Vibration switch
Vibration switch is not really a vibration sensor,but a shock sensor.It is relay that will latch over when it senses a large enough bump (or shock) in a specific direction.The output of the relay is used to trigger an alarm or a shutdown sequence.
The original idea is to shutdown the machine down before it damages itself.This is useful for machines that tend to abruptly lose or gain weight,like a propeller fan in a cooling tower.If a blade become loose,or if it picks up a rag,a fan would be so far out of balance that a severe shock pulse would be sensed by the switch.The relay would trip,disconnecting power to the motor and gearbox and prevent further damage to the fan.