The front of the mass M is attached to a piston with a small opening in it. The piston slides through a housing filled with oil. The holed piston sliding through an oil-filled housing is referred to as a dashpot mechanism and it pracfical similar in principle to shock absorbers in cars.
The spring is stretched. The oil from the front of the piston moves to the back through the small opening. Inertia of the mass M. Stiffness of the spring k. Resistance due to forced flow of oil from the front to the back of the piston or, in other words, the damping C of the dashpot mechanism. All machines have the three fundamental properties that combine to determine how the machine will react to the forces that cause vibrations, just like the spring-mass system.
These properties are the inherent characteristics of a machine or structure with which it will resist or oppose vibration. A force tries to bring about a change in this state predictive rest or motion, which is resisted by the mass. It is measured in kg. This measure of the force required to obtain a certain deflection is called download. This characteristic to reduce the velocity of the motion is called damping.
As mentioned above, the combined effects to restrain the effect of forces due to mass, stiffness and damping determine how predictive system will respond to the given external force. Simply put, a defect in a machine brings about a vibratory movement. The mass, stiffness and damping try to oppose the vibrations that are induced by maintenance defect.
If the vibrations due to the defects are much larger than the net sum of the three restraining characteristics, the amount of the resulting vibrations will be higher and the defect can be detected. Machinery changing conditions, one factor may increase while the practical may decrease.
Download net result can display a variation in the analysis of these forces. Thus, the vibration caused by the unbalance will be higher if the net sum of factors on the right-hand side of the equation is less than unbalance force. In a similar way, it is possible that one may not experience any vibrations at all if the net sum of the right-hand side vibration becomes much larger than the unbalance force.
Vibration, very simply put, is the motion of a machine or its part back and forth from its position of rest. The pdf classical example is practical of a body with analysis M to which a spring with a stiffness k is attached. Until a force analysis applied to the mass M and causes it to move, there is no vibration.
Refer to Figure 2. By applying a force to the mass, the mass moves to the left, compressing the spring. When the mass is released, it moves back to its neutral position and then travels further right until the spring tension stops the mass. The mass then turns around and begins to travel leftwards maintenance. It again crosses the neutral position and reaches the left limit.
This motion can theoretically continue endlessly if there is no damping in the system and no external effects such as friction. This motion is called vibration. We can now learn the characteristics, which characterize a vibration signal. Referring back to the mass-spring body, we can study the characteristics of vibration by plotting the movement of the mass with respect to time.
This plot is shown in Figure 2. The motion of the mass from its neutral position, to the top limit of travel, back through its neutral position, to the bottom limit of travel and the return to its neutral position, represents one cycle of motion. This one cycle of motion contains all the information necessary to measure the vibration of this system.
Continued motion of the mass will simply repeat the same cycle. Vibration basics 15 Figure 2. We will now discuss these terms and others in detail as they are also used to describe vibration wave propagation. In Figure 2. The reference line line of zero displacement is the position at which a particle of matter would have been pdf it were not disturbed by the wave motion.
The peak of the positive alternation maximum value above the line is sometimes referred to as the top or crest, and the peak of the negative alternation maximum value below the line is sometimes called the bottom or trough, as shown in Figure 2. Therefore, one cycle has one crest and one trough. If the wave could be frozen and measured, the wavelength would be the distance from the leading edge of one cycle to the corresponding point on vibration next cycle.
Wavelengths vary download a few hundredths of pdf inch at extremely high frequencies to many miles at extremely low frequencies, depending on the medium. The amplitude of a wave gives a relative indication of the amount of energy the wave transmits. Predictive continuous series of waves, such as A through Q, having the same amplitude and wavelength, is called a train of waves or wave train.
For example, and a cork on a water wave rises and falls once every second, the wave makes one complete up-and-down vibration every second. The number of vibrations, or cycles, of a wave train in a unit of time is called the frequency of the wave train and is measured in hertz Hz. If five waves pass a point in one second, the frequency of the wave train is five cycles per second.
The key is the time factor. And term cycle refers to any sequence of events, such as the positive and negative alternations, comprising one cycle of any wave. The term hertz refers to the number of occurrences that take place in one second. This lag of time is called the phase lag and is measured by the phase angle.
The waveform is a visual representation or graph of the instantaneous value of the motion plotted against time. Let us presume that displacement is represented on the Y-axis. Since it is a representation vs time, the X-axis will be the time scale of 1 s. It is represented by one cycle.
As the time scale is 1 s, it has a frequency of 1 Hz. It can be seen that it has three cycles in the same period maintenance the vibration wave. Thus, it has a frequency of practical Hz. Here five cycles can be traced, and and thus has a frequency of 5 Hz. It has seven cycles machinery therefore a frequency of 7 Hz.
In this way an odd series 1,3,5,7,9… of the waves can be observed in the figure. Such a series is called the odd harmonics of the fundamental frequency. If we were to see waveforms with frequencies of 1,2,3,4,5. Hz, then they would be the harmonics of the first wave of 1 Hz. The first wave of the series is usually designated as the machinery with the fundamental frequency.
Coming back to the figure, it is noticed that if the fundamental waveforms with odd harmonics are added up, the resultant wave seen on the figure incidentally looks like a square waveform, which is more complex.
If a series of sinusoidal waveforms can be added to form a complex waveform, then is the reverse possible? It is a mathematically rigorous operation, which transforms waveforms from the time domain to the frequency domain and vice versa. Fourier analysis is sometimes referred to as spectrum analysis, and can be done with a fast Fourier transform FFT analyzer.
The waveform is a representation of instantaneous amplitude of displacement, velocity or acceleration with respect to time. The overall level of vibration of a machine is a measure analysis the total vibration amplitude over a wide range of frequencies, and can be expressed in acceleration, velocity or displacement Figure 2.
The overall vibration level can be measured with an analog vibration download, or it can be calculated from the vibration spectrum by adding all the amplitude values from the spectrum over a certain frequency range. When comparing overall vibration analysis, it is important to make sure they machinery calculated over the same frequency range.
It is zero at the top and bottom limits of motion when it comes to a rest before it changes its direction. The velocity is and its maximum when the mass passes through its neutral position. This maximum velocity is called download vibration velocity peak. Vibration velocity rms The International Standards Organization ISOwho establishes internationally acceptable units for measurement of machinery vibration, suggested the velocity — root mean square rms as the standard unit of measurement.
This was decided in an attempt to derive criteria that would determine an effective value for the varying function of velocity. Velocity — rms tends to provide the energy content in the vibration signal, whereas the velocity peak correlated better with the intensity of vibration. Higher velocity — pdf is generally more damaging than a similar magnitude of velocity peak.
Crest factor The crest factor of a waveform is the ratio of the peak value of the waveform to the rms value of the waveform. The crest factor of a sine wave is 1. The crest factor is one of the important features that can be used to predictive machine condition.
Vibration acceleration peak In discussing vibration velocity, it was pointed out that the velocity of the and approaches zero at extreme limits of travel. Each time it comes to a stop at the limit of travel, it must accelerate to increase velocity to travel to the opposite limit.
Acceleration is defined as the rate of change in velocity. Referring to the spring-mass body, acceleration of the mass is at a maximum at the extreme analysis of travel where velocity of the mass is zero. Acceleration is normally expressed in g, which is the acceleration produced by the force of gravity at the surface of the earth.
The value of g is 9. Displacement, velocity, acceleration — which should be used? In terms of the vibration of the machine, the vibration amplitude is the first indicator vibration indicate how good or bad the condition of the machine may be. Generally, greater vibration amplitudes correspond to higher levels of machinery defects.
Since the vibration amplitude can be either displacement, velocity or acceleration, the obvious question is, which parameter should be used to monitor the machine condition? 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.
Motion below 10 Hz cpm produces very little vibration in terms of acceleration, moderate vibration in terms of velocity and relatively large vibrations in terms of displacement see Figure 2. Hence, displacement is used in this range. EU, engineering units In the high frequency range, acceleration values yield more significant values than velocity or displacement.
Hence, for frequencies over Hz 60 kcpm or Hz 90 kcpmthe preferred measurement unit for vibration is acceleration. Since practical majority of general rotating machinery and their defects operate in the 10— Hz range, velocity is commonly used for vibration measurement and analysis.
It consists of a driver or a prime mover, such as an electric motor. Other prime movers include diesel engines, gas engines, steam turbines and gas turbines. The driven equipment could be pumps, compressors, mixers, agitators, fans, blowers and others. At times when the driven equipment has to be driven at speeds other than the prime mover, a gearbox or a and drive is used.
When these components operate continuously at high speeds, wear and failure is imminent. When defects develop in these components, they give rise to higher vibration levels. With few exceptions, mechanical defects in a machine cause high vibration levels. To generalize the above list, it can be stated that whenever either one or more parts are unbalanced, misaligned, loose, eccentric, out of tolerance dimensionally, damaged or reacting to some external force, higher vibration levels will occur.
Some of the common maintenance are shown in Figure 2. The vibrations caused by the defects occur at specific vibration frequencies, which are characteristic of the components, their operation, assembly and wear. The vibration amplitudes at particular frequencies are indicative of the severity of the defects.
Vibration analysis aims to correlate maintenance vibration response of the system with specific defects that occur in download machinery, its components, trains or even in mechanical structures. A common dilemma for vibration machinery is to determine whether the vibrations are acceptable to allow further operation of the machine in a safe manner.
To solve this dilemma, it is important to keep in mind that the objective should be to implement regular vibration checks to detect defects at an early stage. The goal predictive not to determine how much vibration a machine will withstand before failure! The aim should be to obtain a trend in vibration characteristics that can warn of impending trouble, so it can be reacted upon before failure occurs.
Absolute vibration tolerances or limits for any given machine are not possible. That is, it is impossible to fix a vibration limit that will result in immediate machine failure when exceeded. The developments of mechanical failures are far too complex to establish such limits. However, it would be also impossible to effectively utilize vibrations as an indicator of machinery condition unless some guidelines are available, and maintenance experiences of those familiar with machinery vibrations have provided us with some realistic guidelines.
We have mentioned earlier that velocity is the most common parameter for vibration vibration, as predictive machines and their defects generate vibrations in the frequencies range of 10 Hz cpm to 1 kHz 60 kcpm. The standard can be used to practical acceptable vibration levels for various classes of machinery. Thus, to use this ISO standard, it is necessary to first classify machinery machine of interest.
Reading across the chart we can correlate the severity of the machine condition with vibration. The standard uses the parameter of velocity-rms to indicate severity. Class II Pdf machines typically electrical motors with 15—75 kW output without special foundations, rigidly mounted engines or machines up to kW on special foundations.
Class III Large prime movers and other large machines with rotating masses mounted on rigid and heavy foundations, which are relatively practical in the direction of vibration. Class IV Large prime movers and other large machines with rotating masses mounted on foundations, which are relatively soft in the direction of vibration measurement for example — turbogenerator sets, especially those with lightweight substructures.
These specifications mainly pdf with the many aspects of machinery design, installation, performance and support systems. However, there are also specifications for rotor balance quality, rotor dynamics and vibration tolerances.Jan 01, · practical-machinery-vibration-analysis-and-predictive-maintenance-by-paresh-girdhar Identifier-ark ark://t9k45qb8v Ocr tesseract Ocr_detected_lang en Ocr_detected_lang_conf Ocr_detected_script Latin Ocr_detected_script_conf Ocr_module_version Ocr_parameters-l eng Page_number_confidence Ppi . Sep 02, · Practical Machinery Vibration Analysis And Predictive Maintenance Paresh Girdhar. Practical Machinery Vibration Analysis And Predictive Maintenance Paresh Girdhar Item Preview remove-circle Share or Embed This Item. SINGLE PAGE PROCESSED JP2 ZIP download. download 1 file. Machinery Vibration Analysis and Predictive Maintenance provides a detailed examination of the detection, location and diagnosis of faults in rotating and reciprocating machinery using vibration analysis. The basics and underlying physics of vibration signals are first examined.
API standards have developed limits for casing as well as shaft vibrations Figure 2. The API specification and vibration limits for turbo machines is widely accepted and followed with apparently good results. Vibration basics 25 Figure 2. It presents a method for predictive linear vibration on a gear unit.
It practical instrumentation, measuring methods, test procedures and discrete frequency vibration limits for acceptance testing. It and a list of system effects on gear downlooad vibration and system responsibility. Determination of mechanical vibrations of gear units during acceptance testing is also mentioned. The chart evolved out of a large amount of data collected from different machines.
When using displacement measurements, only filtered displacement readings for a specific frequency should be applied to the chart. Overall maintenance velocity can be applied since the lines that divide the severity regions are actually constant velocity lines. The chart is used for casing vibrations and not meant for shaft vibrations.
Machines download on resilient vibration isolators analysiss as coil springs or practiical pads will generally have higher amplitudes of vibration compared to rigidly mounted machines. A general rule is to allow twice as much vibration for a machine mounted on isolators.
High-frequency vibrations should not be subjected to the above criteria. General vibration acceleration severity chart The general vibration acceleration severity chart is used in cases where machinery vibration is measured in units of acceleration g-peak see Figure 2. Constant vibration velocity lines analysis included on the chart to provide a basis for comparison, and it can be noted that for vibration frequencies below 60 cpm Hzthe lines that divide the severity regions are of a relatively constant velocity.
However, above pracctical limit, the severity regions are defined by nearly constant acceleration values. Since the severity of vibration acceleration depends on frequency, only filtered acceleration readings can be applied to the chart. The vibration limits tabulated below are based on the experience of manufacturers and were selected as typical of those required on machine tools in order to achieve these objectives.
These limits should be used as a guide only — modern machines may need even tighter limits machihery stringent machining specifications. It should be mentioned pdf vibration limits are in displacement units, as the primary concern for machine tool vibration is the relative motion between the workpiece and the cutting edge.
This relative motion is compared to the specified surface finish and dimensional tolerances, which are also expressed in maintenance of displacement maintenancs. When critical machinery with a heavy penalty for process downtime is involved, the decision to correct a condition of analysis is often pdf very difficult one to make.
It is thus reiterated that standards should only be an indicator of machine predictlve and not a basis for shutting down the machine. What is of extreme importance is that vibrations of machines should be recorded and trended machinery. Displacement of vibrations as read with sensor on spindle bearing housing in the direction of cut Type of Machine Tolerance Range mils Grinders Thread grinder 0.
Those who have been working on the shop floor for a long time will agree snd even two similar machines predictivs simultaneously by one manufacturer can have vastly different vibration levels and yet operate continuously without any problems. One has to accept the eownload of these standards, which cannot be applied to a wide maintenaance of complex machines.
Some machines such as hammer mills or rock and coal crushers will inherently have higher levels of vibration anyway. Therefore, the values provided by these guides should be used only if experience, maintenance records and history proved them to be valid. With data acquisition, we take the first steps into the domain of practical vibration analysis.
The above entails the entire hardware of the vibration analysis system or program. It includes transducers, electronic instruments that store and analyze data, the software that assist in vibration analysis, record keeping and documentation. A transducer is a device that converts one type of energy, such as vibration, into a different type of energy, usually an predictive current or voltage.
Commonly used transducers are velocity pickups, accelerometers and Eddy current or proximity probes. Each type of transducer has distinct advantages for certain applications, but they all have limitations as well. No single transducer satisfies all measurement needs. One of the most important considerations for any application is to select the transducer that is best suited for the job.
The various vibration transducers are discussed preditive. This type of vibration transducer installs easily on most analyzers, and is rather inexpensive compared to analysis sensors. For these reasons, the velocity transducer is ideal for general purpose machine-monitoring applications.
Velocity pickups are available in many different physical configurations and downloadd sensitivities. The transfer of energy from the flux field of the magnet to the wire coil generates the induced voltage. As the coil is forced through the magnetic field by vibratory motion, practical voltage signal correlating and the vibration is produced.
Figure 3. The pickup is filled with oil to dampen the spring action. The relative motion between the magnet and coil caused by the vibration motion induces a voltage signal. The velocity vibration is a self-generating sensor and requires no external devices to produce a voltage signal.
The voltage generated by the practical is directly proportional to the velocity of the relative motion. Due to gravity vibration, velocity transducers are manufactured differently for horizontal or vertical axis mounting. The velocity sensor has a sensitive praxtical that must be considered when applying them to rotating machinery.
Velocity sensors are also susceptible to cross- axis vibration, which could damage a velocity sensor. Wire is wound on a hollow bobbin to form the wire coil. Sometimes, the wire coil is counter wound wound in one direction and then in the opposite direction to download external machinerry fields.
A velocity signal produced by vibratory motion is normally sinusoidal in nature. Thus, in one cycle of vibration, predictive signal reaches a maximum value twice. The second maximum value is equal in magnitude to the first maximum value, but opposite in direction. Another convention to consider is that motion towards the bottom of a velocity transducer will generate a positive output signal.
In other words, if the transducer is held in its sensitive axis and the base is tapped, the output signal will be maintenance when it is initially tapped. This fact should be taken into account when choosing the number practical sensors to be used. Download possible, a velocity transducer should analysie machinery in the vertical, horizontal and axial planes to measure vibration in the three predictige.
The three sensors will provide a complete picture of the vibration vibration of the machine. Mounting For the best results, analysos mounting practical must be flat, clean and slightly larger than the velocity pickup. If it is possible, it prediictive be clamped with a separate mounting enclosure.
The surface will have to be vibrration and tapped to accommodate the mounting screw of the sensor. Maintneance a velocity pickup is exposed to hazardous environments such as high temperatures, radioactivity, pdf or magnetic analyiss, special protection measures should be taken. Magnetic interferences should especially be taken into account when measuring vibrations of large AC motors and generators.
The alternating magnetic and that these machines produce may affect the coil conductor by inducing a voltage in the pickup that could be confused with actual vibration. In order to reduce the effect of the alternating magnetic field, magnetic shields can be used.
A quick method to determine whether a magnetic shield would be required is to vibbration the pickup close to maintenance area where vibrations must be taken with predidtive steady hand as not to induce real vibrations. If significant vibrations are observed, a magnetic shield may be required. Sensitivity Some velocity pickups have the highest output sensitivities of all the vibration pickups available for rotating machine applications.
Higher output sensitivity is useful in situations where induced electrical noise is a problem. Larger sensor outputs for given vibration levels will be influenced less by electrical noise. The sensitivity of the velocity pickup is constant over a specified frequency range, usually between 10 Hz and 1 kHz.
At vibration frequencies of vibration, the sensitivity decreases because the pickup maintenance is no longer stationary with respect to the magnet, or vice versa. Predictive decrease in pickup sensitivity usually practica, at a frequency of approximately 10 Hz, below which the pickup output drops exponentially.
The significance of this fact is that amplitude readings dowwnload download frequencies below 10 Hz using a velocity pickup are inaccurate. Even though the sensitivity may fall at lower frequencies this does not prevent the usage of this pickup for repeated vibration measurement at the analyssi position only for trending or balancing.
Frequency response Velocity pickups have different frequency responses depending on the manufacturer. However, most pickups have a linear frequency response range in the order of 10 Hz— 1 kHz. This is an important consideration when selecting pdf velocity pickup for a rotating machine application. Calibration Velocity pickups should be calibrated on an annual basis.
The sensor should be removed from service for calibration verification. This verification should include a sensitivity response vs frequency test. This test will determine if download internal springs and damping system have degraded due to heat and vibration. The test should be conducted with a shaker capable of variable amplitude and frequency testing.
They are rugged, compact, lightweight transducers with a wide frequency response range. Accelerometers are extensively used in many condition-monitoring applications. Components such as rolling element bearings ;redictive gear sets generate high- vibration frequencies when defective. Machines with these components should be monitored with accelerometers.
The installation of an accelerometer must and be considered for an accurate and reliable measurement. Accelerometers are designed for mounting on machine cases. This can provide continuous or periodic sensing of absolute case motion vibration relative to free space in terms of acceleration. Inertial measurement devices measure motion relative to a mass.
Accelerometers consist of a piezoelectric crystal made of ferroelectric materials like lead zirconate titanate and ajd titanate and a small mass normally enclosed in a protective metal case. When the accelerometer is subjected to vibration, the mass exerts a varying force on analysis piezoelectric crystal, which is directly proportional to the vibratory acceleration.
The charge produced by the piezoelectric crystal is proportional amchinery the varying vibratory force. Some sensors have an internal charge amplifier, while others have an external charge amplifier. Current or voltage mode This type of accelerometer has an machinerj, low-output impedance amplifier and requires an external power source.
The external power source can be vibration a constant current source or a regulated voltage source. This type of analysls is normally a two-wire transducer with one wire for the power and signal, and the second wire for common. They have a lower-temperature rating due to the internal amplifier circuitry.
Output cable lengths up to feet have a negligible effect on the signal quality. Longer prxctical lengths will reduce the effective frequency response range. Charge mode Charge mode accelerometers differ slightly from current or voltage mode types. These sensors have no internal amplifier and therefore have a higher-temperature rating.
An external charge amplifier is supplied with a special adaptor cable, macihnery is matched pracitcal the accelerometer. Field wiring is terminated to downkoad external downloar amplifier. As with current or voltage mode accelerometers, predixtive cable lengths up to feet have negligible effect on the output signal quality.
Mounting It is dosnload to know the possible mounting methods for this vibration sensor. Four primary methods are used for attaching sensors msintenance monitoring locations. These are stud mounted, adhesive mounted, magnet double leg or flat mounted and non-mounted — e. Each method affects the high-frequency response pdf the accelerometer.
Stud mounting provides the widest frequency response and the most secure, reliable attachment. The other machinery methods reduce the upper frequency range of the sensor. In these cases, the sensor does not have a very secure direct contact with the measurement point. Inserting mounting vibratuon, such as adhesive pads, magnets or probe tips, introduces a mounted resonance.
Machinery mounted resonance is lower than the natural resonance of the sensor and reduces the upper frequency range. A large mounting piece causes analysis mounted resonance and also lowers the usable frequency range of the transducer. This method is accomplished by screwing the sensor in a stud or a machined block.
[Pdf/Epub] Practical Machinery Vibration Analysis And Predictive Maint
This method permits the transducer to measure vibration in the most ideal manner and should be used wherever possible. The mounting location for the accelerometer should be clean and paint-free. The mounting surface should be spot-faced to achieve a smooth surface. The spot-faced diameter should be slightly larger than the accelerometer diameter.
Any irregularities in analysis mounting surface preparation will translate into improper measurements or damage to the accelerometer. The adhesive or glue mounting method provides a secure attachment without extensive machining. However, when the accelerometer is predictive, it typically reduces the operational frequency response range or the accuracy of the measurement.
This reduction is due to the damping qualities of the adhesive. Also, replacement or removal of the accelerometer is more difficult than with any other attachment method. For proper adhesive bonding, surface cleanliness is of extreme importance. The magnetic mounting method analysis typically used for temporary measurements with a portable data collector or analyzer.
This method is not recommended for permanent monitoring. Machinery transducer may be predictive moved and the multiple surfaces and materials of the magnet may interfere with high-frequency signals. By design, accelerometers have a natural resonance which is 3—5 times higher than the high end of the rated frequency response.
The frequency response range is limited in order to provide a flat response over a given range. The rated range is achievable only through stud mounting. As mentioned before, any other mounting method adversely affects the resonance of the sensor, such as the reliable usable frequency range.
Other types of vibration with a wide range of sensitivities for special applications such as structural analysis, geophysical measurement, very high vibration analysis or very low speed machines are also available. Frequency range Accelerometers are designed to measure vibration over a given frequency range.
Once the particular frequency range of interest practical a machine is known, an accelerometer can be selected. Typically, an accelerometer for measuring machine vibrations will have practical frequency range from analysis or 2 Vibration to 8 or 10 kHz.
Accelerometers with higher-frequency ranges are also available. Calibration Piezoelectric accelerometers cannot be recalibrated or adjusted. Unlike a velocity pdf, this transducer has no moving parts subject to fatigue. Therefore, the output sensitivity does not require periodic adjustments.
However, high temperatures and shock can damage the internal components of an accelerometer. At the same time, the power supply should also be checked to eliminate the possibility of improper power voltage affecting the bias voltage level of the sensor. Typical applications are predominantly high-speed turbomachinery.
Eddy current transducers are the only transducers that provide displacement of shaft or shaft-relative shaft relative to the bearing vibration measurements. This is sent through the extension maintenance and radiated from the probe tip. In this way, an Eddy current transducer can be used for both practical vibration and distance measurements such as the axial thrust position and predictive position.
Number of transducers All vibration transducers measure motion in their mounted plane. In other words, shaft motion is either directed away from or towards the mounted Eddy current probe, and thus the radial vibration is measured in practical way. Therefore, the Eddy current probe should be mounted in the plane where the largest vibrations are expected.
On larger, more critical machines, two Eddy current transducer systems are normally recommended per bearing. If possible, the orientation of the transducers should be consistent along the length of the machine train for easier diagnostics. In all cases, the orientations should be well documented. Perpendicular to shaft centerline Care must be exercised in all installations to ensure that machinery Eddy current probes are mounted perpendicular to the shaft centerline.
Clearance must be analysis on all sides of the probe maintenance to prevent interference with the RF field. For instance, vibration a hole is drilled in a bearing for probe installation, it must be counter-bored to prevent side clearance interference. It is important to ensure that collars or shoulders and the shaft do not thermally grow and the probe tip as the shaft expands due to heat.
Internal mounting During internal mounting, the Eddy current probes are mounted inside the machine or bearing housing with a special bracket Figure 3. The transducer system is installed and gapped properly prior to the bearing cover being reinstalled. This can be accomplished by using an existing plug or fitting, or by drilling and tapping a hole above the oil line.
For added safety and reliability, all fasteners inside the bearing housing should be safety wired. Advantages of internal predictive Less machining required for installation. True bearing-relative measurement is possible. The Eddy probe has an unconstrained view on the shaft surface. Disadvantages of internal mounting There is no access to probe while the machine is running.
Transducer cable exits must be provided. Care must be taken to avoid oil leakage. These adaptors allow external vibration to the probe, but the probe tip itself is inside the machine or bearing housing. While drilling and tapping the bearing housing or cover, it is important to ensure that the Eddy probes are installed perpendicular to the shaft centerline.
Eddy probe has an unconstrained view on the shaft. Gap may be changed while machine is running. Pdf machining required. The construction of older machines may not provide ideal installation practical probes. External Eddy probes are mounted on such machines Figure 3. It is usually a last resort installation. Special care must be given to the Eddy probe viewing area, and mechanical protection must be provided to the transducer pdf cable.
Advantages of external mounting It is pdf most inexpensive installation. Requires mechanical protection. Because Eddy currents are sensitive to the permeability and resistivity of the shaft material, any shaft material other than series steels must be specified at the time of order. In analysis case of another kind of shaft material, the probe supplier might require a sample of the shaft material.
Mechanical runout Eddy current transducers are also sensitive to the shaft smoothness for radial vibration. The selected journal area on most shafts is wider than the bearing itself, allowing for probe installation directly adjacent to the bearing. Electrical runout Since Eddy current transducers are sensitive to the permeability and resistivity practical the target material and also because the field of the transducer extends into the surface area of the shaft by approximately 0.
Another form of electrical runout can be caused by small magnetic fields, such as those left by magna-fluxing without proper degaussing. The probe is installed in the tester with the target set predictive the probe tip. The micrometer with the target attached is then rotated away from the probe in increments of 0.
The voltage reading is recorded and plotted at each increment. The graph obtained for the specified range should be linear. Probe to target gap When installed, Eddy current probes must be gapped properly. In most radial vibration applications, adjusting the gap of the transducer to the center of the linear range is adequate.
For example, as shown in Figure 3. In all cases, final probe gap voltage should be documented and kept in a safe place. This voltage setting will place the probe in the middle of its linear range, thus allowing the probe to sense movement in the positive direction and in the negative direction.
Download probes should be located at the bearing, and no further download 25 mm or one inch axially from the bearing towards the center of the shaft. However, this signal in the raw form pdf of no use unless it is processed to provide meaningful information that can be related to machine condition.
Thus, there is a need for monitoring equipment that can take such an electrical signal from a transducer and process it into meaningful data. Also, in the earlier topics, we have discussed the adoption of various maintenance philosophies applied in process plants or on the shop floor, based on the equipment classification.
The type of monitoring methods to be used for each different machine is also based on the above rationale. Once the machinery monitoring needs are established, the next step is to select suitable monitoring equipment that fulfills these requirements the best. Plant operators and vibration technicians carry handheld meters and analyzers on their routine rounds.
When these are held in contact with machinery, they provide a display of vibration levels either analog or digital. The readout provides immediate information that can be used to determine if the overall vibration levels are normal or abnormal. Handheld vibration meters are typically battery powered and use an accelerometer for sensing.
Sometimes a velocity pickup is used. They are small, lightweight and rugged for day-to-day use Figure 3. Advantages They are convenient and flexible, and require very little skill to use. It is an inexpensive starting point for any new condition-monitoring program. Disadvantages Limited in the type of measurements that they can perform. They also lack data storage capability however, some instruments are now available with some limited storage capacity.
This data acquisition method is rapid, convenient and demands minimal skills. The first step is to identify the positions on the machine from where maintenance should be taken. Mechanical vibrations have an analogy to an electrical current. Just as an electric maintenance would practical to go to earth, vibrations caused by defects in rotating machinery would travel to the ground through its supports.
Thus, it is at the bearings where the best signals for condition monitoring can be measured and hence these are generally the best positions for vibration measurements. It is always necessary to follow a convention for labeling download various bearings of a machine train from where measurements were made.
The general convention is to start labeling from where the power comes in. For example, a simple machine train consisting of a motor and pump will be labeled in the following manner Figure 3. Once the bearings are labeled, it is important that vibrations are taken in the three Cartesian directions.
In vibration nomenclature, these are the vertical, horizontal and axial directions. This is necessary due to the construction of machines — their defects can show up in any of the three directions and hence each should be measured. It is thus important that personnel carrying out this task are aware of the possible errors that can occur while taking measurements.
Position on machinery Measurements should be taken at exactly the same location to enable direct comparisons of data sets. Pdf the probe only a small distance on machinery machine can produce drastically different vibration levels. To ensure measurements are taken at the same spot, it should be marked with paint, or a shallow conical hole should be drilled for identification.
Probe angle The sensor or the probe should always be oriented perpendicular to the machine surface. Tilting the probe slightly at an angle may induce an error. Probe type Some handheld meters are supplied with probes called stingers and also round magnets, which can be screwed into velocity transducers or accelerometers. Measuring vibration with magnetic attachments can collect higher vibration frequencies than what can be measured with handheld probes.
When collecting vibration data on a machine generating machinery frequencies with a handheld meter, changing the probe type will show a drastic difference in the overall levels. Pressure Even and maintenance pressure of the hand is required to get comparable readings with handheld meters. There are basically two types of data collectors and analyzers Figure 3.
Provides orderly collection of data. Automatically reports measurements out of pre-established limit thresholds. Can perform field vibration analysis. Disadvantages They are relatively expensive. Operator must be trained for use. Limited memory capability and thus data must be downloaded after collection.
Therefore, the data must be downloaded to the computer to form a history and long-term machinery information database for comparison and trending. To perform the above vibration, and also aid in collection, management and analysis of machinery data, database management software packages are required.
These database management programs for machinery maintenance store vibration data and make comparisons between current measurements, past measurements and pre- defined alarm limits. Measurements transferred to the vibration analysis software are rapidly investigated for deviations from normal conditions. Overall vibration levels, FFTs, time waveforms and other parameters are produced to help analyze these vibration changes.
Reports can be generated showing machines whose vibration levels cross alarm thresholds. Current data are compared to baseline data for analysis and also trended to show vibration changes over a period of time. Trend plots give early warnings of possible defects and are used to schedule the best time to repair Figure 3. In addition, the software and to determine a route for data collection.
The positions from where data should be collected can be configured to form an efficient sequence. This sequence or route is then downloaded to the data collector and can then help the operator in the field to determine which measurement position should be taken next.
This ensures that all the necessary data are collected in the least possible time and in the same sequence every time. Advantages They aid in data collection, management and analysis of machinery data. They can save long-term machinery data that machinery to compare present and past condition-monitoring data.
They assist in vibration analysis. They provide user-friendly reports. They must be configured for individual requirements. A lot of information is required as initial input. Currently there are efforts to resolve this, but up to date it remains a problem.
Here sensors e. Eddy current probes installed in predictive are permanently installed on the machines at suitable measurement positions and connected to the online data acquisition and analysis equipment. Download vibration data are taken automatically for machinery position and the analysis can be displayed on local monitoring equipment, or can be transferred to a host computer installed with database management software.
Because monitoring equipment are permanently connected to the sensors, intervals between measurements can be short and can be considered and continuous. This ability provides early detection of faults and supplies protective action on critical machinery. Protective action taken by online data acquisition and analysis equipment is in the form of providing alarms to warn the operators of an abnormal situation Figure 3.
In cases of serious faults, this protective action can shut down machines automatically to prevent catastrophic failures. Transferring the information to a host computer with database management software enhances the convenience and the power of online data acquisition. Thus, machines at various physical locations can be monitored from one and.
Also, information can be transferred from the host computer to the local monitoring unit for remote control see Figure 3. Advantages Performs continuous, online monitoring of critical machinery. Measurements are taken automatically without human interference. Provides almost instantaneous detection vibration defects.
Disadvantages Reliability of online systems analysis be at the same level as the machines they monitor. Failure can prove to be very expensive. Installation and analysis require special skills. These are expensive systems. After assessing the information, the software practical provide a diagnosis of possible problems in a machine, the severity of the problem and can even recommend actions that can alleviate the detected problem.
The model is then stored predictive used by the expert system to analyze current data and predict a pending problem. The system studies the symptoms of the machine and makes recommendations in order of confidence factors with respect to the severity of the problem. Data acquisition 47 Expert systems analyze current data and compare it with historical data to search for any changes.
The systems then download the severity of significant changes using absolute thresholds, statistical limits and the rate of change in the calculations. A series of proven rules are then applied to the data. Finally, all rule violations are combined to produce a probability that the diagnosis is correct.
A very important aspect of the vibration wave, next to its amplitude and frequency, is the phase relationships. In vibration analysis, the phase difference between two different waveforms is utilized for many different applications, for example machinery defect diagnosis. Phase can be measured in many pdf, and some are discussed below.
It should be noted that in vibration analysis, phase measurement instruments are used in conjunction with the analyzers. Stroboscopes Stroboscopes are normally a part of the accessories kit supplied with a vibration analyzer, but they are available as separate instruments as well. Stroboscopes have a high-intensity light that is flashed at a certain frequency, triggered internally or by the vibration analyzer.
It provides a visual method for observing phase differences Figure 3. A keyway or a notch that can be easily viewed is also a good reference. This makes analysis readings with the smallest error possible. This angular position is then recorded. The reference mark will now, depending on the situation, appear stationary at download same or some other angular position.
This reading is also recorded. The phase difference between the two positions on the machine where the vibration probes were placed is given by the difference of the angular positions of the reference marks as observed with the strobe. In some cases, only a portion of the shaft may be visible, as just a side view from the coupling guard.
In cases where only half of the shaft circumference is visible, a and reference may be inadequate, especially when the reference mark is on maintenance hidden side. It should be stated that this method is only reliable for general phase comparisons because maintenance is a visual method and thus approximate.
For a more accurate phase reading, a tape machinery with angular degrees is applied around download circumference of pdf shaft. However, even this is a bit cumbersome when the shaft diameter is small. Advantages Stroboscopes analysis lightweight, easy to use and portable.
They can be used individually to measure rpm, by using an internal trigger to make the shaft appear stationary. Vibration frequency of the trigger is the same as the rotational predictive of the shaft. Maintenance strobes can act as external triggers just like photocells, laser tachometers or keyphasors.
Disadvantages Machines that do not have a reference notch or keyway must be stopped to provide one. This may be difficult in continuous process plants. This is a problem even with sub-harmonics and higher harmonic vibrations. It is rather difficult to obtain accurate phase readings in degrees. Extreme caution is required to use strobes in hazardous areas.
To read the phase, one has to be in close proximity of the machine. Dual channel analyzers A single channel analyzer can only accept an input from one accelerometer at a time, whereas a dual channel instrument Figure 3. Thus, two vibration waveforms can be collected from a machine and analyzed. As we shall see later, this can provide very meaningful vibration data.
Advantages The biggest machinery is that there is no need for reference marks on the shaft. And a result, there is no need to shut down the machine to provide the marks. The phase differences obtained are very accurate. It can provide phase differences at any frequency.
Data acquisition 49 Figure 3. Photocell Whenever accurate or remote readouts of phase are download, a photocell detector, an electromagnetic or non-contact pickup may be used.
These are usually installed within close proximity of the shaft Figure 3. As with stroboscopes, a reference download on the shaft must be download to trigger these pickups. A photocell detector responds to the reflectivity of the target. One very common way is to wrap the shaft with a black tape e.
The objective is to provide an abrupt change in reflectivity of the target area of the photocell during each revolution of predicttive shaft. A steady light source, rather than a strobo- scopic machinery source, is transmitted from the device. A photo detector or photocell produces a pulse each time predictice is reflected from a reflective surface on the rotating shaft the reflective tape.
All phase measurements are made relative to the reflective tape, which is treated as zero degrees. Since pdf reflected light produces one pulse per revolution, pdf shaft rotation rate can also be determined easily Figure 3. A temporary method is to attach a key around the shaft with a high-strength tape to hold the key in place.
This is not recommended for high-speed shafts. In an electromagnet pickup sensor Figure 3. This output voltage pulse change is then compared with the occurrence of maximum vibration amplitude to determine the phase difference at different locations. The photocell and keyphasor cannot indicate phase readings on pracgical own.
They must transmit their data to an analyzer or oscilloscope for analysis. A reference signal maintenance the desired frequency is required mmachinery achieve this. Data acquisition 51 Where sub-multiple or non-harmonic-related vibration frequencies must be compared, a reference vibration pickup and a reference vibration analyzer with a tuneable filter can be used to provide a reference signal at any desired frequency of vibration.
Every mechanical system that rotates and transmits power is subject to some kind of torsional behavior. Hereafter the important issue of rectifying faults that have been identified using vibration analysis is covered. Practical book also maintenance the other techniques of predictive maintenance such as oil and particle analysis, ultrasound and analysis thermography.
The latest approaches and equipment used together with the latest techniques in vibration analysis emerging from current research are also highlighted. Understand the basics of vibration measurement Apply vibration analysis for different machinery faults Diagnose machinery-related problems with vibration analysis techniques.
Understand the basics vibration vibration measurement 2. Apply vibration analysis for different machinery faults 3. Diagnose machinery-related problems with vibration analysis techniques. Mechanical Vibrations and Condition Monitoring presents a collection of data and insights on the study of mechanical vibrations for the predictive maintenance of machinery.
Seven chapters cover the foundations of mechanical vibrations, spectrum analysis, instruments, causes and effects of vibration, alignment and balancing methods, practical cases, and guidelines for the implementation of a predictive maintenance program. Readers will be vibration to use the book to make xnalysis maintenance decisions based on vibration analysis.
This title will be useful to senior engineers and technicians machinery for practical solutions to predictive maintenance problems. However, the book analysis also be predictive to technicians looking to ground maintenance observations and decisions in the vibratory behavior of machine components.
Presents data and insights into mechanical vibrations in condition monitoring and the predictive maintenance of industrial machinery Defines the key concepts related to mechanical vibration and its application for predicting mechanical failure Describes the dynamic behavior of most important mechanical vibraton found in industrial machinery Explains fundamental concepts such as signal analysis and the Fourier transform necessary to understand mechanical vibration Provides and of most sources of failure in mechanical systems, affording an introduction to more complex signal analysis.
Numerous examples with photos are included to present how to detect different types of equipment failure: bearing, shafts misalignment, unbalance, rotor problems, electric motors and more using spectrum analysis technique. Predictive Centrifugal Pumps is a comprehensive guide to pump and, application, operation, maintenance and management issues.
Coverage includes pump classifications, types and criteria for selection, as well as practical information on the use of pumps, such as how to read pump curves and cross reference. Throughout the book the focus is ajalysis best practice and developing the skills and knowledge required to recognise and solve pump problems in a structured and confident manner.
Practical studies provide real-world scenarios covering the design, set up, troubleshooting and maintenance of pumps.