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Here's the setup, going through those four valves will be some fluid, probably water, but I need some fishing line going through this cylinder to be sealed. The line is anchored at two points, the top and bottom, and will be used to draw two points together. This apparatus is to heat and cool wound up fishing line, as that can be used as a muscle fiber. When hot water is applied, it contracts, when cool water is applied, it expands. This does mean that the line will be moving some percentage of the length, which may also mean that the diameter will change slightly.

enter image description here

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    $\begingroup$ Not an answer, but: is regular nylon OK left in water permanently? I thought there were swelling/absorption problems if it's left in water for very long periods. You might want to check this out if possible. $\endgroup$ – Andy Oct 12 '16 at 9:15
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    $\begingroup$ Oh! I had no idea that it absorbed water, thank you! While searching I found this. intechpower.com/material-information/… I guess it depends on the type of nylon being used. $\endgroup$ – FlippantSloth Oct 12 '16 at 9:27
  • $\begingroup$ Why does the casing need to be rigid? It seems to have no structural role. Why not allow its length to change with the line? Think bellows, or even a completely flexible bladder of some sort. $\endgroup$ – Dave Tweed Oct 12 '16 at 10:44
  • $\begingroup$ "some percentage of the length" is the problem. You may need something like a rubber "boot" or a rollsock seal, at least at one end. Like that round a gearstick, or the roll surround on a loudspeaker. Are there significant pressure differences inside and outside the cylinder? $\endgroup$ – user_1818839 Oct 12 '16 at 10:45
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    $\begingroup$ All the more reason to use a flexible bladder ... suck the warm water out, then pump the cold water in, etc. $\endgroup$ – Dave Tweed Oct 12 '16 at 11:06
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A high-quality seal that diameter would be very difficult to fabricate. You always want to avoid seals in designs when you can help it. For example, you could fix one end inside the cylinder so that you only have one moving end to worry about.

If you allow for some leakage and run the water at low pressure (less than 12" of head) you could probably get away with melting a small cup of wax around the line. Have the wax be as tall as the diameter of your primary tube. The longer distance will reduce water leakage. Pull the line taught while it cools so the path is straight. It should pull free with very little force, and with little movement during operation, it should provide a fair amount of operating life.

A more robust solution would be to put a diaphragm (as mentioned in the comments) on the end cap instead of a seal. The diaphragm will act like a soft spring, and can be compensated for with other spring components if necessary.

A design that you may or may not have considered; would be to use a single tube as your tension member and run hot and cold fluid through this tube. The design would be simpler, the volume of fluid per tension force would be lower, and the heat transfer would be quicker. Something to think about.

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  • $\begingroup$ I'm open to multiple designs! I may not know too much, but I'm definitely going to try what you all have suggested, thank you for the help! $\endgroup$ – FlippantSloth Oct 13 '16 at 3:57

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