Vibration is the movement or mechanical shock of a mechanical or structural part around a fixed position. Vibration is measured in acceleration and can be expressed in physical units m/s2 or the gravitational unit constant g, where 1g is 9.81m/s2. The vibration of an object, machine or structure can be measured using different types of sensors, such as accelerometers, proximity probes (eddy current sensors) or displacement sensors (such as LVDTS).
For device condition monitoring, a distinction must be made between relative and absolute sensors.
Relative vibration sensors, such as proximity probes or eddy current sensors, are commonly used to measure shaft vibration in plain bearings where the rotating shaft is not in direct contact with the bearing itself, thereby transmitting the vibration as a mechanical force into the bearing box. For these liquid film sleeve bearings, relative vibration is measured using the non-contact sensor described above.
The absolute vibration sensor is an accelerometer, which in the mechanical monitoring example is mounted on the bearing box of a standard roller bearing. There are several types of accelerometers that provide raw acceleration output or convert measured acceleration directly into vibration velocity (mm/s).
