CHAPTER-I OBJECTIVE AND SCOPE OF THE PROJECT1.1 INTRODUCTIONIn the early days, equipment maintenance was conducted only when equipment actually failed. The work was more fix it than maintenance. Shortly thereafter, failures. came the recognition that performing regular maintenance and refurbishment tasks on equipment could keep equipment operating longer between This became known, variously, as Periodic Maintenance, Calendar Based Maintenance or Preventive Maintenance (PM). The goal was to have most of the equipment be able to operate most of the time until the next scheduled maintenance outage. This approach is also outdated. Now Condition monitoring has made good progress in recent years maintenance is being carried out based on condition of machine which reduces the cost of unnecessarily opening of equipment. Most of the defects encountered in the rotating machinery give rise to a distinct vibration pattern (vibration signature analysis techniques)Vibration Monitoring is the ability to record and identify vibration Signatures which makes the technique so powerful for monitoring rotating machinery. Vibration analysis is normally applied by using transducers to measure acceleration, velocity or displacement. The choice largely depends on the frequencies being analyzed. Condition monitoring has made good progress in recent years in identifying any types of deterioration in plant machinery, so that pro-active maintenance can be performed, improving overall plant productivity. Vibrations are found almost Everywhere in power plants. Rotating machinery misalignments and imperfect vibrates due to unbalances, bearings; Vibration, in general, reduces equipment

life and, in extreme cases, can result in equipment damage or even catastrophic failures. On the other hand, existence of vibration can also be used to diagnose equipment problems and provide.

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1.2 OBJECTIVE OF THE PROJECTThe new generation of condition monitoring and diagnostics systems differs by the detailed solution of diagnostic problems that allows making a step from machine vibration state monitoring to the monitoring of the machine technical condition. Most rotating machine defects can be detected by such a system much before dangerous situations occur. The aim of vibration monitoring is the detection of changes in the vibration condition of the object under investigation during its operation. The cause of such changes is mainly the appearance of a defect. The number of such points can be reduced to one or two for each object to be monitored if there is a common casing. The main objective of this project is to identify the causes of significant vibrations developed in the main pump driving end and main pump non driving end and to rectify those vibrations by proper action and to develop an simple ANN for the fault diagnosis boiler feed pump.

1.3 ORGANIZATION OF THE WORKThe project is organized into following activities: Chapter I highlights the importance, objective of the study and methodology of the work. Chapter II deals with the information from the research papers. Chapter III gives the introduction of maintenance strategies, predictive maintenance procedure, and condition monitoring techniques and briefly about the vibration monitoring. Chapter IV describes the theory regarding basics of vibration, vibration instrumentation, and DATAPAC 1500 the instrument used for the vibration analysis. Chapter V deals with the vibration analysis procedure and fault diagnosis of the machinery. Chapter VI gives the overview of the artificial neural networks, back propagation algorithm and application ANN to vibration analysis. Chapter VII deals with the case study of the BOILER FEED PUMP. The vibration spectrum analysis and the experimentation and the fault diagnosis of the machine. Chapter VIII presents the application of ANN for the fault recognition on BFP. Chapter IX lists out the results, conclusions and future scope of the work. 2

CHAPTER-II LITERATURE REVIEW2.1 INTRODUCTIONA literature survey was taken up to review present status of research in the field of condition monitoring. Machine condition monitoring is gaining importance in industry because of the need to increase reliability and to decrease the possibility of production loss due to the machine breakdown. The use of vibration and acoustic emission signals is quite common in the field of condition monitoring of rotating machinery. By comparing the signals of a machine running in the normal and faulty conditions. Detection of faults like unbalance, rotor rub, shaft misalignment, gear failure and bearing defects is possible. These signals can also be used to detect the incipient failures of the machine components, through the online monitoring system, reducing the possibility of catastrophic damage and the down time. Some of the recent works in this area are. R.K BISWAS, [1] Scientist and head, condition monitoring group, CMERI, DURGAPUR. Presented paper on Vibration based condition monitoring of rotating machines states that Condition Monitoring is defined as the collection, comparison and storage of measurements defining machine condition. Almost everyone will recognize the existence of a machine problem sooner or later. One of the objectives of Condition Monitoring is to recognize damage that has occurred so that ample time is available to schedule repairs with minimum disruption to operation and production. In this aspect vibration is probably the best operating parameter to judge dynamic condition of machines. Condition monitoring is essentially a screening process in which measurements and other data are compared to pre-established norms for the purpose of recognizing abnormal variations. A machine seldom breaks down without warning. The signs of impending breakdown are almost present long before the catastrophic failure. Vibration signals define the dynamic property of the machine including various faults of machine like bearing instability, unbalance, coupling misalignment, looseness, rubs, etc. Vibration 3

characteristics also define early indication of defects on components such as rolling element bearing and gears. M.Todd, S.D.J.McArthur, G.M.West, J.R.McDonald, S.J.Shaw. J.A.Hart [2] paper on the design of a decision support system for the vibration monitoring of turbine generators they discussed about the Condition Monitoring (CM) systems monitor the health of expensive plant items such as turbine generators. They interpret turbine parameters by signaling an alarm when pre-defined limits are breached. This is a time consuming and laborious process due to the volume of data interpreted for each alarm. In order to reduce the burden of alarm assessment, a Decision Support System (DSS) is proposed. The DSS will feature a Routine Alarm Assessment (RAA) module which provides an initial analysis of the alarms, highlighting those with no further operational consequence and enabling the expert to focus on those which indicate a genuine problem with the turbine. The implementation of an RAA prototype is discussed along with how this will act as a foundation for a full alarm interpretation and fault diagnostic system. David Clifton,[3] St. Cross College, December, 2005 made research on Condition Monitoring of Gas-Turbine Engines This report describes preliminary research into condition monitoring approaches for modern gas-turbine aircraft engines, and outlines plans for novel research to contribute to machine learning techniques in the condition monitoring of such systems. A framework for condition monitoring of aircraft engines is introduced, using signatures of engine vibration across a range of engine speeds to assess engine health. Inter- and intra-engine monitoring approaches are presented, in which a model of engine normality is constructed using vibration data from other engines of its class, or from the test engine itself, respectively.T.W. Verbruggen[4] a book on Wind Turbine Operation & Maintenance

based on Condition Monitoring. This report is part of the project entitled WT_ (WT_OMEGA = Wind Turbine Operation and Maintenance based on Condition Monitoring) which has been carried out in co-operation withLagerwey the Wind Master, Siemens Nederland, and SKF.

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A Ramachandra, S B Kandagal, [5] worked on Prediction of Defects in Antifriction Bearings using Vibration Signal Analysis Condition monitoring of antifriction bearings in rotating machinery using vibration analysis is a very well established method. It offers the advantages of reducing down time and improving maintenance efficiency. The machine need not be stopped for diagnosis. In order to prevent any catastrophic consequences caused by a bearing failure, bearing condition monitoring techniques, such as, temperature monitoring, wear debris analysis, oil analysis, vibration analysis and acoustic emission analysis have been developed to identify existence of flaws in running bearings. Among them vibration analysis is most commonly accepted technique due to its ease of application. Sadettin Orhan, Nizami Akturk, Veli C elik,[6] worked on Vibration monitoring for defect diagnosis of rolling element bearings as a predictive maintenance tool: Comprehensive case studies Vibration monitoring and analysis in rotating machineries offer very important information about anamolies formed internal structure of the machinery. In this study, the vibration monitoring and analysis case studies were presented and examined in machineries that were running in real operating conditions. Failures formed on the machineries in the course of time were determined in its early stage by the spectral analysis. It was shown that the vibration analysis gets much advantage in factories as a predictive maintenance technique. Peter W. Hills, Mechanalysis (India) Limited, India, A more intelligent approach to rotating equipment monitoring,[7]. Proactive condition management of rotating machinery is not new to the power sector and is applied widely but with varying degrees of success. The financial benefits have long been recognized and widely reported, but the cost of implementation, required expertise and continuity of the systems remain as constraints to its broader use. To date, the focus of condition monitoring of rotating equipment has been on detecting the mechanical aspects of a machine, such as imbalance, alignment, etc, with little attention being paid to the on-line detection of its electrical system. Cornelius [8], Scheffer, describes the paper on Pump Condition Monitoring through Vibration Analysis It is well-known that vibration analysis is a powerful tool 5

for the condition monitoring of machinery. This especially applies to rotating equipment such as pumps. Through the years a variety of vibration-based techniques have been developed and refined to cost-effectively monitor pump operation and the onset of failures. This paper is an overview of a variety of vibration-based condition monitoring techniques for pumps. In some instances these techniques are also applicable to improve the operation and efficiency of pumps. Specific aspects to consider when taking vibration measurements on pumps are for instance where to take readings, which type of probe to use, what frequency range should be used, what the settings on the analyzer should be, etc. Sheng Zhang, Joseph Mathew, Lin Ma, Yong Sun and Avin Mathew,[9] presented a paper on Statistical condition monitoring based on vibration signals. Designing control limits for condition monitoring is an important aspect of setting maintenance schedules and has been virtually ignored by researchers to date. This paper proposes a novel statistical process control tool, the Weighted Loss function CUSUM (WLC) chart, for the detection of condition variation. The control limit was designed using baseline condition data, where the process was fitted by an autoregressive model and the residuals were used as the chart statistic. The condition variation is reflected by the changes of mean and variance of the statistics distribution against baseline condition, which can be detected by a single WLC chart. The approach was evaluated using a case study which showed that the chart can detect faulty conditions as well as their severity. The proposed approach has the advantage of requiring healthy baseline data only for the design of condition classifiers. It is applicable in numerous practical situations where data from faulty conditions are unavailable. P. Caselitz, J. Giebhardt,[10], presented a paper on Condition Monitoring and Fault Prediction for Marine Current Turbines. This paper introduces the concept of condition monitoring and fault prediction for marine current turbines. It will describe the required hardware to perform condition monitoring measurements and some appropriate fault prediction algorithms specific for marine current turbines. Furthermore, concepts for communication and data base handling will be introduced. For the above mentioned items, some relevant standards and technical guidelines will be addressed.

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Steven M. Schultheis,[11], Charles A. Lickteig, presented a paper on RECIPROCATING COMPRESSOR CONDITION MONITORING. This paper will discuss risk-based decision making in regard to measurements and protective functions, online versus periodic monitoring, proven and effective measurement techniques, along with a review of both mechanical- and performance-based measurements for assessing machine condition. Case histories will also be presented to demonstrate some of the concepts. Peter W. Hills,[12] Mechanalysis (India) Limited, India presented a paper on A more intelligent approach to rotating equipment monitoring in the journal The article is based on the paper Intelligent Condition Management On-line The majority of condition monitoring regimes for power plants rotating equipment is focused the detection of mechanical faults, with little attention paid to electrical faults in equipment. This could be about to change with the introduction of an on-line monitoring system that learns to detect both types of fault. L. B. Jack, A. K. Nandi,[13] presented a paper on Feature Selection for ANNs using Genetic Algorithms in Condition Monitoring. The work presented in this paper the work presented in this paper is based around experimental results per- formed on vibration data taken from a small test rig which was tested with a number of interchangeable faulty roller bearings. This is used to simulate the type of problems that can commonly occur in rotating machinery. Rolling elements, or ball bearings, are one of the most common components in modern rotating machinery; being able to detect accurately the existence of a fault in a machine can be of prime importance in certain areas of industry. Ms S Wadhwani, Dr S P Gupta, Dr V Kumar, [14] Wavelet Based Vibration Monitoring for Detection of Faults in Ball Bearings of Rotating Machines this paper describes the application of wavelet transform (WT) for detection of bearing damage from the vibration signal of the bearing. The wavelet transform approach enables instant to instant observation of the contribution of different frequency components over the full spectrum from. Actually, wavelet transform acts as a mathematical microscope in which one can observe different parts of the signal by 7

adjusting the focus. A new technique combining the WT with neural network for detection and classification of ball bearing fault in a three phase, 3.75 kW inductions motor is presented in this paper. The method is tested successfully for three faulty bearing conditions: crack in inner race, crack in outer race and defect in balls. Zhigang TIAN, [15] An Artificial Neural Network Approach for Remaining Useful Life Prediction of Equipments Subject to Condition Monitoring. Accurate equipment remaining useful life prediction is critical to effective condition based maintenance for improving reliability and reducing overall maintenance cost. An artificial neural network (ANN) based method is developed for achieving more accurate remaining useful life prediction of equipment subject to condition monitoring. The ANN model takes the age and multiple condition monitoring measurement values at the present and previous inspection points as the inputs, and the life percentage as the output. Techniques are introduced to reduce the effects of the noise factors that are irrelevant to equipment degradation. The proposed method is validated using real-world vibration monitoring data. N.M. ROEHL C.E. PEDREIRA" H.R. TELES DE AZEVEDO [16] presented a paper on Fuzzy art neural network approach for incipient Fault detection and isolation in rotating machines. A neural network approach for on-lie detection and isolation of faults in rotating machines is proposed. The methodology is based on clustering of shaft vibration monitoring data by using fuzzy art neural networks. Fault isolation is obtained by retrieving stored associations among known physical faults and clusters. The proposed scheme is implemented to detect and isolate different operation modes in a hydro generator. P.A.L. Ham, B.Sc.C.Eng..F.I.E.E. [17] Trends and future scope in the monitoring of large steam turbine generators Current practices in the monitoring of large steam turbine generators are briefly discussed, consideration being given to the traditional range of turbine supervisory equipment, and the more extended facilities which are sometimes now associated with rotating machinery, such as vibration monitoring, together with the more generalized data logging systems now specified by some Utilities. Consideration is given to the possible range of parameters and equipment 8

areas which may now be incorporated into a monitoring scheme, and attention is drawn to the advances in display technology and operator interfaces which are now possible at moderate cost. In a concluding section, a range of monitoring functions which could be of wide general application in the field of steam turbine generators is discussed.

2.2 SUMMURYThis literature review presents an overview of the vibration based condition monitoring of the rotating equipments in the thermal power plants. This literature review also contains the review methodologies of the predictive maintenance technology.

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CHAPTER-III MAINTENANCE STRATEGIES3.1 INTRODUCTIONGeneral industrial profitability is effected both by on stream functions and maintenance costs. Any system must account for the optimum maintenance that can be performed by an organizational setup. Maintenance besides trying to better its own efficiency and mechanical performance must solve the problem of failure.

3.2 CLASSIFICATION OF MAINTENANCE STRATEGIESMaintenance strategies are classified by three developmental stages: 1. Break down maintenance 2. Preventive maintenance 3. Predictive maintenance 3.2.1 Break Down Maintenance This provides the replacement of defective part or machine after the machine becomes incapable of further operation. Break down maintenance is the easiest method to follow and it avoids the initial costs on training personnel and other related upfront costs. Draw backs of the break down maintenance are 1. Failures are untimely. 2. Since machine is allowed to run till to failure repair is more expensive, sometimes total replacement is required. 3. Failures may be catastrophic. Hence loss will be more. 4. Production loss will be more, as it requires more time to restore normalcy. 5. It reduces the life span of the equipment.

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3.2.2 Preventive Maintenance In preventive maintenance, maintenance is scheduled on calendar or hours to run and is performed irrespective of machine conditions. Advantages: 1. Damage to machine is less. 2. Down time of machine is reduced by 50-80%. 3. Lower expenses of overpay may same as much as 30%. 4. Increases the equipment life expectancy. 5. Reduces maintenance cost by reducing the I. Capital spending by 10-20%. II. Labor cost by 10%. III. Material cost by 30% 6. Improve the employees safety. 7. Preventive maintenance results in a catastrophic failure and down time is required to complete all schedule maintenance costs. Disadvantages: 1. periodically dismantling of each and every critical machine is expensive and time consuming. 2. It may lead to unnecessary inspections even on healthy machine also which may further lead to more complications. 3. It is difficult to predict time interval between inspections, which ultimately may lead to break down maintenance.

Fig: 3.1 Failure rate or bath tub curve 11

Preventive maintenance alone cannot eliminate break down. The causes of equipment failure change with the passage of time fig: shows the failure rate curve which is also called as life span characteristic curve or bath tub curve. Failure rate is taken on ordinate and time is taken on abscissa. When the equipment is new there is a high failure rate due to design and manufacturing errors. Failure rate is increases once again since the equipment approaches the end of its failure. 3.2.3 Predictive Maintenance Predictive maintenance (PdM) techniques help determines the condition of inservice equipment in order to predict when maintenance should be performed. This approach offers cost savings over routine or time-based preventive maintenance, because tasks are performed only when warranted. Predictive maintenance or condition-based maintenance, attempts to evaluate the condition of equipment by performing periodic or continuous (online) equipment condition monitoring. The ultimate goal of PdM is to perform maintenance at a scheduled point in time when the maintenance activity is most cost-effective and before the equipment loses optimum performance. This is in contrast to time- and/or operation count-based maintenance, where a piece of equipment gets maintained whether it needs it or not. Most PdM inspections are performed while equipment is in service, thereby minimizing disruption of normal system operations. Adoption of PdM can result in substantial cost savings and higher system reliability. Trending and analyzing machinery parameters we can detect the developing problems in early stages. Hence repair works can be carried out before failure of a machine Advantages: Shut down can be done at convenient times. Work schedule can be prepared for mobilizing men, tools and replacement parts before shut down reducing machinery down time. Identifying problem, costly trial and error procedures to solve a problem can be avoided. Machine in good running condition can run continuously as long as problem develops. 12

Disadvantages: Requires skilled labor. It is costly affair. For all machine common characteristic is vibrations and hence vibrations become a powerful tool in implementing predictive maintenance program. The vibration predictive maintenance program has four steps: 1. detection 2. analysis 3. correction 4. confirmation Detection First select all available critical machines in the plant. prepare a schedule for all these machines for data collection identify bearing locations of the machine train motor non drive end, MNDE, FNDE, FDE, PNDE, PDE, etc. identify the directions where vibration data is collected like h, v, a etc. define which vibration parameters are to be collected via displacement, velocity, acceleration etc. after doing all these, start collecting vibrating data and related data and record them. Collect the data for every fortnight or monthly or so .by trending and interpreting the data identify source of vibrations. Analysis After identifying the source of vibrations analyze to pin point the root cause for vibrations. This can be achieved by eliminating process. Follow confirmative procedures in support of analysis Correction Open and inspect the machine at a convenient time and make necessary corrections. Confirmation After corrections put the machine in service and again collect vibration data and look for elimination of the source.

Predictive Maintenance Program:

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3.3 CONDITION MONITORING3.3.1 Introduction Condition monitoring pre-supposes knowledge of machines condition and its rate of change, which can be ascertained by selecting a suitable parameter for measuring deterioration and recording its value at intervals either on a routine or continuous basis. This is done while the machine is running. The data obtained may then be analyzed to give a warning on failure. This activity is called as condition monitoring. Condition monitoring essentially involves regular inspection of equipment using human sensory facilities and a mixture of simple aids and sophisticated instruments The central emphasis is however on the fact that most inspections should be preferably done while the machine is running. Condition monitoring is concerned with the analysis and interpretation of signals from sensors and transducers installed on operational machinery, employing sensors positioned outside the machine, often remove from the machine components being monitored, normally does the monitoring of a machine condition and health, using established techniques, the analysis of information provided by the sensor output and interpretation of the evaluated output is the needed to establish what actions to be taken. Condition monitoring can also be a test and quality assurance, system for continuous processes as well as discrete component manufacture. It maximizes the performance of the companys assets by monitoring their condition and ensuring that they are installed and maintained correctly, it aims of detecting condition leading to catastrophic breakdowns and loss of service, reducing maintenance overhauls, fine turning of operating equipment increasing production and operating efficiency and minimizing the replacement parts inventory. This is because a readily monitor able parameter of deterioration can be found in every plant, Machinery and probabilistic element in future prediction is highly reduced or almost eliminated thus maximizing the items life by minimizing the effect of failure

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3.3.2 Condition Monitoring Techniques There are only seven main techniques of condition monitoring. They are: a) Visual monitoring b) Contaminant or debris monitoring c) Performance and behavior monitoring d) Corrosion monitoring thermograph e) Sound monitoring. f) Shock pulse monitoring. g) Vibration monitoring. a. Visual Monitoring Visual monitoring involves the inspections and recording of surfaces to detect Such as surface cracks and their orientation. Oxide films, weld defects and the presence of potential sources such as sharp notches or misalignment. b. Contaminant Monitoring Debris analysis is well proving in all types of industrial and works on the principle of taking or known quantity. Sample example: a gear box, then for analyzing the amount and type of foreign particles present in the sample. This will be show such problems, as gear wear, to the sample detects particles of gear material .oil analysis differs from debris analysis so for as this technique allows an assessment of the actual condition of the oil in use. That is whether the oil quality is good enough for the application after period of use or it is burnt or exceeded its useful use. c. Performance and Behavior Monitoring Performance and behavior monitoring involves checking the performance of machine or component to see whether it is behaving correctly. Monitoring the performance of the bearing by measuring its temperature to see whether it is carrying out its function.

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d. Corrosion Monitoring Corrosion monitoring has actually applied to the fixed plane containing aggressive material to monitor the rates of internal corrosion of walls of the equipment. It is the system systematic measurement of corrosion or degradation of an a item of equipment, with the aim of assisting and understanding the correct corrosion process of obtaining information for the use of controlling corrosion e. Thermography Thermograph is a rapidly developing; it provides color cameras and videos, clean indicator of heat loss, hot spot, cold spot, such as switchgear or any piece of plant or production where temperature or its effects is important, it can be used both as maintenance tools or a quality assurance tool. Shock pulse method is unique technique for monitoring the true operation of the bearing by measuring the pressure wave generated by the instantaneous mechanical impact.

f. Sound Monitoring: Human operators are normally highly sensitive to the detection of defects as a result of sudden change of sound due to the looseness of component results of wear or slackening of fastening are particularly susceptible to such forms monitoring The most widely available micro phones for sound or piezoelectric moving coils and condensers.

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g. Vibration Monitoring: Vibration monitoring measures the frequency and amplitude of vibrations. It is Known that readings will change as machinery wear sets in. such readings can be interpreted as indicators of the equipments condition, and timely maintenance actions can be scheduled accordingly. Electrical machines and mechanical reciprocating or rotating machines generate their own vibration signatures (patterns) during operation. However such raw signals contain a lot of background noise, which makes it difficult or even impossible to extract useful, precise information by simply measuring the overall signal. It is thus necessary to develop an appropriate filter to remove the operationally and environmentally contaminated components of signals (the background noise) so as to reveal the clear signals generated by the events under study. To capture useful condition monitoring data, vibration should be measured at carefully chosen points and directions. Vibration monitoring is a well established method for determining the physical Movements of the machine or structure due to imbalance mounting an alignment this method can be obtained as simple. Easy to use and understand or sophisticated real time analysis, vibration monitoring usually involves the attachment of a transducer to a machine to record its vibration level special equipments is also available for using the output from sensor to indicate nature vibration problem and even its precise cause. Transducers for the measurement of vibrations employ electromagnetic electrodynamics, capacitive, piezoelectric, or strain gauge principles out of these piezoelectric accelerometers is most widely used since the recent past, Among the monitoring techniques vibration monitoring as gained considerable importance because of following fundamental factors 1) All rotation and reciprocating machines vibrate either to a smaller or greater extent machines vibrate because of defects or incurrence in system 2) When inaccuracies or more it results in increased vibration each kind of defect provides a vibration characterized in the unique way. Therefore vibration characteristics reveal the health condition of machine.

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CHAPTER-IV BASICS OF VIBRATION

Definition Vibration can be defined as simply the cyclic or oscillating motion of a machine or machine component from its position of rest. Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.

Fig 4.1 basic vibration representation Vibration is occasionally "desirable". For example the motion of a tuning fork, the reed in a woodwind instrument or harmonica, or the cone of a loudspeaker is desirable vibration, necessary for the correct functioning of the various devices. More often, vibration is undesirable, wasting energy and creating unwanted sound noise. For example, the vibration motions of engines, electric motors, or any 18

mechanical device in operation are typically unwanted. Such vibrations can be caused by imbalances in the rotating parts, uneven friction, the meshing of gear teeth, etc. Careful designs usually minimize unwanted vibrations.

4.1 What Causes Vibration?Forces generated within the machine cause vibration. These forces may be one that Change in direction with time, such as the force generated by a rotating unbalance. Change in amplitude or intensity with time, such as the unbalanced magnetic forces generated in an induction motor due to un equal air gap between the motor armature and stator (field). Result in friction between rotating and stationary machine components in much the same way that friction from a rosined bow causes a violin string to vibrate. Cause impacts, such as gear tooth contacts or the impacts generated by the rolling elements of a bearing over flaws in the bearing raceways. Cause randomly generated forces such as flow turbulence in fluid handling devices such as fans, blowers and pumps, or combustion turbulence in gas turbines or boilers.

4.2 What is Machine Vibration?Most of us are familiar with vibration; a vibrating object moves to and fro, back and forth. A vibrating object oscillates. We experience many examples of vibration in our daily lives. A pendulum set in motion vibrates. A plucked guitar string vibrates. Vehicles driven on rough terrain vibrate, and geological activity can cause massive vibrations in the form of earthquakes.

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Fig 4.2 Examples of vibration There are various ways we can tell that something is vibrating. We can touch a vibrating object and feel the vibration. We may also see the back-and-forth movement of a vibrating object. Sometimes vibration creates sounds that we can hear or heat that we can sense. What is machine vibration? Machine vibration is simply the back and-forth movement of machines or machine components. Any component that moves back and forth or oscillates is vibrating. Machine vibration can take various forms. A machine component may vibrate over large or small distances, quickly or slowly, and with or without perceptible sound or heat. Machine vibration can often be intentionally designed and so have a functional purpose. At other times machine vibration can be unintended and lead to machine damage. Most times machine vibration is unintended and undesirable. This book is about the monitoring of undesirable machine vibration. Shown below are some examples of undesirable machine vibration.

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Fig 4.3 vibrating parts

4.3 Vibration and Machine LifeYour first question may be: "Why worry about a machine's vibration?" obviously, once a machine is started and brought into service, ft will not run indefinitely. In time, the machine will fail due to the wear and ultimate failure of one or more of its critical components. And, the most common component failure leading to total machine failure is that of the machine bearings, since it is through the bearings that all machine forces are transmitted. Of course, the next question is: "How long will be bearings last?" Although an exact answer to this question is impossible, the manufacturers of rolling element bearings attempt to estimate bearing life using the following calculation: L 10 LIFE (HOURS) = 16.666/ RPM X (RATE / LOAD)3 Where: RPM Load = Machine rotating speed in Revolutions per Minute = the actual load to which the bearing is subjected.

RATE = the rated load capacity of the bearing (lbs.) This includes not only the static load due to the weight of the rotor, but the dynamic load due to forces of unbalance, misalignment, etc., FORCES THAT CAUSE VIBRATION. According to this calculation to estimate bearing life, doubling the rotating speed from, say 1800 RPM to 3600 RPM, would cut bearing life in half. However, by cutting the load on the bearing by one-half would increase its service life by eight times (2-cubed 21

or 2 x 2 x 2 = 8). Of course, this estimate of bearing life does not take into consideration other factors such as inadequate lubrication, lubricant contamination or damage from improper storage or installation techniques. From the above calculation, it can be seen that bearing load, including dynamic load from vibratory sources such as unbalance and misalignment, has a significant effect on bearing life and, ultimately, machine life. Further, the amount of vibration exhibited by a machine is directly proportional-to the amount of force generated. In other words, if the unbalance force is doubled, the resultant vibration amplitude will be doubled also. Or, if the unbalance force is cut in half the unbalance -generated vibration will be cut in half also. Therefore, the answer to the question: "Why worry about a machine's vibration?" is simple: 1. Increased dynamic forces (loads) reduce machine life. 2. Amplitudes of machinery vibration are directly proportional to the amount of dynamic forces (loads) generated. If you double the force, you double the Vibration. 3. Logically then, the lower the amount of generated dynamic forces, the lower the levels of machinery vibration and the longer the machine will perform before failure It's that simple. Low levels of vibration indicate low vibratory forces which, in turn, results in improved machine life. With few exceptions, when the condition of a machine deteriorates, one of two possibly both things will generally happen: The dynamic forces generated by the machine will increase in intensity, causing an increase in machine vibration. Wear, corrosion or a build-up of deposits on the rotor may increase unbalance forces. Settling of the foundation may increase misalignment forces or cause distortion, piping strains, etc. The physical integrity (stiffness) of the machine will be reduced, causing an increase in machine vibration. Loosening or stretching of mounting bolts, a broken weld, a crack in the foundation, deterioration of the grouting, increased bearing clearance through wear or a rotor loose on its shaft will result in reduced stiffness to control even normal dynamic forces, Thus, it should be obvious that an increase in machinery 22

vibration is a positive indicator of developing problems.

In addition, each

mechanical or operational problem generates vibration in its own unique way. As a result, it is also possible to identify the specific nature of the problem by simply measuring and noting its vibration characteristics. The techniques of identifying specific defects and problems are presented in the section on VIBRATION ANALYSIS.

4.4 Characteristics of VibrationWhenever vibration occurs, there are actually four forces involved that determine the characteristics of the vibration. These forces are: The exciting force, such as unbalance or misalignment. The mass of the vibrating system, denoted by M. The stiffness of the vibrating system, denoted by the symbol K. The damping characteristics of the vibrating system, denoted by the symbol C.

The exiting force is trying to cause vibration, where as the stiffness, mass and damping forces are trying to oppose the exiting force and control or minimize the vibration. The characteristics needed to define the vibration include: Frequency Displacement Velocity Acceleration Spike energy Phase

4.4.1 Vibration Frequency The amount of time required to complete one full cycle of the vibration is called the period of the vibration. If, for example, the machine completes one full cycle of vibration in 1/60th of a second, the period of vibration is said to be 1/60th of a second. Although the period of the vibration is a simple and meaningful characteristic, a characteristic of equal simplicity but more meaningful is the vibration frequency. 23

Vibration frequency is simply a measure of the number of complete cycles that occur in a specified period of time such as "cycles-per-second" (CPS) or "cycles-per-minute" (CPM). Frequency is related to the period of vibration by this simple formula: Frequency = 1/Period In other words, the frequency of a vibration is simply the "inverse" of the period of the vibration. Thus, at the period or time required to complete once cycle is 1 / 60th of a second, then the frequency of the vibration would be 60 cycles-per-second or 60 CPS. Given a frequency expressed in Hz, you can convert it to CPM: CPM = Hertz x 60 Seconds/Minute Given a frequency expressed in CPM, you can convert it to Hz: Hertz = CPM/60 Seconds/Minute Significance of Vibration Frequency There are literally hundreds of specific mechanical and operational problems that can cause a machine to exhibit excessive vibration. Obviously, when a vibration problem exists, a detailed analysis of the vibration should be performed to identify or pinpoint the specific cause. This is where knowing the frequency of vibration is most important. Vibration frequency is an analysis or diagnostic tool. The forces that cause vibration are usually generated through the rotating motion of the machines parts. Because these forces change in direction or amplitude according to the rotational speed (RPM) of the machine components, it follows that most vibration problems will have frequencies that are directly related to the rotational speeds. To illustrate the importance of vibration frequency, assume that a machine, consisting of a fan operating at 2400 RPM and belt driven by a motor operating at 3600 RPM, is vibrating excessively at a measured frequency of 2400 CPM (1 x fan RPM), this clearly indicates that the fan is the source of the vibration and not the motor or belts. Knowing this simple fact has eliminated literally hundreds of other possible causes of vibration. Predominant Frequency: Predominant frequency is the frequency of vibration having the highest amplitude or magnitude. Synchronous Frequency: Synchronous frequency is the vibration frequency that occurs at 1 x RPM. 24

Sub synchronous Frequency: Sub synchronous frequency is vibration occurring at a frequency below 1 x RPM. A vibration that occurs at 1/2 x RPM would be called a Sub synchronous frequency. Fundamental Frequency: Fundamental frequency is the lowest or first frequency normally associated with a particular problem or cause. For example, the product of the number of teeth on a gear times the RPM of the gear would be the fundamental gearmesh frequency. On the other hand, coupling misalignment can generate vibration at frequencies of 1 x, 2x and sometimes 3 x RPM. In this case, 1 x RPM would be called the fundamental frequency. Harmonic Frequency: A harmonic is a frequency that is an exact, whole number multiple of a fundamental frequency. For example, a vibration that occurs at a frequency of two times the fundamental gear mesh frequency would be called the second harmonic of gear mesh frequency. A vibration at 2 x RPM due to, say, misalignment, would be referred to as the second harmonic of the running speed frequency (1 x RPM). Order Frequency: An order frequency is the same as a harmonic frequency. Sub harmonic Frequency: A sub harmonic frequency is an exact submultiples (1/ 2, 1/3, 1/4, etc.) of a fundamental frequency. For example, a vibration with a frequency of exactly 1/2 the fundamental gear-mesh frequency would be called a sub harmonic of the gear mesh frequency. Vibration at frequencies of exactly 1/2, 1/3 or 1/4 of the rotating speed (1 x RPM} frequency would also be called . Sub harmonic frequencies; and these can also be called Sub synchronous frequencies. However, not all Sub synchronous frequencies are sub harmonics. For example, a vibration with a frequency of 43% of the running speed (1 x RPM) frequency is a Sub synchronous frequency but it is not a sub harmonic. 4.4.2 Vibration Amplitude As mentioned earlier, vibration frequency is a diagnostic tool, needed to help identify or pinpoint specific mechanical or operational problems. Whether or not a vibration frequency analysis is necessary, depends on how "rough" the machine is shaking. If the machine is operating smoothly, knowing the frequency or frequencies of vibration present is not important. The magnitude of vibration or how rough or smooth the machine vibration is, is expressed by its vibration amplitude. Vibration amplitude can 25

be measured and expressed as: Displacement Velocity Acceleration SPIKE ENERGY Vibration Displacement The vibration displacement is simply the total distance traveled by the vibrating part from one extreme limit of travel to the other extreme limit of travel. This distance is also called the "peak-to-peak displacement". Peak-to-peak vibration displacement is normally measured in units called mils, where one mil equals one-thousandth of an inch (1 mil = 0.001 inch). Measured vibration amplitude of 10 mils simply-means that the machine is vibrating a total distance of 0.010 inches peak-to-peak. In Metric units, the peak-to-peak vibration displacement is expressed in micrometers (sometimes called microns), where one micrometer equals one-thousandth of a millimeter (1 micrometer = 0.001 millimeter). Vibration Velocity The vast majority of machine failures caused by vibration problems are fatigue failures, & the time required to fatigue failure is determined by both how far an object is deflected.(displacement) and the rate at which the object is deflected (frequency), of course, displacement is simply a measure of distance traveled and frequency is a measure of the number of times that trip is taken in a given period of time such as a minute or second, if it is known how far one must travel in a given period of time, it is a simple matter to calculate the speed or velocity required. Thus, a measure of vibration velocity is direct measure of fatigue in short Fatigue=displacement * frequency Velocity=displacement *frequency Thus: velocity=fatigue Vibration velocity is measurement of the speed at which a machine or machine component is moving as it undergoes oscillating motion. Vibration velocity is expressed in inches-per-second peak (in/sec-pk) for English units in metric units, vibration velocity is expressed in millimeters-per-second peak. 26

Vibration Acceleration VIBRATION ACCELERATION is another important characteristic of vibration that can be used to express the amplitude or magnitude of vibration. Technically, acceleration is simply the rate of change of velocity. The acceleration of the weight is maximum or at its peak value at the upper limit of travel where the velocity is zero (0). As the velocity of the weight increases, the rate of change of velocity or acceleration decreases. At the neutral position, the weight has reached its maximum or peak velocity and at this point, the acceleration is zero (0). After the weight passes through the neutral position, it must begin to slow down or "decelerate" as it approaches the lower limit of travel. At the lower limit of travel the rate of change of velocity (acceleration) is, again, at its peak value. Expressed in This can also be written as; in/sec/sec = in/sec2 Or mm/sec/sec = mm/sec2 4.4.3 Spike Energy When flaws or defects appear in a bearing, the resulting vibration will appear as a series of short duration spikes or pulses such .The duration or "period" of each pulse generated by an impact depends on the physical size of the flaw; the smaller the flaw, the shorter the pulse period will be. As the size of the defect increases, the period of the pulse becomes longer. A short-term (40 millisecond sec) time waveform that was taken on a ball bearing with a small nick purposefully ground on the bearing inner race way. It can be seen that the pulse period lasts only a few microseconds (1 microsecond = 1 millionth of a second). Of course, if the period of a vibration signals is-known, the frequency of the vibration can be found by simply taking the inverse of the period. For example, if it takes 1/3600 minute to complete one cycle of a vibration, then the vibration frequency is 3600 cycles per minute (CPM) or the inverse of the period. In the case of the pulses generated by the bearing defects, since the pulse periods are so short, the period inverses (frequencies) are typically very high. To illustrate, a 27 in/sec/sec-peak or mm/sec/sec-peak.

MICRO-FLAW is generally defined as a defect that is so small that it is essentially invisible to the naked eye. The pulses generated by a micro-flaw are typically less than 10 micro-seconds (i.e. 10 millionths of a second). By taking the inverse of a 10 microsecond pulse, the fundamental frequency becomes 100,000 Hz (TOOK Hz) or 6,000,000 CPM. As bearing deterioration progresses, the flaw gets larger. The next stage is a MACRO-FLAW or one that is detectable with the naked eye. Since the macro-flaw is larger, the duration or period of the pulse generated is longer and, thus, the fundamental pulse frequency is lower. Typically, a macro-flaw will generate a pulse with a period exceeding 20 microseconds, resulting in a fundamental pulse frequency of 50K Hz (3,000,000 CPM) or less. Of course, as the bearing defects continue to increase in size, the resultant pulse periods become even longer resulting in a decrease in fundamental pulse frequency. Experimentation has revealed that by the time the fundamental pulse frequency has reduced to approximately 5k Hz (300,000 CPM), bearing deterioration has generally reached severe levels. With the above facts in mind, the following outlines the basic features of the SPIKE ENERGY (abbreviated gSE) approach developed by 1RD Mechanalysis 1. Since the frequencies of bearing vibration are very high, utilize a vibration acceleration signal from an accelerometer transducer. Vibration acceleration tends to emphasize higher frequencies as shown by the comparison in Figure. 2. Incorporate a "band-pass" frequency filter that will electronically filter out frequencies above 50K Hz (3,000,000-GPM) -and below 5K Hz (300,000 CPM). By eliminating frequencies above 50K Hz/, micro-flaws, defects that are undetectable with the naked eye, will not affect the measurement. In other words, when the SPIKE ENERGY (gSE) measurements reveal a significant increase, a visual inspection of the bearing should provide confirmation with a visible flaw. For most predictive maintenance programs, detecting micro-flaws is of little concern since deterioration to the macro-flaw stage may take several months. The lower cut-off frequency of 5K Hz (300,000 CPM) filters out or ignores most other inherent sources of vibration including unbalance, misalignment, aerodynamic and hydraulic pulsations, electrical frequencies, etc., that tend to dominate or "hide" the vibration from bearing defects. 28

3. Since the spike-pulse signals generated by bearing defects have very low RMS values, incorporate a true peak-to-peak detecting circuit instead of an RMS detecting circuit. 4.4.4 Phase Phase, with regards to machinery vibration, is often defined as "the position of a vibrating part at a given instant with reference to a fixed point or another vibrating part. Another definition of phase is: "that part of a vibration cycle through which one part or object has moved relative to another part". The concept of "phase" is often the most confusing to newcomers to the field of vibration detection and analysis; however, from a practical standpoint, phase is simply a convenient mean of determining the "relative motion" of two or parts of a machine or vibrating system. The units of phase are degrees, where one complete cycle of vibration equals 360 degrees.

4.5.

INSTRUMENTS

FOR

VIBRATION

DETECTION

AND

ANALYSIS4.5.1 Introduction Instruments for measuring and analyzing machinery vibration are available in a wide array of features and capabilities, but are generally categorized as: 1. Vibration meters. 2. Vibration frequency analyzers 4.5.2 The Vibration Transducer Regardless of the vibration instrument being used, the "heart" of every instrument is the vibration transducer. This is the device that is held or attached to the machine to convert the machine's mechanical vibration into an electrical signal that can be processed by the associated instrument into measurable characteristics of vibration amplitude, frequency and phase. Many different varieties of vibration transducers have been used over the years. However, with few exceptions, the transducer provided as standard with nearly all present-day vibration meters, analyzers and data collectors is the vibration accelerometer. An accelerometer is a self-generating device that produces a voltage output proportional to vibration acceleration (G's). The amount of voltage generated per unit of 29

vibration acceleration (G) is called the sensitivity of the accelerometer and is normally expressed in milli volts-per-G (mv/G), where 1 milli volt equals one-thousandth of a volt (1 mv = 0.001 volt). Accelerometers are available with sensitivities ranging from less than 1 mv/G to 10,000 mv/G; however, most accelerometers for general purpose vibration detection and analysis applications will have sensitivities ranging from 10 to 100 mv/G.

Fig: 4.3 Basic construction of an accelerometer Theory of Operation Figure 4.1 shows a simplified diagram of typical accelerometer construction. The component of the accelerometer that generates the electrical signal is called a "piezoelectric" element. A piezoelectric material is a non-conducting crystal that generates an electrical charge when mechanically stressed or "squeezed". The greater the applied stress or force, the greater the generated electrical charge. Many natural and man-made crystals have piezoelectric properties. There are also a number of ceramic (polycrystalline) materials which can be given piezoelectric properties by the addition of certain impurities and by suitable processing. These are called "Ferro-electric" materials. Most commercially available accelerometers used today incorporate Ferro-electric materials because they can be fabricated in a variety of shapes and their piezoelectric properties can be controlled more easily than crystals to suit many applications. Referring to the diagram in Figure 4-1 the accelerometer consists of a mass (usually a stainless steel disk) compressed against a "stack" of piezoelectric disks. The size and number of piezoelectric disks used in an accelerometer determines not only its 30

sensitivity (mv/G), but its usable frequency range as well. When the accelerometer is held or attached lo a vibrating object, the piezoelectric elements will be subjected to resultant "inertia" forces of the mass. Thus, a forces proportional to the vibration acceleration is applied to the piezoelectric elements, resulting in an electrical charge signal proportional to vibration acceleration. The operation of an accelerometer used for measuring and analyzing machinery vibration is exactly the same as that of a ceramic cartridge used on phonographs and record players, where the vibration of a phonograph needle riding in the grooves of a record is converted to an equivalent electrical signal. The amount of electrical signal generated by the piezoelectric element is relatively small and many times must be transmitted by an interconnecting cable to the vibration instrument or analyzer which may be some distance away. For this reason, a common practice is to incorporate an electronic amplifier directly inside the accelerometer to amplify the signal so it can be transmitted through long cables without worrying about signal loss or interference Horn radio frequencies (RF interference) or high voltage electro-static interference or high voltage transformers, electrical fields around motors, etc. loss or interference. Where to Take the Readings Accelerometers built-in amplifiers can normally be used with interconnecting cables up to 1000 feet (330 meters) in length without appreciable signal

Fig: 4.4 Direction for placing the sensor Since vibratory forces generated by the rotating components of a machine are passed through the bearings, vibration readings for both detection and analysis should be 31

taken directly on the bearings whenever possible. FFT means: The term "FFT" stands for "Fast Fourier Transform". Nearly 200 years ago, French mathematician, Baron Jean Baptist Joseph Fourier established that any periodic function (which includes machinery vibration signals) can be represented mathematically as a series of sines and cosines. In other words, it is possible to take a vibration time waveform, whether simple or complex, and mathematically calculate the vibration frequencies present along with their amplitudes. The process is called a "Fourier Transform". Although a Fourier Transform can be done manually, the process is extremely time consuming. However, with the introduction of digital technology, the process can be carried out very fast. Hence the term: Fast Fourier Transform or FFT. Digital vibration analyzers and data collectors actually include a computer chip programmed to perform the FFT function.

Analog Signal The FFT process begins with an analog signal from a vibration transducer. Normally, the transducer will be a vibration accelerometer; however, signals from other types of transducers can be processed as well such as microphones, pressure transducers, current transformers, etc.

Input Since a vibration accelerometer is normally used for vibration detection and Analysis, it may be necessary to convert the acceleration signal to velocity by "single integration" or to displacement by "double integration". These functions are carried out at the input section. Calibration of the analog signal, based on transducer sensitivity, is also performed at the input.

32

4.5.3 DATA PAC 1500Instrument Details: ENTEK IRD (company) Data Pac 1500 Portable data collector \ analyzer. Data Pac 1500 is part of Entek IRD is complete range of monitoring products and services to all industry segments world wide .the data pac 1500 is a fully featured portable data collector \ analyzer designed in a small lightweight package that monitors the conduction of the equipment found in many process industries such as power generators petrochemical pulp and paper and primary metals. This easy-to use instrument features high frequency range and true zoom capabilities normally only found in high priced, bulky real-time analyzers .The data Pac 1500 collects field data, including vibration information and process variables with a frequency range of 10cpm -4518000cpm (.18hz)-75.3khz .it also includes true zoom capability ,screen capture and print utilities.

Fig: 4.5 DATA PAC 1500 The data Pac 1500 utilizes the latest advances in analog and digital electronics including digital signal processing (DSP) and industry highest resolution A\D converter to provide both speed and accuracy in the data collection process. The instrument incorporates a large VGA resolution screen for easy reading and comprehensive data presentation online context sensitive help is building to all applications so they are easy 33

to use and require minimum training .

The data Pac 1500 accepts industry standard

type 1 or type 2 PC memory cards to provide both unlimited and reliable data storage and is powered by long life, rechargeable, easily removable Ni-cad battery cells. Features: Inputs/outputs: Single data channel, constant current interface standard +/- 10volts engineering units (EU), providing for vibration inputs and process inputs (temperature prop is optional). Reference input channel, supports a variety of externally powered TTL compatible inputs. Including photocells, electromagnetic transducers or ENTEK IRD lace tach. Frequency: Frequency response: 10CPM to 4518000CPM (0.18Hz to 75.3 kHz) non integrated 21CPM to 4518000CPM (0.36Hz to 75.3 kHz) integrated Frequency ranges: 42 ranges between 600CPM and4518000CPM (10Hz and 75300Hz) Frequency resolution: upto12800 lines GSE corner frequencies: 100, 200, 500, 1000, 2000, 5000Hz Amplitude range /resolution: 18 bit A\D converter is incorporated for a solid 96db dynamic range Auto ranging capability sets full scale in 1,2and5 increments Last hardware range is stored for each measurement to improve measurement speed Supported measurement: Acceleration Velocity Displacement Spike energy Temperature Thrust or axial position DC voltage AC voltage 34

Adb, Vdb Phase (1x-99x) Speed Time synchronous FFTs Time synchronous wave form Amplitude vs. RPM (optional) Start up (coast down FFT waterfall plots (optional)), NY quist plots (Optional), speed Profiling plots (optional). Signal processing: A wide range of options are available and controllable by the host software including RMS, peak, peak to peak, and DC meter types Linear, exponential, RMS and peak hold averaging FFT processing (hamming, hanning, Kaiser-bessel flat top and rectangular window) 12.5 KHz real time data collection and processing rate Time domain data collection Automatic amplitude ranging

35

CHAPTER-V VIBRATION ANALYSIS

5.1 INTRODUCTIONThere are literally hundreds of specific mechanical and operational problems that can result in excessive machinery vibration. However, since each type of problem generates vibration in a unique way, a thorough study of the resultant vibration characteristics can go a long way in reducing the number of possibilitieshopefully to a single cause. A simple, logical and systematic approach that has been proven successful in pinpointing the vast majority of the most common day-to-day machinery problems.

5.2 DEFINE THE PROBLEMThe following lists some of the reasons for performing a vibration analysis: 1. Establish "baseline data" for future analysis needs. At the beginning of a

predictive maintenance program, even machines in good operating condition should be thoroughly analyzed to establish their normal vibration characteristics. Later, when problems do develop, this baseline information can be - extremely useful in performing a follow-up analysis to show precisely the vibration characteristics that have changed. 2. Identify the cause of excessive vibration. Referring to the vibration severity guidelines machines in service that have vibration levels in the "rough" regions or greater should be thoroughly. Analyzed to identify existing problems for immediate correction. Once corrections have been made, a follow-up analysis should be performed to insure that problems have been solved and the machine returned to satisfactory condition. If all significant problems have been solved, the follow-up analysis data will serve as the baseline data for future analysis as outlined in (1) above. 3. Identify the cause of a significant vibration increase. Once a developing problem has been detected by routine, periodic checks, the obvious next step is to perform a detailed vibration analysis to identify the problem for correction. Here also, a follow-up analysis will verify that the problems have been corrected and provide a baseline for 36

future comparison 4. Identify the cause of frequent component failures such as bearings, couplings, seals, shafts, etc. 5. Identify the cause of structural failures such as the structure or foundation, piping etc. 6. Identify the source of a noise problem.

5.3 DETERMINE MACHINE DETAILSSome of the important detailed features of the machine that need to be known for Accurate analyses include: 1. The rotating speed (RPM) of each machine component: Of course, direct-coupled machines have only one rotating speed (RPM) that needs to be known. However, machines that include gear drives will have more than one.' For single gear increasers or reducers, both the input and output speeds are needed. For multiple gear increasers or decreases, the rotating speeds of the various intermediate gears must be known along with the input and output speeds. 2. Types of bearings: Of course worn or defective sleeve or plain bearings will have different vibration characteristics than defective rolling-element bearings. Therefore, it is most important to know whether the machine has plain or rolling element bearings. If the machine has rolling-element bearings, it is also beneficial to know the number of rolling elements and other details of bearing geometry; with this information, the vibration analyst can actually calculate the frequencies of vibration caused by specific bearing defects such as flaws on the outer and inner raceways, rolling elements, etc. Details on determining specific bearing defect frequencies are presented in the ANALYSIS OF ROLLING ELEMENT BEARINGS section of this chapter. 3. Number of fan blades: Knowing the machine RPM and number of blades on a fan will enable the analyst to easily calculate the "blade-passing" frequency. This is simply the product of the number of fan blades times fan RPM. This frequency of vibration is also called the "aerodynamic pulsation frequency. 4. Number of impeller vanes: Similar to fans and blowers, knowing the number of vanes on a pump impeller allows the analyst to calculate the vane-passing frequency, also 37

called the "hydraulic-pulsation" frequency. 5. Number of gear teeth: The rotating speed and number of teeth on each gear must be known in order to determine the possible "gear-mesh" frequencies. 6. Type of coupling: Gear and other lubricated types of couplings can generate some unique vibration characteristics whenever their lubrication breaks down or if lubrication is inadequate. 7. Machine critical speeds: Some machines such as high speed, multi-stage centrifugal pumps, compressors and turbines are designed to operate at speeds above the natural or "resonant" frequency of the shaft. The resonant frequency of the shaft or rotor is called its "critical" speed, and operating a near this speed can result in extremely high vibration amplitudes. Therefore, knowing the rotor critical speed relative to machine RPM and other potential exciting force frequencies are very important. 8. Background vibration sources: Many times the vibration being measured on a machine is actually coming from another machine in the immediate area. This is particularly true for machines mounted on the same foundation or that are interconnected by piping or other structural means. Therefore, it is important to be aware of potential "background" contributions. This is especially true with machine tools, due to the low levels of vibration required! If possible, the machine under analysis should be shut down and readings taken to directly determine the amount and significance of background vibration.

5.4 VISUAL INSPECTIONBefore collecting data, the vibration analyst should first make a visual check of the machine to determine if there are any obvious faults or defects that could contribute to the machines condition. Some obvious things to look for include; 1. Loose or missing mounting bolts 2. Cracks in the base, foundation or structural welds 3. Leaking seals 4. Worn or broken parts 5. Wear, corrosion or build-up of deposits on rotating elements such as fans.

38

Slow Motion Studies Another test that is helpful in a visual inspection of the machine is slow motion observation of the various rotating elements of the machine with a high-intensity stroboscopic light. The strobe light must be one that has an adjustable flash rate. Simply adjust the strobe to flash at a rate which is slightly faster or slightly slower than the rotating speed (1 x RPM) of the machine. This will make the rotating components appear to rotate slowly. Slowing down the rotating motion of the machine makes it possible to visually detect problems that may be difficult to detect any other way. Visual run out of a shaft may pinpoint or verity a bent shaft condition. Eccentricity of "V" belt sheaves and pulleys can be easily detected in slow motion. Slow motion studies are especially useful in evaluating problems with belt drives. Worn grooves in pulleys or belts with variations in thickness can easily be seen by observing the action of the belt riding up and down in the pulley grooves. On multiple-belt drives, belt slippage can be determined by observing the belts in slow motion.

5.5 PROBING STUDIESThe tendency in vibration analysis is to concentrate on analyzing vibration data taken at the bearings of the machine. While this data is definitely an important part of any vibration analysis, in many cases the vibration that is occurring at the machine's bearings is actually the result of problems elsewhere in the "system. For example, in one case a vertical pump had a vibration of 0.7 in/sec measured at the top bearing of the pump motor. However, overall vibration readings taken on the pump base, foundation and piping revealed that the discharge piping was vibrating at a level of 3.0 in/sec or over four times higher than the pump motor itself. The problem turned out to be resonance of the discharge piping and not a problem with the pump itself. The pump and drive motor in this case were simply responding to the piping problem. The only way other problems in the system can be detected, such as the piping resonance described above, is to go looking for them. Depending on the anticipated vibration frequencies, select overall displacement, velocity or acceleration for measurement. Some of the areas that should be checked include: 39

1. Suction and discharge piping on pumps: Take overall measurements in three directions. On long piping runs, take readings at several locations along the piping. 2. Externally mounted components such as exciters, lube-oil pumps, surge bottles, etc. Here also, take overall measurements in three directions 3. Take overall measurements on nearby machines that may contribute background vibration. If a nearby machine has higher vibration amplitudes than the one being analyzed, it is very likely some of the vibration is coming from the background source. 4. Compare overall vibration readings across all mounting interfaces to detect obvious signs of looseness or weakness. 5. In addition to taking comparative overall readings across the mounting interfaces To detect Obvious looseness problems, the vibration, amplitudes taken vertically at The mounting points of a machine, such as the four feet of a motor, can be compared to reveal the possibility distortion or "soft-foot or distortion conditions caused by uneven mounting or foot will usually be indicated if one or more of the feet reveals a significantly higher amplitude than the other feet. If this is defected, the condition should be verified and Corrected before further analysis is carried out. Soft-foot conditions can be checked by placing a dial indicator directly on the foot and carefully loosening the mounting bolt while observing the indicator reading. Any movement or "spring" in excess of 0.002 - 0.003 inch is generally considered excessive and should be corrected.

5.6

OBTAIN HORIZONTAL, VERTICAL AND AXIAL SPECTRUMS (FFTS) AT EACH BEARING

OF THE MACHINE TRAIN

In many cases, the analysis steps carried out thus far may be sufficient to pinpoint the specific problem causing excessive vibration. If not, the next step is to obtain a complete set of amplitude-versus-frequency spectrums or FFTs at each bearing of the machine train. For a proper analysis, the machine should be operating under normal conditions of load, speed, temperature, etc. In order to insure that the analysis data taken includes all the problem-related vibration characteristics and, yet, is easy to evaluate and interpret, the following recommendations are offered;

40

Interpreting the Data Once horizontal, vertical and axial FFTs have been obtained for each bearing of the machine train, the obvious next question is: "What is this data telling me?" Essentially, amplitude-versus-frequency spectrums or FFTs serve two very important purposes in vibration analysis: 1. Identify the machine component (motor, pump, gear box, etc.) of the machine train that has the problem And 2. Reduce the number of possible problems from several hundred to only a limited few.

Identifying the Problem Component Based On Frequency Figure 5-1 shows a fan operating at 2200 RPM, belt driven by an 1800 RPM motor. The rotating speed of the belts is 500 RPM. Assume that a vibration analysis was performed on this machine and the only significant vibration detected had a frequency of 2200 CPM or 1 x RPM of the fan. Since the vibration frequency is exactly related to fan speed, this clearly indicates that the fan is the component with the problem. This simple fact eliminates the drive motor, belts and possible background sources as possible causes. Most problems generate vibration with frequencies that are exactly related to the rotating speed of trip in trouble. These frequencies may be exactly 1 x RPM or multiples (harmonics) of 1 x RPM such as 2x, 3x, 4x, etc. In addition, some problem's may cause vibration frequencies that are exact sub harmonics of 1 x RPM such as 1/2x, l/3x or 1/4 x RPM. In any event, the FFT analysis data can identify the machine component with the problem based on the direct relationship between the measured vibration frequency and the rotating speed of the various machine elements. Identifying the Problem Component Based On Amplitude Identifying the fan as the source of vibration based on vibration frequency was quite easy in the above example because of the notable differences in the rotating speeds of the various machine components. The obvious question, of course is: What about direct-coupled machines that is operating at exactly the same speed?" In this case, the component with the problem is normally identified as the one with the highest amplitude. For example, consider a motor direct coupled to a pump. Examining the analysis data, it is noted that the highest vibration amplitude on the motor is 1.0 in/sec compared to 0.12 41

in/sec on the pump. In this case, the motor is clearly the problem component since its vibration amplitude is nearly 8 times higher than that measured on the pump. In general, the machine component that has the problem is usually the one with the highest amplitude of vibration. The forces that cause vibration tend to dissipate in strength at increased distances from the source. However, there are exceptions to this rule such as the example given earlier where a vertical pump was vibrating excessively due to a resonance problem with the discharge piping. In this case, the exciting force was actually generated by the motor/pump but was being amplified by the resonant condition of the piping. Another exception to this rule involves misalignment of direct coupled machines. Sir Isaac Newtons third law of physics slates that "whenever one body exerts a force on another, the second always exerts on the first a force which is equal in magnitude but oppositely directed." In other words, "for every action, there is an equal but opposite reaction." In the case of coupling misalignment, the vibratory force (action) is generated at the coupling between the driver a driven components. As a result, the "reaction" forces on the driver and driven unit; will be essentially equal, resulting in reasonably comparable vibration amplitudes. The only reason one component may have a slightly higher or lower amplitude than the other is because of differences in the mass and stiffness characteristics of the two components. But, in most cases with the coupling misalignment, the vibration is fairly uniformly "shared" by the driver and driven units.

Fig 5.1 Different components generate different vibration frequencies Reducing the List of Possible Problems Based On Frequency In addition to identifying the problem machine component based on frequency 42

and/or amplitude characteristics, the second purpose of FFT analysis data is to limit or reduce the list of possible problems based on the measured vibration frequencies. As stated earlier, each mechanical and operational problem generates its own unique vibration frequency characteristics. Therefore, by knowing the vibration frequency, a list of the problems that cause or generate that particular frequency can be made, which greatly reduces the long list of possibilities. The chart lists the most common vibration frequencies is they relate to machine rotating speed (RPM), along with the common causes for each frequency. To illustrate how to use the chart, assume that the belt-driven fan pictured in Figure 4-1 has excessive vibration at 2200 CPM which is 1 x RPM of the fan. Of course, this clearly indicates that the fan is the component with the problem and not the drive motor or belts. In addition, since the vibration frequency is 1 x RPM of the fan, the possible causes listed on the chart are: 1. Unbalance 2. Eccentric pulley 3. Misalignmentthis could be misalignment of the fan bearings or misalignment of the fan and motor pulleys. 4. Bent shaft 5. Looseness 6. Distortionfrom soft foot or piping strain conditions 7. Bad beltsif belt RPM 8. Resonance 9. Reciprocating forces 10. Electrical problems Using this simple chart, along with the fact that the vibration frequency is 1 x RPM of the fan has reduced the number of possible causes from literally hundreds to only ten (10) likely causes, A little common sense can reduce this list even further. First, since the vibration frequency is not related to the rotating speed (RPM) of the drive belts, possible belt problems can be eliminated as a possible cause. Secondly, since this is a reciprocating machine such as a reciprocating compressor or engine, the possibility of reciprocating forces can be eliminated from the remaining list. Finally, since the 43

frequency is not related to the drive motor or AC line frequency. In any way, the possibility of electrical problems can be eliminated. Now, the number of possible causes of excessive vibration has been reduced to only seven (7) by simply knowing that the vibration frequency in this case is 1 x RF of the fan. Table 5.1: VIBRATION FREQUENCIES AND THE LIKELY CAUSES Frequency in Most Likely causes Other possible causes & Remarks Terms Of RPM 1x RPM Unbalance Eccentric journals, gears or pulleys Misalignment or bent shaft if high axial vibration 3) bad belts if RPM of belt 4} Resonance 5) Reciprocating forces 6) Electrical problems Mechanical looseness 1) Misalignment if high axial vibration 2) Reciprocating forces 3) Resonance 4) bad belt if 2 x RPM of belt Misalignment Usually a combination of misalignment and excessive axial clearance (looseness). Oil Whirl {Less than 1) Bad drive belts 1/2 x RPM 2) Background vibration 3) Sub-harmonic resonance 4) "Seat" Vibration Electrical Problems Common electrical problems include broken rotor bars, eccentric rotor, and unbalanced phases in poly-phase systems, unequal air gap. Torque Pulses Rare as a problem unless resonance is excited Gear teeth times RPM of bad gear Number of fan blade times RPM Number of impeller vane times RPM May occur at 2, 3, 4 and sometimes higher harmonics if severe looseness 1) Bearing vibration may be unsteady amplitude and frequency 2) Capitation, recirculation and flow turbulence causes random high frequency vibration 3)Improper lubrication of journal bearings 4)rubbing 44 1) 2)

2 x RPM

3 x RPM Less than 1x RPM Synchronous (A.C line frequency) 2xSynch. Frequency Many Times RPM (Harmonically Related Freq.)

Bad Gears Aerodynamic Forces Hydraulic forces Mechanical Looseness Reciprocating Forces High Frequency Bad Anti-Friction (Not Harmoni- bearing cally Related)

Comparing Tri-Axial (Horizontal, Vertical and Axial) Data Not only can specific vibration problems be recognized by their specific frequency characteristics, but in many cases by the direction in which the vibration occurs. This is why it is necessary to take analysis data in the horizontal, vertical and axial directions - to further the process of elimination. Table 5.1 shows a typical "set" of tri-axial data taken on one bearing of a belt driven fan operating at 2200 RPM. Of course similar data would be taken on the, other fan bearing as well as the motor bearings. "Stacking" the horizontal, vertical and axial data for a particular bearing on the same sheet as shown, greatly simplifies the comparison. Note That the same full-scale amplitude range (0 to 0.3 in/sec) was used for all the data to further simplify the comparison. There are basically two comparisons that need to be made from the data in Figure 5-2. First, how do the horizontal and vertical readings part; and secondly, how do the radial readings (horizontal and vertical) compare1 to the axial readings.

Fig 5.2: Typical tri-axial data taken on a belt-driven fan

45

Comparing Horizontal and Vertical Readings When comparing the horizontal and vertical data, it is important to take note of how and where the machine is mounted and also, how the bearings are mounted to the machine. Basically, the vibration analyst needs to develop a "feel" for the relative stiffness between the horizontal and vertical directions in order to see whether the comparative horizontal and vertical readings indicate a normal or abnormal situation. Machines mounted on a solid or rigid base may be evaluated differently than machines mounted on elevated structures or resilient vibration isolators such as rubber pads or springs. To explain the significance of machine stiffness, assume that the fan in Figure 5-1 is mounted on a rigid, solid concrete base which, in turn, is mounted on a solid foundation located at ground level. This would be regarded as a "rigid" installation and under normal conditions the vertical stiffness would be greater than the horizontal stiffness. If such is the case/one would expect that normal problems, such as unbalance, would cause higher amplitude of vibration in th2 horizontal direction than the vertical direction, if a rigidly mounted machine has higher vibration in the vertical direction than the horizontal direction, this would generally be considered as 'abnormal', and may indicate a looseness or weakness condition. On the other hand, if this same machine was mounted on springs or rubber pads, a higher amplitude in. the vertical direction may not be considered unusual or an indication of structural problems. Another factor that needs to be considered is the "ratio" between the horizontal and vertical Amplitudes. As explained, it is not unusual for rigidly mounted machines to have higher amplitudes of vibration in the horizontal direction, compared to the vertical direction. However, the ratio between the horizontal and vertical amplitudes should be checked to see if it is normal or indicative of some unusual problem. As a normal unbalance response, it is not unusual for machines to exhibit ratios between the horizontal and vertical amplitudes of 1:1, 2:1, 3:1 or 4:1, depending on the particular installation. In other words, it would not be unusual for a rigidly mounted fan, motor or pump to have a vibration amplitude at 1 x RPM as much as 4 times higher in the horizontal direction than the vertical direction due to unbalance. Ratios beyond 4:1 somewhat unusual and typically indicate an abnormal condition such as looseness or resonance. 46

Comparing Radial (Horizontal & Vertical) Data to Axial Data The second important comparison that needs to be made to tri-axial analysis data is how the radial (horizontal and vertical) readings compare to the axial readings. Relatively high amplitudes of axial vibration are normally the result of: 1. Misalignment of couplings 2. Misalignment of bearings 3. Misalignment of pulleys or sheaves on belt drives 4. Bent shafts 5. Unbalance of "overhung" rotors such as the fan in Figure 5.1 A general rule, any time the amplitude of axial vibration exceeds 50% of the highest radial (horizontal or vertical) amplitude, the possibility of a misalignment or bent shaft condition should be considered.) CM course, extremely high amplitudes of axial vibration may also be due to resonance or unbalance of an overhung rotor. Verifying the cause of a high axial vibration using "phase analysis" techniques will be covered in the sections to follow. Examining the axial vibration in the examples given in Figures, it can be seen that in neither instance is the amplitude of axial vibration greater than 50% of the highest radial amplitude. As a result, misalignment or bent shaft Conditions are not indicated examples. 'Where Do Multiple Harmonic Vibration Frequencies Come From? Something that often worries or confuses the beginning vibration analyst is the appearance of numerous "harmonic" frequencies that sometimes appear in their FFT analysis data. A good example is the frequency analysis data presented in Figure -5-3. Although the predominant vibration is clearly 2200 CPM (1 x RPM of the fan), vibration frequencies can also be seen at 4400 CPM (2 x RPM), 6600 CPM (3 x RPM) and 8800 CPM (4"x RPM). Although their amplitudes are considerably lower than that at 1 x RPM, these "harmonic" frequencies are very important and should not be ignored, as will be explained in the following Paragraphs. The presence of multiple or "harmonically" related vibration frequencies is not uncommon, and their presence in the FFT data can be easily explained by examining the frequency characteristics of various vibration waveforms. Figure 5.3 illustrates four (4) 47

different types of vibration waveforms a sinusoidal is a sine wave, a square wave, s triangular or "saw-tooth" wave and a spike pulse. These waveforms can be readily generated by various machinery problems, depending on the nature of the problem and the extent of the exciting forces. The '1 Linda -mental" frequency of each of the waveforms in Figure 5-3 is the same; however, the frequencies presented in the FFTs will be considerably different.

Fig 5.3: Different wave forms result in different frequency characteristics A sinusoidal or "sine" wave could be the result, of a simple unbalance or misalignment problem. If a frequency analysis (FFT) is performed on a true sinusoidal waveform, the result will be a single frequency of vibration with certain amplitude and NO multiple frequencies. By comparison, a frequency analysts (FFT) of & square waveform will not only display the fundamental frequency (1x), but the odd multiple or harmonic frequencies as well (i.e. 3x, 5x, 7x, etc.). The number of odd multiple frequencies present in the FFT data will depend on how close the waveform is to a true square wave, the intensity or amplitude of the vibration and the response characteristics (peak or RMS) of the instrument as well as its dynamic range. Figure 4-8 shows a 6000 CPM (1 00 Hz) square waveform signal obtained from an electronic signal generator along with the FFT 48

frequency analysis. Note that the frequency analysis not only includes the fundamental frequency of 6000 CPM, but the odd multiples as well (i.e. 18,000 CPM, 30,000 CPM, 42,000 CPM, etc.). One possible explanation (or a square wave vibration would be an unbalance condition combined with system looseness. If the unbalance force was great enough, the machine could literally be lifted off the foundation and held to the limit of looseness until the unbalance force has rotated to a position where the upward force is reduced, allowing the machine to drop. Another possibility is a mild rubbing condition that might "flatten" the unbalance sine wave whenever the rub occurs. The fundamental (1x) frequency accompanied by the odd multiple or harmonic frequencies, similar to a square wave. However, the amplitudes of the odd harmonics of a triangular waveform decrease more quickly at higher frequency than do those of a square waveform as shown in Figure 5-3. Triangular or saw tooth waveforms can also be generated by conditions such as looseness or excessive bearing clearance that result in "distortion" of an unbalance sine we Here also, the number of odd multiple frequencies that accompany the fundamental frequency will depend not only on the amplitude of the fundamental frequency, but the dynamic range and circuit response characteristics (peak RMS) of the analysis instrument. Some problems such as a cracked or broken tooth on a gear, or a flaw on a bearing raceway or rolling element, will generate vibration in the form of impact or spike-pulses. A frequency analysis or FFT of a spike-pulse signal will reveal the fundamental impact frequency, followed by the entire multiple or harmonic frequencies (i.e. 2x, 3x, 4x, 5x, 6x, etc.) as shown in Figure 4-3. As before, the number of harmonic frequencies evident in the FFT will depend on the amplitude of the fundamental component and the dynamic range and circuit response characteristics (peak or RMS) of the analysis instrument. The presence of multiple, harmonic frequencies in an FFT are definitely important and should not be ignored, even though their amplitudes may be considerably less than that of the fundamental frequency. Their mere existence indicates that the vibration is not a true sine wave, and may provide clues to other significant problems such as looseness conditions, gear tooth problems, bearing problems, etc. In the case of the belt driven fan in Figure 5-1, the harmonic frequencies only appeared at the drive-end bearing (bearing 49

C). Ultimately, the problem was found to be a loose pulley on the fan shaft, which was allowing the pulley to "rattle" on the shaft during rotation. This caused a spike-pulse distortion of the unbalance sine wave, resulting in the harmonic vibration frequencies. Once the set-screws were tightened securing the pulley to the shaft, the multiple harmonic frequencies totally disappeared, leaving only the 1 x RPM unbalance vibration frequency. Distortion of a sinusoidal vib