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Cable test results letting users down in real world applications

Cable test results letting users down in real world applications

Engineers are being badly let down by existing standards for testing flexible cables, with neither of the regularly quoted standards being meaningful in real world dynamic applications. So says Richard Habering, global product manager for chainflex cables at Igus, adding: “The only way to reliably certify cables for dynamic applications within energy chains is to test them within energy chains.”

The two quoted standards on cable manufacturer data sheets tend to be IEC 60811 and VDE 281 Part 2. Part 504 of IEC 60811 defines a procedure for performing bending tests at low temperatures on extruded insulations and outer jackets. In this so-called mandrel test, the cable specimen and a metal rod are cooled to a set temperature for a minimum period of 16 hours. After 16 hours of chilling, the cable is wound round the rod, and the assembly is brought up to ambient temperature. The cable is then examined with the naked eye for damage.

This is the test quoted to certify cables as suitable for operation in dynamic applications at low temperatures, but since the cable performs no dynamic movements as part of the test and the inspection is purely visual, its suitability as a test for dynamic applications is questionable. “The value for temperature can be given in a cable manufacturer’s datasheet for ‘flexible’ applications, but it does not show the stress the cable will be loaded with when used in e-chain applications,” says Habering.

VDE 281 – part 2, the so-called rollover test, allows cable manufacturers to define a bend radius in their datasheets when the cable has completed 30,000 double strokes without any visible signs of damage. But again it is a long way from a genuine energy chain implementation. Both tests fail to take into account the real world conditions of dynamic applications, and neither measures the electrical performance of the cable before and after the test.

For engineers specifying cables for dynamic applications based on the quoted results from these tests, there is a real risk of premature cable failure and expensive machine downtime. Indeed, Igus began manufacturing its own cables precisely because customers were reporting problems with using existing cables in energy chains. Typical problems included jacket wear and jacket breaks, corkscrewing and cable core breaks – all issues that were getting more pronounced as applications became faster and the bend radius got smaller.

“When we looked, we found there was no specialist in the world making cables for energy chains,” says Habering. “So Igus began making its own.” The designs are innovative, with a host of materials and features specifically intended to prevent the common problems experienced when using conventional cables in dynamic applications. But what really sets Igus apart is its testing produces for dynamic cables. Not only are the cables batch tested to ensure production quality is maintained, but they are also comprehensively tested in dynamic applications, within energy chains, over millions of cycles. The Igus test lab for dynamic cables also includes a cold chamber where the behaviour of products is tested at temperatures down to –40°C.

“We also test other manufacturers’ cables,” says Habering. “We tested a cable from one manufacturer that quoted 2.1 million double-strokes in its test report. However, this cable failed in the Igus rollover test setup at the same radius after 157,000 double-strokes when operating inside an energy chain.” The cables failed via the corkscrew effect. The Igus chainflex cable equivalent in tests, on the other hand, delivered 2.4 million double stokes at the same radius.

“Igus has also tested cables from other manufacturers that have passed the IEC 60811 mandrel test, but which failed after a couple of thousand double strokes working at same temperature in an energy chain in the cold chamber,” adds Habering.

With 26 years of cable manufacturing experience, Igus designs its products to extend lifetime and to make lifetime predictable, with all products carrying guarantees over a defined number of cycles.

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