A much simpler alternative to continuous vibration sensors (displacement or acceleration) and monitoring equipment suitable for less critical applications is a simple mechanical switch actuated by a machine’s vibration. These switches cannot, of course, quantitatively analyze machine vibrations, but they do serve as qualitative indicators of gross vibration.
The following photograph shows a Robertshaw “Vibraswitch” unit:

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This switch works on the principle of a weighted lever generating a force when shaken. A pair of magnets located at the weighted end of the lever hold it in either the “reset” (normal) or “tripped” position:

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When reset, the lever is pre-loaded by spring tension to flip to the “tripped” position. All it needs to make that transition is enough acceleration to generate the “breakaway” force necessary to pull away from the holding magnet. Once the acceleration force exceeds that threshold, the lever moves toward the other magnet, which holds it securely in position so that switch will not “reset” itself with additional vibration.

This pre-loading spring is adjustable by a small screw, making it possible to easily vary the
sensitivity of the switch:

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