Racquet Stringing Machine

Tensioning devices on stringing machines come in three types:Drop weight:  These are used on the cheaper machines and can be awkward to use accurately because the same weight suspended at slightly different angles can give significantly different tensions. Such devices do however give a constant pull to the string – explained further below.Spring loaded lockout:  On these devices tension is wound up using a handle and when it matches the pre-set tension value on the spring the device "locks out" (locks into a set position) preventing the operator adding any more tension.Electronic constant pull:  These are used on more expensive machines and use an electronic pulling device with applies a constant pull to the string at the pre-set tension. Such devices maintain the pull on the string - at a constant tension - even as the material of the string starts to relax. As the string relaxes and stretches further (creeps - for an explanation of “creep” see the glossary of terms in the "Technical - Introduction & Glossary" page) the device maintains the tension at the same level by pulling in more string.

Loss of tension due to creep
Why do the different machine types deliver different tensions to the strings when set to the same nominal tension (the tension setting on the machine)? The answer is because of "creep" and the fact that on a lockout machine the tension is set before initial creep occurs and on the constant pull machine the tension is set after initial creep has occured. By initial creep I mean the creep of the first 5-10 seconds.
The materials from which all strings are made suffer “creep” when held under tension. Different materials “creep” at different rates but most of the materials used in racquet strings creep quite a bit for the initial few seconds during which they are placed under tension. You can watch this on a tension measuring device and, depending on the string material, it amounts to typically around 10% of the overall string tension over 5-10 seconds. ie: you can watch 10% of the tension disappear over the first 5-10 seconds of the string being held in place. The material is relaxing at a molecular level and will not try and regain its original position.

Before or after the string is clamped
Once a string is pulled to tension it is then clamped in place to hold the tension while it is threaded through the next hole ready to be tensioned again. On a constant pull tension head the string will be at its nominal tension at the time the clamp is engaged to hold the string in place. As long as this is around 5 seconds after the tension was first applied it will not drop significantly further while it is clamped and threaded through the next hole. On a lock out tension head the head locks into poisiton at the nominal tension and creep starts from that point. Typically 10% of tension will be lost by the time the clamp is applied to hold the string. In order to compensate for this I allow the head to lock out, then set the clamp in position which typically takes 5 - 10 seconds, then pull the tension in the string again so as to take up the tension lost to creep and then clamp the string. I thus "emulate" the performance of a constant pull head.

This difference in string tensions between tension heads is not a problem. If you use the same stringer you will get the same result (or more or less tension if you ask for it) each time. If you move between stringers talk to your new stringer and agree if any compensating action is required in order to maintain consistency of string tension.

Don’t be alarmed by this loss of tension through creep. The differences between machines can be compensated for as explained above. Furthermore all strings will continue to lose more tension through creep after they have been clamped. Typically a further ~10% of string tension will be lost through creep during the first 24 hours after the racquet has been strung, regardless of the tension head used. It’s going to happen and can’t be avoided. The best thing you can do to avoid any negative effect is not play with your racquet in the first 24 hours after it has been strung as it'll feel very different - very tight!