On 2014-02-14, Ecnerwal wrote:
In article ,
"DoN. Nichols" wrote:

I have no experience with these, but I suspect that there is
either a spring or a weight which pushes the rod out the 18" or so, and
the wax simply locks the rod in place as long as the temperature is low
enough. When it melts, it does not *push*, but rather *allows* the rod
to move.

And how does it *close again* under your suspicion?

There was no mention of it closing again in the original
posting, so I did not cover that at the start. I was figuring manual
reset, and the vent opening to be a catastrophe prevention measure.

He asked how it could possibly work, and I threw out one
possibility -- before going on to do more searching.

If you ever cast wax candles, you have seen the considerable shrinkage
of wax as it changes from liquid to solid. That's all that's going on
here, and it provides plenty of force in both directions (in a
well-designed system, anyway.)

Did you not read all of what I posted? I described that very
shrinkage -- and did a web search for the term to see what some makers
might say about it.

They are simple hydraulic mechanisms, the only part that's voodoo is a
wax that melts at ~70F rather than the more easily obtainable waxes that
melt around 100-120F, and good (or bad, more often) basic
hydraulic/mechanical design under the somewhat different parameters
where the volume of the cylinder is important to the distance of
actuation. Assembly without trapping air pockets will also help.

Someone else posted a pointer to a site which included a
cut-away view which showed how it was made. A bit more complex than I
first thought. Wax cylinder with a flexible diaphragm at one end, which
bulges and presses on hydraulic oil, which then pushes the piston. The
return force for closing (based on the first site I found) comes from
springs in the mounting, not from the shrinking of the cooling wax. The
shrinkage simply *allows* the springs to re-close the vents.

Enjoy,
DoN.

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