Vibration Analysis 101
A simple guide to understand vibration
Why we talk about vibration?
Vibration provides information about the health of the equipment so an action can be taken to prevent unscheduled breakdown and costly repairs. It is the most common condition monitoring and diagnostic method. It is a technique capable of early detecting symptoms of many different equipment problems and failures. The most common problems detected by vibration are unbalance, misalignment, looseness, bearing and gear problems, electrically induced vibrations, pipe vibration, pressure pulsation etc.
Vibration can detect bearing related problems months before we see detect the problem by our human senses.
What is vibration?
Vibration in a simple terms is an oscillatory motion. Every machine or component subjected to rotational forces or impacts will vibrate. The simplest vibration pattern will be an oscillation “up and down”. If we observe this motion over time it will it will look like a wave. The wave has amplitude (now much the object is moving from its rest position) and period (how long takes to complete one cycle).
What is vibration amplitude?
If we measure from the bottom to the top of the oscillatory motion, we call that Peak to Peak amplitude.
If we measure from the equilibrium position, then we call that 0-Peak amplitude.
It is a common practice to express the amplitude in RMS (root mean square). This is because it better relates to the vibration energy. In a simple oscillatory motion this amplitude is 0.707 of the 0-Peak amplitude.
It is very important to know what is the vibration amplitude expressed in (Peak to Peak, 0-Peak or RMS).
What is vibration frequency?
While vibration amplitude is a measure of the vibration intensity, the frequency is the measure of how often the object is oscillating per unit time. Most common ways to express frequency are:
- CPM — Cycles (number of oscillations) per minute
- Hz — Hertz which is cycles (number of oscillations) per second.
Frequency in CPM can be converted in Hz by dividing with 60.
What is vibration spectrum?
We understood that vibration as a wave can be described with its amplitude and frequency. So, rather then looking the oscillatory motion in amplitude versus time plot, we can express the same information in amplitude versus frequency plot. This amplitude vs frequency plot is called vibration spectrum. Scale, we can simply.
In the spectrum, a simple time waveform can be represented just with two numbers — amplitude and frequency.
How the vibration of real machine may look like?
Imagine if we can isolate each vibration source and Vibration of real machines is more complex. Real machines have different components and shafts that may be rotating at different speeds generating vibration with different amplitudes and frequencies. When all these vibration signals are added together, the resulting waveform looks much more complex then the wave we analyses so far.
Real machines have complex vibration waveform.
How to understand more complex vibration patterns?
To be able to identify each vibration sources in amore complex vibration pattern, we will need to find out each individual simple wave that contributed to the final vibration signal.
In this example, the pattern on the left is a sum of the simple pat-terns on the right.
By breaking down complex vibration signal in its simple components, we can better understand what is causing the vibration.
Each vibration component now can be easily converted in its spectrum. We can now see different amplitude at different frequencies.
How is complex vibration transformed in a simple spectrum?
A mathematical process called Fast Fourier Transform (FFT) is used to convert complex vibration pattern into a spectrum that shows all vibration components (amplitudes and frequencies). This process is critical part in a modern vibration analyser.
The FFT is used to convert complex vibration pattern in a simple spectrum.
Understanding vibration spectrum
After the vibration of the real machine is presented in a spectrum, we can analyse the spectrum by looking each of the amplitude peaks and interpreting what may be casing them, are they high or low, are they changing over time etc. In the most cases, problems present themselves in the vibration spectrum at characteristic frequencies which makes diagnostic easier.
Hopefully this simple guide answered some of the basic questions you might have regarding the vibration analysis. We will be publishing more material here on medium.com or through our website VibCloud | Powerful, Affordable Vibration Monitoring & Analysis.
