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Condition Monitoring & Custom ProductsVibration Monitoring
and Machine Protection Systems
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STI Field Application Note Bearing Failure Modes
This application note presents specificfailure modes associated
with machineryhaving journal bearings. Signalmeasurements presented
here are timewaveforms that are collected with twoorthogonally
located Eddy Probes, or proximity type sensors. Although
certainfaults can be analyzed with spectrumanalyzers, the majority
of these faults canbe diagnosed using orbit analysis alone.
BALANCE
Diagnosis of a degrading balancecondition is performed by
concentratingon the synchronous amplitude whichcoincides withthe
rotor speed.This can beaccomplished byviewing thespectra from
anysingle EddyProbe sensor. A similar diagnosis can bemade by
viewing the filtered signals fromtwo orthogonally mounted Eddy
Probessensors as orbits. As the balancecondition deteriorates the
size, andsometimes the shape, of the orbit willgrow larger until
the peak-to- peakamplitude exceeds acceptable limits.
CRACKED SHAFT
A crack in a rotor, or shaft, can generateseveral different
effects on how themachine behaves: a change in thevibration level,
a change in the operatingphase angle, and/or a change in
theresonance frequency as the machinestarts or stops. Spectral
analysis can beused to identify this fault, but
observingfiltered,synchronousorbits with thephase anglesuperimposed
onthe orbit allowsrapididentification of
this condition. Changes in the filtered amplitude can
bedetermined using orbits analysis. Bysuperimposing the phase angle
inputsignal onto the orbit a shift in thisparameter can be easily
determined. Bynoting the operating speed at which theresonance
frequencies occur, a change inthis frequency may indicate
the"possibility" of a crack existing.
The "possibility" must be emphasized andcarefully analyzed
because many other causes can produce these changes, suchas, a
damaged or loose bearing support,foundation problems, loose
rotatingparts...basically anything that caninfluence the "system"
mass, damping,and/or stiffness.
LOOSE ROTATING PART
A loose rotating part can generateunusual vibration signals.
They may
OIL WHIP
Oil whip occurs during the later stages of an oil whirl
condition and it has adistinctive orbital display. The display,with
the phase input superimposed on thedisplay, appears to have several
phasemarks. This display will be round in shapeand theamplitude
willgreater that theamplitude notedduring oil whirl.
The size of the orbit be will larger becausethe shaft uses up
the entire bearingclearance as an oil wedge can no longer be
established by the rotor and the shaftis in direct metal-to-metal
contact with thebearing. The orbit display will no longer rotate
because the oil whirl frequency hascoincided with the first natural
resonance,or critical speed, and has "locked" ontothis frequency.
Oil whip is a dangerouscondition because the rotor uses up
theentire bearing clearance and is in directmetal-to-metal contact
that will wear awaythe bearing rapidly and destroy the rotor if not
corrected.
EXCESSIVE PRELOAD
All journal bearing machines have someamount of preload so that
a stable oilwedge can be established. The preloadmay be internally
or externally produced.Internal sources of preloads are from gear
meshing or hydraulic loading duringpumping actions. External
preloads maybe from coupling misalignment or pipingand support
system thermal changes.These sources of preload create anelliptical
orbit that is flattened in thedirection of the preload vector.
As the preload increases the orbit isfurther flattened. As
excessive preloadincreases further the orbit begins tocollapse to
forma "banana"shape as theshaft tries tocontinue itsnormal
rotationpattern anddirection.
After the orbithas been flattened into the "banana"shape a 2X
frequency is present onspectra displays. Heavy preloads further
distort the orbit into a figure eight shape.As preload increases
the shaft centerlinewill shift in the direction of the
preloadvector.
RUB
A common problem in newly rebuilt or modified rotors is a slight
rubbingcondition as the rotor is initially operated.Rotor rubs are
not a phenomenon whichcontinues over an extended period;
theyusually increase the clearances until therub has been cleared
or, if not corrected,
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fluctuate in amplitude and the phaseangle may shift, also. This
fault isdiagnosed easiest using filtered,synchronous orbit
analysis. Imagine amass, such as an impeller, which hascome loose;
it can rotate freely on theshaft independently.
As the loose part rotates it influences thebalance condition of
the rotor which
appears as acyclical increaseand decrease inthe
synchronousamplitude. This isobservable usinga spectrumanalyzer,
but the
changes may be too rapid for thesampling rate of the instrument.
Anoscilloscope set up to observe a filteredorbit will sample
continuously so that thechanges can be seen. The phase shiftingcan,
also, be observed using anoscilloscope.
The inception of a loose part condition willproduce a "nervous"
filtered, synchronousorbit. The orbit will appear to
vibrateslightly as this condition is created; thepart may be
slipping and then sticking on
the shaft just prior to becoming a fullfledged loose rotating
part.
OIL WHIRL
Oil whirl and oil whip are sometimes listedas a single machine
fault, but closer observation of the vibration signals andthe
machine conditions causing thesesignals will produce different,
distinctsignal displays for each condition. Thisfault is caused by
a condition whichprevents the rotor from creating a stableoil wedge
on which ride. An improperlydesigned bearing is the usual source
for oil whirl conditions, but a change in thefluid viscosity or
machine alignment stateare other possibilities.
Generally, an oil whirl condition precedesan oil whip condition.
Spectral and orbitanalysis can be used to identify either
condition. This phenomenon creates anindividual sub synchronous
frequencywhich can occur within a frequency rangefrom 35% to 48% of
rotor speed,depending upon the machine/bearingdesign or
construction. As the machineaccelerates the whirl frequency
willincrease as machine speed increases.
Observing oil whirl as a filtered,synchronous orbit produces a
distinctivedisplay. The orbitwill be more or less round inshape
with anamplitude thatnearlyapproximates thebearing clearance, and
when the phaseangle is superimposed upon the display,the orbit will
appear to have two phasemarks on it. This characteristic is due
tofiltering at shaft speed and the fault beinggenerated at a
subsynchronousfrequency. The two phase marks will notbe displayed
symmetrically on the orbitbecause the whirl frequency is not
atexactly machine speed.
they will wear away the internalclearances until the machine
cannot beoperated. The shape of the orbit displaywill differ
depending upon the relationshipof the shaft speed to the first
naturalfrequency and the severity of the rub.
Spectra displays of rub conditions arecharacterized by distinct
frequencies thatoccur at multiples of a fundamentalfrequency.
Thefundamentalfrequency willdepend upon therelationship of the
shaft speedto the firstnaturalresonance frequency. At shaft speeds
upto twice the natural resonance frequency,the fundamental rub
frequency willcoincide with the shaft speed withmultiples at 2X,
3X, etc. Between twiceand three times the first natural
resonancefrequency, the fundamental rub frequencywill be shaft
speed with multiples at 1X,3/2X, 2X, 5/2X, etc. Between three
andfour time the natural resonancefrequency, the fundamental rub
frequencywill be shaft speed with multiples at 2/3X,1X, 4/3X, 5/3X,
2X, 7/3X, etc.
The severity of the rub will affect theshape of the orbit. A
light rub will producea "tear drop"shaped orbit,with the pointof
the tear dropcoinciding withthe impactspot. As therub getsheavier
theorbit willflattened andmay appear as an excessive preload.
At higher machine speeds (above twicethe first natural
frequency) the unfilteredorbits will begin to have internal loops
withthe fundamental rub frequency inverselyproportional to the
number of internalloops. These internal loops will have their own
phase marks displayed and the loopswill be located symmetrically on
thedisplay.
Bearing Failure Modes-JournalBearings Checklist
1. Balance 2. Cracked Shaft 3. Loose Rotating Parts 4. Oil Whirl
5. Oil Whip
6. Excessive Preload 7. Rub
STI HomepageCopyright @ 1989-1999 by Sales Technology, Inc. ALL
RIGHTS RESERVED
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