The main methods used to supply power to overhead cranes that have been used up until recently have been festoon systems, cable drums and conductor bars.
Metreel, a UK-based supplier of all these alternatives, explains that festoon systems comprise a series of wheeled trolleys running either in an enclosed track or on an I beam. They can handle either round or flat cable and support the cable in loops that open and close as the trolleys move with the crane. The ability to use such a system depends upon the space available to accommodate the storage required when all the trolleys are bunched together, as well as the depth of the cable loops.
With a cable drum system, the cable is wound round a tensioned spool and is reeled in or out as the crane moves. Tension is applied either by a spring or a motor and, according to Metreel, this can be a cost-effective solution for small cable and short travel distances. However, cable drums tend to be less widely used on standard overhead cranes because of the size of the reel required and its associated cost.
In the case of single bar conductor systems, a steel or copper conductor is used for each phase and for earth, each with its own individual plastic shroud for electrical insulation. For many years, this has been the standard method of supplying power to indoor cranes, mainly because of its relatively low cost. Another variation is the enclosed conductor, which uses a single extruded plastic profile with internal segregated slots to accommodate copper conductors. According to Metreel, it offers some technical and space advantages over traditional single bar options but its higher price has led to slow growth in demand.
However, the company recently introduced 4-Ductor, which is claimed to offer all the advantages of an enclosed conductor system with the economy of traditional single bar systems. The 4-Ductor overcomes the cost penalty by using a channel housing with four slots to accommodate the copper conductors. It offers current carrying capacities from 35 to 160A and uses continuous conductors to minimise voltage drop.
The company says: “The system provides for easy installation because a single profile houses all the conductors. There are no worries about having to allow for expansion as the 4-Ductor has been designed with clearance between the conductors and the profile to allow for free movement of the conductors within their channels.” Stemmann-Technik of Germany specialises in trolley systems. Its KW 310 system comprises a series of cable trolleys and towing trolleys capable of operating at speeds up to 240m/min.
The cable trolleys are used to support cables and hoses weighing a maximum of 500kg and individual trolleys are linked using a combination of pvc-sheathed steel cables and rubber ropes. Towing trolleys are the same size as the cable trolleys but are fitted with a moving saddle instead of a rubber buffer at their front end, enabling connection between the trolley and the crane. The trolleys are available in three support radii: 130, 180 and 230mm and four widths from 300 to 600mm.
In many installations, self-powered and free-running trolleys are used, to enable long distances to be covered and high speeds to be achieved. In these cases, the driven cable trolleys are controlled by a drive signal from the crane and the speed of each one is specified by a frequency converter. A typical example of such an installation is a container crane in Bremerhaven, where the crane travel distance is 115m. In this case, the cable support system comprises a towing trolley on the crane, a fixed end support at the end of the track, 15 free-running cable trolleys and three driven trolleys.
The latest alter- native to these traditional systems is the energy chain‚ or cable chain, which uses a flat chain made up of hollow plastic links. Power cables, hydraulic hoses and fibre optics can be enclosed within the chain, which lies alongside the crane and is folded upon itself as the crane moves along the track.
Major suppliers of these systems include Igus and Cavotec. The Igus range has recently been extended by the introduction of the roller energy chain, where links with additional rollers are fitted at pre-determined distances along the chain. This reduces friction as the upper and lower runs move relative to one another and is claimed to offer several benefits over the earlier design, including a maximum travel distance of 800m at a speed of up to 10m/s.
The lower friction means that significantly less power is needed to drive the chain and the rollers help keep friction low even when it is used in dirty environments that would cause problems for the older design. The inner height of the chain is 56mm and widths range from 75 to 600mm.
Cavotec specialises in power supply systems for cranes and other industrial equipment. The Cavotec Group comprises six manufacturing companies, in Australia, Canada, Germany, Italy, Sweden and the UK, as well as 18 sales companies. In the early 1960s it started manufacturing motor-powered cable reels. Since then Specimas, the Cavotec company that specialises in this field, has expanded its range with the production of slipring columns and power cable protection systems.
More recently, Cavotec started working with manufacturing partners, one of which is Brevetti Stendalto. Brevetti has been a pioneer in the application of cable chains, following the development in 1968 of the first nylon cable chain by company founder Gianni Mauri.
They have been used for a wide range of industrial applications, including robots, conveyor systems and special vehicles but their use on cranes is relatively new. In 1997, Brevetti introduced a dedicated range of cable chains for overhead cranes as an alternative to cable trolleys.
The two companies started working together in 1982, when Cavotec sales units started selling Brevetti cable chains in several markets around the world. Today, Cavotec handles marketing, distribution and servicing of Brevetti cable chains in most countries except Italy, France and Austria.
