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The kites are very big by ordinary standards and the pull on the kite line is between 10 and 70 kg depending on the wind strength. Even with gloved hands the line is so slippery and thin it would be impossible to hold the line even for a few seconds. Then we have the task of releasing and winding in thousands of meters of line. It is clear that some method of storing and winding the line must be used. This is the role of the winch. The first version of the winch used one drum or reel to wind and store the 10,000 meters of Spectra UHMWPE line. However, the reel sides buckled and split with the pressure of the line being wound in under tension from the kite pull. Further study of large scale line retrieval in the fishing and mining industries gave a clear picture of what was required, that is, a capstan and reel system.
The capstan is just a rotating drum that grips the line, to control it’s release and retrieval. The line is wound around the drum and the number of turns required depends on the friction cooefficient between the drum surface and the line. Large diameter hemp ropes may only need 2 turns and UHMWPE lines such as Dyneema may need 12 turns to give enough surface area contact to prevent slipping. The number of turns also depends on the exit line tension required. In this case the line tension between the capstan and the storage reel was less than 1 kg so 15 turns was enough to transfer over 95% of the line tension to the capstan and eliminate slipping of the line on the capstan. The line tension between capstan and reel is a function of slipper clutch pressure which in turn is controlled by spring pressure . The aim of the system is to control line release and retrieval in a smooth manner and minimise pressure buildup on the storage reel by accumulated line tension.
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