Cable stretch, pre-stretching, proof and load testing
Overhead view of lanyard being proof tested.
Cable / Wire Rope
Cable / Wire Rope stretches as a load is applied. The amount of stretch will increase as the load is increased. There are two types of stretch occurring here. The first type of stretch is called CONSTRUCTIONAL STRETCH. The second type is referred to as ELASTIC STRETCH.
Constructional Stretch occurs as the outer wires of a strand close around its core and as the outer strands of the cable close around its core under tension. This closing action is the filling up of the spaces between the individual wires and the spaces between the strands. Unfortunately, this type of stretch is not consistent (because of all of the variables involved in the manufacture of the cable) and cannot be quantified into a formula. Constructional stretch can be removed by prestretching or prestressing (whichever term you prefer). We accomplish this in accordance with MIL-DTL-5688. Specifications can be downloaded from the Summarized Military Specifications section of this Web Site.
Elastic Stretch occurs as the individual wires elongate under load. This is a quantifiable factor until the yield point of the cable is exceeded. The factor varies according to load applied and the construction of the cable or strand. In short; the fewer wires and strands in a given diameter, the lower the stretch factor. If you require a calculation for elastic stretch for a particular cable, Contact at 1-800-851-2961 and one of our sales staff will acquire this information for you.
Proof and Load Testing
Whether you need to proof test a single sample, one of every X assemblies, or every last one, we have the tools to do it. We can also load test to destruction if you need. Certifications are, of course, available upon request.
Measuring strand, cable and wire rope properly is done in the following manner. Using a pair of calipers or micrometers, move the anvils into contact with the peaks of two opposing strands (see illustration below). DO NOT HAVE THE ANVILS MAKE CONTACT WITH FOUR STRANDS SIMULTANEOUSLY. This will give a false undersize reading. All cable is manufactured to a plus tolerance. Please see the charts in the Strand and Cable section of this Web Site for bare cable tolerances.
See also the Wire Rope Jacket Specifications section for coated cable diameters.