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?

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.)

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.

One possible source for this type of wax is refills for "wax cooling
vests" which have a similar melting/freezing temperature. Not exactly
cheap; but accessible; read the MSDS if you want to try the "direct
ordering from a chemical factory" route, but that's generally not all
that cheap or easy in small quantities without an established account.

Piston area is still directly related to force/pressure. The total
volume of wax in the cylinder is what sets how far it will actuate, as
the expansion when it melts and contraction when it freezes is the
"motor" in this system. Providing the cylinder with fins or heat pipes
can improve response time to temperature changes. Not overloading the
thing will improve service life...

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