Plastic torsion spring

Could anyone give me any links to images/sites/existing products that
show how a lid can pop/spring open about a pivot just using the
plastic parts themselves and not an extra metal torsion spring?

Any help would be much appreciated.

Thanks.

 

Many of those things are hard to find.

The best examples I've seen are from the flip-top lids on shampoos and
food container bottle caps. You just have to pick the style you like
and model it yourself.

There have been some patented items, and you can see one type at:

www.hyper-cap.com

Some designs just use two pieces molded with strong enough connecting
hinges to open the item when a latch is "popped".

It takes some creativity and practical smarts and a good dose of
prototyping with the Polypropylene like latest versions of the SLA
epoxies that can somewhat mimick PP's flexible hinge capability.

Bo

 

Could anyone give me any links to images/sites/existing products that

Will,
Sorry I don' have any links to sites handy. But I (hope I) can give
you some advice.

Most of the products out in the 'world' that I have seen that do this
don't use a torsion spring - they use the 'over center' principle.
This is used on shampoo bottles I've seen, but also draw latches and
even on the articulating gussets that hold folding table legs in
place.
The basic principle is simple:
You have two rigid linear segments connected by a hinge that in their
relaxed state look like '^' (sorry about my ascii approximation -
that's an upside down 'V').
If you apply force, you can get those two components into super-
compression that eats up all tolerance in the system and even flexes
the members to look like this: -- (that's ascii for in line with each
other, but really pressing against each other with some amount of
resistance/interference).
This is a high energy state - the linkages don't want to stay here,
and will either fall back into the '^' or fall the other way to 'go
downhill' to a lower energy state.
If you push the linkages over 'the hill' (from '^' to '--' and then
beyond) they fall 'the other way' into the 'over-center condition'
'V', (note: the over-center condition requires some sort of 'stop' so
the 'V' doesn't turn into 'II').
This is what makes it seem like a spring - instead of using the
typical helical extension/compression spring or the torsion/leaf
spring we are used to and learned in basic physics classes, over
center locks take advantage of the natural tendency of a compressed
linear element to 'spring' the system to one side or another of an in-
line, high energy situation. This only works if you designed your
faux-spring to incorporate the sytem tolerances and flex of the
materials so the in-line situation has all parts in interference when
inline so they need to pop to one side '^' or the other 'V' in order
to 'fall-back' to a lower energy statewhen they reach 180° to each
other (this is the case with the flip-top shampoo bottle, not the
folding table leg which uses gravity to maintain the over-center
pressure and lower-energy state)

If that doesn't make sense, try this as another vector to an
explanation:

Stand with your knees locked, and have someone apply a little pressure
to the back of your knee (where your thigh meets your calf). You
fall, right? Quickly, right? And it doesn't take much force at all to
make you fall because the locked leg is in a high energy state in a
'center', knee locked position. Your calf is a rigid element, your
thigh is a rigid element, but there is a lot of pressure between the
mass of your body and the mass of the earth. Oddly, it doesn't take
much force - maybe 5 pounds or so- to 'spring' this pressure and make
you drop.
That is how over-center systems work( and its also a fun way to haze
friends when you sneak up on them).
That sort of pressure can also be achieved between rigid element that
are not subject to gravity - if some sadist laid you on your back and
attached a machine to your ankle and the other part of the machine to
your hip and set the *distance* right so your calf/hip system was
enduring some force, eventually it would buckle one way or the other.
Now, imagine if your knee could bend the other way and your assistant/
sadist pushed on the front of your knee to get you to an 'over-center
postion' (don't try this at home or you will limp for life) You will
fall the other way, right? And it would take a ton of muscle (or in
this case, surgury) to go the other way and stand back up.
That is how an overcenter 'spring' works, and it can be done easily
from plastic. IUn my experience, for linear elements about 1.0" long,
0.03" interference provides a really nice spring (but make it steel
safe so you can dial it)

Ed

ptoential thread hijacking digression:

I do a lot of over-center latches and systems in my design work and
have found them to be a nightmare to mate because, since there are two
valid solutions for any over center latch (before they go over-center
and after they go over-center), SolidWorks always picks the wrong one
eventually as you cycle through configurations of the system. Anyone
have a way to beat this?
In an attempt to get some control, I have adopted putting the over-
center linkages into a subassembly with multiple configs to drive the
various positions and use those in my top-level configs, but even then
there is anotehr solution that I did not want and the system will
never see, or SWx goes into 'overdefining mate mode' when if it just
thought a few more degrees it would find the correct orinetation to
satisfy the mates.
Anyone figure out how to beat this?

 

Will, with respect to springs, I have use a wide array of pure leaf
springs, torsion springs, and then over center type elements, some of
which have also been hinges, made from partial spherical segments,
like on the top of coffee cup lids that invert, conical segments that
invert, and ones like Ed talked about as used on shampoo & condiment
flip top bottle caps.

Some of the plastics companies like Dow and GE issue design guides and
spec out general forms of springs and their sizing, and much of that
is often associated with hinges.

There is a ton of stuff you can find with Google. Some good, some
excerpts from expensive books, and some college lecture PPTs.

FlexureLecture.ppt by Brian Trease of Univ. of Michigan in 2004 is
probably in the internet archive and has 109 slides and recommends 4
books on the 3rd slide for reference.

It is a 10 meg ppt, and if you get me your email, I can send you a
copy, if you can't find it.

Bo

 

Start here:

http://www.efunda.com/DesignStandards/plastic_design/hinge.cfm


And here:

http://www.profilesmagazine.com/p15/tips_spinal.html

http://engr.bd.psu.edu/pkoch/plasticdesign/living_hinge.htm

If you take this basic hinge and make it narrow and with a slight
curvature it will snap into one of two positions.

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