Cables: Testing to extremes
28 June 2017
In the production industry, energy chains help protect moving machine cables and hoses from tensile and torsional forces, as well as external influences such as impact or welding sparks. Even then, the demands on the cables remain high. Justin Leonard, the-chain director at igus, believes that, in order to ensure reliability and predictability, continuous technological advances in materials, test standards and procedures are needed
The energy chain is the umbilical cord of a machine – feeding it with vital energy, data and media. The degree of movement can range from simple linear strokes, such as those found on pick-and-place machines, to six-axis robotic applications. For cables to withstand stresses millions of times (ie to not work harden), without corkscrews, core breaks or jacket ruptures after a few thousand cycles, the material and structure of the cable must be perfectly matched to each other.
Experience shows that even highly flexible cables can often reach their stress limits quite rapidly when used in moving applications, especially when housed in energy chains, so is their service life predictable?
The standard tests performed by VDE, IEC or UL do not offer a clear statement; only a long-term test in the energy chain itself offers this. Relevant standards use other means that merely simulate the wear regardless of the chain or the chain material, again offering little accuracy for cable lifetime in energy chains.
To predict the service life reliably, igus operates a 2750m2 test lab for cables moving in energy chains. To characterise the resilience of products, they undergo continuous operation tests in 58 different rigs. Since the exact reproduction of the real working conditions is crucial, numerous test axes are available with different travel distances and accelerations or environmental conditions.
For testing large energy chain systems, such as those used in crane facilities, an outdoor test site with a travel distance of up to 240m is available - here products can travel at 4m/s and with a fill weight of 8kg/m for a total lifetime of 25,000km.
Likewise, igus tests its products in temperature conditions from -40 to +60°C using its two climatic test chambers. Unlike standard cold winding tests, in which cables are wound up on a mandrel and cooled to the test temperature conditions just once, here the cables and chains are put under appropriate test temperatures and realistic motion conditions. They must withstand millions of strokes to prove they will endure the expected bending stress in a real application. A product passes the test only when no jacket ruptures occur, and the product is proven to have the necessary cold flexibility.
Tests are not always about extreme temperatures - the most popular minimum operating temperature specified is at -5°C. igus offers cable jackets made from an oil-resistant PVC (PolyVinyl Chloride) compound, which provides high abrasion resistance over a wide temperature range. igus believes this to be unique to the industry, since the common PVC compounds used for ‘chain-suitable’ cables do not normally meet these requirements. Another benefit is that in moderate temperature ranges, it is not necessary to rely on expensive jacket materials such as PUR or TPR.
Bundle instead of layer
The findings obtained from the ongoing analysis of all tests over a period of 25 years help steer new product developments. This has led to, among other things, the introduction of winding cores in bundles, similar to the concept used in steel cables.
In an elaborate bundle winding process, the cores are stranded in individual bundles with three, four or five wires; these are then wound with each other again into an overall bundle. For large super structures, this is done around a strain relief element. The result is a cable that is durable in motion and suitable for chains because, in contrast to a layered cable, each of the cores moves similarly in the inner and outer radius with the motion in the energy chain and thereby prevents relative stretching and compression.
In even more extreme movements, cables with a similarly complex cable structure are used. The so-called ‘robot cable’ range is primarily used in industrial robots and must follow extreme movements, bends and torsion. Special damping elements give the cores the necessary freedom of movement in the interior of the cable. Because, the more twisted the cable is approaching its load limit, the more difficult it becomes for the cable to twist. Special shields and exterior materials also ensure an optimum durability of the cables.
The service life of a cable used in an energy chain will depend on a variety of variables, which dictate the structure and choice of materials. Thus, the chainflex product family currently offers 1381 different cables. How long the cables will last in a given application is easy to find out. Using the results from 2 billion test cycles each year in the laboratory, the free online tool on the igus website predicts the service life of cables.