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Vibration spectrum analysis pdf

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always relative to the vibration level of a “good”;. “new” machine! DETECTION VS . ANALYSIS. VIBRATION. (AMPLITUDE VS. FREQUENCY). Overview / How To. However, the basic processes of measurement and analysis have remained . Frequency analysis is the essence of vibration analysis and enables the. 5. Time Waveform Analysis complex time waveform. Individual vibration signals combine to form a complex time waveform showing overall vibration frequency.


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1. Introduction. A vibration FFT (Fast Fourier Transform) spectrum is an incredibly useful tool for machinery vibration analysis. If a machinery problem exists. Vibration Spectrum Analysis for Indicating Damage on Turbine and Steam Generator Amurang Unit 1. Article (PDF Available) · December Vibration Analysis Techniques 4. Definitions. 3. 2. Level Measurement. 3. 3. Time Waveform. FFT Spectrum and Phase Analysis.

Swapping out a bearing that is showing wear by vibrating heavily may or may Part 1 not solve your problem. SEE is the best tool for detecting bearing problems in their earliest stages, when the Sometimes with bearing defects there are missing pulses due to defect is subsurface or microscopic and not causing any the physical randomness of a defect as well as sub-harmonic measurable vibration signal. The frequency at which the vibration occurs indicates the type of fault. Scaled values are calculated from the RMS factors used in overall vibration measurements are Peak, Peak- value. However, because of gravity and abnormally high amplitudes. Click here to sign up. There are two measurable derivatives of displacement:

On machines with varying speeds or loads, perform most common machinery problems and imbalance measurements at all extreme rating conditions in addition to produces a radial vibration, that is, part vertical and selected conditions within these limits. However, misalignment and bent shaft problems do create vibration in the A trend plot is a line graph that displays current and past overall axial plane.

Past values should include a base-line known good reading. Viewing the oscillation as position over time produces a sine wave. The starting point when the mass is at rest is the zero point. One complete cycle of the mass displays a positive and a negative displacement of the mass in relation to its reference zero. Displacement is the change in distance or position of an object relative to a reference. The magnitude of the displacement is measured as amplitude. There are two measurable derivatives of displacement: For base-line to determine machinery changes.

For example, some components have a Thus, vibration has three measurable characteristics: Although these three for most machines, but is normal for them. The current reading characteristics are related mathematically, they are three by itself might lead an analyst to believe that a problem exists, different characteristics, not three names for the same quantity.

It is necessary to select a vibration measurement and sensor type that measures the vibration most likely to reveal the expected ISO Standards are good for a start until you develop a machine failure characteristics. Vibration is exampled using a spring-mass setup. Measured in mils or micrometers, displacement is the change in distance or position of an object relative to a reference.

Displacement is typically measured with a sensor commonly known as a displacement probe or eddy probe. A displacement probe is a non-contact device that measures the relative distance between two surfaces.

Displacement probes most often monitor shaft vibration and are commonly used on machines with fluid film bearings.

Displacement probes measure only the motion of the shaft or rotor relative to the casing of the machine. This relationship is used for balancing and shaft orbital analysis reference the Phase Section. It is the most common machine vibration measurement. Historically the velocity sensor was one of the first electrical sensors used for machine condition monitoring.

This because for an equal amount of dynamic motion being generated, velocity remains constant regardless of frequency. However, at very low frequencies under 10 Hz velocity sensors lose their effectiveness. Likewise at higher frequencies above 2 kHz. Today, with the arrival of low cost The above time waveform plot illustrates how the signal from an and versatile accelerometers, most velocity values are obtained accelerometer or velocity probe appears when graphed as by integrating an acceleration reading into the velocity domain.

This type of vibration plot is also called a time domain plot or graph. Vibration in terms vibration. Though typically not as useful as other analysis of acceleration is measured with accelerometers. An formats, time waveform analysis can provide clues to machine accelerometer usually contains one or more piezoelectric crystal condition that are not always evident in the frequency spectrum elements and a mass.

In non-mathematical terms, this means that the signal is Base Connector broken down into specific amplitudes at various component frequencies.

When the piezoelectric crystal is stressed it produces an electrical output proportional to acceleration. The crystal is stressed by the mass when the mass is vibrated by the component to which they are attached. Accelerometers are rugged devices that operate in a very wide frequency range from almost zero to well above kHz.

However, since velocity is the most common measurement for monitoring vibration, acceleration Frequency scale showing measurements are usually integrated either in the accelerometer component vibration signals itself or by the data collector to get velocity.

Acceleration units at various frequencies. However, this vibration signal is of such low frequency contained in the signal. These values or amplitudes energy that, with overall vibration monitoring, it is lost in the are then plotted over the frequency scale. The resulting plot is called an FFT spectrum. Spall An FFT spectrum is an incredibly useful tool.

Following are a examples of alternate processing methods. For that matter, SEE measurements are also very effective for measuring any machine condition that produces acoustic emissions such as corrosion and SEE technology Spectral Emitted Energy provides very early friction due to fretting, cavitation, sliding or friction events, etc.

SEE detection specific conditions. Circumstances that can cause acoustic provides enough pre-warning for the maintenance person to emissions include: See Appendix A for a more detailed description of measuring phase.

This type of signal is not considered HFD vibration as much as it is considered high frequency sound, however, vibration is the commonly used industrial term. HFD provides early warning of bearing problems.

The High Frequency Detection HFD processing method displays a numerical overall value for high frequency vibration generated by small flaws occurring within a high frequency band pass 5 kHz to 60 kHz.

One competitors technology is very similar to HFD and gives These technologies enhance the repetitive components of a the same indications. Measured as Vibration Turn off machine power and monitor vibration.

If the vibration immediately drops, the problem is electrical. On an overhung machine, imbalance and Note: Unsatisfactory — Unacceptable Levels. Use phase measurements to differentiate between the two. Radial 1 and Radial 2 positions differ by 90 Note: No radial Displacement phase shift across the machine or coupling. Account Radial Displacement for change in sensor orientation when making axial measurements.

Bearing defect SEE frequencies are non- integer multiples of running speed i. In other words, what are the possibilities? When adjacent machine. Vibration from one machine can measuring displacement, velocity, or acceleration travel through the foundation or structure and affect vibration, all data collectors should produce very similar vibration levels on an adjacent machine.

Electric motors usually run at these speeds. As mentioned earlier, an FFT spectrum is an incredibly useful analysis tool. Look for this peak and check for peaks at two until the problem becomes critical. Harmonics FFT spectra allow us to analyze vibration amplitudes at various usually cause vibrations at multiples of the running component frequencies on the FFT spectrum.

In this way we speed frequency although they might be very small. Certain information is needed before attempting to diagnose an FFT spectrum. Most experts agree that over half of all machinery problems are caused by misalignment. The two types analyze the spectrum. Analysis usually follows a process of of misalignment are angular and parallel, or a combination of elimination. Eliminate what is not on the spectrum and what is both. Try to find out if they are electrical or mechanical.

These peaks may or may not represent the indicated fault. Look for harmonics to determine if the identified frequencies were generated from the indicated fault. Parallel misalignment occurs when the shaft centerlines are parallel but displaced from one another.

Possible causes of misalignment are: Most machines are aligned times the fundamental fault frequency, then this also cold, then as they operate and heat up, thermal growth represents a strong indication that the indicated fault is causes them to grow misaligned.

A higher than normal reading than its design specification, which in turn causes bearing failure indicates a problem. If possible, measure the phase shift between axial readings on opposite ends of the machine. With static imbalance only one force is involved. The term direction compared to the radial direction.

Misalignment forces are seldom the same in both positions. If higher than normal forces are present in the vertical plane, misalignment is indicated. In reality, almost all imbalance is dynamic imbalance. Dynamic imbalance is the combination of static and couple imbalance. If the axial 1x amplitude is abnormally high, On simple machines, there is usually more static imbalance than and there is a coupling or belt, couple imbalance. On more complex machinery, with more than then there may be misalignment.

When balancing for coupling imbalance, the user is forced to balance in multiple planes. With pumps, uneven wear on impellers is indicated as imbalance. Key characteristics of vibration caused by imbalance are: Imbalance usually causes the bearing to carry a higher dynamic load than its design specification, which in turn causes the bearing to fail due to fatigue.

Fatigue is the result of stresses applied immediately below the load carrying surfaces and is observed as spalling away of surface metal. On an FFT spectrum, this appears as a higher than normal 1x amplitude.

While other faults can produce a high 1x amplitude they usually produce harmonics as well.

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In general, if the signal has harmonics above once per revolution, the fault is not imbalance. However, harmonics can occur as imbalance increases or when horizontal and vertical support stiffness differ by a large amount. However, because of gravity and abnormally high amplitudes. These harmonics may be sporadic. Radial and axial overall vibration readings will compare with or at 3x, 3.

It is important to note increasing imbalance forces place increasing loads on nearby bearings. If both the above conditions are true, the problem is probably a bent shaft.

If the machine is rigidly connected no coupling or belt , and the radial 2x is high, then there may be mechanical looseness. However, phase measurements from the axial position help differentiate the two. With overall vibration and spectral analysis, a bent shaft problem usually appears identical to a misalignment problem. Phase measurements are needed to distinguish between the two. EFFECTS As with imbalance, a bent shaft usually causes the bearing to carry a higher dynamic load than its design specification, which in turn causes the bearing to fail due to fatigue.

If the phase readings among the above four sensor locations vary considerably, a cocked bearing is indicated. It is important to reiterate that most often the bearing NOTE: For example, if a bearing is very heavily loaded, When a bearing defect is detected you should repetitive vibration energy is small and enveloping automatically look for other fault symptoms like measurements become less effective.

As previously mentioned, there are many machinery problems NOTE: Others A bearing may fail for a number of reasons; ineffective include: As a rolling elements pass over these cracks, fragments break away.

This is known as spalling or flaking. The spalling progressively increases and eventually makes the Velocity vibration measurements are typically performed on bearing unusable. This type of bearing damage is a relatively most machinery. These measurements are very useful for long drawn-out process, and makes its presence known by detecting and analyzing low frequency rotational problems such increasing noise and vibration.

No radial Displacement phase shift across the machine or coupling. Account Radial Displacement for change in sensor orientation when making axial measurements. Bearing defect SEE frequencies are non- integer multiples of running speed i.

In other words, what are the possibilities? When adjacent machine. Vibration from one machine can measuring displacement, velocity, or acceleration travel through the foundation or structure and affect vibration, all data collectors should produce very similar vibration levels on an adjacent machine.

Electric motors usually run at these speeds.

Analysis vibration pdf spectrum

As mentioned earlier, an FFT spectrum is an incredibly useful analysis tool. Look for this peak and check for peaks at two until the problem becomes critical. Harmonics FFT spectra allow us to analyze vibration amplitudes at various usually cause vibrations at multiples of the running component frequencies on the FFT spectrum.

In this way we speed frequency although they might be very small. Certain information is needed before attempting to diagnose an FFT spectrum. Most experts agree that over half of all machinery problems are caused by misalignment. The two types analyze the spectrum. Analysis usually follows a process of of misalignment are angular and parallel, or a combination of elimination. Eliminate what is not on the spectrum and what is both.

Try to find out if they are electrical or mechanical. These peaks may or may not represent the indicated fault. Look for harmonics to determine if the identified frequencies were generated from the indicated fault. Parallel misalignment occurs when the shaft centerlines are parallel but displaced from one another.

Possible causes of misalignment are: Most machines are aligned times the fundamental fault frequency, then this also cold, then as they operate and heat up, thermal growth represents a strong indication that the indicated fault is causes them to grow misaligned.

A higher than normal reading than its design specification, which in turn causes bearing failure indicates a problem. If possible, measure the phase shift between axial readings on opposite ends of the machine. With static imbalance only one force is involved. The term direction compared to the radial direction.

Misalignment forces are seldom the same in both positions. If higher than normal forces are present in the vertical plane, misalignment is indicated. In reality, almost all imbalance is dynamic imbalance. Dynamic imbalance is the combination of static and couple imbalance. If the axial 1x amplitude is abnormally high, On simple machines, there is usually more static imbalance than and there is a coupling or belt, couple imbalance. On more complex machinery, with more than then there may be misalignment.

When balancing for coupling imbalance, the user is forced to balance in multiple planes. With pumps, uneven wear on impellers is indicated as imbalance.

Key characteristics of vibration caused by imbalance are: Imbalance usually causes the bearing to carry a higher dynamic load than its design specification, which in turn causes the bearing to fail due to fatigue. Fatigue is the result of stresses applied immediately below the load carrying surfaces and is observed as spalling away of surface metal. On an FFT spectrum, this appears as a higher than normal 1x amplitude.

While other faults can produce a high 1x amplitude they usually produce harmonics as well. In general, if the signal has harmonics above once per revolution, the fault is not imbalance. However, harmonics can occur as imbalance increases or when horizontal and vertical support stiffness differ by a large amount. However, because of gravity and abnormally high amplitudes. These harmonics may be sporadic. Radial and axial overall vibration readings will compare with or at 3x, 3.

It is important to note increasing imbalance forces place increasing loads on nearby bearings. If both the above conditions are true, the problem is probably a bent shaft.

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If the machine is rigidly connected no coupling or belt , and the radial 2x is high, then there may be mechanical looseness. However, phase measurements from the axial position help differentiate the two. With overall vibration and spectral analysis, a bent shaft problem usually appears identical to a misalignment problem.

Phase measurements are needed to distinguish between the two. EFFECTS As with imbalance, a bent shaft usually causes the bearing to carry a higher dynamic load than its design specification, which in turn causes the bearing to fail due to fatigue.

Spectrum analysis pdf vibration

If the phase readings among the above four sensor locations vary considerably, a cocked bearing is indicated.

It is important to reiterate that most often the bearing NOTE: For example, if a bearing is very heavily loaded, When a bearing defect is detected you should repetitive vibration energy is small and enveloping automatically look for other fault symptoms like measurements become less effective. As previously mentioned, there are many machinery problems NOTE: Others A bearing may fail for a number of reasons; ineffective include: As a rolling elements pass over these cracks, fragments break away.

This is known as spalling or flaking. The spalling progressively increases and eventually makes the Velocity vibration measurements are typically performed on bearing unusable. This type of bearing damage is a relatively most machinery. These measurements are very useful for long drawn-out process, and makes its presence known by detecting and analyzing low frequency rotational problems such increasing noise and vibration. Another type of bearing failure is initiated by surface distress.

Surface distress is usually caused by excessive The following chart illustrates the ISO Standard for load or improper lubrication. In both cases the failing bearing produces noise and vibration signals that, if detected, usually gives the user adequate time to correct the cause of the bearing problem or replace the bearing before it fails completely.

Since SEE technology measures the ultrasonic noise acoustic emissions created when metal deteriorates, it is the best tool for detecting bearing problems in their earliest stages, when the defect is subsurface or microscopic and not causing any measurable vibration signal.

For that matter, SEE measurements detect any machine condition that produces acoustic emissions, such as lack of lubrication, contaminated lubrication, corrosion and friction due to fretting, cavitation, sliding or friction events, etc. In very early stages this hump may not amplitudes in the vibration spectrum, there is probably a bearing exist, or may appear as non-synchronous peaks.

For example, if a gear box NOTE: If bearing analysis software is not available, bearing defect frequencies should be mathematically calculated. If no enveloping signals are initially and present, use the following tables as guidelines for evaluating the severity of the enveloped value. SEE measurements monitor the ultrasonic frequencies at which Amplitudes are not absolute.

The amplitude depends on loading these acoustic emissions occur kHz and filter out all and defect conditions; new, old, etc. For example, if a bearing is very heavily loaded, repetitive vibration energy is small and enveloping measurements become less effective.

In cases like this, SEE might be a more useful measurement. Example of an Enveloped Spectrum Outer Race. This allows for early detection of specific machinery problems that may not show under normal monitoring conditions and provides more ways to measure deviations from normal signals.

Multiparameter monitoring has proven very effective for monitoring bearing condition. For example, if a rolling element bearing has a defect on its outer race, each roller will strike the defect as it goes by and cause a small, repetitive vibration signal.

Use a multiparameter approach to best detect bearing problems like the one above. Overall Vibration — Monitors low frequency machine vibrations. The difference of between the inner race and outer race displays Detects rotational and structural problems like imbalance, is the signal modulation caused by the defect rolling in and out misalignment, shaft bow, and mechanical looseness.

Detects of the load zone. The modulation frequency is running speed 1x and is Enveloping — Filters out low frequency vibration noise and found at the far left of the FFT spectrum. Ball defects react the enhances high-frequency, repetitive bearing and gear mesh same, except the modulation frequency occurs at the cage speed.

Pdf analysis vibration spectrum

Has proven very effective for early detection and diagnoses of bearing problems. These modulation signals appear as sideband spikes on either side of the spectrum peaks. Inner race defects have sideband SEE — Measures the ultrasonic noise acoustic emissions peaks spaced at shaft rotational speed.

SEE is the best tool for detecting bearing problems in their earliest stages, when the Sometimes with bearing defects there are missing pulses due to defect is subsurface or microscopic and not causing any the physical randomness of a defect as well as sub-harmonic measurable vibration signal. The impact depends on the condition of the defect. When the defect grows and the With bearings, SEE and Enveloping technologies provide ample edges fade out the defect manifestation it is more like a local pre-warning time, allowing the maintenance person to take waviness than an impact, which behaves more like looseness.

When there is a fresh defect, the signal grows again. For discussion purposes, this setup uses an accelerometer sensor to sense the force of the imbalance, and a tachometer to sense shaft position. A notch is placed in the rotating shaft. This pulse initiates data collection. At this point the imbalance force produces the highest the start of data collection to when the sensor experiences negative reading from the sensor. EXAMPLE A The heavy spot rotates 90 additional degrees to complete its The tachometer senses the notch in the shaft and triggers data degree revolution, the force experienced by the sensor is again collection.

At this point force equals zero. The heavy spot rotates 90 degrees to the sensor position. At this point the imbalance force produces the highest positive reading from the sensor. Because the heavy spot is approaching the sensor position, its force is considered to be in the positive direction.

Often which operates Axis — The reference plane the rotor speeds which continuously at a fixed used in plotting routines. The Y-axis is the reference event. Cycle — One complete 2x, 3x, etc. Usually refers to Acceleration — The time rate mass approaches the an object relative to a upper frequency limit of of change of velocity.

Gear Mesh Frequency — whose output is directly to upper cutoff frequencies.

Analysis vibration pdf spectrum

The frequency generated The width of the band is Engineering Units — by two or more gears proportional to determined by the Physical units in which a meshing teeth together. Relatively large damage on considered unacceptable or Selected by the user.

For example RPM is vibration. Harmonics are terms of peak-to-peak, band pass filter, the center Examples of fields are produced either by an event zero-to-peak, or RMS. Condition Monitoring — Transform. Asynchronous — Vibration Determining the condition Hertzian Contact Zone — In Frequency — The repetition components that are not of a machine by a bearing, the area at which rate of a periodic event, related to rotating speed interpretation of the ball transfers the load usually expressed in cycles non-synchronous.

Orders are frequency and extending to measurements to improve commonly referred to as theoretically infinite statistical accuracy or to CPS — Cycles per second. Imbalance — A condition Axial — In the same direction such that the mass of a Frequency Domain — An shaft and its geometric as the shaft centerline. FFT graph amplitude vs. Nevada trade name. A resonance is amplitude vs.

Phase Response — The increase, and related phase Waveform.