The need to produce faster, more efficient container cranes has led to the examination of each component of crane design to determine how and where improvements in performance and productivity can be found. There are many parts that make up the whole operation of a container crane and of course, each part, big or small is vital. But perhaps one element that can claim to make a major impact on performance is the energy supply cable. The main target for power supply cables for cranes has always been to increase the mechanical stresses resistance in order to permit higher accelerations, and to reduce the cable dimensions in order to save weight (that means costs) in the reeling system; unfortunately, in terms of design, these two elements are not going in the same direction. Generally speaking, reeling and unreeling movements create tensile stresses inside the cable because of the relative motion between the different inside elements (conductors, inner sheath, anti-torsion protection and outer sheath); this motion generates an uneven elongation and therefore it is possible that the conductors on the outer layer could be damaged or broken. In order to guarantee maximum performances, the cable manufacturer has to design improved products whilst at the same time keeping the design in compliance with recognised Standard References – usually in accordance with VDE Standards. Great effort must be taken to get continuous improvement in the flexibility of the cable through a special construction of the copper conductors. In this case the standard requirements (EN 60228 in Europe) are quite “large”, requiring simply a maximum electrical resistance value and a maximum diameter of the elementary wires, so it is really the designer’s “know how” that can make the difference in the final result. Reducing the wire diameter or the stranding pitch are just easy ways of getting more flexibility, and usually result in higher costs and sometimes an increased conductor diameter; indeed, a well designed and tested conductor stranding can really make a difference, in terms of flexibility as well as in terms of maximum tensile load the conductor can be submitted to.
The maximum tensile load of the cable is usually increased by using, for example, Kevlar® support or other special fibers in a central position inside the cable. This applies because of the general construction design rule (also following VDE requirements) of shortening the laying-up pitch cores, or by tightening the structure against the internal relative movement. This construction allows the cable elements to transfer the mechanical stresses to the central element that is thus reinforced to give more resistance to the whole product. Of course, part of this effect is lost if the cable connection (the fixed point of the cable) is not made in the right way, especially if twisted cores are not tightened around the central support.
Another way to increase the mechanical resistance is to improve the sheath material features. Special compounds (as inner and outer sheath) with increased mechanical features are always used in the cable design, usually referring to the best standardised qualities (for example 5GM5 quality in accordance with VDE 0207 part 21), with much better breakdown tensile loads than the usual jackets for mobile applications (as for cable type H07RN-F). A very important consideration is how long the materials keep their original features during cable use. Over the years, a cheaper chlorinated material, sometimes with a certain content of PVC, was introduced that would allow the compound to achieve a standard performance in laboratory tests, but the behaviour of the compound during the cable lifetime could not really be guaranteed.
These details in design are not standardised by the VDE requirements, or by any other standards concerning cables for these applications (generally for mobile use), and so in this case the designer’s know how and skills (and the use of the best materials in the production) are the key to success for these reeling cables.
Below is a comparison of 3 materials: rubber compounds (5GM3 and 5GM5 quality) and polyurethane:
From the table it is simple to conclude that polyurethane is a valid solution: to obtain a smaller cable than the rubber version but with the same tensile load and to obtain a lighter cable than the rubber version with the same outer diameter. The textile braid inside the outer sheath is also very important. In this case the standards don’t require any performance at all about this element. The geometry of the braid, coupled to the elongation features of the textile fibers is essential to the successful working of the cable. Considering the much lower elongation of these fibers, compared with the outer sheath material, it is important to understand when the braid is starting its anti-torsional work and when it is arriving at the higher limit of its elastic strain.
These mechanical stresses on the cables are directly connected with the typology of drum where they work. An example is the comparison between a Mono-spiral Drum (single spire multi-layers – typical for harbour cranes) and a Cylindrical Drum (multi-spire multi-layers random wound, typical for mine equipment). The mono-spiral drum is ideal to guarantee the heat dissipation and the control of irregular twisting during unwinding (the limit could be the cable’s length in relation to the reel’s diameter), the cylindrical drum is the cheaper reel but the reeling and unreeling of the cable may generate torsional stresses higher than other reels. It also doesn’t guarantee the control over the layers of cable: the cable could be stacked, for example, on one side of the drum and the stresses could increase and damage it.
To avoid these problems the crane manufacturer has to rely on the experience of the internal designer, continuous tests of every component and the testing equipment in order to simulate the expected working conditions of a cable. Finally, correct installation of the cable is very important for its lifetime; the mechanical stresses can be very high and a failure of the cable could generate a short circuit or even an explosion. In order to get the best cable installation and operation, customers should always verify the crane system features with the cable manufacturer in order to choose the right cable for the crane as often it is the small details that are the basics for a good cable lifetime.