Vibration analysis of
rotating machinery
Vibration signal
• Vibration :
It is oscillating motion of body about its neutral position.
• Vibration analysis is time varying, real world signal from
transducer or signal.
• The raw vibration signal is processed mathematically or routed
to mathematical integrator for post-processing.
Time domain graphs
• In this amplitude vs time graph is plotted.
• Amplitude comprises parameter like acceleration, velocity and
displacement.
• Reading of time domain graph is complicated task.
Frequency domain graphs
• In this graph is between amplitude vs frequency.
• Graph reading is easy.
• Mathematically time waveform is converted in to frequency
waveform.
FFT is method of taking real world time varying signal and splitting it with
amplitude, phase, and frequency.
Types of faults in Rotating
machinery
1.
2.
3.
4.
5.
Rotor unbalance
Shaft misalignment
Mechanical Looseness
Eccentric rotor
Faulted rolling element bearing
Rotor unbalance
 Static unbalance :
It is an unbalance condition where the central principal axis is displaced parallel to the shaft axis.
 It can be detected by simply placing knife edge or roller and allowing the heavy spot to swing to the
bottom.
 In this vibrational displacement amplitude and phase reading about both edges is equal.
 It is single plane problem.
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Couple unbalance :
It is condition in which central principal axis intersects the shaft axis at the rotor center of gravity.
It represented by equal heavy spot at each end of rotor but directly opposite (180 degree) to one another.
In this vibration amplitude is same but phase reading at each end is differ by 180 degree.
This problem can be solved by single plane solution.
 Quasi-static unbalance :
 Combination of both static and couple unbalance is quasi-static unbalance.
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Dynamic unbalance :
It is most common type of unbalance.
It is simply random combination of static, couple, and quasi-static unbalance.
Dynamic problem need to solve by two plane problem
Misalignment
• Factors that affect alignment:
1. Operating temperature
2. Settling of the base or foundation
3. Deterioration or shrinkage of foundation
• Vibration frequencies due to misalignment are usually 1X 2X
or 3X RPM, and may appear in combination depending on the
type and extent of misalignment.
• Angular misalignment occurs at 1X RPM, where as offset or
parallel misalignment at 2X RRM.
Mechanical looseness
• It simply loss of reduction in the normal stiffness of the machine
or system.
• Looseness causes to vibrate at 2X, 3X, 4X RPM
Sometimes sub-harmonic frequencies ( i.e 1 2 RPM, 1 3 RPM, or
1 RPM).
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Vibration due to eccentricity
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Some eccentricity or out of roundness is present on every
rotating assembly.
Eccentric gear will cause highly directional vibration at 1X
RPM.
Faulted rolling bearing
• There are four types of bearing defect frequencies :
1. Ball passing frequency for outer raceways (BPFO):
frequency at which ball passes point on outer race
2. Ball passing frequency for inner raceway (BPFI):
Frequency at which ball passes point on inner raceway.
3. Ball spin frequency (BSF): it is ball rotating frequency
4. Fundamental train frequency (FTF): it is frequency of rotation of
cage.
Depending upon frequency at which vibration occurs defect of ball
bearing can be identified.
Vibration frequencies and their likely causes
Frequency in terms of RPM
Possible causes
1X RPM
Unbalance, Eccentric journals (gears or pulley),
Resonance, reciprocating forces, Bad belt (If RPM of belt )
2X RPM
Mechanical looseness ( if high axial vibration),
Reciprocating forces,
Resonance.
3X RPM
Misalignment, Combination of misalignment and excessive axial
clearance(looseness)
Less than 1X RPM
Oil whirl (less than 0.5 RPM),
Bad drive belts,
Background vibration,
Sub harmonic resonance, beat vibration.
Synchronous (A.C Line frequency)
Electrical problem
(like broken rotor bars, eccentric rotor, unbalanced phases in
poly phase system, unequal air gap.
2X Synchronous frequency
Torque pulses.
Many times RPM
( Harmonically related frequencies)
Bad gear, aerodynamic forces, hydraulic forces, mechanical
looseness, reciprocating forces.
High frequency
( Not harmonically related)
Bad anti friction bearing, cavitation, recirculation, flow
turbulence, improper lubrication of journal bearing, rubbing.
Thank you