Bi-stable spring loaded mechanism examples?

[Sungod3000]

Hi, 

im designing a latch that has one "engaged" position and one "out of the way" position. The latch itself if rather simple and pivots around one anchor point https://a360.co/2WP23ws 

Ideally I only have one spring that enables the latch to be fixed and stable in both positions. I dont need much tension or holding force. But I cant really come up with a good placement for the spring that would allow this because both position are ca 120deg apart from each other so something like this https://www.youtube.com/watch?v=3v5xfWkp9Ys&t=314s doesnt apply.

I looked through http://507movements.com for an example but there are no spring loaded mechanism on that page. There is also this YT channel https://www.youtube.com/user/thang010146 but he has only very complex mechanism like the internal working of push buttons. 

I wonder if you guys know other sources for demonstration or example for spring loaded mechanical examples, that I can get inspired by.

Cheers 

[chrisplyler]

Okay, I've represented the spring with a sliding rod here. This is for a spring in tension (don't need a rod/guide to prevent the spring buckling). If you want to use a spring in compression, put the fixed arm above the hinge point instead of below it, such that the distance between the spring's ends gets longer towards the ends of the latch's rotation instead of shorter as it does here. Of course you can mess around with the mounting locations for the spring's endpoints within reason, as long as you maintain the shorter-spring-at-ends-of-rotation condition.

[Sungod3000]

Not bad, not bad at all. I had focused on having the spring the in the middle but thats quite limiting. 

Ill try that. Thanks

[chrisplyler]

 Well, if you use a spring under compression, you can put the endpoint above the hinge axis instead of below it, so in theory you can put one endpoint on top of my orange cylinder, and put the other endpoint on the crossbar. Probably have to move the crossbar up to make room, and perhaps change the geometry of the latch to keep the same points of contact against whatever it is going to stop against, but that could possibly work.

[Sungod3000]

So i tried this in a small model with a compression spring and it works, but when I scale this to real dimensions I would need a custom spring, which is hard to come by. I have one of these kits with 100 little springs and the tension spring seem much more flexible because the have more range and force. 

Any idea how to design this mechanism the other way around so a tension spring would be used here? 😄

[chrisplyler]

Well...the example I showed in my video previously was for a tension spring. So it's difficult for me to understand why you're asking for a design idea using a tension spring.

Perhaps if you detail the dimensions and constraints/requirements more specifically? Is there something that my example doesn't accomplish?

[Sungod3000]

I think i figured it out. Im working on it. Ill write again when I have it done.

[chrisplyler]

Also...

If the range of motion allowed by a particular, desired size of spring (tension OR compression) doesn't satisfy the motion requirements of your mechanism, it can be multiplied via use of a lever or linkage.

You could put in a partial cam surface that the end of the spring must ride over, such that it kind of "detents" into the two positions you want.

You can make a compression spring out of a tension spring by, for example, using the tension spring to pull two opposite corners of a trapezoid/square/rectangle together, driving the other two corners apart. Or, more simply you know, just with a lever.