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Practical machinery vibration analysis and predictive maintenance. Machinery — Vibration 2. Vibration — Measurement 3. Machinery — Maintenance and repair I. Title Ltd, Pondicherry, India www. The basics and underlying physics of vibration signals are first examined. The acquisition and processing of signals are then reviewed followed by a discussion of machinery fault diagnosis using vibration analysis. Hereafter the important issue of rectifying faults that have been identified using vibration analysis is covered.
The book is concluded by a review of the other techniques of predictive maintenance such as oil and particle analysis, ultrasound and infrared thermography. The latest approaches and equipment used together with current research techniques in vibration analysis are also highlighted in the text. These categories are briefly described in Figure 1. This approach works well if equipment shutdowns do not affect production and if labor and material costs do not matter.
When unexpected production interruptions occur, the maintenance activities require a large inventory of spare parts to react immediately. Without a doubt, it is the most inefficient way to maintain a production facility. Futile attempts are made to reduce costs by purchasing cheaper spare parts and hiring casual labor that further aggravates the problem. The personnel generally have a low morale in such cases as they tend to be overworked, arriving at work each day to be confronted with a long list of unfinished work and a set of new emergency jobs that occurred overnight.
Here the repair or replacement of damaged equipment is carried out before obvious problems occur. This is a good approach for equipment that does not run continuously, and where the personnel have enough skill, knowledge and time to perform the preventive maintenance work. The main disadvantage is that scheduled maintenance can result in performing maintenance tasks too early or too late. Equipment would be taken out for overhaul at a certain number of running hours. It is possible that, without any evidence of functional failure, components are replaced when there is still some residual life left in them.
It is therefore quite possible that reduced production could occur due to unnecessary maintenance. In many cases, there is also a possibility of diminished performance due to incorrect repair methods. In some cases, perfectly good machines are disassembled, their good parts removed and discarded, and new parts are improperly installed with troublesome results.
Predictive maintenance basics 3 1. Mechanical and operational conditions are periodically monitored, and when unhealthy trends are detected, the troublesome parts in the machine are identified and scheduled for maintenance. The machine would then be shut down at a time when it is most convenient, and the damaged components would be replaced.
If left unattended, these failures could result in costly secondary failures. One of the advantages of this approach is that the maintenance events can be scheduled in an orderly fashion. It allows for some lead-time to purchase parts for the necessary repair work and thus reducing the need for a large inventory of spares.
Since maintenance work is only performed when needed, there is also a possible increase in production capacity. A possible disadvantage is that maintenance work may actually increase due to an incorrect assessment of the deterioration of machines. To track the unhealthy trends in vibration, temperature or lubrication requires the facility to acquire specialized equipment to monitor these parameters and provide training to personnel or hire skilled personnel.
The alternative is to outsource this task to a knowledgeable contractor to perform the machine-monitoring duties. If an organisation had been running with a breakdown or preventive maintenance philosophy, the production team and maintenance management must both conform to this new philosophy. It is very important that the management supports the maintenance department by providing the necessary equipment along with adequate training for the personnel.
The personnel should be given enough time to collect the necessary data and be permitted to shut down the machinery when problems are identified. Each failure is analyzed and proactive measures are taken to ensure that they are not repeated. RCFA detects and pinpoints the problems that cause defects. It ensures that appropriate installation and repair techniques are adopted and implemented. It may also highlight the need for redesign or modification of equipment to avoid recurrence of such problems. As in the predictive-based program, it is possible to schedule maintenance repairs on equipment in an orderly fashion, but additional efforts are required to provide improvements to reduce or eliminate potential problems from occurring repeatedly.
Again, the orderly scheduling of maintenance allows lead-time to purchase parts for the necessary repairs. This reduces the need for a large spare parts inventory, because maintenance work is only performed when it is required. Additional efforts are made to thoroughly investigate the cause of the failure and to determine ways to improve the reliability of the machine.
All of these aspects lead to a substantial increase in production capacity. It is also possible that the work may require outsourcing to knowledgeable contractors who will have to work closely with the maintenance personnel in the RCFA phase. Proactive maintenance also requires procurement of specialized equipment and properly trained personnel to perform all these duties.
Presently, the predictive and proactive maintenance philosophies are the most popular. Breakdown maintenance was practiced in the early days of production technology and was reactive in nature. Equipment was allowed to run until a functional failure occurred. Secondary damage was often observed along with a primary failure. This led to time-based maintenance, also called preventive maintenance. In this case, equipment was taken out of production for overhaul after completing a certain number of running hours, even if there was no evidence of a functional failure.
The drawback of this system was that machinery components were being replaced even when there was still some functional lifetime left in them. This approach unfortunately could not assist to reduce maintenance costs. Due to the high maintenance costs when using preventive maintenance, an approach to rather schedule the maintenance or overhaul of equipment based on the condition of the equipment was needed. This led to the evolution of predictive maintenance and its underlying techniques. Predictive maintenance requires continuous monitoring of equipment to detect and diagnose defects.
Only when a defect is detected, the maintenance work is planned and executed. Today, predictive maintenance has reached a sophisticated level in industry. Till the early s, justification spreadsheets were used in order to obtain approvals for condition-based maintenance programs.
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Luckily, this is no longer the case. The advantages of predictive maintenance are accepted in industry today, because the tangible benefits in terms of early warnings about mechanical and structural problems in machinery are clear. The method is now seen as an essential detection and diagnosis tool that has a certain impact in reducing maintenance costs, operational vs repair downtime and inventory hold-up. In the continuous process industry, such as oil and gas, power generation, steel, paper, cement, petrochemicals, textiles, aluminum and others, the penalties of even a small amount of downtime are immense.
It is in these cases that the adoption of the predictive maintenance is required above all. Through the years, predictive maintenance has helped improve productivity, product quality, profitability and overall effectiveness of manufacturing plants. Predictive maintenance in the actual sense is a philosophy — an attitude that uses the actual operating conditions of the plant equipment and systems to optimize the total plant operation.
It is generally observed that manufacturers embarking upon a predictive maintenance program become more aware of the specific equipment problems and subsequently try to identify the root causes of failures. This tendency led to an evolved kind of maintenance called proactive maintenance. This ensures that they eliminate the causes that may give rise to defects in their equipment in the future. This evolution in maintenance philosophy has brought about longer equipment life, higher safety levels, better product quality, lower life cycle costs and reduced emergencies and panic decisions precipitated by major and unforeseen mechanical failures.
This makes the maintenance work faster and smoother. As machines are stopped before breakdowns occur, there is virtually no secondary damage, thus reducing repair time. For instance, vibration in paper machines has a direct effect on the quality of the paper. In all probability, the proactive and predictive maintenance philosophy is adopted for critical equipment. Vibration-monitoring instruments are provided with continuous, full-time monitoring capabilities for these machines.
Some systems are capable of monitoring channels simultaneously so that rapid assessment of the entire machine train is possible. For example, centrifugal fans in corrosive service. In many cases, the preventive maintenance philosophy, and at times even a less sophisticated predictive maintenance program is adopted for such equipment.
These essential machines do not need to have the same monitoring instrumentation requirements as critical machines. Vibration-monitoring systems installed on essential machines can be of the scanning type, where the system switches from one sensor to the next to display the sensor output levels one by one. Usually it is acceptable to adopt the breakdown maintenance philosophy on general purpose equipment. However, in modern plants, even general purpose machines are not left to chance. These machines do not qualify them for permanently installed instrumentation or a continuous monitoring system.
They are usually monitored with portable instruments. Industrial or in-plant average life statistics are not used to schedule maintenance activities in this case. Predictive maintenance monitors mechanical condition, equipment efficiency and other parameters and attempts to derive the approximate time of a functional failure. Predictive maintenance basics 7 A comprehensive predictive maintenance program utilizes a combination of the most cost-effective tools to obtain the actual operating conditions of the equipment and plant systems.
On the basis of this collected data, the maintenance schedules are selected. Predictive maintenance uses various techniques such as vibration analysis, oil and wear debris analysis, ultrasonics, thermography, performance evaluation and other techniques to assess the equipment condition. Predictive maintenance techniques actually have a very close analogy to medical diagnostic techniques.
Whenever a human body has a problem, it exhibits a symptom. The nervous system provides the information — this is the detection stage. Furthermore, if required, pathological tests are done to diagnose the problem. On this basis, suitable treatment is recommended see Figure 1. Figure 1. However, this may or may not be easily detected on machinery systems with human perceptions. It is here that predictive maintenance techniques come to assistance. These techniques detect symptoms of the defects that have occurred in machines and assist in diagnosing the exact defects that have occurred.
In many cases, it is also possible to estimate the severity of the defects. The specific techniques used depend on the type of plant equipment, their impact on production or other key parameters of plant operation. Of further importance are the goals and objectives that the predictive maintenance program needs to achieve. Collection and analysis of this debris provides vital information on the deterioration of these components.
The method can detect thermal or mechanical defects in generators, overhead lines, boilers, misaligned couplings and many other defects. It can also detect cell damage in carbon fiber structures on aircrafts. The efficiency of machines provides a good insight on their internal conditions. Despite all these methods, it needs to be cautioned that there have been cases where predictive maintenance programs were not able to demonstrate tangible benefits for an organisation. The predominant causes that lead to failure of predictive maintenance are inadequate management support, bad planning and lack of skilled and trained manpower.
Upon activating a predictive maintenance program, it is very essential to decide on the specific techniques to be adopted for monitoring the plant equipment. The various methods are also dependent on type of industry, type of machinery and also to a great extent on availability of trained manpower. It is also necessary to take note of the fact that predictive maintenance techniques require technically sophisticated instruments to carry out the detection and diagnostics of plant machinery.
These instruments are generally very expensive and need technically competent people to analyze their output. The cost implications, whether on sophisticated instrumentation or skilled manpower, often lead to a question mark about the plan of adopting predictive maintenance philosophy. However, with management support, adequate investments in people and equipment, predictive maintenance can yield very good results after a short period of time.
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 deteriorating or defective bearings, mechanical looseness and worn or broken gears. Vibration analysis can also detect misalignment and unbalance before these conditions result in bearing or shaft deterioration.
Trending vibration levels can identify poor maintenance practices, such as improper bearing installation and replacement, inaccurate shaft alignment or imprecise rotor balancing. Predictive maintenance basics 9 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 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.
Recent advances in technology allow a limited analysis of reciprocating equipment such as large diesel engines and reciprocating compressors. These machines also need other techniques to fully monitor their operation. A vibration analysis system usually consists of four basic parts: 1.
Signal pickup s , also called a transducer 2. A signal analyzer 3. Analysis software 4. A computer for data analysis and storage. These basic parts can be configured to form a continuous online system, a periodic analysis system using portable equipment, or a multiplexed system that samples a series of transducers at predetermined time intervals. Hard-wired and multiplexed systems are more expensive per measurement position. The determination of which configuration would be more practical and suitable depends on the critical nature of the equipment, and also on the importance of continuous or semi- continuous measurement data for that particular application.
In order to determine if a serious problem actually exists, they could proceed with a vibration analysis. If a problem is indeed detected, additional spectral analyses can be done to accurately define the problem and to estimate how long the machine can continue to run before a serious failure occurs. Vibration measurements in analysis diagnosis mode can be cost-effective for less critical equipment, particularly if budgets or manpower are limited. Its effectiveness relies heavily on someone detecting unusual noises or vibration levels.
This approach may not be reliable for large or complex machines, or in noisy parts of a plant. Furthermore, by the time a problem is noticed, a considerable amount of deterioration or damage may have occurred. Another application for vibration analysis is as an acceptance test to verify that a machine repair was done properly.
The analysis can verify whether proper maintenance was carried out on bearing or gear installation, or whether alignment or balancing was done to the required tolerances. Additional information can be obtained by monitoring machinery on a periodic basis, for example, once per month or once per quarter. Periodic analysis and trending of vibration levels can provide a more subtle indication of bearing or gear deterioration, allowing personnel to project the machine condition into the foreseeable future. The implication is that equipment repairs can be planned to commence during normal machine shutdowns, rather than after a machine failure has caused unscheduled downtime.
These can include improper bearing installation and replacement, inaccurate shaft alignment or imprecise rotor balancing. Trending vibration levels can also identify improper production practices, such as using equipment beyond their design specifications higher temperatures, speeds or loads. These trends can also be used to compare similar machines from different manufacturers in order to determine if design benefits or flaws are reflected in increased or decreased performance. Ultimately, vibration analysis can be used as part of an overall program to significantly improve equipment reliability.
This can include more precise alignment and balancing, better quality installations and repairs, and continuously lowering the average vibration levels of equipment in the plant. Figure 2. There is a mass M attached to a spring with a stiffness k. 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 is 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 of 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 stiffness. 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 a 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 the 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.
With changing conditions, one factor may increase while the other may decrease. The net result can display a variation in the sum 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 factors 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 most classical example is that of a body with mass M to which a spring with a stiffness k is attached. Until a force is 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 again. 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 if 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 the next cycle. Wavelengths vary from a few hundredths of an 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. A 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, if 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. The 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 of the first wave.
Thus, it has a frequency of 3 Hz. Here five cycles can be traced, and it thus has a frequency of 5 Hz. It has seven cycles and 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 wave 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 of 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 meter, or it can be calculated from the vibration spectrum by adding all the amplitude values from the spectrum over a certain frequency range.
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When comparing overall vibration levels, it is important to make sure they were 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 at its maximum when the mass passes through its neutral position.
This maximum velocity is called as vibration velocity peak. Vibration velocity rms The International Standards Organization ISO , who 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 — rms 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 trend machine condition. Vibration acceleration peak In discussing vibration velocity, it was pointed out that the velocity of the mass 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 limit 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 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. 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 kcpm , the preferred measurement unit for vibration is acceleration. Since the 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 belt 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 defects 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 the vibration response of the system with specific defects that occur in the machinery, its components, trains or even in mechanical structures. A common dilemma for vibration analysts 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 is 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 the 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 analysis, as most 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 determine acceptable vibration levels for various classes of machinery.
Thus, to use this ISO standard, it is necessary to first classify the 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 Medium-sized 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 stiff 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 deal 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. API standards have developed limits for casing as well as shaft vibrations Figure 2. The API specification on vibration limits for turbo machines is widely accepted and followed with apparently good results. Vibration basics 25 Figure 2. It presents a method for measuring linear vibration on a gear unit. It recommends instrumentation, measuring methods, test procedures and discrete frequency vibration limits for acceptance testing.
It annexes a list of system effects on gear unit 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 vibration 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 mounted on resilient vibration isolators such as coil springs or rubber 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 are included on the chart to provide a basis for comparison, and it can be noted that for vibration frequencies below 60 cpm Hz , the lines that divide the severity regions are of a relatively constant velocity. However, above this 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 for stringent machining specifications. It should be mentioned that 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 terms of displacement units. When critical machinery with a heavy penalty for process downtime is involved, the decision to correct a condition of vibration is often a very difficult one to make. It is thus reiterated that standards should only be an indicator of machine condition and not a basis for shutting down the machine.
What is of extreme importance is that vibrations of machines should be recorded and trended diligently. 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 that even two similar machines built simultaneously by one manufacturer can have vastly different vibration levels and yet operate continuously without any problems. One has to accept the limitations of these standards, which cannot be applied to a wide range 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 electric 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 below. This type of vibration transducer installs easily on most analyzers, and is rather inexpensive compared to other 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 output 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, a voltage signal correlating with 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 pickup is a self-generating sensor and requires no external devices to produce a voltage signal.
The voltage generated by the pickup is directly proportional to the velocity of the relative motion. Due to gravity forces, velocity transducers are manufactured differently for horizontal or vertical axis mounting. The velocity sensor has a sensitive axis 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.
Catalog Record: Vibration of structures and machines | HathiTrust Digital Library
Sometimes, the wire coil is counter wound wound in one direction and then in the opposite direction to counteract external electrical fields. A velocity signal produced by vibratory motion is normally sinusoidal in nature. Thus, in one cycle of vibration, the 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 positive when it is initially tapped.
VIBRATION OF STRUCTURES AND MACHINES.- Practical aspects, 3rd edition eBook
This fact should be taken into account when choosing the number of sensors to be used. Where possible, a velocity transducer should be mounted in the vertical, horizontal and axial planes to measure vibration in the three directions. The three sensors will provide a complete picture of the vibration signature of the machine. Mounting For the best results, the mounting location must be flat, clean and slightly larger than the velocity pickup. If it is possible, it should be clamped with a separate mounting enclosure. The surface will have to be drilled and tapped to accommodate the mounting screw of the sensor.
Whenever a velocity pickup is exposed to hazardous environments such as high temperatures, radioactivity, water or magnetic fields, 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 field 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 hang the pickup close to the area where vibrations must be taken with a 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 low frequencies of vibration, the sensitivity decreases because the pickup coil is no longer stationary with respect to the magnet, or vice versa. This decrease in pickup sensitivity usually starts at a frequency of approximately 10 Hz, below which the pickup output drops exponentially. The significance of this fact is that amplitude readings taken at 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 same 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 a 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 the 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 or gear sets generate high- vibration frequencies when defective. Machines with these components should be monitored with accelerometers. The installation of an accelerometer must carefully 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 barium 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 the piezoelectric crystal, which is directly proportional to the vibratory acceleration. The charge produced by the piezoelectric crystal is proportional to 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 internal, low-output impedance amplifier and requires an external power source.
The external power source can be either a constant current source or a regulated voltage source. This type of accelerometer 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 cable 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, which is matched to the accelerometer. Field wiring is terminated to the external charge amplifier. As with current or voltage mode accelerometers, signal cable lengths up to feet have negligible effect on the output signal quality. Mounting It is important to know the possible mounting methods for this vibration sensor. Four primary methods are used for attaching sensors to 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 of the accelerometer. Stud mounting provides the widest frequency response and the most secure, reliable attachment. The other three 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 pieces, such as adhesive pads, magnets or probe tips, introduces a mounted resonance. This mounted resonance is lower than the natural resonance of the sensor and reduces the upper frequency range. A large mounting piece causes lower mounted resonance and also lowers the usable frequency range of the transducer.
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This method is accomplished by screwing the sensor in a stud or a machined block. 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 the 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 glued, 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 is typically used for temporary measurements with a portable data collector or analyzer.
This method is not recommended for permanent monitoring. The transducer may be inadvertently 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 accelerometers with a wide range of sensitivities for special applications such as structural analysis, geophysical measurement, very high frequency 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 for a machine is known, an accelerometer can be selected. Typically, an accelerometer for measuring machine vibrations will have a frequency range from 1 or 2 Hz to 8 or 10 kHz. Accelerometers with higher-frequency ranges are also available. Calibration Piezoelectric accelerometers cannot be recalibrated or adjusted. Unlike a velocity pickup, 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 cable and radiated from the probe tip. In this way, an Eddy current transducer can be used for both radial vibration and distance measurements such as the axial thrust position and shaft 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 this 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 the Eddy current probes are mounted perpendicular to the shaft centerline. Clearance must be provided on all sides of the probe tip to prevent interference with the RF field. For instance, if 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 on the shaft do not thermally grow under 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 mounting 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 access 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. More machining required. The construction of older machines may not provide ideal installation of 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 and cable. Advantages of external mounting It is the most inexpensive installation. Requires mechanical protection.