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I'm not sure if this is the right place to ask, but I'm curious about what contributes to the asymmetry of force-distance graphs provided by keyboard switch manufacturers?

For example, it is common to see graphs that look like this online: enter image description here

Accompanied by gifs of the mechanism like this: enter image description here

The mechanism here and what I've seen of actual keys look symmetrical in that there's no reason for any difference between pressing the key and the return of the key. So why is there the reduced amount of force on the return?

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  1. Friction is the cause of "hysteresis-like" difference over the whole stroke. Friction always acts in direction opposite than movement, so on the way "down" you act against spring strength + friction, while on the way "up" you act against spring strength - friction.

  2. force hysteresis coming from the switch construction. The schematics don't show the switch accurately - there's a point where the switch "latches" requiring more force in the negative direction to disengage, than in the positive direction, to engage. In simpler keys, like typical microswitches, this is usually achieved by a small sheet of metal pressed to buckle in the middle; pressing upon it flips the bulge to the opposite side - a bit like a "clicker lid" used on some juices etc, but the amount of engineering that goes into these things when designing keyboards of pleasant key action goes far beyond solutions this simple.

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Because on the "pressing" stroke you are compressing the spring requiring your energy / force and on the return the spring is now assisting the return.

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