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Pre-stretch — manual vs. machine

Q: After I upgraded from my old lockout machine to my new electronic stringing machine, I started using the built-in machine pre-stretch function (set at 5 percent) because I thought it would help recover some of the clamp drawback, and produce a more even end-to-end tension on the crosses. Then, I talked with a stringer I admire, and he said that you get less consistent results using the built-in machine pre-stretch, and that I should always do pre-stretching by hand, as described in The Stringer’s Digest. If the built-in machine pre-stretch isn’t that good, why do stringing machine manufacturers even bother to offer it?

A: Pre-stretch is defined as applying tension lengthwise to the string prior to installation. This tension helps to align the molecules within the string. This molecular alignment serves to lengthen the molecules on the micro level, and to straighten the string on the macro level. The more times you apply tension and then release it, the more alignment you’ll achieve.

Comparing hand pre-stretching to machine pre-stretching is comparing apples to oranges, although it’s understandable given the fact that both techniques share the same name. It would be nice if the “pre-stretch” function on stringing machines was called something different, but at this point we’re probably stuck with it.

Manual pre-stretching is most effective at reducing coil memory, and it allows you to tame some of the wilder strings in order to make installation easier. Even though the tensions used in manual pre-stretching are relatively low, you do also get better tension maintenance and a slightly higher stringbed stiffness, albeit at the expense of slightly lower longevity.

Machine pre-stretching is most effective at recovering clamp drawback, as the additional tension helps to overcome the friction of the string going around the outside of the frame. When tensioning crosses, pre-stretching can help overcome the friction of the crosses against the mains, so that the far end of the cross string (the one away from the tension head) is closer to the reference tension than it otherwise would be. In other words, the built-in machine pre-stretch is more an over-tension than a pre-stretch.

However, it is impossible to tension strings without some machine pre-stretching, no matter what kind of machine you have. Each time you pull tension on the portion of the string inside the frame, you are also pulling on the portion of the string between the outside of the frame and the tension head. Therefore, some portion of each successive string has already been pre-stretched during installation, whether or not you performed manual pre-stretching, and because reference tensions are almost always higher than the tensions you use during manual pre-stretching, there is more molecular alignment resulting from this type of pre-stretch.

Therefore, manual pre-stretching and machine pre-stretching are not interchangeable: Each has its function, and you are free to use one, or both, or neither. The important thing is to find a technique that allows you to deliver consistent results to your customers time after time.

From the November 2007 issue
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