Cable Geometry and Coil Cord Performance

March 16, 2011 - PRLog -- CABLE GEOMETRY AND COIL CORD PERFORMANCE

Whether you call it a coil cord, coiled cable or retractile cord we have all seen the spiral cords used in many applications.  From garage doors, to cell phone car chargers, to medical equipment these cables are critical in achieving many design goals.  Whatever the design goal the primary reason for using this type of solution is to put a lot of cable or wire into confined space and have it available to extend when necessary.  Coil cords are a great way to accomplish this goal.

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.  

For many applications the time required for a coiled cable to retract from its full working length is critical.  A number of factors influence the retraction speed of a coil cord but one important consideration is the raw cable’s geometry.  This geometry is typical round, but in special it can be rectangular or even ovate.  The advantages of each shape and the corresponding “bean effect” are explored below.

Every coil cord requires forces to extend it.  That force comes from the unwinding of the coils themselves but also from the bending of the raw cable along its length.  If viewed from the individual coil perspective what is happening can best be described as beam being bent.  In a round geometry the force required in any given direction is equal.  An easy way to visualize this is with a metal coat hanger.  If a straight section is held tight at one end and the other end pushed from the side, from the side, the force required to bend it in any direction would be very similar.  While other factors such as fabrication stresses, molecular orientation or grain do have an effect to some degree it is quite minimal in this application.  This design creates a coil cord with a typical retraction speed.

In order to achieve a faster retraction speed a coiled cable made from a rectangular shape cable can be used.  The rectangular shape adds force that can be best visualized using a ruler or a regular 2 X 4 board.  When these are laid flat with something supporting just the ends the middle is free to bend, and depending on the length it can bend quite easily.  However if the direction is changed and these are placed on edge they will not bend much at all.  When a coiled cord is made using a cable with a rectangular cross section this resistance to bending, or “beam effect”, augments the spring effect of the coils and creates a coil cord with a higher retraction speed.

Sometimes a round geometry is too slow and a rectangular geometry too fast.  In these instances a cable with an oval geometry can be used.  Since this cable is not round it has a higher beam effect, however since the cable is not fully rectangular the beam effect is less.  As one can imagine there are many variations between round and rectangular.  All the options lead to many design opportunities when designing a custom coil cord.

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.  Visit http://www.meridiancable.com/coil-cords/standard-coil-cords/ for details.