Resonance
Resonance is a condition that occurs when a resonant frequency (sometimes called a natural frequency) is excited by an external forcing frequency.
The forcing frequency is amplified by the resonant frequency- the frequency at which a machine will naturally amplify vibration when excited.
Just because a machine has a resonant frequency, does not mean that the machine is faulty. All structures possess resonance characteristics. Resonance is only a problem when a forcing frequency, such as one times the turning speed (1xTS), coincides with a resonant frequency. A special test, such as a bump test using a dead blow hammer and a response sensor such as an accelerometer, will prove or disprove the presence of a resonant frequency. Another test which will provide evidence of a resonance is a transient time waveform performed during a coast-down or start-up.
The rule of thumb has always been that a resonant frequency measured with machinery shut off should be at least 20 percent away from any forcing frequency.
Mass, Stiffness, and Damping
Mass, stiffness and damping are the three parameters that effect the frequency and amplification of a resonance. Mass is the property that creates the vibration forces. Stiffness is the property that counteracts the inertia due to the mass forces. Damping is the property that converts mechanical energy to thermal energy, actually absorbing the vibration.
Increasing the mass of a structure will decrease the resonant frequency, increasing the stiffness of a structure will increase the resonant frequency, increasing the amount of damping in a structure will decrease the amplitude of the resonance. At resonance, damping is the only property that controls the amplitude of the vibration.
The added damping also reduced the resonant frequency slightly. If adding mass will decrease a resonant frequency then taking mass away will certainly increase the resonant frequency. Likewise, if adding stiffness increases a resonance then decreasing the stiffness will decrease the resonant frequency.
An analogy could be made to a guitar string, the tighter the string (the more stiffness) the higher the tone (resonant frequency). If a thicker string is used (more mass) the lower the tone will be.
Mass Imbalance
Mass Imbalance occurs when the center of mass differs from the center of rotation.
It will appear in spectral data as a high one times running speed peak.
Fan Imbalance is a very common fault. Fans can become out-of-balance due to contamination, broken parts, loss of an old balance weight or even wear.
Looseness
There are 2 main types of looseness which occur in rotating machinery.
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Structural Looseness
This occurs when fasteners such as bolts, grout and welds become loose or broken. The severity of such looseness can be identified by the number of Harmonics seen in the spectral data. The use of Phase Analysis can help to identify the point of the looseness.
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Bearing Fit Looseness
This occurs when the fit of a bearing becomes oversized and the bearing is allowed top move around inside a housing or on a shaft. This condition is easily identified using a patented technique used by REDLINE. The severity of this type of fault can also be characterized.
Misalignment
There are 2 main types of Misalignment which occur in rotating machinery. Misalignment creates added loads to bearings and can cause early failure.
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Coupling Misalignment
This occurs when the two shaft center lines are not the same. This condition can usually be identified by a high amplitude vibration in the axial direction with the 2 times and 4 times harmonics being higher than the 1 times and, can be further diagnosed using Phase Analysis.
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Belt Misalignment
This occurs when the two belt sheaves are not inline with one another. This condition can usually be identified by a high amplitude vibration in the axial direction and can be further diagnosed using Phase Analysis.
Harmonics
The harmonic is a mathematical definition, generally used when talking about frequencies. The harmonic series is important in vibration analysis applications because many machines faults produce vibrations that contain harmonic frequencies. Analyzing these harmonics can tell us much about a specific fault.
A frequency is harmonic if it is an integer multiple of the fundamental frequency. The fundamental is the first harmonic (although it's generally referred to as the fundamental). The second harmonic is two times the frequency of the fundamental, the third harmonics is three times the fundamental, and so on. So with a fundamental of 1800 cycles per minute CPM, the second harmonic is 3600 CPM, the third is 5400 CPM, the fourth is 7200 CPM, etc.
Frequency
The term Frequency is often used in vibration analysis. When we know the frequency of something (a vibration) we identify how often it occurs. When we know how often a particular vibration occurs we can relate it to a specific fault.
Frequency:
- Christmas = Once per year
- Full Moon = Once per month or 12 times per year
Some frequency terms used in vibration analysis:
- Cycles per Minute CPM
- Revolutions per Minute RPM
- Cycles per Second CPS or Hz
Sidebands
Sidebands are the result of amplitude modulation.
Imagine a set of gears and one of the gears is not centered on it's shaft. This non-centered gear has 20 teeth and rotates at 100 RPM.
In our vibration spectrum we would expect to see a frequency peak at 20 teeth X 100 RPM or 2000 CPM (Gear Mesh Frequency). But since the gear is not centered on it's shaft this frequency of 2000 CPM will be modulated (have a higher amplitude) once per revolution. That is to say, the teeth will impact one another with more force on one side of the gear than the other side. Resulting in sidebands of 100 CPM around the gear mesh frequency (2000 CPM).
The bottom line is that by identifying these sidebands we can identify a problem gear or bearing and, by with trending we can determine the severity of a fault.
