vibration measurement calibration
Vibration is a physical phenomenon of the parts of rotating equipment and machines.
It is a periodic movement of mass about the point of equilibrium. All rotary equipment tend to vibrate. Pumps, compressors, gearboxes, motors, fans, engines of low speed as well as high speed do vibrate to some extent. It is due to the following facts:
- Unbalance of the parts.
- Misalignment.
- Wear on bearing.
- Shaft cracks and rubs.
- Process load changes etc.
Vibration Measurements are taken for variety of reasons.
- For evaluating the condition of equipment.
- For safeguarding against breakdown.
- For trouble shooting machine defects.
The overall mechanical condition of healthiness of a rotating machine is indicated by the Shaft radial vibration amplitude and axial position.
Displacement and frequency are the fundamental measurements required describing the motion of the mass. This is true for any vibrating object.
- “Displacement” is the distance of the mass from the start or rest position at any given time, See figure 28.
- “Frequency” of the vibration is the number of cycles the mass completes during a given time (normally I second).
- Apart from the displacement measurement of a vibrating mass, it is also possible to measure the “Velocity” of vibration at any given time.
- “Velocity” is a measure of the speed at which the mass is moving or vibrating during its oscillation.
As shown in Figure 29, as the displacement reaches a maximum value, the mass Movement reverses its direction. The moment the direction changes the velocity of Vibration will be zero. Conversely the velocity of the vibration is at maximum, when the Vibration displacement is at minimum.
Units of Measurment
Displacement (relative and absolute) is normally measured in “micro metres”. 1 micrometer = 1/1000 of a millimeter. It can also be measured in “mils”.
One mil equal 1/1000 of an inch. 1 mil = 25.4 microns. 1 micrometer is 1 micron. Also I mil is 1 thou.
Displacement is expressed as peak to peak value or peak value only.
Velocity is normally measured in millimeter / second (mm/s). It can also be measured in In/Sec.
It is expressed as RMS or Veff. RMS is root mean square. Veff is effective velocity.
Velocity is measured as peak or average value.
- “Acceleration” of the movement of vibrating mass also can be measured.
Acceleration of vibration is defined as the rate of change in the vibration velocity.
Acceleration is normally measured in m/S2,(meters per second/ second). But it can also be measured in ft/S2 or g’s. 1 g = 9.81 m/S2 or 32.2 ft/S2. Acceleration can be expressed as a peak value or as RMS value.