Decided to try and make one from an old magnetic stirrer i have with a blown motor. I often want to stir things in the ultra-low freezer and this
would be a solution. I never did win the rubber band powered car competition at school so i'm negative about my chances on this project already. Do
the spring steels used in windup mechanisms fail to perform at -85, if someone knows i could then discount it before wasting time on it.
Any one have any alternative ideas other than the stocko winding mechanism( using springs), batteries, drilling a hole into my fridge or doing the
reaction on the surface of pluto.
How does one vary the speed on a winding mechanism without simply applying greater load (and therefore limiting the time one wind will stir for), is
it simply with gears or is there a tricky way.
Any good links would be appreciated.aonomus - 1-1-2010 at 20:42
You may want to consider whether the bearings and other mechanical bits in your stirrer function at -85degC. I'm sure you can store it in the freezer
for a day or so, pull it out, and try to spin the magnet by hand. Lubricants might be too viscous (or freeze), or different metal sleeves might
contract and seize.
A windup mechanism seems like an awkward idea simply because you will need a huge amount of tension to maintain spinning and speed. Opening the
freezer to wind it back up might just allow enough heat to enter to make the whole thing warm up.watson.fawkes - 2-1-2010 at 06:46
I would recommend this one, as it would seem to put the least extra heat load
on your refrigerator. Just consider the stir platform to be a new, permanent modification to the freezer. You shouldn't need to use a metal axle with
high thermal conductivity; a plastic one should do fine.densest - 3-1-2010 at 22:15
If you decide to use a metal axle, stainless steel is a remarkably bad heat conductor for a metal. A dovetailed plastic insert between metal sections
could provide a thermal break and the plastic would only be in compression rather than torsion = much stronger. Panache - 4-1-2010 at 02:40
So as i understand you guys are advocating a hole in the freezer for the purposes of running a shaft into it that would drive the stirrer, not running
a power lead in into which i could plug any device i wanted. I currently have a heavy duty mag stirrrer (geared) chilling nicely in the freezer, i
will take it out later and give it a whirl.densest - 4-1-2010 at 05:36
The lubrication in your motor and gearbox is not designed to work below (say) -10C, so it may spin, but it may seize up just when you don't want it
to. A geared stirrer will put out a fair amount of heat moving its grease around. If you wanted to change the grease for an arctic-rated blend, it
might help. The clearances for the gears and bearings may not be correct once they are chilled that much. That could cause either much heat from
unexpected friction or failure from galling or simply jamming. The gears may become brittle enough to shatter. There are a lot of unknowns putting the
stirrer to work in that environment.
It's about trade-offs. The external powered version will -probably- work longer with fewer surprises and with less load on the freezer compressor.
Think of it this way: cooling 1L of water (once frozen) to -85C over an hour requires removing about 100 watts. Adding (say) 25W of motor heat is
significant in that environment. If you wanted to be really paranoid about heat leakage, you could use a magnetic coupling through a plastic barrier
so no air would leak either way. You may be able to find out from the freezer manufacturer how much heat it can pump, what the heat leakage under
standard conditions is, and judge from that. not_important - 4-1-2010 at 07:31
-85 is fairly tough on a lot of things, read one of the books on the Antarctic research expeditions; there were a couple that covered the IGY ones
back in `57-58.
A shaft to on external drive sounds like the least troublesome route to me.
Panache - 5-1-2010 at 18:03
lol, i took out the stirrer after leaving it in overnight, it had a nice covering of dry ice condensed from the air. Plugged it straight in and turned
it on, absolutely nothing happened, i put my head close to it and there was a slight hum. Thirty minutes later it was running fine again.
I had a thought about a windup mouse (cat toy), i wonder how long they would spin around the bottom of 5 litre beaker for, i'll buy one i'm at the
supermarket next.aonomus - 5-1-2010 at 19:37
Realistically, a windup spring would need to be under a fair amount of tension, and usually under very cold temperatures, elastic materials lose their
elasticity. Perhaps somewhere in a materials science text you could find out where regular spring steel loses its elasticity?densest - 5-1-2010 at 20:40
Cold hard steel shatters. Bicycle thieves love LN2. dann2 - 6-1-2010 at 18:42
Hello,
This may not suit you but if you do not want to drill a hole and put in a shaft going into the freezer you can position a rotating magnet(s) close to
the inside skin of the freezer (magnets are outside the freezer chamber) and place your vessel to be stirred above the rotating magnets. The skin will
have to be made from plastic or Al (not Iron) for the magnets to spin the stirrer bar. You will have to cut away the outside skin + the insulation
where you are going to place the spinning magnet(s).
