Vibration diagnosis

Mechanical vibrations show high information content in several respects:

  • Indicator for asset condition

  • Reference to dynamic loads of machines, foundations, close-by machine parts

  • Reference to operational safety, service life and economy of machines

  • Basis of machine diagnosis and vibration damping

Vibration diagnosis is the basis for mechanical condition monitoring of an asset. Based on the vibration diagnosis the following errors can be detected:

  • Changes in the airborne sound can be detected

  • Displacements of machine parts can be detected

  • Altered mechanical vibration behavior can be detected

  • Mechanical wear can be detected

Vibration diagnosis also offers the following opportunities:

  • Individually planning of maintenance intervals

  • Increased machine availability

  • Increased productivity of machines

In a detailed analysis, the vibrations in the frequency range are examined. A Fast Fourier Transform (FFT) is used to transform a time signal to the frequency range. This FFT separates the total vibration into sine and cosine components with corresponding amplitudes and frequencies. The time signal is either the vibration signal itself, or the envelope of the vibration signal. This depends on the problem definition and which machine part is monitored.

With the vibration signal, the following errors can be detected:

  • Unbalance

  • Misalignment

  • Soft Foot

With the envelope of the vibration signal, the following errors can be detected:

  • Damage to rolling bearing

  • Damage to gears

The frequency components generated by individual machine parts are usually known. If they are not known, they must be calculated. If a particular frequency component is overrepresented, a fault can be assumed. The frequency components in a vibration can therefore be used to identify a large number of faults. For example, vibrations may be caused by the following faults:

  • Changes to balance condition

  • Changes to alignment

  • Wear or damage to journals or rolling bearing

  • Damage to gears (meshing) and couplings

  • Fissures in critical components

  • Operating faults

  • Disruptions to the flow in hydroelectric machines

  • Transient excitations in electric machines

  • Friction

  • Mechanical loosening

For example, an imbalance causes a higher amplitude at single speed, whereas a misalignment is characterized by single and double speed in the frequency range. In the case of damage to rolling bearing, the damage frequencies in the vibration signal are not directly visible. Shock pulses are generated as the rolling elements move over the location of the fault. These must be used to create an envelope of the vibration signal. This signal is then transformed into the frequency range.