Monday, April 26, 2021

Coil Cords - How to Specify the Physical Shape

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. Whatever the application knowing how to specify a coil cord is critical in achieving the design goal. Coil cords are a great way to attach two items where one or both need to move and this article will give you the basics needs to specify one.

All coil cords have the same general shape, a straight section, followed by a coiled or curly section, followed by yet another straight section. Both the straight and coiled sections have key features that need to be specified correctly.

The straight sections of a coil cord are called tangents and can come in two configurations, perpendicular and axial. Perpendicular tangents are most common since they are the easiest to manufacture. In this arrangement the end of cable keeps following the coiled path of the cable and stops turning. This means the cable exits the coiled section perpendicular to the center line of the coils. A good way to visualize this is to slide the coil cord onto a rod. The rod would be along the center line of the coils and the tangents would hang down from the rod perpendicularly. Interestingly this is very close to the manufacturing process.

Axially tangents exit the coiled section along the center line. These require different tooling and take a little longer to make. If a coil cord with axial tangents slid on a rod the tangents would lay along side the rod. This means that the tangents turn ninety degrees when exiting the coiled section. For many applications this is preferred since the tangents are in the same line as the stretching cable.

When specifying a coil cord the most important physical feature is the coiled section. This is the part that provides the cable with its unique ability, extending and retracting, and these are the terms used to describe this section. In its natural state the coils would be retracted and measuring the length of the coiled section would determine the retracted length. If pulled to its maximum operational length, or extended, and then measured this would determine the extended length.

The retracted length is the most common way to specify the coiled section of a coiled cable. However the purpose of a coiled cord is to extend so it is good to know the extended length, and sometimes this is the main criteria used in the specification. The retracted length and extended length are related by a ratio which ranges from 3:1 to as high as 6:1. Which ratio is correct to use depends on the diameter of coils which is the final feature needed to specify the physical shape of coil cables.

Coiled cords are made up of a bunch of circles all in a row. If each circle was straightened out the length of cable used to make the circle would be pi times the coil diameter. The diameter to use is the outside coil diameter minus the diameter of the raw cable. This mean diameter represents the center line of the raw cable, and the larger the circle the more cable and hence the longer the retracted length. Coil diameters can range from 1/4 of an inch up to 4 or 5 inches. If the raw cable is eliminated this give a range, when multiplied by pi, from 3/4 of an inch up to 15 inches!

So, for small coil OD's of 1/4 of an inch the ration of extended to retracted length is around 3 to 1. This means a coil cord with a 1/4 inch coil OD and 1 inch retracted length would extend about 3 inches. On the other hand a coiled cable with a 5 inch coil OD and 1 inch retracted length would extend about 6 inches. As you can see this is a wide variation but the correlation is fairly linear, as the coil OD gets larger than the retracted length gets longer.

Using these three features, tangents, retracted/extended length and coil diameter it is easy to specify the physical shape of a coil cord. These are the basic physical properties and will get the designer fairly close. Other issues such as cable construction and materials can play an important part in the coiled cable's performance. So if the function of the cable needs to be fine tuned it is always best to contact an expert.

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