Chapter4 Evaluation of vibration

1. Evaluation of facility importance

Evaluation of facility importance

  • A:High value machinery
    (Private power generator, High pressure compressor etc.)
    Mechanical failure is directly linked to stop of operation
  • B:Important machinery
    (Boiler, Pump,Compressor etc.)
    Mechanical failure may cause a decrease of production
  • C:Ordinary machinery
    (Intake or exhaust fan etc.)
    Mechanical failure may cause inconvenient for operation
  • D:Low value machinery
    (Small general purpose motor etc.)
    Mechanical failure is not linked to operation and repair/replacement is much cheaper

2. What is simple diagnosis?

The aim is to diagnose facilities and clearly show the degree of deterioration by measuring vibration. Judgment value will be needed for precise evaluation. There are three main judgment methods in simple diagnosis. Using these three methods will ensure more accurate measurement.

1. Absolute value judgment

Facilities are classified into several types (small type, medium type, large type). It is a method that if measured vibration exceed a certain level , it is diagnosed as abnormality.

  • Advantage
    Judgment is easy because diagnosis standard is already established.
  • Disadvantage
    Judgment may change depending on types or parts of facilities and makers.
Judgement example

ISO vibration evaluation standard

This standard is widely used for synthetic judgment
of rotational machinery.
The current ISO standard 10816-3:2009 describes the absolute value judgment by velocity rms

Objective machinery is classified into 2 major groups

Machinery Group1
Large machinery
Output: 300kW~50MW, Shaft height:> 315mm
Machinery Group 2
Medium machinery
Output: 15kW~300kW, Shaft height: 160mm~315mm
ISO 10816-3:2009 Vibration Standard Evaluation

2. Relative value judgment

The method of setting up the standard velocity RMS value and comparing the normal value with some absolute values for judgment. Normal value of velocity RMS which is originally set up by someone is multiplied by x times to be regarded as "alert value" or "dangerous value". For example, when the velocity RMS value of motor shows 5.00 most of times after more than ten times measurement, "5.00" can be set up to be the normal value. 10.00 (two times as high as normal value) is set up to be alert value, 25.00 (five times as high as normal value) is "dangerous value." The normal value should be decided after more than ten times measurement.

  • Advantage
    More precise than absolute value judgment
  • Disadvantage
    Because the normal value is decided according to each personal experiences, so ambiguity remains in the validity of the judgment values.
Judgement example
This is a case which alert value is set to be two times as high as normal value.
Dangerous value is set to be five times as high as normal value.
V(RMS) Trend Analysis Chart

3. Intercomparison value judgment

A method of judgement by comparing vibration amount at the same measuring point of the same machine. If numerical value is more than twice as big as equivarent facilities, there is a possibility of abnormality.

  • Advantage
    More precise than relative value judgment
  • Disadvantage
    Applicable machinery is limited
Judgement example
When there is over two times difference between the two same machinery, there may be an abnormality.
Intercomparison value judgment

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3. What is precise diagnosis?

The aim is to analyze waveform with FFT and identify machine part that have deteriorated with frequency distribution, producing more reliable results than simple diagnosis. When a normal machinery and an abnormal machinery are compared with FFT, it may show a big change at a feature frequency value. The following chart is about the feature frequency and the kinds of abnormal state.

What is precise diagnosis?

Bearing abnormalities and feature Frequency

N=Number of circulation(rpm), M=Multiple(1, 2, 3…), P=Pitching frequency
*Abnormality described above can be recognized correctly by che cking the specified mode.

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