CONDITION MONITORING OF ROTARY EQUIPMENTS BY VIBRATION ANALYSIS
A Seminar Report
Submitted by RENJITH M
MECHANICAL ENGINEERING DIVISION SCHOOL OF ENGINEERING
COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY Cochin 682 022 SEPTEMBER 2006
MECHANICAL ENGINEERING DIVISION SCHOOL OF ENGINEERING COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY Cochin 682 022
CERTIFICATE Thi This
is
to
certi rtify
that the Semina inar
Rep Report ort
entitled “CONDITION
MON MONIT ITOR ORIN ING G OF ROTA ROTARY RY EQUI EQUIPM PMEN ENTS TS BY VIBR VIBRAT ATIO ION N ANAL ANALYS YSIS IS” ”
submitted by Mr. RENJITH M to the Mechanical Engineering Division towards the partial fulfillment of the requirements for the VII Semester of the B. Tech Degree course in Mechanic Mechanical al Engineer Engineering ing of Cochin Cochin Universi University ty of Science Science and Technolo Technology, gy, is a bonafide record of the seminar talk presented by him/her on
Seminar Coordinator
Seminar Supervisor
ACKNOWLEDGEMENT I express my deep gratitude to almighty, the supreme guide, for bestowing his blessings up on me in my entire endeavor. I would to like to express my sincere thanks to Dr. P.S Sreejith Head of Department of Mechanical engineering for all his assistance Gireesh Kumaran Kumaran I wish wish to expr expres esss my deep deep sens sensee of grat gratit itud udee to Lect Lectur urer er Mr . Gireesh Thampi. Departm Department ent of Mechani Mechanical cal Engine Engineerin ering g who who guided guided me throug through h out the
seminar. His overall direction and guidance has been responsible for the successful completion of the seminar. I would also like to thank Lecturer Mr. Ajith Kumar for his valuable suggestions. Finally, Finally, I would like to thank all the faculty members of the department of mechanical mechanical engineering and my friends for their constant support and encouragement.
ABSTRACT
The main function of condition monitoring is to provide the knowledge of machine condition and of its rate of change which is essential to the operation of this method. The knowledge may be obtained by selecting suitable parameters such as vibration for measuring and reading its value at intervals. Signals from vibration sensors are measured and then compared with reference measurements in order that they may be interpreted. This involves some analysis of the signals ranging from simple RMS amplitude measurement to vibration signature or spect spectra rall anal analys ysis is,, poss possib ibly ly incl includ udin ing g wave wavefo form rm plot plotss and and exten extendi ding ng in its its most most sophisticated form to data processing and a range of physico-mathematical concepts. Other techniques such as orbital analysis, time waveform and phase analysis have signifance as methods for study of particular dynamic characteristics.
CONTENTS 1. Intr Introd oduc ucti tio on 2. Cond Condit itio ion n Mon Monit itor orin ing g 3. Classi Classifica ficatio tion n of rotary rotary equipm equipment entss 4. Vibration 5. Tran ransducers 6. Vibr Vibrat atio ion n analy analysi siss 7. Data Data acqu acquis isit itio ion n 8. Data Data inte interp rpre reta tati tion on 9. Conclusion 10. Referen Reference ce
LIST OF TABLES
Table 1: Vibration Vibration Severity Severity Range Range Table 2: Vibration Vibration Range Range for Diesel Generating Generating set for Diesel Diesel Generators Generators Table 3: Vibration Trouble Trouble Shooting Chart
LIST OF FIGURES
Figu Figure re 1: Vibr Vibrati ation on Velo Veloci city ty Figu Figure re 2: 2: Vibr Vibrati ation on Acc Accele elera rati tion on Figure Figure 3: General General Machin Machinery ery Vibr Vibratio ation n Chart Chart Figure 4:
Vibration Acceleration general severity chart
Figure 5:
Moving Coil Type
Figure 6:
Direct Prod Transducer
Figu Figure re 7: Acce Accele lero rome mete terr Transd Transduc ucer er Figu Figure re 8: 8: Shaf Shaftt Ride Riderr Acce Access ssor ory y Figu Figure re 9: Tri-a Tri-axi xial al Read Readin ing g Figure Figure10: 10: Amplit Amplitude ude Vs Vs Freque Frequency ncy Figure Figure 11: Short term term Amplitude Amplitude Vs Time Time data Figure Figure 12: Long term term Amplitude Amplitude Vs Time Time data Figure 13: Vibration amplitude amplitude Vs frequency data recorder identifies unbalance unbalance Figure 14: Frequency analysis of vibration showing showing a bad bearing
1
1. INTRODUCTION Machin Machinery ery distre distress ss very very often often manifes manifests ts itself itself in vibrati vibration on or a change change in vibration pattern. Vibration analysis is therefore, a powerful diagnosis tool, and trouble shooting of major process machinery would be unthinkable without modern vibration analysis. It is natural for machines to vibrate. Even machines in the best of operation cond condit itio ion n will will have have some some vibr vibrat atio ion n beca becaus usee of mino minorr defe defect ctss as a resu result lt of manufacturing tolerances. Therefore each machine will have a level of vibration which may be regarded as normal or inherent. When machinery vibration increases or becomes excessive, some mechanical trouble is usually the reason. Machinery vibration levels just do not increases or become excessive for no reason at all. Something causes it unbalance, looseness etc. Each mechanical defect generates vibration in its own unique way. This makes it possible to positively identify a mechanical problem by simply measuring and studying its vibration characteristics. The success of a process industry often depends on the continued, safe and productive operation of rotating machinery. An effective maintenance programme is vital to this kind of success. The quality of the company’s maintenance programme determines how long the machines will run, how safe they are for the people working around them. The benefits of a good maintenance programme are: 1.
Prolonged machinery life.
2.
Minimizes un unscheduled do down ti time.
3.
Eliminates unn unnecessary ov overhaul.
4.
Eliminates standby equipment.
5.
Prov rovides more effic ficient operati ations.
6.
Increases ma machinery sa safety.
7.
Improves quality performance.
8.
Improves cu customer sa satisfaction.
2
2. CONDITION MONITORING The main functi function on of condit condition ion monito monitorin ring g is to provid providee the knowle knowledge dge of machine condition and of its rate of change which is essential to the operation of this method. The knowledge may be obtained by selecting suitable parameters such as vibration for measuring and reading its value at intervals. With condition monitoring, repair repairss are carried carried out only only when when the conditio condition n of machin machinee has deterior deteriorate ated d to a predetermined level. Thus repairs or replacement of parts take place only when it has definitely been proved that a fault exists and if it left unrepaired would result in unsatisfactory operation or breakdown with possible damage to other machine parts and disruption of production.
2.1 Condition Monitoring of Rotary Equipments:
1. Continuous monitoring of vibration and bearing temperature of critical machines. 2. Vibration and noise measurement and analysis on all rotary equipments. 3. Bearing temperature monitoring by surface thermometers. 4. Condition monitoring of anti-friction bearings using shock pulse meters. 5. Measurement of RPM by stroboscope/tachometer. 6. Measurement of shaft residual magnetism. 7. Detection of cavitations in pumps by SPM.
3
3. CLASSIFICATIONS OF ROTARY EQUIPMENTS
Rotary equipment will be classified into three categories depending upon critically for on-stream condition monitoring as described below. A) Category Category-I(C -I(Criti ritical cal Machine Machines) s)
These are vital machines which will be generally of high cost, high speed, too large and complex in their design and duties and does not justify the economics of having another spare set and breakdown of which result in immediate and serious interruption in production. Thes Thesee equi equipm pmen ents ts will will have have cont contin inuo uous us on-l on-lin inee vibr vibrat atio ion n and and bear bearin ing g temperature monitoring systems. These machines will be monitored for vibration on bearing housings once a week using portable monitoring instruments. B) Category-II Category-II (Semi-critical (Semi-critical Machines) Machines)
These are essential machines which will be needed for normal operation of the plant, but having stand by set, and also the running speed of which will not be very high. Failure of such equipments will not cause immediate production loss as the stand by set will come in line in case of failure. These machines will be monitored for vibration on bearing housing once in two weeks. However, some of the important machines may be monitored once in a week depending upon the requirement and equipment behavior. C) Category-III Category-III (Non-critical (Non-critical Machines) Machines)
These are desirable auxiliary and general purpose machines which, owing to its functi function, on, can be allowe allowed d to remain remain tempor temporari arily ly out of operat operation ion withou withoutt having having a serious effect on operations. These equipments will be normally having spare sets. These machines will be monitored for vibration housings once in a month.
4
4. VIBRATION Vibration Vibration is simply simply the back and forth movement of an object object from its position of rest. It is like an oscillatory oscillatory motion. motion. Vibrations Vibrations in machines machines above certain limits are harmful to their functioning. The most common causes of vibration are: Unbalance of motor. Looseness Misalignment Bend shaft Eccentrical Bad belt drive and drive chains Electromagnetic forces Hydraulic forces 4.1) Vibration Characteristics Characteristics
Machines condition and mechanical problems are identified by simply noting its vibration characteristics are: 1) Amplitude (Displacement, Velocity, Acceleration) 2) Frequency 3) Phase
4.1.1) Vibration Displacement (peak to peak)
The total distance traveled by the vibrating part from one extreme limit of travel to the other extreme limit of travel is referred to peak to peak displacement. The vibration displacement is usually expressed in micrometer where one micrometer equals one thousandth of a millimeter (0.001mm). 4.1.2) Vibration Velocity
The velocity of the motion is definitely a characteristic of the vibration but since it is constantly constantly changing changing throughout throughout the cycle, the highest peak velocity is selected selected for measurement. Vibration velocity is expressed in millimeter per second peak.
5
Fig 4.1 Vibration velocity 4.1.3) Vibration Acceleration Acceleration
Vibr Vibrat atio ion n accel acceler erati ation on is anot anothe herr impo import rtan antt char charac acter teris isti ticc of vibr vibrat atio ion. n. Technically acceleration is the rate of change of velocity. It is normally expressed in “g” “s” peak, where one “g” is the acceleration produced by the force of gravity at the surface of the earth
Fig 4.2 Vibration Acceleration 4.1.4) Vibration Frequency
The amount of time required to complete one cycle of a vibration pattern is called the period of vibration. Vibration frequency is the measure of complete cycles that occur in a specified period of time. Frequency =
1/period
The frequency of vibration is usually expressed as the number of cycles that occur in each minute or CPM (cycles per minute) or number of cycles per second or Hertz (Hz). 4.1.5) Vibration Phase
Phase is defined as the position of a vibrating part at a given instance with referen reference ce to a fixed fixed point point or anothe anotherr vibrati vibrating ng part. part. Phase Phase measur measureme ements nts offer offer a
6 convenient way to compare one vibration motion with another or to determine how one part is vibrating relative to another part.
Vibration Severity
There are no realistic figures for selecting a vibration limit which, if exceeded will result in immediate machinery failure. The events surrounding the development of a mechanical failure are too complex to set any reliable limits. On the other hand we must have some general indications of machinery condition that can be evaluated on the basis of vibration amplitude.
Fig. 4.3 General machinery vibration severity chart On the fig 4.3 the horizontal axis is scaled in terms of vibration frequency and the the vert vertica icall axis axis in terms terms of disp displa lacem cement ent.. The The area area betw betwee een n the the diag diagon onal al line liness represents levels of vibration severity from extremely smooth to very rough.
7 The fig 4.4 is a severity chart which works much the same way but uses velocity and acceleration parameters and covers a higher CPM range
Fig.4.4 Vibration acceleration general severity chart
8
5. TRANSDUCERS Transducer is a sending device which converts one form of energy into another. The Vibration Transducer (Pick-Up) converts mechanical vibration into an electrical signal. There are mainly three types of Vibration Transducers. 1) Velo Veloci city ty Tran Transd sduc ucers ers.. 2) Acceler Accelerome ometer ter Transd Transduce ucers. rs. 3) Prox Proxim imit ity y Tran Transd sduc ucer ers. s. Velo Velocit city y Tran Transd sduc ucer er and and Acce Accele lero romet meter er Tran Transd sduc ucer er are are call called ed Seis Seismi micc Transducers. Proximity Transducer is called Non-contact Transducer. 5.1) Velocity Transducers Transducers
Veloci Velocity ty transdu transducer cerss respon respond d direct directly ly to vibrati vibration on veloci velocity. ty. Most Most vibrat vibration ion measurement instruments have provision for processing the electrical signal from a velocity pick up to show vibration displacement as well. In theory it is also possible to convert signals from velocity pickups to units of acceleration, however, this is not done in practice, because the results have been found to be unreliable. 5.1.1) Moving coil type
Fig 5.1 Moving coil type The fig 5.1 is a simplified diagram of a seismic velocity vibration transducer. The system consists of a coil of fine wire supported by soft spring. A permanent magnet, firmly attached to the case of the transducer, provides a strong magnetic field around the coil. Whenever this transducer is fixed or held tightly against a vibrating object, this permanent magnet vibrates while the spring suspended coil of wire remains stationary in space. When the coil of wire cuts magnet lines of force, a voltage is
9 generated in that wire. The voltage is proportional to the velocity of motion, the strength of the magnetic field, and number of turns of wire in the coil. The voltage generated is transmitted by cable to a vibration meter, monitor or analyzer. 5.1.2) Direct Prod Transducer
Many times it is necessary necessary to measure measure the vibration vibration of a small light weight weight part or structure. However, holding or attaching the standard velocity pickup to a small part can actually reduce the vibration. We can solve this problem by using a direct prod pickup such as the one shown in figure.
Fig 5.2 Direct prod transducer The principle of operation of a direct prod pickup is identical to that of a seismic velocity pickup. With the direct prod pickup, a thin prod extends through the end cap of the pickup and is attached directly to the movable coil inside. To measure vibration with a direct prod pickup, we should fasten the main body of the unit to a rigid structure to serve as a point of reference. The tip of the prod is then attached to the vibrating part, using a threaded tip or a special magnetic tip. We should hold the direct prod unit by hand movements that naturally result; we should use an analyzer whose filter is tuned to the vibrat vibration ion frequen frequency cy of interes interest. t. The low frequen frequency cy vibrati vibration on dye to the hand hand movements are thus eliminated from the measurements. One of the advantages advantages of the advantages advantages of this type pickup is that it adds only the weight of the weight of the prod and moving coil to the vibrating part. This makes the pickup especially useful on small, light weight objects where the mass of a standard seismic velocity pickup can affect the actual vibration. It is often selected for use on balancing machines where parts may be balanced at speeds as low as 50 RPM with excellent results.
10 5.1.3) Piezoelectric Piezoelectric velocity transducer
These transducers transducers have an output output that is proportional proportional to velocity, velocity, but have no internal moving parts. Stresses due to vibrational forces applied to the pickup cause a crystal or special ceramic material to produce an electric charge. These are designed specifically for low frequency applications. It can measure down to 60 CPM. 5.2) Accelerometer Transducer
An accelero acceleromet meter er is a self self generat generating ing devise devise with with a voltag voltagee charge charge output output proportional to vibration acceleration. Vibration acceleration is the measure of the rate of change of velocity and is normally expressed in terms of “g’s”. Acceleration is a function displacement and frequency. As a result Accelerometer is extremely sensitive to vibration occurring at high frequencies. 5.2.1) Piezo-electric Piezo-electric with built in amplifier
Fig 5.3 Accelerometer Transducer The figure shows a simplified simplified diagram of piezo-electric piezo-electric with built in amplifier. When this pickup is fixed or held against a piece of vibrating machinery, the mechanical vibrations are passed through the frame to a piezo-electric material. This material has the ability to generate an electrical electrical charge in response response to a mechanical force applied to it. In this this instan instance ce mechan mechanical ical vibrat vibration ion produc producers ers the force force and the piezo-e piezo-elec lectric tric material responds by generating an electrical charge that is proportional to the amount of vibration acceleration. 5.3) Non-contact (Proximity) Transducers
Many high speed machines consist of relatively light weight motors mounted in massiv massivee cases cases and rigid rigid bearin bearings. gs. Because Because of weight weight and stiffn stiffness ess of the massiv massivee machine case and bearings, externally mounted vibration and acceleration pickups often
11 show little outward evidence of motor or shaft vibration. It is necessary to measure the actual shaft vibration in order to know when bearing clearances are in danger. It is displacement transducer measuring the shaft displacement relative to it fixing object. 5.4) Shaft Rider Accessory
A shaft stick is usable for periodic vibration checks and some analysis and in place balancing operations. When it is necessary to monitor shaft vibration for extended periods of time, it is recommended that we use a shaft rider. The figure figure shows shows that that a shaft shaft rider is perman permanent ently ly instal installed led in the bearing bearing housing. It consists of a spring loaded probe that is held firmly against the rotating shaft so that is held firmly against the rotating shaft so that it accurately follows shaft motion.
Fig 5.4 Shaft rider accessory
12
6. VIBRATION ANALYSIS Vibration Analysis is a two step process involving the ACQUISITION and INTERP INTERPRET RETATI ATION ON of machin machinery ery vibrati vibration on data. data. Its purpos purposee is to determ determine ine the mechanical condition of a machine and specific mechanical or operational defects. The The Data Data Acqu Acquis isit itio ion n proc proced edur uree is a mean meanss of syst system emat atic ic meas measur urin ing g and and recording of the vibration characteristics needed to analyze a problem. The Data Interpretation involves comparing the recorded data with the details of the machine, like its speed or speeds, its foundations, the construction details etc. then the the char charact acter eris istic tic of vibr vibrati ation on typi typica call of vari variou ouss defec defects ts are are comp compar ared ed with with the the characteristics that have been measured. By this, one can pinpoint the trouble and take corrective measures.
13
7. DATA ACQUISITION Data acquisition is the essential first step in vibration analysis, since the right data must be acquired under the right conditions to completely interpret a machine’s condition. Data Data acqu acquis isit itio ion n can can be done done in seve several ral ways ways depe depend ndin ing g on the the avai availa labl blee instruments. Apart from data acquisition, additional data acquisition procedure such as semi-automatic, automatic and real time analysis are employed where the job can be quicker and more accurate. In the semi-automatic method, the operator manually adjusts the filter through the frequency ranges, while the data is automatically recorded in a recorder. These types of plots are records of vibration amplitudes in the ‘Y’ axis and the frequencies in the ‘X’ axis. Such a plot is called Machinery Vibration Profile (Signature) and the analysis of the same is called as Signature Analysis. Auto Automa mati ticc data data acqui acquisi siti tion on is the the term term used used to desc describ ribee the the proc proced edur uree of obtaining the data, where the instrument automatically plots the vibration profiles. This type of instrument incorporates and electronically swept filter as well as provisions for simultaneous plotting of data with the recorders. 7.1) Selection of Measurement Parameters
The various measurement parameters are displacement, velocity, acceleration:7.1.1) Displacement
Displacement can be measured with both velocity and acceleration pickups. This is accomplished by means of integrator circuits that are normally included in the circuit of vibr vibrat atio ion n mete meters rs and and anal analyz yzer ers. s. Pick Pickup upss that that resp respon ond d dire direct ctly ly to vibr vibrat atio ion n displacement are readily available, but are usually used in the non-contact pickups. 7.1.2) Velocity
Velocity can also be measured with both velocity and acceleration pickups. Seismic and piezoelectric velocity pickups obtain vibration directly. The output from an accelerometer can be integrated to produce the equivalent of a velocity measurement, down to about 3Hz, or 180 CPM.
14 7.1.3) Acceleration
Acceleration should be measured only with an accelerometer. It is theoretically possi possible ble to differ different entiate iate signal signalss from from a veloci velocity ty transdu transducer cer to produc producee acceler acceleratio ation n readings, but this would be needlessly complicated and expensive. 7.2) Common Types of Measurements
The common types of measurements are:1) Overall Overall vibrat vibration ion amplit amplitude ude measure measuremen ments. ts. 2) Amplit Amplitude ude Vs Frequ Frequenc ency y measurem measurement ents. s. 3) Amplit Amplitude ude Vs Vs Time Time measu measurem rement ents. s. 4) Phas Phasee mea measu surem remen ents ts.. They are described below:7.2.1) Overall vibration amplitude measurements.
Overall vibration amplitude measurements provide a quick check of general mach achiner inery y con conditio ition n. A vibra ibrati tion on mete meterr or anal analyz yzer er can can be used sed for for thes thesee measurements. This measurement is generally manually recorded in tabular form, or the data automatically stored in memory for computer based automated instruments. 7.2.2) Amplitude Vs Frequency measurements. measurements.
Amplitude Vs Frequency measurements provide frequency spectrum which is used to pinpoint the problem to a specific frequency or range of frequencies. Full capacity or advanced check analyzers are required to take these measurements. Data can be recorded manually in tabular form, or by semi automatic or automatic swept filter analysis with tabular or graphic hard copy recording of the data. FFT type analyzer can also provide tabular/graphic hard copy of visual display of the data. It is estimated estimated that over 85% of the mechanical mechanical problems problems occurring occurring on rotating machinery can be identified by displaying the vibration Amplitude Vs Frequency data. Importance of tri-axial readings
It is common practice to record the Amplitude Vs Frequency data measured in the horizontal, vertical and axial pickup directions at each bearing of the machines being being analyz analyzed. ed. Obtain Obtaining ing measur measureme ements nts in all the three three directi directions ons is extrem extremely ely important important for distinguis distinguishing hing between between various various mechanical mechanical problems. problems. eg. Unbalance, Unbalance, Misali Misalignm gnment ent,, bend bend shaft shaft struct structura urall weakne weakness ss (loose (loose parts) parts) will will genera generally lly cause cause vibr vibrat atio ion n at a freq freque uenc ncy y 1X RPM. RPM. Unba Unbala lanc ncee will will almo almost st alway alwayss prod produc ucee high high amplitudes in the horizontal direction while lower amplitudes in the axial direction. Misalignment of couplings and bearings or a bend shaft will generally show relatively
15 high high ampl amplit itud udee of vibr vibrat atio ion n in the the axial axial dire directi ction on.. Ampl Amplit itud udes es due due to stru struct ctur ural al weakness, loose parts are shown in Vertical direction.
Fig 7.1 Vibration are normally taken in horizontal, vertical and axial directions on a machine bearing
16
Fig 7.2 Amplitude Vs Frequency 7.2.3) Amplitude Vs Time measurements. measurements.
Time measurements can be made during machine operation to detect vibrations that would not be apparent from Amplitude Vs Frequency analysis. Amplitude Vs Time measurements can be made for very fast transient vibrations or for slowly occurring vibrations. For fast transient vibrations use an oscilloscope with the horizontal axis scaled in milliseconds. For slowly varying vibrations use a recorder with the horizontal axis scaled in seconds. It can be taken with a DC recorder connected to an analyzer with that built-in-capability.
17
Fig 7.3 Short term Amplitude Vs Time data.
Fig 7.4 Long term Amplitude Vs Time data
7.2.4) Phase measurement
Phase Phase measur measureme ements nts are import important ant when when analyz analyzing ing mechan mechanical ical proble problems ms in machinery. machinery. Phase is defined as the position position of a vibrating part at a given instance with referen reference ce to a fixed fixed point point or anothe anotherr vibrati vibrating ng part. part. Phase Phase measur measureme ements nts offer offer a convenient way to determine how one part is vibrating relative to another part. To obtain obtain phase phase measur measureme ements nts,, an analyz analyzer er with with a strobe strobe light or remote remote reference pickup is required. The use of strobe light necessities visual observation of the rotating rotating shaft and the capability capability to fire the strobe light with vibration signal in order to
18 obtain phase. The remote phase pickup, which is usually an electromagnetic pickup, non-contact transducer or photocell must be installed so that to observe mechanical protrusion (depression) or a reflective mark on the shaft. The strobe light measurement involves observing the angular position of the reference mark that appears under the strobe light, while the remote reference pickup provides phase readout (digital or analog) using a meter on the analyzer.
9. DATA DATA INTERP INTERPRET RETATI ATION ON
Once Once the necess necessary ary inform informati ation on have have been been collec collected ted by manual manual,, or semisemiautomatic or automatic, the next step is to review and compare the reading with the characteristics of vibration typical of various types of troubles. A key to this comparison is the frequency. If a machine part has some defect, the frequency of vibration resulting from this defect will some multiple of the RPM. The multiple is different for different
19 defects. Also there are some defects which will produce vibration frequencies that are not related with the RPM. 7.1) Causes Of Vibration
The major causes of vibration on Rotary machines are:1) Unbalance
The horizontal, vertical and axial vibration signatures presented in the figure given below illustrate typical Amplitude Vs Frequency analysis data resulting from an unbalance condition. It can be noted that, the predominant vibration occurs at 2200 CPM corresponding to the 2200 RPM fan speed. Since the amplitude of vibration in the axial direction is relatively low compared to the radial amplitudes, a bent shaft or misalignment is not indicated. The appearance of small amplitudes at the harmonic frequencies is common and does not necessarily indicate any unusual problems such as mechanical looseness.
Fig 7.1 Vibration amplitude Vs frequency data recorder identifies unbalance 2) Mechanical looseness
The vibration may be the result of loose mounting bolts, excessive bearing clearance, a crack or break in the structure or bearing pedestal, a rotor which is loose on the shaft, shaft, or some some other other loose loose machin machinee compo componen nent. t. The vibrati vibration on charact characteris eristic tic of mechanical looseness will not occur unless there is some other exciting force such as unbalance or misalignment can result in large amplitudes of looseness vibration. The
20 vibration due to looseness can be detected from Amplitude Vs Frequency when taking the reading in vertical direction. 3) Misalignment
Misali Misalignm gnment ent is an extrem extremely ely common common proble problem. m. Misali Misalignm gnment ent,, even even with with flexib flexible le coupli couplings ngs,, result resultss in two forces forces,, axial axial and radial radial vibrat vibration ion.. The signif significan icantt characteristic of misalignment and bent shafts is that vibration will be noted in both the radi radial al and and axia axiall dire direct ctio ions ns.. As a resu result lt,, a comp compar arati ative ve axia axiall vibr vibrat atio ion n is the the best best indication of misalignment or a bent shaft. 4) Defective antifriction bearing
Flaws on the raceways, balls or rollers of rolling element bearings cause highfrequency vibrations and the frequency is not the multiple of the shaft RPM. The amplitude of vibration depends on the extent of the bearing fault. The natural frequency vibrations typically occur as vibration peaks in the 10,000 to 100,000CPM. Defects in the bearin bearing g compon component entss can generat generatee vibrati vibration on peaks peaks at frequen frequencie ciess related related to the bearing geometry. The vibration generated by the bearing is not normally transmitted to other points of the machine.
Fig 7.2 Frequency analysis of vibration showing a bad bearing The other reasons for the vibration are:1) Defective sleeve bearing. 2) Defective gears. 3) Eccentricity. 4) Oil whirl. 5) Bad drive belts or chain.
21 6) Electrical defects. 7) Rubbing. 8) Bend shaft. 9) Cavitation. 10) Flow turbulence.
7.2) Recommended Method of Vibration Classification (As Per ISO 2372)
The machines are classified into five groups as per ISO 2372. They are: Class I
Individual parts of engines and machines integrally connected with the complete machine in its normal operating condition (Production electrical motors of up to 15KW are typical examples of machines in this category). Class II
Medium Medium sized sized machin machines es (typic (typicall ally y electri electrical cal motor motorss of 15-75 15-75KW KW output output)) without special foundations rigidly mounted engines or machines (up to 300KW) on special foundations. Class III
Large prime movers and other large machines with rotating mass mounted on rigid rigid and heavy heavy founda foundatio tions ns which which are relativ relatively ely stiff stiff in the directi direction on of vibrati vibration on measurement. Class IV
Large prime movers and other large machines with rotating masses mounted on foun founda dati tion onss whic which h are are rela relati tivel vely y soft soft in the the dire direct ctio ion n of vibr vibrati ation on meas measur urem emen entt (eg.Diesel-generator sets, especially those with light weight substructures).
Class V
Machines and mechanical drive systems with unbalancable inertia effects (due to reciprocating parts) mounted on foundations which are relatively stiff in the direction of vibration measurement. Class VI
22 Machines and mechanical drive systems with unbalanceable inertia effects (due to reciprocating parts) mounted on foundations which are relatively stiff in the direction of vibration measurement. Machines with rotating slack-coupled masses such as beater shafts in grinding mills, machines like centrifugal machines with varying unbalances capable capable of operating operating as self contained units without without connecting connecting components, components, vibrating vibrating screens, dynamic fatigue-testing machines and vibration exciters used in process plants.
In practice, instead of good/Allowable/Just permissible, the following colloquial is used to stipulate the health condition of machines. Good Satisfactory Just satisfactory Unsatisfactory Dangerous
VIBRATION SEVERITY RANGE (In accordance with ISO 2372) Velocity –mm/sec Class A-Good B-Usable C-Still ac acceptable
D-Un acceptable
23 0-15KW
Class I
0.71
1.8
4.5
45
15-75KW
Class II
1.12
2.8
7.1
45
>75KW
Class III
1 .8
4.5
11.2
45
Turbocharge r
Class IV
2 .8
7.1
18
45
Table 7.1 Vibration Severity Range
DIESEL GENERATING SET FOR DIESEL GENERATORS (WARTSILA,ANMAR,FUJI,DAIH (WARTSILA,ANMAR,FUJI,DAIHATSHU,MERLESS,BL ATSHU,MERLESS,BLACKSTONE,SKL,SK ACKSTONE,SKL,SK ODA,CATTERPILLER ETC) Limit parameter is peak-velocity in mm/sec. Sl. No LOCATION S.LIMIT J.S.LIMIT MACHINE BASE FIXING 1. 10 20 LOCATION CRANK CASE CENTRE 2. 10 20 LINE CRANK CENTRE LINE 3. 10 20
CYLINDER HEADS
4.
TURBOCHARGER
5. 6. 7.
PILLOW BLOCK BEARING ALTERNATOR BEARING GENERATOR BEARING
8.
10
20
15
30
15
25
05
15
05
15
Table 7.2 Vibr Vi brat atio ion n Rang Ra ngee for fo r Dies Di esel el Gener en erat atin ing g Set For Diese ie sell Gene Ge nera rato tors rs
VIBRATION TROUBLE SHOOTING CHART
Frequency of Nature of fault
domain vibration (RPM)
Direction
Remarks
24 Rotating members out of balance Misalignment &
1X RPM
Radial
Usually 1X RPM.
Radial &
Bent shaft Damaged Rolling
Often 2X RPM. Impact rates for the
Axial
Element Bearings
individual bearing
(Ball, Roller etc)
components. Sub harmonics of
Journal bearing loose in housing Oil film whirl or whip in journal bearing
shaft rpm,exactly ½
A common cause of excess vibration in machinery A common fault
Radial &
Uneven vibration level, often
Axial
with shocks, impact rates Looseness may only develop at
Radial
operating speed and high
or 1/3 rpm
temperature(eg.turbomachines)
Slightly less than
Applicable to high speed
half speed
Radial
machines. Vibrations exited when passing
Hysteresis wh whirl
Shaft cr critical sp speed
Radial
through critical speed are maintained at higher shaft speeds.
Tooth meshing Damaged or worn gear
frequency (shaft rpm X number of teeth) and
Side bands around tooth Radial &
meshing frequencies detectable
Harmonics
with very narrow band analysis and spectrum
harmonics Mechanical looseness
2X RPM
Radial &
Also sub harmonics for loose
Axial
journal bearings. The precise problem can
Faulty belt drive
1,2,3,4 X RPM
Radial
usually be identified with the help of a strobe light.
Unbalanced reciprocating
1X RPM
Radial & Axial
Easily felt by hand touch
Increased
Blade passing
turbulence or
frequencies and
recirculation
harmonics
Cavitations
1X RPM
Radial
Electrical induced
1X RPM or 1 or 2
Radial &
Disappear immediately when
vibrations
times synchronous
Axial
turn-off the power
Radial &
Increased level indicate
Axial
increasing turbulences No phase difference with strobe light
25 frequency
Table 7.2 Vibration Trouble Shooting Chart
8 CONCLUSION Cond Condit itio ion n moni monito torin ring g by vibr vibrat atio ion n analy analysi siss give givess info inform rmat atio ion n abou aboutt the the changing condition of the equipment thus enabling the avoidance of total breakdown and can have reduced time. Prediction of residual life enables the equipment to be stop stoppe ped d befo before re they they reach reach crit critica icall cond condit itio ion n and and thus thus safe safe oper operati ation on is ensu ensure red. d. Improved production quality is achieved through condition monitoring. By condition
26 monitoring, detection of incipient faults and determination of plant condition enable forecasting of maintenance demands.
9 REFERENCE
1. S.S. S.S.R Ratta attan n, Theory of Machines , Tata McGraw-Hill, New Delhi, 2004. 2. Vibrotech ech, Training Manual , Chennai.
27 3. IRD Mechanalysis, Instruction Manual .
