machinery vibration analysis and maintenance
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Maintenance Philosophies
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Vibration in everyday life
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Useful Vibration
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Vibration Monitoring
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Vibration Basics
Any external force has to overcome structure’s properties:
- Mass- Stiffness- Damping A force cause vibration!
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Vibration Basics
Vibration can be described in terms of:- Acceleration- Velocity- Displacement
Single degree of freedom system:- Forced vibration response- Free vibration response- Resonance is free vibration
Frequency is the number of vibration cycles / time!
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Vibration Basics
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System Response
2g10N
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Nature of Vibration - SHM
• X = Xo sin w t• X = Disp. at instant t• Xo = Maximum disp.• w = 2 .p. f (rad/s)• f = frequency (Hz)• t = time (seconds)
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Wave Terminology
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Wave Terminology - Phase
• A time lag of T is a phase angle of 360º.• A time lag of T/4 will be a phase angle of 90º.
The two waves are out of phase by 90º !
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A Fast Fourier Transform
Also called the Frequency Domain or Vibration Spectrum
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Harmonics
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Frequency and Time Domains
• FFT is the Frequency Domain
• Time Waveform is the Time Domain
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Frequency Analysis
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Overall Amplitude
• It is the total vibration amplitude over a wide range of frequencies.
• Acceleration, Velocity, or Displacement.
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Which to choose?
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“Real world” vibration levels
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Vibration Terminology
• Displacement [peak-peak]• Velocity [peak]• Velocity [rms]
– Velocity rms tends to provide the energy content in the vibration, whereas the Velocity peak depicts more of the intensity of vibration.
• Acceleration [peak]
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Machinery Fault Diagnosis• Vibration analysis is used to monitor
the state of a machine.• Detailed analyses can be made
concerning the health of the machine and any faults, which may be arising or may have already arisen.
• The need for higher reliability and availability of critical machinery forces the use of this technique of PdM.
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Common Machinery Faults
• Unbalance• Bent shaft• Eccentricity• Misalignment• Looseness• Belt drive problems• Gear defects• Bearing defects
• Electrical faults• Oil whip / whirl• Cavitation• Shaft cracks• Rotor rubs• Resonance• Hydraulic + aerodynamic
forces
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Unbalance - Static• Amplitude due to
unbalance will vary with the square of speed
• The FFT will show 1 rpm frequency of
vibration.• It will be predominant• Phase difference is as
shown
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Unbalance - Couple• Amplitude varies with
square of speed• Predominant 1 peak• May cause high axial
along with radial vibrations.
• Phase difference is 180º on shaft ends in both planes.
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Unbalance - Overhung Rotors• Amplitude varies with
square of speed.• Predominant 1 peak• May cause high axial
along with high radial vibrations.
• Axial plane phase difference is 0º. Radial direction phase is unsteady.
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Bent Shaft
• Bend near centre: 1 is predominant• Bend at ends: 2 is predominant• No phase difference in
radial direction at one location.
• 180º phase difference in axial plane.
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Misalignment
• After unbalance, misalignment is the major cause for high vibrations.
• Two kinds of misalignment:– Angular - shaft ends meet an angle.– Parallel - shaft ends are parallel but have an offset.
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Angular Misalignment
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• Predominant peak is 1
• 1, 2 , 3 may be present.
• High axial vibration with 1 and 2
• Axial phase difference across the coupling is 180º.
Angular Misalignment
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Bent Shaft - Angular Misalignment
• Difference between angular misalignment and a bent shaft is differentiated only by phase difference.
• In misalignment, phase difference is 180º across the coupling.
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Parallel Misalignment
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• The predominant peak is at 2
• Vibrations in radial direction are higher than in the axial direction.
Parallel Misalignment
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• Phase difference in radial direction across the coupling is 180º.
Parallel Misalignment
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Misaligned Bearing
• Cocked bearing show high axial vibrations
• Opposite ends have an axial plane phase difference of 180º.
• FFT may show peaks of 1, 2 and 3
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Misalignment - Orbits
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Mechanical Looseness• Internal assembly looseness:
– Bearing liner in its cap.– Sleeve or rolling element bearing.– Impeller on a shaft.
• Looseness at machine to base plate interface:– Loose bolts.– Cracks in the frame structure or bearing pedestal.
• Structure looseness:– Weakness of machine feet, baseplate or
foundation.– Loose hold-down bolts, distortion of frame or base.
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Internal Looseness
• Phase is unstable.• Radial vibrations taken
at 30º. Will see different spectrums for each (directional).
• FFT will show harmonics of or even .
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Looseness at Machine to Base Plate interface
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Structure Looseness
• Measure each bolt, one at a time.
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Resonance• Every body has a resonance frequency.• Frequency is dependant on mass, stiffness and
damping.• Forced and free vibrations (e.g. ringing of bell).• Resonance is free vibrations.• Bump test is simple technique to find resonant
frequency of stationary mass or system.
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Bump Test
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Thank You For Your Interest
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