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Stacked spring refers to an arrangement where a helical spring is placed on top of a snap-action spring (sometimes referred to as a "plate spring"). This snap-action component can be a rubber dome, which achieves the same objectives with lower cost.
The stacked spring arrangement allows a snap-action actuator to gain full travel motion, pretravel and overtravel, and hysteresis. Although rubber domes inherently offer full travel by themselves, the remaining characteristics are only enabled by the use of a helical spring.
- When depressing the switch, the helical spring will take up the initial pressure and compress in a linear fashion; this lead-in period provides the pretravel that a snap-action spring cannot offer by itself, which will only snap by a fraction of this distance.
- Once the pressure in the helical spring reaches the buckling point for the snap-action spring or dome, that component will snap down into its unstable state, and actuate the switch.
- The buckling action will allow the helical spring to expand, reducing its pressure and providing a tactile event; the snap-action component will not release, because the force required to hold it down is lower than that required to buckle it, so the switch will sit comfortably in its actuated state without jitter.
- Although the snap-action component is held in place, the helical spring can be compressed further; this extended period provides overtravel (achieved without any motion in the snap-action component, which is now held firm).
- When releasing the slider back to the point where the switch actuated, it won't release, per step 3; the slider must be released further before the snap-action component snaps back up, providing hysteresis.
The switch can actuate using different methods, including:
- Integrated contact assembly ("switchplate"), as found in Alps plate spring yielding a fully modular switch
- Three-layer pressure membrane (either with a rubber dome or plate spring)
- Conductive rubber dome over PCB or single-layer membrane
- Contact bridge: Olivetti snap action keeps one side of the spring permanently held on a pad on the PCB, and the centre of the spring is brought down over a second pad, connecting them together
The best known example is Fujitsu Peerless. No other rubber dome variety has yet to be reported as clicky, while the plate-spring types all click due. The Olivetti dome with slider patent explains the principle in detail.