On 2014-02-13, Butter wrote:

Most of the descriptions say it has some kind of wax in a cylinder.
When temp gets to about 55 to 70 degrees it melts forcing rod against
greenhouse vent. I'm wondering how to make something instead of paying
$60 ea. What kind of wax could this be. Doesn't seem like melting would
make much pressure. Some of these automatic vents push out 18in

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.

Are those temperatures in F or C? 55 Sounds rather low to be
opening a greenhouse -- but I don't know what you are growing. And 55 C
is equivalent to 131 F which is a point at which I would expect many
plants to be uncomfortable. 70 C is 158 F, which seems even more
uncomfortable for most plants.

Anyway -- once you know what kind temperature you are looking
at, then experiment with Paraffin (used for sealing "canned" foods) and
other waxes (I save a lot of wax from Gouda cheese which melts at lower
temperatures than the Paraffin does. Find out what temperature you get
while there are still significant chunks of solid swimming around in the
melted wax. If you find two, and one is higher than you need, and the
other is lower -- experiment with mixing them in various proportions
(mix well while molten) and see what melting point you get as a result.

At a suggestion, I would make the rod have a number of grooves
in the area which is in the wax holding cylinder so it is less likely to
slip if bumped when cold. Once the wax melts, it should move out
freely. But -- you would have to re-heat it to reset it.

O.K. -- looking at this web site:

http://www.littlegreenhouse.com/accessory/vent2.shtml

I see that their openers work between 60 F and 75 F (apparently
adjustable), and they don't say "wax" but rather "a mineral which
expands when heated". *And* they self reset when it cools off. Perhaps
the design converts a high force from the expansion to long travel via
levers. Then again -- wax does expand significantly when it melts. The
Gouda wax which I save I melt to about 7/8 full in a soup can, and when
it cools (at the outside first) it leaves a cone (funnel shape) in the
top surface where the level retreats as it solidifies. So you could get
a significant amount of force as it melts. But the real range shown by
the web site 60-75F is rather low as a melting point for the waxes which
I have experience with. Hmm ... what does beeswax melt at?

Do I assume that you have a lathe? That would be helpful in
making the cylinder in which the rod operates and grooving the rod,
along making with the spring or weight and linkage to provide the
operating force. Since you are posting in rec.crafts.metalworking, I
consider it a reasonable thing to expect you to have at a minimum.

And later in the web page it does say that it uses wax and some
of the cylinders to have your 55-70F range, so ignore my conversion from
C above.

So -- find a wax which melts at the right temperature, and
enclose it in a cylinder with a piston. The larger the diameter, the
more force you will get, and you can use leverage to convert that force
to travel.

O.K. The adjustment of temperature is simply an adjustment of
how far the piston can move before it starts moving the vent. Download
the instruction sheet from them to get an idea as to what the linkage
looks like.

Good luck,
DoN.

--
Remove oil spill source from e-mail
Email: | (KV4PH) Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---