Vibration analysis is a process that monitors the levels and patterns of vibration signals within a component, machinery or structure, to detect abnormal vibration events and to evaluate the overall condition of the test object.
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Vibration analysis is a process that monitors vibration levels and investigates the patterns in vibration signals. It is commonly conducted both on the time waveforms of the vibration signal directly, as well as on the frequency spectrum, which is obtained by applying Fourier Transform on the time waveform.
The time domain analysis, on chronologically recorded vibration waveforms, reveals when and how severe the abnormal vibration events occur, by extracting and studying parameters including but not limited to root-mean-square (RMS), standard deviation, peak amplitude, kurtosis, crest factor, skewness and many others. Time domain analysis is capable of evaluating the overall condition of the targets being monitored.
In real world applications, especially in rotating machinery, it is highly desirable to incorporate the frequency spectrum analysis in addition to time domain analysis. A complex machine with many components will generate a mixture of vibrations, which is a combination of vibrations from each rotating components. Therefore, it is difficult to use only time waveforms to examine the condition of the critical components such as gears, bearings and shafts in a large rotating equipment. Frequency analysis decomposes time waveforms and describes the repetitiveness of vibration patterns, so that the frequency components corresponding to each components can be investigated. Additionally, the well-established Fast Fourier Transform (FFT) technique facilitates fast and efficient frequency analysis, as well as the design of various digital noise filters.
Vibration is a physical phenomenon that presents itself in operational rotating machineries and moving structures, regardless of the condition of their health. Vibration can be induced by various sources, including rotating shafts, meshing gear-teeth, rolling bearing elements, rotating electric field, fluid flows, combustion events, structural resonance and angular rotations. Because of its ubiquity, vibration is highly applicable for investigating the operational conditions and status of rotating machinery and structures.
Vibrations can be represented in different forms, including displacement, velocity and acceleration. Displacement describes the distance that the measuring point has moved; velocity describes how fast the movement is; and acceleration is self-explanatory. The three types are all widely used, specifically acceleration, which offers the widest frequency range and is extensively applied for dynamic fault analysis.
Vibration can be measured through various types of sensors. Based on different types of vibrations, there are sensors designed to measure displacement, velocity and acceleration, with different measuring technologies, such as piezoelectric (PZT) sensors, microelectromechanical sensors (MEMS), proximity probes, laser Doppler vibrometer and many others.
PZT sensors, the most commonly used sensor, generate voltages when deformed. The voltage signals can be digitalised and translated to represent the vibrations. When selecting suitable vibration sensors, the vibration levels/dynamic range and maximum frequency range/bandwidth should be considered, as well as the other operating environment such as temperature, humidity and pH level.
Sensor installation is critical for ensuring that high quality data is recorded. The recommended method for installing sensors is to stud mount the sensor on a flat and clean surface on the machine. This ensures that a broad and smooth frequency spectrum is captured. When stud mount is not applicable, magnet holders, wax or glue can be adopted as substitutions with vibration levels and frequencies considered.
Vibration signals are usually below 20 kHz, except for certain vibration resonances that can reach beyond that. In practice, the sampling rate should be carefully chosen, to make sure that the bandwidth containing frequencies of interest are captured. Additionally, the recording length for one measurement should be at least several periods of the lowest speed of the machines.
Vibrations can be described both in intensity by amplitude and in periodicity by frequency. Figure 1 shows the vibration time waveform captured from a moving mechanism. The time waveform is complicated by its speed-varying movement. The peak amplitude can be observed to be approximately 0.12 g, which was induced when the mechanism started to move. The root-mean-square (RMS) value, which represents the “effective” signal level, is roughly 0.007 g, as labelled in the graph. Figure 2 demonstrates the frequency spectrum of the same signal. The dominant frequency is 30 Hz, which means the majority part of the mechanism movement vibrated 30 times per second.
Figure 1. The vibration time waveform captured from a moving mechanism.
Figure 2. The frequency spectrum of the same signal.
Time domain vibration analysis is able to monitor vibration levels. Acceptable operation vibration limits can be pre-defined either through long-term operation and maintenance history or through referring to established standards. If the limit is breached, this could be that the overall health condition of the machine is deteriorating and defects have developed.
Frequency domain vibration analysis excels at detecting abnormal vibrating patterns. For instance, a crack that has developed on a roller bearing outer race will lead to periodic collisions with bearing rollers. In time waveform, this information is usually hidden and masked by the vibration from other sources. By studying the frequency spectrum, the periodicity of the collisions can be discovered and thus detect the presence of bearing faults.
A vibration monitoring system is a complete system that is capable of acquiring vibration signals according to pre-determined parameters such as sampling frequency, vibration level, recording length, recording intervals and frequency bandwidths. The system should be able to process the recorded vibration and translate the information to intuitive indications for the machine operators, maintenance staff or asset managers.
The system should not interfere the normal operation of the machines or structures that are being monitored and the benefits of the system should be higher than the cost of implementing the system.
Vibration analysis is predominantly applied for the condition monitoring on machineries and their key rotating parts, including but not limited to:
- Bearings, gears, shafts, free wheels
- Rotating machines such as gearboxes, motors, fans and drive-trains
- Reciprocate machines such as piston engines, reciprocate compressors, pumps and door mechanisms
Vibration analysis has also been employed in structural health monitoring, including but not limited to:
- Bridges
- Pipes
- Turbine blades
- Real-time reaction to the change of health conditions
- Supports remote condition monitoring
- Well-established processing and signal analysis methods/algorithms for predictive maintenance
- Supported by various sensors commercially available for different operational conditions
- Difficult to conduct fault localisation
- Difficult to monitoring crack propagation
- High requirements for proper system setup