today there is a tendency towards faster crane movements, higher duty cycles and therefore a high demand for reliable systems.
Additionally a drift away from festoon systems towards energy chains is appearing. For the main gantry travel there is no real alternative to the HV motor reel although some applications are in operation with energy chains or conductor bars, but these are single solutions.
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