how to design a variable torsional spring

April 19, 2008 3:53 AM

how do i design a cheap spring to attach to a small plastic lever, that will only act over a portion of the rotation of that lever?

i am designing something with a small straight plastic lever, about 40mm long. it is attached to a plastic door by fitting into the shaft at the end, and the lever rotates over about 40 degrees to open and close the door. the door opens from a flat bottom position to a top position, and seals at both ends.
the lever is rotated by a mating pivot lever, that has a pin. the pin runs down a straight slot in the lever. the ratio is just slightly amplified, but the rotation of each lever is near enough to equivalent.
i want to add a torsional spring to the lever to help lift weight of the door from the bottom position. the spring is a simple 3 coil torsional spring, with one fixed end, and the other end rotating with the lever.
but when i do, of course, the door doesnt seal well at the bottom most point (because the mechanism force is too low). the spring works against the pin force, to open the door slightly.

is there a type of torsional spring that can be free of tension at the bottom point, but then help lift the lever through most of the travel.
or alternatively, is there a simple way to fix the end of the torsional spring to have the same effect?

posted by edtut to science & nature (4 comments total)

Maybe I'm misunderstanding, but it sounds like you need a counterweight rather than a spring.
posted by Lanark at 5:12 AM on April 19, 2008

I don't have a good picture based on your description, but my guess would be you need a lever arm to redirect the DIRECTION the spring is pulling so that it's pulling in a neutral direction when the door is closed. (By a neutral direction, I mean either a direction that helps keep the door closed or in a perpendicular direction where it has no impact.

Consider, as an example, the mechanism behind the ashtray or similar compartment in some cars or on airplanes (back when they had ashtrays.
posted by JMOZ at 6:05 AM on April 19, 2008

I've seen springs that sort of buckle so that they have very little force past a certain point. Not being a mech.e I have no idea what they'd be called.

JMOZ' suggestion seems good too — arrange the mechanism so that the spring has zero mechanical advantage as the door shuts. You could probably even arrange it so it has zero advantage shortly before the door shuts, and actually helps hold the door closed once it's all the way shut. I'm sure I've used mechanisms set up like that (extension springs, not torsion springs, though).
posted by hattifattener at 12:47 PM on April 19, 2008

counterweight wont do it because the mechanism force is too low. however the ashtray style spring could be just the trick. sorry i couldnt work a way to get a pic in here, would have made it a lot easier i guess.
posted by edtut at 6:38 PM on April 19, 2008