National Instruments Order Analysis Toolset - Manual (Page 18)

Chapter 2 Order Analysis
© National Instruments Corporation 2-3 LabVIEW Order Analysis Toolset User Manual
Assuming that speed remains constant during data acquisition, you can use
the following equations to switch between the frequency domain and the
order domain.
Orders often reflect the physical characteristics of rotating machines. As in
classical harmonic analysis, by analyzing the phase and amplitude
relationships between different orders, you often can discover a great deal
about the system in which you are interested. For example, order analysis
has enabled the observation of the following relationships:
• Imbalance results in a spectral peak at the first order.
• Misalignment or bending of the shaft generates a large second order.
• Oil whirl might lead to strong fractional orders.
• Gears, belts, and blades might enhance high orders.
Figure 2-2 shows the order spectrum of the vibration signal measured from
a PC fan with seven blades and four coils.
Figure 2-2. Order Spectrum of a PC Fan with Seven Blades and Four Coils
The vibration signal depicted in Figure 2-2 contains strong fourth and
seventh orders. The four coils inside the fan drag and push the shaft
four times per revolution, causing the strong fourth order. The seven blades
of the fan pass the position of the sensor seven times per revolution and
cause the strong seventh order.
Frequency
RPM
60
------------- Order ×
=
Order Frequency
60
RPM
------------- ×
=
Orders
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0
4 Coils 7 Blades