What Is Vibration?
The vibration occurs within operational machinery and moving structures, even those that appear to be in good working order. Moving parts, such as rotating shafts, gear teeth and fluid flows, cause vibrations that can be both felt and heard. Vibration is a core measurement for assessing and investigating the condition and status of both rotating machinery and rotating structures. Vibration monitoring seeks to assess and analyse the vibration levels and patterns therein.
What Types Of Vibration Are There?
There are three main forms of vibration that are measured: velocity, displacement and acceleration.
Velocity defines the speed of the movement; displacement measures the distance that a given point has moved from, and acceleration refers to the rate at which velocity changes with time, in terms of both speed and direction. These forms of vibration are often used when looking for dynamic fault analysis.
How Can Vibration Monitoring Be Used?
Vibration monitoring assesses vibration levels and looks at any patterns that arise in vibration signals. Often, monitoring is carried out directly on the time waveforms of the vibration signal alongside the frequency spectrum. When monitoring the frequency spectrum, the Fourier Transform is applied to the time waveform.
Monitoring can evaluate the performance of the assets being targeted and can offer a reliable assessment of their condition. Often time domain analysis is used in the first instance.
When inspecting a complex machine, frequency spectrum analysis is also used alongside time domain analysis. A machine that’s made up of many different moving parts and components will create more than one type of vibration as each of the components moves. Frequency analysis breaks down time waveforms to assess the repetitiveness of patterns of vibration so that each can be investigated. Done at speed, this is known as the Fast Fourier Transform (FFT) technique.
Measuring Vibration
Different vibrations are measured by using different sensors. These types of sensors include displacement, velocity and acceleration measurement sensors, and utilise different measuring technologies, such as proximity probes, piezoelectric (PZT) sensors, laser Doppler vibrometers and microelectromechanical sensors (MEMS) to name a few.
Sensor installation is required to ensure that quality data is recorded. Vibration signals should usually fall below 20 kHz, with a few exceptions. It is usual for a sample to include a range of frequencies and should also try to capture vibrations when the machine is operating at its lowest speed.
How Does Vibration Monitoring Work And What Can It Detect?
Vibrations are described in terms of noise and regularity, otherwise known as intensity by amplitude and periodicity by frequency. A measurement should tell us how loud a vibration is and how many times it happens over a time period, like a second.
A set of vibration measurements can be compared with other sets taken previously. Levels can be set according to acceptable limits or by using recommended standards. If a new reading shows that the limit has been breached, the machine can be assessed for changes.