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Vibration Monitoring

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  • 1. Machinery Fault Diagnosis A basic guide to understanding vibration analysis for machinery diagnosis. 1
  • 2. Unbalance Vibration Monitoring ω m Unbalance is the condition when the geometric centerline of a rotation axis doesn’t coincide with the mass centerline. Funbalance = m d ω2 1X Radial MP MP A pure unbalance will generate a signal at the rotation speed and predominantly in the radial direction. 3
  • 3. Static Unbalance Vibration Monitoring Static unbalance is caused by an unbalance mass out of the gravity centerline. S m U The static unbalance is seen when the machine is not in operation, the rotor will turn so the unbalance mass is at the lowest position. The static unbalance produces a vibration signal at 1X, radial predominant, and in phase signals at both ends of the rotor. 4
  • 4. Pure Couple Unbalance Vibration Monitoring U = -U -m Pure couple unbalance is caused by two identical S unbalance masses located at 180° in the transverse area of the shaft. Pure couple unbalance may be statically balanced. m When rotating pure couple unbalance produces a vibration signal at 1X, radial predominant and in opposite phase signals in both ends of the shaft. U b 5
  • 5. Dynamic Unbalance Vibration Monitoring U Dynamic unbalance is static and couple unbalance at the -m same time. S m U In practice, dynamic unbalance is the most common form of unbalance found. When rotating the dynamic unbalance produces a vibration signal at 1X, radial predominant and the phase will depend on the mass distribution along the axis. 6
  • 6. Documentation of balancing Vibration Monitoring Frequency spectra before/after balancing Balancing diagram and balancing diagram. .. after balancing before .. 7
  • 7. Overhung Rotors Vibration Monitoring A special case of dynamic unbalance can be found in overhung rotors. The unbalance creates a bending moment on the shaft. Dynamic un balance in overhung rotors causes high 1X levels in radial and axial direction due to bending of the shaft. The axial bearing signals in phase may confirm this unbalance. 1X Radial Axial 8
  • 8. Unbalance location Vibration Monitoring The relative levels of 1X vibration are dependant upon the location of the unbalance mass. 9
  • 9. Misalignment Vibration Monitoring Misalignment is the condition when the geometric centerline of two coupled shafts are not co-linear along the rotation axis of both shafts at operating condition. 1X 2X MP MP Axial A 1X and 2X vibration signal predominant in the axial direction is generally the indicator of a misalignment between two coupled shafts. 10
  • 10. Angular Misalignment Vibration Monitoring Angular misalignment is seen when the shaft centerlines coincide at one point a long the projected axis of both shafts. The spectrum shows high axial vibration at 1X plus some 2X and 3X with 180° phase difference across the coupling in the axial direction. These signals may be also visible in the radial direction at a lower amplitude and in phase. 1X 2X 3X Axial 11
  • 11. Parallel Misalignment Vibration Monitoring Parallel misalignment is produced when the centerlines are parallel but offset . The spectrum shows high radial vibration at 2X and a lower 1X with 180° phase difference across the coupling in the radial direction. These signals may be also visible in the axial direction in a lower amplitude and 180° phase difference across the coupling in the axial direction. 1X 2X 3X Radial 12
  • 12. Misalignment Diagnosis Tips Vibration Monitoring In practice, alignment measurements will show a combination of parallel and angular misalignment. Diagnosis may show both a 2X and an increased 1X signal in the axial and radial readings. The misalignment symptoms vary depending on the machine and the misalignment conditions. The misalignment assumptions can be often distinguished from unbalance by: • Different speeds testing • Uncoupled motor testing Temperature effects caused by thermal growth should also be taken into account when assuming misalignment is the cause of increased vibration. 13
  • 13. Alignment Tolerance Table Vibration Monitoring The suggested alignment tolerances shown above are general values based upon experience and should not be exceeded. They are to be used only if existing in-house standards or the manufacturer of the machine or coupling prescribe no other values. 14
  • 14. Shaft Bending Vibration Monitoring A shaft bending is produced either by an axial asymmetry of the shaft or by external forces on the shaft producing the deformation. A bent shaft causes axial opposed forces on the bearings identified in the vibration spectrum as 1X in the axial vibration. 2X and radial readings can also be visible. 1X 2X Axial 15
  • 15. Rotating Looseness Vibration Monitoring Rotating looseness is caused by an excessive clearance between the rotor and the bearing Rolling element bearing: Rotation frequency 1X and harmonics Radial Journal bearing: Rotation frequency 1X Harmonics and sub Harmonics. Radial 16
  • 16. Structural Looseness Vibration Monitoring Structural looseness occurs when the machine is not correctly supported by, or well fastened to its base. MP • Poor mounting • Poor or cracked base MP MP MP • Poor base support • Warped base Condition Monitoring 1X Structural looseness may increase vibration amplitudes in any measurement direction. Radial Increases in any vibration amplitudes may indicate structural looseness. Measurements should be made on the bolts, feet and bases in order to see a change in the amplitude and phase. A change in amplitude and 180° phase difference will confirm this problem. 17
  • 17. Resonance Vibration Monitoring Resonance is a condition caused when a forcing frequency coincides (or is close) to the natural frequency of the machine’s structure. The result will be a high vibration. 1st form of natural 2nd form of natural flexure 3rd form of natural flexure flexure v v v tx tx t tx t t t Shaft 1st, 2nd and 3rd critical speeds cause a resonance state when operation is near thesecritical speeds. f1st nat, flexuref2nd nat, flexuref3rd nat, flexure f no harmonic relationship 18
  • 18. Resonance Vibration Monitoring • Resonance can be confused with other common problems in machinery. • Resonance requires some additional testing to be diagnosed. 1 2 Resonance Step-up rev/min Grad Phase jump MP MP by 180° rev/min 2 1 φ1= 240° 2 φ2= 60...80° 1 1. 1. O. O. Amplitude at rotation frequency fn by residual Strong increase in amplitude of the rotation unbalance on rigid rotor. frequency fn at the point of resonance, step-up dependent on the excitation (unbalanced condition) and damping. 19
  • 19. Resonance Diagnosing Tests Vibration Monitoring Run Up or Coast Down Test: • Performed when the machine is turned on or turned off. • Series of spectra at different RPM. • Vibration signals tracking may reveal a resonance. The use of tachometer is optional and the data collector must support this kind of tests. 20
  • 20. Resonance Diagnosing Tests Vibration Monitoring Bump Test: 3 s 1 2 Excitation - force pulse Response - component vibration F/a 3 D t t ouble beat 5 ms 1 2 Decaying functionShock component, natural vibration, vertical Frequency response, vertical Frequency response, horizontal 2 Natural frequency, nd mod. vertical 1st mod. Natural frequency, horizontal t 21
  • 21. Journal Bearings Vibration Monitoring Journal bearings provides a very low friction surface to support and guide a rotor through a cylinder that surrounds the shaft and is filled with a lubricant preventing metal to metal contact. High vibration damping due to the oil film: • High frequencies signals may not be transmitted. • Displacement sensor and continuous monitoring recommended Clearance problems (rotating mechanical looseness).0,3-0,5X 1X Oil whirl Radial • Oil-film stability problems. • May cause 0.3-0.5X component in the spectrum. 22
  • 22. Rolling Element Bearings Vibration Monitoring 1. Wear: Wear • Lifetime exceeded • Bearing overload • Incorrect assembly • Manufacturing error • Insufficient lubrication Wear The vibration spectrum has a higher noise level and bearing characteristic frequencies can be identified. Increased level of shock pulses. 23
  • 23. Rolling Element Bearings Vibration Monitoring 2. Race Damage: 4 3 Roller bearing geometry and damage frequencies: 1 - Outer race damage 2 2 - Inner race damage 3 - Rolling element damage Angle of contact 4 - Cage damage Dw D Arc diameter 1 d Rolling element diameter Example of rollover frequencies: Dpw Z Number of rolling elements Ball bearing SKF 6211 n Shaft RPM RPM, n = 2998 rev/min Z n d Rollover frequenciesBall pass frequency, outer race: BPFO = ( 1- cos ) Dimensions D 260 Z d BPFO = n / 60 4.0781 = 203.77 HzBall pass frequency, inner race: BPFI = ( 1+ cos ) d =77.50 mm n D 260 d 2 D =14.29 mm BPFI = n / 60 5.9220 = 295.90 HzBall spin frequency: BSF = D n ( 1- cos ) D d60 2.fw = n / 60 5.2390 = 261,77 Hz ) Z n d = 10Fundamental train frequency: TFT = ( 1- cos 2 60 D = 0 fK = n / 60 0.4079 = 20.38 Hz 24
  • 24. Rolling Element Bearings Vibration Monitoring Outer race damage: Inner race damage: (Ball passing frequency, outer range BPFO) (Ball passing frequency, inner range BPFI) fn BPFO 2 BPFO 3 BPFO 4 BPFO Sidebands at intervals of 1X BPFI 2 BPFI 3 BPFI 4 BPFI Inner race damage frequency BPFI as well as Outer race damage frequency BPFO as well as numerous sidebands at intervals of 1X. harmonics clearly visible 25
  • 25. Rolling Element Bearings Vibration Monitoring Rolling element damage: Cage damage: (Ball spin frequency BSF) (Fundamental train frequency FTF) Sidebands in intervals of FTF FTF and 2nd, 3rd, 4th harmonics 2 BSF 4 BSF 6 BSF 8 BSF Cage rotation frequency FTF and harmonics Rolling elements rollover frequency BSF with visible harmonics as well as sidebands in intervals of FTF 26
  • 26. Rolling Element Bearings Vibration Monitoring Lubrication Problems: Major fluctuation in • Race damage level of shock pulsesLubricant contamination and damage • Defective sealing frequencies • Contaminated lubricant used • Insufficient lubricantInsufficient lubrication • Underrating Subsequent small temperature increase Over-greasing • Maintenance error Large temperature • Defective grease increase after regulator lubrication • Grease nipple blocked 27
  • 27. Rolling Element Bearings Vibration Monitoring Incorrect mounting. Shock pulse Air gapBearing rings out of round, deformed. • Incorrect installation Damage frequencies • Wrong bearing storage envelope • Shaft manufacturing error • Bearing housing overtorqued. Dirt Bearing forces on floating bearing. • Incorrect installation • Wrong housing calculation • Manufacturing error in bearing Fixed housing Floating Severe vibration bearing bearing Bearing temperature increases Cocked bearing. Axial 1X, 2X and 3X. • Incorrect installation 28
  • 28. Blade and Vanes Vibration Monitoring MP A blade or vane generates a signal frequency called blade pass frequency, fBP:MP f BP =B n N Bn = # of blades or vanes N = rotor speed in rpm Identify and trend fBP. An increase in it and/or its harmonics may be a symptom of a problem like blade-diffuser or volute air gap differences. fBPF Example characteristic frequency: Radial 3 struts in the intake; x=3. 9 blades; Bn=9. fBP x = N Bn x Characteristic frequency = N 27 29
  • 29. Aerodynamics and Hydraulic Forces Vibration Monitoring MP MP There are two basic moving fluid problems diagnosed with vibration analysis: • Turbulence • Cavitation Cavitation: Turbulence: 1X fBPF Random Random 1X 30
  • 30. Belt Drive Faults Vibration Monitoring MP MP Belt transmission a common drive system in industry consisting of: • Driver Pulley • Driven Pulley MP MP • Belt The dynamic relation is: Ø1 ω1 = Ø2 ω2 Belt frequency: ØØ1 ω 1 2 ω2 fB 3,1416 ω1 Ø1 l l: belt length 31
  • 31. Belt Drive Faults Vibration Monitoring Belt Worn: fn The belt frequency fB and first two Radial (or even three) harmonics are 1X,2X,3XfB visible in the spectrum. 2 fB generally dominates the spectrum Pulley Misalignment: 1X of diver or driven pulley visible and predominant in the axial reading. Offset Angular Twisted 32
  • 32. Belt Drive Faults Vibration Monitoring Eccentric Pulleys: The geometric center doesn’t coincide with the rotating center of the pulley. High 1X of the eccentric pulley visible in the spectrum, predominant in the radial direction. Belt direction Easy to confuse with unbalance, but: • Measurement phase in vertical an horizontal directions may be 0° or 180°. • The vibration may be higher in the direction of the belts. Belt Resonance: If the belt natural frequency coincides with either the driver or driven 1X, this frequency may be visible in the spectrum. 33
  • 33. Gear Faults Vibration Monitoring Spur Gear: Worm Gear: gear wheel Driving gear Gear Driven gear gear wheel pair gear train Gear (wheel) Pinion Worm gear Planet Gear: Bevel Gear: Ring (cone) Bevel gear Sun gear Bevel gear Planet gear Carrier 34
  • 34. Gear Faults Vibration Monitoring Gear Meshing: Gear meshing is the contact pattern of the pinion and wheel teeth when transmitting power. Left flank End point of tooth meshing Working flank 85 The red dotted line is the contact path where the 89 meshing teeth will be in contact during the 86 rotation.Right flank 88 4 87 3 5 2 6 1 Pitch point Non working flank Flank line Top land Starting point of tooth meshing Pitch line Tooth space Tip edge Pitch surface Tooth Root flank Gear mesh frequency fZ can be calculated: Fz = z fn Root mantel flank Where z is the number of teeth of the gear rotating at fn. Flank profile 35
  • 35. Gear Faults Vibration Monitoring Incorrect tooth meshing MP fz fz 2 fz 3 fz 4 fzfn1 z2 z1 MP fn2 Wear fz 2 fz 3 fzMP MP Detail of X: X fz 36
  • 36. Gear Faults Vibration Monitoring Incorrect tooth shape fz MP fz Detail of X: X MP Tooth break-outMP MP fz and X Detail of X: harmonics z2 Sidebands z1 fz fn1 fn2 37
  • 37. Gear Faults Vibration Monitoring Eccentricity, bent shafts fz and MP MP Detail of X: harmonic X sidebands fz “Ghost frequencies" or machine frequencies fz f M “Ghost freque Gearwheel being manufactured Cutting tool Worm drive part of the gear cutting machine zM 38
  • 38. Electrical Motors Vibration Monitoring Electromagnetic forces vibrations: Twice line frequency vibration: 2 fL fL : line frequency Bar meshing frequency: fbar = fn nbar nbar: number of rotor bars p: number of poles Synchronous frequency: fsyn = 2 fL / p Slip Frequency: fslip = fsyn - fn Pole pass frequency: fp=p fslip • Stator eccentricity • Eccentric rotor • Rotor problems • Loose connections 39
  • 39. Electrical Motors Vibration Monitoring Stator Eccentricity: Loose iron Shorted stator laminations Soft foot MP MP 1X 2X Radial 1X and 2X signals 2 fL fL without sidebands Radial predominant High resolution should be used when analyzing two poles machines 40
  • 40. Electrical Motors Vibration Monitoring Eccentric Rotor: Rotor offset Misalignment Poor base MP MP fp 1X 2X Tslippage R adial 2 fL t [ms] fp, 1X, 2X and 2fL signals. Modulation of the vibration time signal with the slip 1X and 2fL with sidebands at fP. frequency fslip Radial predominant. Tslip 2-5 s High resolution needed. 41
  • 41. Electrical Motors Vibration Monitoring Rotor Problems: 1. Rotor thermal bow: 1X Radial Unbalanced rotor bar current Unbalance rotor conditions Observable after some operation time f [Hz]2. Broken or cracked rotor bars: 1X 2X 3X 4X 1X and harmonics with sidebands at fP Radial High resolution spectrum needed Possible beating signal 42
  • 42. Electrical Motors Vibration Monitoring 3. Loose rotor bar: 1X 2X fbar 2f bar Radial fbar and 2fbar with 2fL sidebands 2fbar can be higher 1X and 2X can appearLoose connections: f [Hz] fn 2f L 2fL excessive signal with sidebands at 1/3 fL Radial Electrical phase problem Correction must be done immediately 43

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