Broad band vibration measurement is the most widely used and cost-efficient method for the diagnosis of general machine condition.
There are two ISO recommendations concerning machine condition monitoring by this type of measurement, the much used ISO 2372 and the more recent ISO 10816, which is a replacement of the older standard.
Features of ISO 10816
Measurements are made in three direction (horizontal, vertical, axial).
Machine condition is generally diagnosed on the basis of broad band vibration measurements returning an RMS value. ISO 10816 keeps the lower frequency range flexible between 2 and 10 Hz, depending on the machine type. The upper frequency is 1000 Hz.
ISO 10816 operates with the term vibration magnitude, which, depending on the machine type, can be an RMS value of vibration velocity, acceleration or displacement. If two or more of these parameters are measured, vibration severity is the one returning the relative highest RMS value. For certain machines, ISO 10816 also recognises peak-to-peak values as condition criteria.
The standard consists of several parts, each treating a certain type of machines, with tables of limit values differentiating between acceptable vibration (green range), unsatisfactory vibration (yellow range), and vibration that will cause damage unless reduced (red range).
Vibration analysis (EVAM®)
EVAM stands for Evaluated Vibration Analysis Method, the method generates three sets of machine condition data:
- Condition parameters, which are measured and calculated values describing various aspects of machine vibration.
- Vibration spectra where significant line patterns are found, highlighted and evaluated with the help of preset fault symptoms.
- Machine specific condition codes (green, yellow, red) and condition values, based on a statistical evaluation of the condition parameters and symptom values.
For each measuring point, the user can make an individual selection and define the type of data best suited for the surveillance of an individual machine. Alternatives include:
- time synchronous averaging
- band alarms and averaging of measurement results for improved alarm reliability
Random high readings caused by resonance or other sources of disturbance are filtered out, minimizing the number of false alarms.
Condition parameters are measured for a selected frequency range. They can be individually activated and are shown in measuring result tables and as diagrams. Available are:
Spectrum analysis with ‘symptoms’
For easy pattern recognition in spectra, EVAM supplies a range of ready made ‘fault symptoms’. These are instructions to highlight a spectrum line pattern and display the sum of the lines’ RMS values as a symptom parameter (which can be evaluated and trended). Most symptoms are automatically configured by using the rpm as a variable, for some an input is needed, e. g. the number of vanes on a rotor. Suitable symptoms and symptom groups are selected from a menu in Condmaster when the measuring point is set up.
Machine specific condition codes
In Condmaster, alarm limits can be set on all active parameters. Once measuring results are collected, an EVAM ‘criterion’ can be created that compares new parameter values with the statistical mean value and displays a dimensionless condition value against a green – yellow – red scale.
A phase is a time delay expressed in degrees of rotation. We calculate the time delay between the passage of the tachometer pulse and the peak of the frequency component of interest from the vibration transducer at the speed of rotation. The value presented is a relative angle, not an absolute, because there is no compensation for phase lag in the transducer or the electronic circuits.