We have all seen them, those curly wires that are used to connect things and stretch like a spring.  Coil cords are found in garage doors, your cell phone car charger, defibrillator paddles and simple key chains.  Coil cords are a great way to attach two items where one or both need to move.  Whatever the application knowing how and what mechanical tests are needed to verify a coil cord design is critical in achieving proper performance.

All coil cords have the same general shape, a straight section, followed by a coiled or curly section, followed by yet another straight section.  The extension capability of a coil cord comes from the coiled or curly section.  When the cable is pulled the coiled section acts like a spring and stretches.  It is this mechanical stretching that makes this type of cable assembly critical in certain applications.

Since a coil cord is a dynamic assembly care must be taken to ensure the design will last the expected life of the product.  Advanced life testing is one way to verify the design meets the required specifications and can perform as expected.  The testing discussed in the article are Cycles to Failure, Retraction Compliance and Cable Flexing to Failure.

Coiled cables are springs made from raw cable that is typical composed of copper and plastic.  As we have all seen, if copper or plastic is repeatedly flexed it can fatigue and eventually fail.  When a coiled cable is extended and then retracted flexing is occurring.  This flexing, over time, will fatigue the materials.  The best way to determine if the design can withstand this process is to perform a Cycles to Failure test.

Cycles to Failure Test

In a Cycles to Failure test a coiled cord or cords is attached to a test machine that can be programmed to extend and retract the cable. The test machine’s extended length, cycle speed, extension dwell and retraction dwell can all be programmed. For example, the machine could be set to 110% of designed extended length, a cycle speed of 30 per minute, an extension dwell of 10 seconds and a retraction dwell of 20 seconds.  The machine would then cycle the cable or cables until an open circuit occurs indicating failure. All the parameters can be changed to align properly with the intended application.

Retraction Compliance Test

In a Retraction Compliance test, the cords are attached to the cycling machine and cycled.  However, before cycling the coil cable’s initial retracted length is measured. After cycling, the cable is allowed to rest for a period of time and then the cycled retracted length is again measured. Retraction Compliance is defined as a percentage of cycled retracted length to initial retracted length. These percentages can range from 50% for a very poor or heavily cycled coiled cable to 95% plus for a well-designed assembly.

Cable Flexing to Failure Test

The Cable Flexing to Failure test is used to make sure the raw cable used itself can withstand fatiguing.  The coil cord is made and sections of the cable, both the coiled section and straight section, are attached to programmable cable flexing machine.  The machine’s flex radius, cycle speed and rotation angle can all be customized to ensure the cable meets the desired requirements.  For example, the flex radius could be ½”, the cycle speed could be 60 per minute and the rotation angle could be 180 degrees. The machine would then cycle the cable until an open circuit occurs indicating failure.  All the parameters can be changed to align properly with the intended application.

Designing a coiled cable properly is critical.  Over engineered and the cost is too high; under engineered and the performance is inadequate.  However, the only way to ensure the design is correct is through testing.  There are two types of testing.  Lab based, programmable testing which Meridian Cable performs, or field testing which everyone who buys a coil cord eventually does.  In many cases, the lab based testing before field launch will help ensure a proper cable is designed, manufactured and delivered.

Should you need a standard coil cord or a custom design coiled cable Meridian Cable can help.  Meridian Cable has over 30 years of experience designing and manufacturing coil cords, coiled cables and cable assemblies. Designed – Built – Delivered, Meridian Cable is ready to help create a solution for you.

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