Practical Solutions To Machinery And Maintenance Vibration Problems Pdf
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- Practical Machinery Vibration Analysis and Predictive Maintenance
- Machine Fault Signature Analysis
- Practical Machinery Vibration Analysis and Predictive Maintenance
- Online Textbook
One of the first points to make regarding vibration due to rolling element bearings is that generally speaking, the bearing is not a source of mechanical trouble but merely a result of other problems.
Practical Machinery Vibration Analysis and Predictive Maintenance
One of the first points to make regarding vibration due to rolling element bearings is that generally speaking, the bearing is not a source of mechanical trouble but merely a result of other problems. Most often, when symptoms of bearing defects arise, they are accompanied by other symptoms, such as for misalignment, unbalance or lubrication.
When dealing with rolling element bearing analysis, a good rule is to not only analyze the symptoms of bearing defects but to also determine why the bearing is defective. Look for symptoms of such sources as unbalance, misalignment, poor assembly, etc. There are many different symptoms of distressed rolling element bearings, and much good information has been published on the subject.
It is, therefore, not the purpose of this section to supersede information that is likely already known, but instead to highlight some of its more practical applications and strengthen certain portions. One of the most difficult Bearing problems to identify is incipient failure failure that is just about to happen. Vibration velocity readings help us to identify bearing defects in the latter stages of bearing life.
However, in order to get the earliest warning of impending trouble, it is necessary to monitor the ultrasonic vibration levels. Most instruments now have the capability to measure very high frequency vibration in the 5 kHz - 50 kHz range, Kcpm - Kcpm. The terminology for the particular unit of measurement varies from manufacturer to manufacturer, such as "spike energy," shock pulse," "bearing defect energy," "high frequency detection," etc.
However, they all measure pretty much the same thing. The instrument manufacturers will be able to furnish a detailed definition and description of their specific units of measurement. For the purposes of this text, general terms will be used. These high frequency measurements sense the low energy, repetitive, metal-to- metal impacts that occur in the earliest stages of an incipient bearing failure.
The rate, amplitude and frequency are all combined to give a single "numerical" output. To maintain impartiality, this text will identify all these units with the generic term IBF Incipient Bearing Failure number. The problem with all these units is that it is almost impossible to accurately determine bearing condition based on a single measurement. Measurements are very sensitive to a host of external influences such as differences in installation, load, lubrication, pickup mounting, location and number of interfaces.
Other problems, such as cavitation, gear mesh and steam leaks, also have significant effects in IBF numbers i. The use of IBF readings excels when trended over a period of time. If used correctly, IBF units will provide the earliest warning system for impending failure. In this manner, change becomes more significant than absolute values. If this technique is to be successful, accurate repeatable data has to be acquired.
Of all the vibration parameters measured, IBF units are the most sensitive to transducer location and mounting. Notice that the IBF number actually drops either before or as the bearing fails. If a single IBF reading of, say, 0. This is not known unless enough plots have been taken to show a trend. It is not recommended that a diagnosis be made based on IBF units alone. Instead, always verify the diagnosis with other vibration units such as velocity. Remember, trending IBF units provides only an early warning system.
Whether plotted by hand or by computer, trends of IBF units or vibration units usually velocity vs calendar dates are certainly valuable for monitoring how fast a bearing may be deteriorating.
However, such trends do not always accurately predict the approximate time for failure. Usually trend plots of IBF units show increases as the bearing deteriorates, but as actual failure occurs such as cracking , the IBF units may show a sharp decrease.
When IBF units are still increasing, most careful analysts monitor trends of velocity amplitudes as well. But again, amplitude increase is not always present as a bearing deteriorates. The difficulty is due to the entirely different nature of vibration amplitude increases due to bearing defects, as compared to amplitude increases due to other vibration sources such as unbalance, misalignment, bent shaft and so on.
For example, as fly ash builds up on a fan, its unbalance may progressively increase. Increased unbalance shows increased amplitude at 1 x rpm.
Or, for example, greater shaft-to-shaft misalignment will give higher amplitudes for its frequencies. Therefore, the analyst is tempted to think that greater defects at the source always produce larger amplitudes. True for almost all vibration sources. However, not necessarily true for bearing defects. Sometimes bearing defect amplitudes increase as the defect gets worse -- but not always. Amplitudes at bearing defect frequencies may not increase much at all and, in some situations, may actually decrease as the bearing gets worse.
Instead of the bearing defect amplitudes increasing, they may instead increase the number of sidebands. What may have started out as a relatively sharp peak may appear to be spreading out to cover a wider frequency range.
As with trends of IBF units, just before or right at bearing failure, the vibration amplitude suddenly decreases. Therefore, trend plots based on amplitude, may or may not give an accurate picture of what is happening in the bearing. Vibration amplitudes measured on the machine's case that are considered as acceptable or even smooth if originated from, for example, unbalance, are not acceptable for amplitudes due to bearing defects.
For example, a pump's case reading of 0. Vibration amplitudes at bearing defect frequencies are usually very low. For a "regular speed" machine, such as rpm or rpm, a bearing is considered mildly bad at only 0. Although this amplitude appears mild, the bearing can still immediately fail. For very slow speed machines, such as papermachine dryer rolls under to rpm , a bearing defect velocity amplitude of only 0. However, this textbook is for Practical Solutions to Machinery and Maintenance Vibration Problems Part I and the information provided is sufficient for analysis of bearing defects in about 90 percent of all situations.
In order to accurately monitor and trend IBF units over time, it is necessary to acquire accurate readings. Of all the various vibration units available, IBF units are the most sensitive to pickup location and mounting techniques.
The preferred location is as physically close to the bearing as possible and preferably but not essentially in the "load zone. However, for IBF measurements, each interface decreases the accuracy of the reading.
Therefore, try to choose a location with the least amount of interfaces between pickup location and the bearing. As with all maintenance work, compromises may be necessary. For initial maintenance setup on large machines, it may be necessary to review the engineering drawings in order to find an appropriate pickup location that will provide a good transmission path. The location must be clearly marked on the machine so that subsequent readings will be acquired in the same location.
Stud mounting is the best, but obviously more expensive. A practical alternative would be the magnetic holder. Use of an extension probe "stinger" or stem usually does not produce as good results unless of short length and adequately tightened to the pickup.
Update also suggests small drill point holes on the case's measuring points. One of the challenges of maintenance departments worldwide is to develop effective lubrication programs for the many grease lubricated rolling element bearings.
In an effort to ensure that a bearing is not lost due to lack of lubrication, there is a strong tendency to over-lubricate rolling element bearings. Over- lubrication is probably responsible for more bearing failures than under- lubrication. There have been several approaches over the years to solve lubrication problems of rolling element bearings. Some special instruments have built-in computers. Strong claims are made for their effectiveness.
Others use certain features of their regular vibration instruments, such as measuring IBF units. However, none of these approaches address the question of how much grease the bearing needs. A fully sealed bearing does not require regreasing through the course of its life, which if correctly installed in a machine that is well balanced and well aligned, can run in excess of 10 years. Why is it, therefore, that relubricating bearings can be called for every few months?
The usual answer is, "To flush the bearing of contaminants. This is not a section on bearing lubrication. Its focus is only on how some specialists use IBF units to accurately determine the quality and quantity of bearing lubrication in rolling element bearings. IBF measurements are made on a regular basis, watching for significant change. When significant change occurs, the technician verifies the initial data.
Then, while monitoring the IBF reading, grease is applied. After applying two or three pumps of the grease gun, the technician pauses for a few seconds while observing the IBF reading. Typically, the reading decreases. Grease is applied until no further reduction in the IBF reading can be obtained. At that point, the bearing is considered to be properly lubricated.
The bearing should be monitored for 10 to 15 minutes after the process to ensure that the IBF reading does not increase. If it does, the process is repeated. Sometimes overgreasing will show an IBF increase. The bearing should then be checked 24 to 48 hours later to ensure no further increase has occurred. A return to previous levels probably means the bearing is starting to fail.
It is possible to temporarily reduce the IBF readings on severely defective bearings by applying grease. One of the keys to the successful analysis of distressed bearings is pattern recognition.
Since there are many different configurations, types and sizes of rolling element bearings, it is very difficult to accurately determine one specific frequency and amplitude that will be generated by a bearing defect. Therefore, it is necessary to observe the familiar "patterns" developed by about 80 percent of distressed bearings rather than their absolute amplitudes and frequencies. Stage One: Fig. An increase in IBF units has occurred.
Machine Fault Signature Analysis
Practical Solutions Vibration Course. Online Course for Practical Solutions Vibration. This text has been written for the person working with already-built machinery rather than for the designer. Details covered are needed by the person new in vibration as well as the vibration specialist who already has hands-on experience. This text is not a complete vibration textbook with all varieties of terminology needed by the reader completely new to the subject. Instead, it has been written to help and assist those who have already attended Update's machinery vibration seminars. Link to Seminar Schedule.
Practical Machinery Vibration Analysis and Predictive Maintenance and diagnostic techniques • The ability to diagnose simple machinery related problems with vibration analysis The real solution depends on the purpose of the analysis.
Practical Machinery Vibration Analysis and Predictive Maintenance
This book offers professionals working at power plants guidelines and best practices for vibration problems, in order to help them identify the respective problem, grasp it, and successfully solve it. Accordingly, the case studies discussed here will equip power plant engineers to quickly evaluate the vibration problem at hand by deciding whether the machine is at risk or can continue operating and find a practical solution. His research activities focused on the development of mechatronic systems with applications to rotating machinery, machine tools and automotive systems.
Prevention is a far better goal than trying to solve problems as they arise. While this strategy may be a little costly at first, it is not nearly as expensive as allowing problems to occur. Maintenance problem-solving is primarily concerned with four areas: maintaining critical systems, fixing the problem quickly and faster than the last time, determining what is causing the breakdown to happen so frequently, and identifying the 20 percent of breakdowns that are consuming 80 percent of your resources. This article focuses on the four common types of maintenance problems with the ultimate goal of helping you to prevent or at least minimize each type.
I hope you will find this handbook both stimulating and informative as you work to develop and implement RCM-based preventive maintenance programs in Navy ships. Ebeling Published Engineering Part 1 Basic reliability models: the failure distribution constant failure rate model time-dependent failure models.
Pratesh Jayaswal, A. Wadhwani, K. The objective of this paper is to present recent developments in the field of machine fault signature analysis with particular regard to vibration analysis. The different types of faults that can be identified from the vibration signature analysis are, for example, gear fault, rolling contact bearing fault, journal bearing fault, flexible coupling faults, and electrical machine fault. It is not the intention of the authors to attempt to provide a detailed coverage of all the faults while detailed consideration is given to the subject of the rolling element bearing fault signature analysis.
As vibration isolation and reduction techniques have become an integral part of machine design, the need for accurate measurement and analysis of mechanical vibration has grown. Using accelerometers to convert vibratory motion into an electrical signal, the process of measurement and analysis is ably performed by the versatile abilities of modern electronics. A body is said to vibrate when it describes an oscillating motion about a reference position. The number of times a complete motion cycle takes place during the period of a second is called the frequency and is measured in hertz Hz. The motion can consist of a single component occurring at a single frequency, as with a tuning fork, or of several components occurring at different frequencies simultaneously, for example, with the piston motion of an internal combustion engine. Vibration signals in practice usually consist of very many frequencies occurring simultaneously so that we cannot immediately see just by looking at the amplitude-time pattern, how many components there are, and at what frequencies they occur.
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