Strain Relieving and Cable Assemblies

April 21, 2011 - PRLog -- Anyone who has used an electronic device has come in contact with a strain relieved wire or cable.  Some strain reliefs are visible and some aren’t, but all of them are vital to the life of an assembly using wire or cable.  This article will explain the reasons why strain reliefs are needed and the basic types and methods used.

Why use a strain relief?  All of us have unplugged something from an AC wall socket - a vacuum cleaner, lamp, extension cord or power saw.  There are really only two ways to do this, one, grab the plug and pull it out of the socket, or two, grab the wire or cable some distance away from the plug and yank.  Now we all now which is the proper way to unplug the cable; but we have used the wrong way too.  The latter method puts all the extraction force onto the contacts inside the plug.  If the wires weren’t connected to the plug strongly enough the cable wouldn’t last long.  This is an extreme example of why strain relieving is necessary, but it illustrates the fact that anytime a connector is on the end of a wire or cable there is the potential for force to be transferred from the wire or cable onto the connector.  Strain relieving is how engineers design cable assemblies, coiled cords and wire harnesses to withstand these forces.

Another purpose for stain reliefs is to help a cable assembly or coil cords withstand flexural forces.  This can be best illustrated using the metal coat hanger as an example.  Most of use have taken a metal coat hanger and wanted to make it shorter.  Usually we don’t grab a pair of scissors or wire cutters for this task.  Usually we find the spot on the coat hanger we want the “cut” to be and start bending the hanger back and forth, and eventually the coat hanger breaks.  This same effect can occur in wire and cable, and just like with the coat hanger once broken it cannot be used for its original purpose.  Here strain relieving is used by engineers to design a cable assembly or coiled cable to withstand bending the product might see during use.

To prevent failure from pull forces designers must “anchor” the wire or cable to the connector.  This is accomplished mechanically along with the electrical connection needed to transfer the signal or power the connector must carry.  This mechanical connection can be accomplished in many ways.  For many contacts a hole is placed in the metal through with the wire is passed and in essence tied onto the contact.  In other contacts the stranding is crimped by metal tight enough to become one with the wire.  These methods are only as strong as the wire itself and will only withstand a breaking force equal to the strength of the wire.

Cables and coil cords often have multiple conductors and the break strength of these is much stronger than that of any of the individual conductors within.  For these assemblies it is often desirable to anchor the whole cable and not just the individual conductors.  One way to accomplish this is to place something around the cable or wires that grabs onto the bundle.  This could be a metal band, a plastic tie wrap or even a knot.  Once the feature is on the cable it is placed against a feature on the connector, a metal housing, a plastic housing or over mold that transfer the force from wire of cable to the body of the connector.  This makes sure the force is not exerted onto the connector’s contacts which are often not able to withstand any great force.

To prevent failure from bending forces it is necessary to change how the cable or wire bends, not how much it bends.  Many coiled cords and cable assemblies are used in dynamic environments, and therefore must bend a lot.  The key here is to design a product that can withstand the bending.  Going back to our coat hanger example we know that the sharper the bend radius the quicker the coat hanger will break.  The same is true for any assembly made with wire or cable.  Here the designer’s goal is to increase the bend radius.  This can be done with an over molded strain relief that forces the cable or wire to bend in a large radius using a tapered shape with slots.  Another method is to design the product such that it contains a built in radius in the metal or plastic that allows the wire or cable to bend in a large radius.  Many times a simple section of heat shrink or a series of heat shrinks can accomplish the same result.

As is quite evident strain relieving is a critical component in most custom cables and wire harnesses.  In further articles we will discuss specific design situations, the solutions used and the testing results seen.  However sometimes it is necessary to rely on expert advice.  If you need help designing strain reliefs for your cable assembly or coiled cord Meridian Cable has over 30 years experience and often has existing solutions that can be used.