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23/08/2013
1
www.eit.edu.au
Practical Machinery
Vibration Analysis and
Predictive Maintenance
By Steve Mackay
Dean of Engineering
Engineering Institute of Technology
www.eit.edu.au
EIT Micro-Course Series
• Every two weeks we present a 35 to 45 minute interactive course
• Practical, useful with Q & A throughout
• PID loop Tuning / Arc Flash Protection, Functional Safety, Troubleshooting conveyors presented so far
• Upcoming: – Electrical Troubleshooting and
much much more…..
• Go to http://www.eit.edu.au/free-courses
• You get the recording and slides
Engineering Institute of Technology (EIT)
Machinery Vibration Analysis and Predictive Maintenance Webinar
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Objectives
• Give some background to vibration measurement
• Give some suggested techniques
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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 Monitoring
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Vibration Monitoring
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Vibration Monitoring
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Vibration Monitoring
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Vibration Monitoring
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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 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 ω t
• X = Disp. at instant t
• Xo = Maximum disp.
� ω = 2 .π. 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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Why do a 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 InterestIf you are interested in further training, please visit:
IDC Technologies Short Courses:
Two-day practical courses available to the public:
http://idc-online.com/workshops
IDC Technologies Conferences:
http://idc-online.com/conferences-main
The Engineering Institute of Technologies
Online Certificate and Advanced Diploma programs:
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