Real-life cable questions answered

Cables are often the last component to be considered when designing moving machines or equipment. It shouldn't be this way, because they supply the electric power and electronic signals vital to almost all modern motion designs. That's why cables compromised by extreme temperatures, chemicals, abrasion, or EMI can bring entire systems to a screeching halt, costing both time and money. Following are some real-world questions from engineers in the field, answered by W. L. Gore & Associates Inc., Landenberg, Pa., for getting the most out of cables that must perform in tough environments.

80% of our cable failures happen at the backshell. Are there any new developments in this area?

The backshell of a cable — an end piece that joins the cable conductors with various connectors — can fail under EMI, excessive strain, a compromised environmental seal, and other issues. EMI is probably the most common issue — particularly because of how some cable shields transition to the backshell. Several new designs and process solutions address these issues. One way to combat environmental sealing problems is with moulds or to pot the inside of the backshell.

Any ideas on how to reduce cable tangling?

Tangling occurs in applications where cables are in motion, particularly if they are in a cable chain or the cable is just moving. Packaging is critical; individual wires should be packaged into a round cable jacket, or at a minimum, a shrink tube or expandable sleeving should be used to hold the wires together. If the application involves continuous motion or repeated flexing with rests in between, be aware that moving cables generate kinetic energy: This kinetic energy results in the wires having to work to eliminate the stress. This can actually cause the cables to move, corkscrew, and tangle unless carefully managed. The most effective way is to package them into a planar cable, also called a flat cable. Planar cables are revolutionizing the linear motion, high-flex arena because they take individual round constructions and eliminate the need to put dividers and shelves into a cable chain to keep round constructions from tangling.

I have an application in which my cables continue to fail even though they are supposedly made for high-flex applications. Any thoughts?

Though a full answer to this question would require more information, here are a few tips. First, identify the mechanical constraints. In a flexing application, the type of motion is the first thing to classify. Is it a rolling flex, torsion, or tick-tock motion, or is a person attached to it? If it's a rolling flex — which encompasses 75% of automated motion applications — the stroke length, length of system travel, acceleration, and velocity are some of the key parameters that help point to the best solution. Next, identify the minimum bend radius of the cable. Most cables use standard copper conductors and shields, but in high-flex applications, say 20 million flex cycles or higher, copper alloys along with engineered PTFE materials can increase cable flex life.

Any tips on how to design or arrange copper conductors to increase bending performance?

When talking about bend radius, engineers are generally discussing an automation application. Bend radius is involved with rolling flex and tick-tock flex, which is a more severe motion. And bend radius has an exponential impact on a cable's longevity. I always advise engineers to use the largest bend radius they can, but they often don't have that luxury. They are usually asked to go smaller and lighter. If the bend radius is below three inches, a flat cable construction is best. That allows the highest flex life for the lowest dollars. If the bend radius is more than three inches, standard round cable constructions will perform well.

I'm working with silicone in a flat cable application, and it's really difficult to prep the cables for connectors. Any suggestions?

First, determine why silicone cables were selected, and if there is a unique application requirement that only silicone can fulfill. One of the drawbacks of silicone cables is in their termination. Keep in mind that cable termination is one of the factors in total cost of ownership. If a design requires very flexible cable and weight is not a factor, silicone is a good material to use. However, it's going to cost more to gain access to the conductors in the cable because it's going to take more time.

Generally speaking, what is the preferred cable insulation material for cryogenic environments?

The best is PTFE because it remains an effective insulator down to -250° C and unlike many materials, it retains some flexibility at cryogenic temperatures. PFA and FEP are also effective choices for environments down to -200° C. PTFE and its copolymers also have the benefit of low outgassing, which is critical for UHV environments. Polyimide is another insulating material that remains stable down to -250° C with very low outgassing properties, but it's not very flexible in comparison.

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How does vacuum affect various cable materials?

It depends on the application and pressure level. Remember: Vacuum is just a very low level of pressure. PTFE materials are best for use in vacuum because they don't have any process additives, oils, lubricants, or plasticizers added to the material. Anything that is added to the cable's insulation, jacket, conductors, or shield wires will outgas under vacuum, and when cable materials outgas, they condense on cooler surfaces — typically the critical surfaces being worked on. This in turn can fog or damage the surfaces. Case in point: A satellite's optics can be fogged during manufacture by silicone oil outgassed from cable. PTFE is chemically inert and has no materials that outgas, so it passes the toughest NASA and ESA requirements.

In aerospace design, some of our biggest cable issues are abrasion and cut-through in doors and landing gear. How can we improve performance?

Cable routing is the most obvious way to reduce abrasion and cut-through, but routing can only go so far because you have to clamp tightly and it is a very cold environment in the wheel well. These cables are also exposed to contaminants such as salt spray and hydraulic fluids. A cable containing engineered PTFE materials is suitable here.

For more information, call (800) 445-4673 or visit www.gore.com.

Improving cable reliability in harsh environments

Want to learn more about specifying cable for difficult environmental conditions? Check out this on-demand webinar presented by W. L. Gore & Associates Inc. Go to www.motionsystemdesign.com and click on the Webinars button. This presentation investigates the impact of harsh environments on cable systems and some of the variables engineers can control to improve performance. Learn about: