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When looking at vibration analysis, we can simplify the process by relating the frequency of the fault in relationship to the running speed of the shaft. We call this order analysis. Order analysis is a technique for analyzing vibration signals from rotating or reciprocating machinery, such as engines, compressors, fans, and pumps. These machines have a variety of parts, and each part contributes unique vibration patterns to the vibration pattern of the whole machine. With order analysis, you can identify and isolate these vibration patterns to analyze the performance and quality of each machine part individually.
Using Vibration signals and applying the Fast Fourier Transform (FFT) analysis we can turn the time waveform signal into the respective frequencies. Most fault conditions can be explained by using the order reference, for example: imbalance usually gives a 1X high amplitude frequency, Or blade pass frequency of a 5 bladed fan is 5X
You can use an FFT spectrum to identify and quantify the frequency components of a vibration signal. The fan rotates at the same speed as it shaft and vibration coming from this are said to be synchronous; However the bearing rolling element by design is running much slower than the shaft and is said to be Non-Synchronoous.The vibration signal in the animation is showing the frequency of the balls hitting the outer race defect just under 4 hits per revolution. We locate the frequency @ 3.9x orders in this example. when ever we see non integer frequencies (with a decimal point) we should be thinking of Bearings. This however is only one part of the diagnosis , because in complex machine, we have multiple shafts at different speeds, which could be transmitting the signal and therefore it is important to identify all the shaft speeds and component,such as Gear teeth, bearing, no of blades..etc before analysing the data. Knowing the SPEED is important. Other factors to take into account is the electrical frequencies. These can, for a four pole motor be very close to bearing frequencies. Sometime dropping the spectrum into Hz helps with this diagnosis.....more in the chapters below.