Vibration is a complex measurement that contains various parameters. Different measurement technologies have advantages and disadvantages depending on the ultimate vibration measurement goals.
What is vibration?
Vibration is the back and forth or repetitive motion of an object from its point of rest. When a force is applied to the mass, it stretches the spring and moves the weight to the lower limit. When the force is removed, the stored energy in the spring causes the weight to move upward through the position of rest to its upper limit. Here, the mass stops and reverses direction traveling back through the position of rest to the lower limit. In a friction-free system the mass would continue this motion indefinitely. All real systems are damped, that is they will gradually come to their rest position after several cycles of motion, unless acted upon by an external force. The characteristics of this vibratory motion are period, frequency, displacement, velocity, acceleration, amplitude and phase. Continued vibration of this spring mass system would only repeat the characteristics shown in this single cycle.
All rotating machines produce vibrations that are a function of the machine dynamics, such as the alignment and balance of the rotating parts. Measuring the amplitude of vibration at certain frequencies can provide valuable information about the accuracy of shaft alignment and balance, the condition of bearings or gears, and the effect on the machine due to resonance from the housings, piping and other structures.
Vibration measurement
Vibration measurement is complex because of its many components: displacement, velocity, acceleration, and frequencies. At the same time, each of these components can be measured in different ways: peak-to-peak, peak, average, RMS; each of which can be measured in the time domain (real-time, instantaneous measurements with an oscilloscope or data acquisition system) or frequency domain (vibration magnitude at different frequencies across a frequency spectrum), or just a single number for “total vibration.”
Vibration measurement is an effective, non-intrusive method to monitor machine condition during start-ups, shutdowns and normal operation. Vibration analysis is used primarily on rotating equipment such as steam and gas turbines, pumps, motors, compressors, paper machines, rolling mills, machine tools and gearboxes. Vibration analysis is used to determine the operating and mechanical condition of equipment. A major advantage is that vibration analysis can identify developing problems before they become too serious and cause unscheduled downtime. This can be achieved by conducting regular monitoring of machine vibrations either on continuous basis or at scheduled intervals.
Regular vibration monitoring can detect in a power plant shaft lines, deteriorations and defects of a number of various structural mechanical conditions, for example cracks, bearing faults, coupling damage, imbalance as well as interruptions in the electric grid.
There are three main parameters that are measured to evaluate the vibration characteristics of any dynamic system as: acceleration, displacement and velocity.
Acceleration
Acceleration places greater importance on high frequencies. However, an acceleration signal is not exclusive. The acceleration signal can be converted to velocity or displacement.
Displacement
Just like acceleration places greater importance on high frequencies, displacement looks at low frequencies. Displacement measurements are generally only used when examining the broad picture of mechanical vibrations. You might use displacement to discover unbalance in a rotating part due to a significant amount of displacement at the rotational frequencies of the machine's shaft.
Velocity
Velocity is related to the destructive force of vibration, making it the most important parameter. It places equal importance on both high and low frequencies. Usually, the RMS (Root Mean Square) value of velocity (measured in the range of 10 to 10,000 Hz) shows the best sign of vibration severity. RMS is calculated by multiplying peak amplitude by 0.707.
The displacement, velocity and acceleration characteristics of vibration are measured to determine the severity of the vibration and these are often referred to as the ‘amplitude’ of the vibration. In terms of the operation of the machine, the vibration amplitude is the first indicator to indicate how good or bad the condition of the machine may be. Generally, greater vibration amplitudes correspond to higher levels of machinery defects. The relationship between acceleration, velocity and displacement with respect to vibration amplitude and machinery health redefines the measurement and data analysis techniques that should be used.
Vibration Measurement Techniques
Vibration Analysis collects vibrations detected from equipment and measures them against known failure vibrations to identify potential failure points.
Vibration Analysis can help maintenance professionals proactively address performance issues before machines break on the job.
Implementing vibration analysis is only the first step in actually using and benefiting from this technique. The complete processes of vibration measurement techniques is described as follows:
Calculate the Vibration Spectrum - For most systems, you can measure various points on its vibration spectrum. Using this data, you can produce a model that illustrates expected vibration behavior.
Establish a Baseline - In order to take full advantage of your vibration data, you must establish baseline data to inform your monitoring. There are normal levels of vibration and acceptable variations that do not indicate failure. Baseline data will set parameters around what performance is acceptables in order to make abnormalities clear.
Generate Signal Models - Vibration data must be translated from the format in which it is collected (typically frequency, amplitude, etc.). The data needs to be digitally reconstructed into a model that you can analyze. The result is typically a waveform diagram, measuring oscillation amplitude over a period of time.
Analyze Your Data - This process begins by looking at the harmonics of your vibration data. Look at synchronous peaks, or high points in your vibration data that should repeat. The synchronous peaks correlate the frequency peak with the physical characteristics of the system, typically indicating a motor rotation or pumping cycle. Spikes in frequencies are completely normal, but peaks at an unexpected time or an unusually high peak are points to watch out for.
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