Machine Survey Vibration Fault Matrix Commonly Occurring Symptoms of Machinery Faults

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Harvey Tucker
5 years ago
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1 Imbalance Shaft Misalignment Looseness / Impacting Resonance High Radial. Distributed all directions radial. Possible high axial w/ couple imbalance. Changing axial or radial. Likely dominant in single direction. Presence may increase or decrease vibe amplitude. High or increasing. May dominate in single direction. Amplitude difference across loose interfaces. Structural - High in direction of resonance mode. Shaft - High radial. Substantial amplitude changes with speed changes. Dominant 1X 1X, possible 2X or multiple harmonics (dependent on coupling type) 1X w/ multiple harmonics. Roller coaster pattern across tops of harmonics in spectra. Half orders in presence of resonant frequencies. Non-synch: Loose stationary part. Commonly 1X. Possibly 2X,3X. W/ motor possible 2XLF, harmonic, or RBPF. W/ pump/fan pssble vane pass. Any freq in the presence of impacting. Sinusoidal "W" or "M" shape or double step in sinusoid Not sinusoidal. Sharp peaks or truncation Sinusoidal in the direction of greatest response. Stable. Rotates w/ transducer. Stable. Does not rotate with transducer. Possible 180 deg phase displacement across coupling axial or radial. Unstable. difference across structural joints. Substantial phase changes w/ speed change. Circle / Open Ellipse Pinched, peanut shaped, figure 8. Reverse precession harmonics (exterior loops). Elongated ellipse along axis of resonant response. Normally forward precession (reverse precession w/ split critical) Radial forces vary proportional to speed squared. Increased bearing housing or coupling temperature (dependent on coupling type). "Nibbling" at joints. Substantial amplitude changes with speed changes. Newly installed rotating element. Residual Imbalance > ISO-1940 requirements. Newly Installed or aligned. Allowable misalignment 1800: <1.8 mils/in 3600: <1.5 mils/in Fretting evidence. Bluing shows insufficient contact. Denting or peening at loose contact surfaces. High balance sensitivity. Condition veritied w/ impact test. Commonly recommend quieting resonance before attempting balance solution. Thermography identifies increased bearing housing or coupling temperature. Oil analysis shows wear if looseness occurring within lubrication boundary. Verified with impact test or modal analysis. Impact test for sleeve brg machine cstdwn. May require "on-line to running" alignment survey. Shim pack coupling intolerant of axial mispositioning. May detune resonance by varying mass or stiffness. May be quieted by damping or dynamic absorber.

2 Rub Strain / Distortion / Softfoot Thermal Variability External Force High. Any direction. Shaft rub may create long period amplitude oscillation (Period from a few minutes to several hours). Changing axial or radial. Likely dominant in one direction. May decrease vibe amplitude. Motor softfoot w/ 2-pole motor may "beat" w/ 2X and 120 HZ. Changing any direction. change occurs during warm-up following cold start Random direction & amplitude Several possible freqs 1x, harmonics, non-synch, sub-synch, broad-banded. Possibly excite natural freqs. 1X w/ possible harmonics. Motor softfoot commonly yields 2XLF. 1X and/or 2X Non-synchronous. Frequency peaks generated by adjacent machinery. Non-sinusoidal. Truncation. "Beating" 2-pole motor may shows beat in time domain. Possibly same as adjacent machinery. Unstable. 1X may create circular pattern on polar plot progressing against shaft rotation over time. may vary during warm-up Truncation, exterior loops. Full annular rub may result in reverse precession. change occurs with change in machine or process fluid temperature NDE evidence at contact point Untorquing of mounting feet or piping attachment results in >3-5 mils displacement. Running softfoot check. >3-4 shims per foot. Thermography - Evidence of high temperature at contact point. Increased bearing housing temperature if distortion causes preload within bearing. Rub may "wipe away" interference restoring normal vibration. May create "chaotic" sypmtoms. Morton Effect-similar symptoms for oil lubricated interface. Observe safety precautions if performing running softfoot check. May implement compromise balance. Allow thermal effects to find equilibrium prior to collecting balance data. Assess likely external sources and compare frequencies.

3 Rolling Element Bearing Degradation Sleeve Bearing Condition Gear Condition Increased amplitude, radial or axial. Early stage evident closest to energy source. Increased Radial Increasing. Observed at monitoring point closest to gear. Stage 1 / or Underlubrication >20K HZ - HFD or Spike Energy - Excite transducer nat frequency. Stage 2 0.5K - 4K HZ - Defect freqs in demod spectra. Stage 3 Defect Freq in Velocity Spectra - Sidebands and/or Harmonics. Stage 4 Increasing 1X, Noise floor. 1X - Excessive clearance, incorrect preload dimensions. 1X directional - Internal misalignment. Sub-harmonics on long vert shaft - wear or whip. Approx 0.44X - Whirl. Sub-synch locked on shaft resonance - Whip 1X and/or 2X of any geared shaft. Gear mesh w/ harmonics or shaft speed sidebands. Hunting tooth. Shaft torsional resonance. Replace: Ball Bearing True Peak > 8g-p Roller Bearing True Peak > 12g-p Broken gear tooth yields sharp peaks at gear mesh intervals. Whirl/Whip - Forward Precession Audible squeal may corroborate under-lube, early degradation, or ball skidding. Ball skidding likely w/ under-loaded bearing. Bearing metal or return oil temp change. Increased return oil flow. Whirl/ship may be initiated due to changes in oil viscosity or pressure. Observe damaged ball paths due to fatigue failure, false brinelling, contamination, or fluting. Confirm damage cause w/ ISO Damaged babbitt. Change in bearing RIC. R/C should be < 0.5 (Ref ISO , Tbl B.1). Broken gear teeth. Excessive or uneven gear wear. Misaligned shaft. Excessive backlash. Oil analysis may confirm bearing damage. Black oil possibly due to low oiler level. Inadequate grease lubrication possible BSF. Oil analysis may confirm bearing damage Oil Analysis - Wear evience in oil. Ref L10 life expectancy for expected service life due to fatigue failure. Under-lubrication may show as BSF. Dirtorted outer race may show BPFO. Cage degradation - FTF and BSF Harms in demod spectra. Whirl/whip may require design modification for correction.

4 Degraded Pedestal High or increasing. May dominate in single direction. > 0.03 ips-p at pedestal base. Locked / Worn Coupling Changing axial or radial. Dominant in single direction. Presence may increase or decrease vibe amplitude. may change dramatically with load. Eccentric Rotor/Sheave High any direction Cracked Shaft High any direction 1X w/ possible harmonics 1X, 2X, and/or 3X. 1X 1X or 2X Double step in sinusoid. "W" or "M" pattern. "Rocking" indicated by 180 deg phase difference betweeen vert measurements on adjacent machine feet. Does not rotate w/ accel. Possible 180 deg disp across coupling axial or radial. may shift after start/stop, load or speed change. Stable changes w/ crack propagation Pinched, peanut shaped, figure 8. Possible reverse precession harmonics or exterior loops. "Nibbling" at pedestal interfaces. Grease expelled inside coupling guard. Increased bearing housing or coupling temperature. Observed damaged grout. Hammer test. Observe worn coupling or hardened lubricant. Mis-directed lubricant spray. Excessive runout axial or radial. Verified by NDE exam Thermography identifies increased bearing housing or coupling temperature (dependant on coupling type). NDE - Evidence of crack Simply retorquing embedded bolts is ineffective. Must re-establish grout bonding. Possible epoxy refill. NEMA MG , para Possible sporadic occurrence. Amplitude or phase may change following load change or start/stop cycle. Compromise balance may decrease vibration in one direction, but increase vibration in the other directions. Commonly diagnosed initially as imbalance.

5 Cocked Rolling Element Bearing Bent Shaft Belt/Sheave Wear Centrifugal Fan Condition High axial. High radial or axial Increasing along direction of belt 1X. 1X or 2X of BPFO. 1X 1X or Belt Frequency. Multiple belt freq harmonics in spectra. Observe lower 1X frequency if belt is slippiing. Vane pass - Common. 0.6X to 0.75X - Rotating stall. 0.5 to 2.0 Hz - Surge. Non synch - Vortex Shedding (likely inlet damper throttled). Sinusoidal Axial phase rotates with transducer Stable 1X. Rotates w/ transducer. Axial 180 deg out of phase across element Circle / Open Ellipse Increased bearing housing temperature. Observe Belt riding low in sheave. Belt elevated temperature. Excessive belt dust. Stall occurs w/ reduced flow or inlet disturbance. unstable opertng point. Vortex Sheding - Inlet damper throttled (30-50%). Newly installed bearings. Observe fretting evidence. Excessive face and/or rim runout. Exposed belt cord. Cup shaped sheave race. Verify sheave pitch matches belt pitch. Thermography - Increased bearing housing temperature Thermography - Increased belt temperature Dynamic pressure monitoring. Duct vibration survey. Perf monitoring by tracking fan curve operating point. Belt related noise can be distinguished from bearing noise by spraying DI water on belt/sheave interface. Fan imipeller modes may become excited. Fan design normally requires several diameters of straight ducting at inlet and outlet of fan.

6 Centrifugal Pump Condition Induction Motor Condition 1X - Impeller or fluid imbalance. Vane pass - Flow <BEP, improper gaps. May excite resonance. 0.1X to 0.3X - Diffuser stall 0.6X To 0.9X - Impeller stall or suction recirculation. Elevated noise floor - Insufficient NPSH. Approx 0.45X - Whirl (@ bearing or impeller shroud) High subsynch - Possible Whip (@ bearing or lmpeller shroud) 1X w/ pole pass SBs - rotor fault. RBPF w/ 2XLF SBs - Common but possible rotor fault? 2XLF - Possible Uneven Air Gap / Softfoot / Loose Terminations. 2XLF with harmonics - Possible loose stator windings (Verified by inspection, corrected by re-wedging). 6XLF and 12XLF - VFD related. Slot pass freq w/ 1X SBs - Loose Stator Coils? Non-periodic impacts due to low-flow axial shuttling Vane pass - Forward precession harmonics (interior loops) Operation away from BEP confirmed by reviewing pump curve. Observed axial shaft shuttling at low flow. Degrading performance indicates wear or excessive clearances. Noise w/ insufficient NPSH. Visually confirm damaged/worn impeller, wear rings, or diffusers. Confirm improper lift or gap clearances. Performance monitoring. Changing pump curve characteristics. generally increases as flow is throttled back (mismatched velocity angles with impeller or diffuser vanes). Wear ring clearance may alter shaft stiffness and damping (Lomakin effect). Impact testing during operation is preferred (vs shutdown). Audible beat w/ rotor fault (1X modulated by PP). Audible 2-pole motor beat between 2XLF and 2X Shaft Speed. "Foghorning" sound - Loose stator weldments. 60 Hz "buzz" w/o turning - Improperly terminated. Measured uneven airgap (softfoot or eccentric rotor). Inspect damaged stator bars/weldments. Electrical tests - PI, surge, MCSA, Baker AWA, Explorer, etc. Bearing insulation test confirms circ current damage. Corroborating electrical tests (PI, surge, MCSA, Baker AWA or Explorer, etc). Resistance test - Failing bearing insulation. Cannot test for stator insulation embrittlement. 2XLF and RBPF are common - Difficult to set action thresholds - May not warrant corrective maintenance. Rotor Fault diagnosis load dependent - requires >80% rated load. Corroborate pole pass SBs by PP in demod spectra.

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