Fast bi-stable structure (washer/belleville/dome...)

[DomP]

Hi all,
I just discovered this forum, great threads!

I would like to continue the discussions of these two previous discussions with a new parameter: the speed of snapping.

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http://www.eng-tips.com/viewthread.cfm?qid=337556

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http://www.eng-tips.com/viewthread.cfm?qid=249650

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Let's say we have a bi-stable structure with an OD of 30mm (1.18in), the structure could be a spherical cap (dome type) with or without center hole, or a belleville washer (conical). As per previous discussion, the ratio of the height over the thickness h/t must be high enough to have a bistable structure.

Question: How the dome/washer should be designed in order to provide the highest speed of snapping with a minimal force of triggering (loaded to unloaded (ie. initially stamped) state? The force to load the dome/washer and the height of the structure do not matter.

We are looking at a very asymmetrical bistable structure, with a high residual stress in the loaded state I guess. How to achieve that?

With an insight in the stress zone, would removing some part of the dome would decrease the trigger force but keeping the speed? eg. the center
How to increase the internal stress to a level just before triggering back the dome, so that the the trigger force is low.

What equations describe the speed of such structures?

A lot of questions One mystery: the speed & low force.

Comments welcome. Thanks!
Cheers

 

[btrueblood]

"What equations describe the speed of such structures?"

Newton's laws of motion. Plot force vs. deflection (ideally do this for a real spring, as Bellevilles have a marked hysteresis curve), then solve for F = ma, where m and a are the mass and accel. of your trigger mechanism, F is the formula value for the spring force.

 

[Compositepro]

A thin washer should have the highest speed and lowest force. This is probably why all clickers are made from thin, spring tempered steel. Spring temper means a high yield strength, so more internal stress can be applied without yielding. Thickness of the washer makes the trigger point less sharp because some of the material is still resisting inversion at the inversion point. Stacking thin washers would work better than a thick washer.