98 Focus | Cable harnesses those that the design will experience during repeated routing, mating and un-mating. Particular attention is paid to the joint between cable and connector interface, as that is assumed to be the part most likely to break first. Beyond that, more specialised tests for high-end requirements include flame testing, water testing, drop testing, strength testing and many other ways of investigating the suitability of a harness to its environment. While this list can become a long one, much of the testing can be automated, with a lot of it being carried out inside programmable machinery to ensure the repeatability of test results. Reports on these tests can be sent to customers and their technicians to show the limits of what their products can endure. Jacket materials There are many outer jacket materials, thicknesses and structures available, with the best choice depending strongly on the environment in which the harness will operate. Understanding local properties such as temperature ranges, what chemicals it will be exposed to and their physical state, or the likelihood of icing or outgassing at altitude are key. It is increasingly common for instance that harnesses in high-end applications need to survive in temperatures from -55 to +175 oC. Also, military integrators increasingly want specific properties, such as requiring that the jacket’s chemistry be smoke-free or sometimes halogen-free; the latter is also being requested by civil and commercial organisations. Despite their excellent flame and heat-resistance properties as additives in a cable harness jacket, halogens are harmful to human health. Technicians can avoid their negative effects by using PPE, but this move by the industry towards halogen-free materials could lead to a growing use of jackets made from plastics such as silicone rubbers, polyurethanes, polyethylenes, polyamides, polypropylenes, thermoplastic elastomers or ethylene propylene diene rubbers. That said, materials such as ETFE (ethylene tetrafluoroethylene), PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy alkanes) and FEP (fluorinated ethylene propylene) are popular across a range of heavy-duty applications and jacket types. Among these, heat shrink tubing is increasingly used, as such materials contract radially when heated. That means wires can be fed through them in their comparatively spacious normal state, and then the subsequent heating enables the jacket to tighten and the overall harness to take up less space. PTFE tubing exhibits low friction, strong resistance to puncturing and chemical damage, and can operate in temperatures from -55 to 175 oC. FEP is less expensive than PTFE, and lacks some of its temperature and mechanical properties, but it is more flexible and easier to manufacture. That trade-off tends to happen with other materials as well. ETFE for instance can exhibit up to 33% higher tensile strength than PTFE, but the latter is again more flexible: it will stretch 66% more than ETFE before breaking. PTFE also retains the advantage in friction, with one-third of ETFE’s coefficient of friction, meaning reduced wear in tight spaces where many enclosures or other cable harnesses might be tightly fitted, and where movement might be induced by vibration or g-forces. Polyurethanes and silicones on the other hand are widely used in harnesses for those seeking simpler, less expensive and halogen-free materials in light-duty applications. While both materials come in many forms, silicones mostly have better thermal and UV resistances, while polyurethanes have the advantage in terms of abrasion resistance and tensile strength. On top of these various, well-established materials, some unconventional jacket choices are gaining in popularity, such as cross-linked polymers, which are October/November 2023 | Uncrewed Systems Technology A range of jacket materials are available for the temperature, chemical, icing or other environmental hazards the cable has to survive (Courtesy of LEMO)
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