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23/08/2013
1
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Practical Machinery Vibration Analysis and Predictive Maintenance
By Steve Mackay
Dean of Engineering
Engineering Institute of Technology
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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
Machinery Vibration Analysis and Predictive Maintenance
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Objectives
Give some background to vibration measurement Give some suggested techniques
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Maintenance Philosophies
Machinery Vibration Analysis and Predictive Maintenance
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Vibration in everyday life
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Useful Vibration
Machinery Vibration Analysis and Predictive Maintenance
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Vibration Monitoring
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Vibration Monitoring
Machinery Vibration Analysis and Predictive Maintenance
IDC Technologies and EIT
23/08/2013
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Vibration Monitoring
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Vibration Monitoring
Machinery Vibration Analysis and Predictive Maintenance
IDC Technologies and EIT
23/08/2013
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Vibration Monitoring
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Vibration Monitoring
Machinery Vibration Analysis and Predictive Maintenance
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23/08/2013
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Vibration Monitoring
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Vibration Basics
Any external force has to overcome structures properties:
- Mass- Stiffness- Damping A force cause vibration!!!
Machinery Vibration Analysis and Predictive Maintenance
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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 t X = Disp. at instant t Xo = Maximum disp. = 2 .pi. f (rad/s) f = frequency (Hz) t = time (seconds)
Machinery Vibration Analysis and Predictive Maintenance
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Wave Terminology
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Wave Terminology - Phase
A time lag of T is a phase angle of 360. A time lag of T/4 will be a phase angle of 90.
The two waves are out of phase by 90 !
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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.
Machinery Vibration Analysis and Predictive Maintenance
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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.
Machinery Vibration Analysis and Predictive Maintenance
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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 180 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 0. Radial direction phase is unsteady.
Machinery Vibration Analysis and Predictive Maintenance
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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.
180 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.
Machinery Vibration Analysis and Predictive Maintenance
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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 180.
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 180 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 180.
Parallel Misalignment
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Misaligned Bearing
Cocked bearing show high axial vibrations
Opposite ends have an axial plane phase difference of 180.
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 30. Will see different spectrums for each (directional).
FFT will show harmonics of or even .
Machinery Vibration Analysis and Predictive Maintenance
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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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