The company igus has developed cables for sensors with thermocouples that use special materials such as constantan and nickel/chromium-nickel compounds instead of copper. The impetus for this was a request from MS Spaichingen who sought suitable thermal compensation cables for the innovative method of heating contact riveting. Basic work had to be done in the process, but the developers were able to draw on extensive experience in the development of cables for special requirements.
The MS Spaichingen GmbH is active in various business areas for the automotive industry. The welding technology business division has specialised in the construction of customised ultrasonic welding systems for the joining of plastic parts. However, there are also limits to ultrasonic welding. If, for example, metal and plastic are to be joined, heating contact riveting is used as an alternative. In this process developed by MS Spaichingen, only temperature is introduced into the material and a gap-free riveted joint is produced. The process also includes the accompanying measurement of the temperature at the riveting points. Each rivet punch, which is used in the MS Spaichingen systems, is therefore equipped with a thermocouple that records the temperature.
Initially, a cable from the heater manufacturer was used for the energy and signal supply. However, after around 100,000 double strokes, which corresponds to a system operating time of one year, cable ruptures occurred, which move up and down with each cycle.
The search for an alternative cable was not very easy, because special materials had to be used here. The process requires that the cable must not have a copper conductor as usual, but the same alloy as the thermocouple. This means that cables made of alloys such as a nickel/chromium-nickel compound are needed - and in large quantities, too. In addition, a flexible and dynamic cable was desired.
The igus developers first had to do basic work by looking for ways to process the comparatively hard and brittle alloys into wires. Furthermore, besides the sensor signal, the cables should also transmit energy to the sensor head. A combination of copper cables and thermocouple cables had to be chosen, whereby the comparatively soft copper cores had to be combined with the hard cores of the thermo alloys in such a way that a long service life was achieved even under dynamic stress.
According to these specifications, igus first developed a movable cable only with thermocouples, which is now included in the product ranges as chainflex CFTHERMO. K.001, as well as a cable with three copper wires and a thermocouple (CFTHERMO. K.002). The cable type K.002 was tested at igus and MS Spaichingen and the MS designers issued the approval after 300,000 double strokes. In the meantime the CF thermo cables have been in use for some years, and the experience is all-round positive. Joachim Wein, in charge of Electronics Development for the Welding Technology Business Unit at MS Spaichingen: "We use the cables with and without energy chains and haven't had a single failure yet."
igus has expanded the CFTHERMO product range and added further alloys that are used, for example, for measuring higher temperatures. Moreover, since last year the [Link pagenr="18150" title=""readycable""] harnessed cables for thermocouples, have been available. The connectors are made of the same material as the cables. The dynamic tests that igus carried out with the first chainflex thermal cables are, still reliable. They started in January 2011 and have now reached over 100 million double strokes, and the resistance measurements still show no irregularities. In the meantime, igus has also encountered other applications for such cables - even in-house, more precisely in the injection moulding plant, where several hundred injection moulding machines produce plain bearings, energy chains and other igus products. In these machines, the temperature of the process must also be monitored, and this task is undertaken by thermocouples. Also, increasingly more movable cables are used in other areas, which have the properties of a thermocouple and at the same time are extremely reliable.