EuroWire January 2016

Technical article

Technique for precise concentricity measurement of a conductor in the insulation and relevant parameters of cables and wires By Harald Sikora and Katja Giersch, Sikora, Germany

Introduction Wire and cable

Early in the assembly process, the cable or wire is cut to lengths and stripped at the ends in order to be subsequently fitted with plugs. The pull-off forces used for the stripping are exactly defined. Deviations of the pull-off forces within the conductor insulation can cause interferences during the stripping. The use of innovative measuring and control technology allow the cables to meet the high demand requirements and also identify potentials for process optimisation of the extrusion line. The requirements for measuring and control technologies in cable extrusion lines are very precise where tolerances within a few micrometres have to be maintained. While the mean value of the diameter or the wall thickness of the insulation is sufficient for controlling the extrusion line, the reliable recording and visualisation of short-term variations, especially of the eccentricity values, is additionally necessary under the aspect of the minimum wall thickness. Slight oscillations of the conductor directly before the crosshead can lead to significant deviations of the concentricity, especially if the conductor is oscillating in one plane or rotating. Such eccentricities cannot be shown with a conventional graphical visualisation. A fast rotating Rotating/oscillating conductor

vector of a constant eccentricity value can only be displayed to the operator by a visualisation beyond the standard display.

manufacturers of automotive and control wires as well as coax and communication cables are using measuring and control technologies in their extrusion lines to maintain specified cable parameters. This is important for process optimisation and quality assurance but is also demanded when products are assembled automatically. In these cases, assembly failure rates are recorded statistically, registered and assigned to the manufacturer. For this reason, cable plants are aiming to supply cables with reliable high quality standards. A challenge in the cable extrusion process is maintaining measurement values with tight tolerances of the conductor concentricity in the insulation, the conductor diameter, the outer diameter of the cable and the ovality of the outer diameter. During the extrusion process deviations of the measuring values can occur, which would remain undetected without special equipment for visualisation. Lumps, neckdowns, faults or bare spots in the cable can lead to malfunctions during assembly. Challenges in cable extrusion

Angle position of a conductor

During the extrusion process it is possible that a cable, with or without guide rollers, has a slightly angled position when running through the measuring plane. Eccentricity measuring systems have to be able to generate accurate measuring values even when such a position occurs (see Picture 6 ).

A sag or bend of the conductor

Even on the assumption that the conductor is stretched when running through an extrusion line, it may still show some bends, which have to be detected when entering the gauge head of an eccentricity system. Without any special measures, even invisible curve radii in the range of 5 to 20 metres can lead to eccentricity errors of up to 40 micrometres. Only in an ideal case, guide rollers can compensate this imperfect stretching of the conductor, or vice versa, even increase the error. Therefore, the aim should be to eliminate guide rollers completely.

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January 2016