Vibration displays different patterns across domains. Because of such, engineers transform waveforms recorded in the time domain for various analyses. For rotational machinery, the angular domain highlights anticipated vibrational patterns such as torsion. Rotational vibration can also be analyzed—or separated—into parts.
What is Order Analysis?
Order analysis can help differentiate a rotational source of vibration from random vibration, such as the rotational vibration from a rotor blade on a helicopter.
Order analysis is a method of analyzing the noise and vibration of machines with rotational or reciprocating components, such as an engine or rotor blade. Machinery is composed of different parts, each with a distinct vibrational pattern. Order analysis helps differentiate the rotational source compared to random vibration in the machine’s overall output.
Orders of Rotational Vibration
Order analysis separates rotational or reciprocating vibration into orders. The first order is the reference value that corresponds with the machine’s rotational speed (RPM). In other words, the first order and the frequency of the rotation are the same value. The subsequent orders (2, 3, 4, etc.) are a multiple of that value.
Orders are a measurement of frequency (Hz) and correspond to the reference RPM (order 1) and its harmonics. A harmonic is a multiple of a fundamental frequency. In this case, a harmonic is a multiple of the reference RPM. When the reference RPM changes, so do the order values.
Frequency = RPM / 60
For example, if an engine rotates at 900RPM (15Hz), then order 2 equals 30Hz, order 3 equals 45Hz, and so on. If the RPM increases to 1,200 (20Hz), the corresponding order values would be 40Hz, 60Hz, 80Hz, etc.
| 900 RPM | 1200 RPM | |
| 1st Order | 15Hz | 20Hz |
| 2nd Order | 30Hz | 40Hz |
| 3rd Order | 45Hz | 60Hz |
| 4th Order | 60Hz | 80Hz |
The number of harmonics depends on the number of events per revolution. For example, many four-cylinder, four-stroke automotive engines have a second-order vibration; for every engine revolution, half—or two—of the cylinders fire.
Watch: How a car engine works by Autotechlabs (via YouTube).
Recording Order Data
In most cases, rotational machinery varies in speed. Engineers must record data over time to understand the potential vibrations of the rotating component. This process helps improve the safety of rotational equipment by analyzing its response to the range of RPMs that may occur in use.
Engineers perform a run-up or coast-down that steadily increases the RPM as a sweep. Order analysis generates a plot of the frequency response versus angular velocity (RPM). This frequency-domain data can be processed in real-time or recorded for post-processing.
A spectrogram is a helpful tool to help identify order lines. It displays the strength of a signal over time at various frequencies. Below is a time-history waveform with an RPM trace and a tachometer-based spectrogram; the dashed red lines indicate the orders with the highest amplitudes.
Time-history file with RPM trace (above) and tachometer-based spectrogram (below).
Order Tracking
Engineers can also employ order tracking, a more advanced process performed in the angular (or order) domain. Order tracking uses resampling to correspond the response to the angular position of the rotating component. The signal is sampled at steady angular increments and is unrelated to speed. Order tracking collects samples per revolution, and the signal’s amplitude and phase are a function of orders.
Order Analysis in ObserVIEW
The ObserVIEW software extracts orders of interest from recordings with a defined tachometer (angular velocity) channel. A tracking filter that closely follows the RPM of the tachometer trace extracts vibration data for the selected order number while rejecting out-of-band data.
The user can manually choose orders or automatically generate the top orders. Using the spectrogram data, the software selects the orders based on the highest peak acceleration levels within the analysis range. With time- and order-based spectrograms, the user can then analyze/manipulate the data and compare amplitudes.
Order analysis in ObserVIEW.
Next, they can employ the Sine Tracking, Analysis and Generation (STAG) software. STAG identifies dominant orders and extracts them from the original recording to generate an accelerated Sine-on-Random test. It uses a combination of order analysis, order extraction, sine tone profile creation, and sine tone profile acceleration to isolate, extract, and create an accelerated sine profile or series of sine profiles from a recording.
STAG dialog in the ObserVIEW software.
From there, the random content is analyzed with FDS and combined with the sine test profile. STAG allows for the analysis, acceleration, and generation of a test representative of real-world environments with dominant sinusoidal components.
Why Order Analysis?
Engineers can use order analysis to identify how the vibration of an individual component contributes to the overall level. Fatigue and damage are often related to harmonics, and analysis of order content helps to locate and prevent product failure. Analyzing order content is critical for a complete evaluation of rotating machinery.