CEMB USA N500 - Manual (Page 34)

6 - 2 FFT analyzer mode
2. Type of measurement
This is the mode in which each component (line) of the spectrum is applied; it can be:
–
RMS (Root Mean Square):
This is the one most typically used, as it is associated with the overall RMS value.
–
PK (Peak):
This is the maximum value reached by the component in question in a certain
interval of time;
it is rarely used because it does not provide information about the overall PK value;
line by line it is simply equal to the RMS value multiplied by 1.41.
–
PP (Peak-to-Peak):
This is the difference between maximum value and minimum value reached by the
vibration in a certain interval of time;
it is rarely used because it does not provide information about the overall PP value;
line by line it is simply equal to the RMS value multiplied by 2.82.
3. Unit of frequency
It can be chosen from:
- Hz – cycles (revs) per second
–
RPM – revs. per minute
N.B.
Obviously the relationship 1 Hz = 60 RPM holds good between the two units
4. Max frequency
This is the maximum frequency of interest in the phenomenon; in practice, it is the
maximum frequency which can be shown in the spectrum. It can be chosen from the
following default values 25, 100, 500, 1000, 2500, 5000, 10000 and 15000 Hz, on the basis
of which the N500 instrument will choose the appropriate frequency for data acquisition.
N.B.
The typical choice, suitable for most situations, is 1000 Hz (60,000 RPM),
coherently with the requirements of ISO 10816-1.
N.B.
One practical consideration normally adopted is that of making sure that the max.
frequency preset is at least 20-30 times that of the frequency of rotation of the shaft
being examined. This allows including in the spectrum also the high frequency zone
where problems relating to the bearings usually occur.
N.B.
With other conditions remaining the same, the choice of a maximum low frequency
(less than 1000 Hz) would cause an appreciable increase in the times required for
acquisition and measurement.