Machinery Vibration Analysis and Predictive Maintenance
23/08/2013
Practical Machinery Vibration Analysis and Predictive Maintenance By Steve Mackay Dean of Engineering Engineering Institute of Technology Technology
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EIT Micro-Course Series • • •
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Ever Every y tw two o wee weeks ks we we prese present nt a 35 35 to 45 minute interactive course Pract Practica ical, l, usefu usefull with with Q & A throughout PID PID loo loop p Tun Tuning ing / Arc Arc Flash Flash Protection, Functional Safety, Troubleshooting conveyors presented so far Upcoming: – Electrical Troubleshooting Troubleshooting and much much more…..
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Go to http://www.eit.edu.au/freecourses You You get get the the reco record rdin ing g and and slides slides
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Machinery Vibration Analysis and Predictive Maintenance
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Objectives
• Give Give some some backgroun background d to vibrati vibration on measu measurem rement ent • Give Give some some sugges suggeste ted d techn techniq ique uess
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Maintenance Philosophies
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Machinery Vibration Analysis and Predictive Maintenance
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Vibration in everyday life
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Useful Vibration
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Machinery Vibration Analysis and Predictive Maintenance
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Vibration Monitoring
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Vibration Monitoring
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Machinery Vibration Analysis and Predictive Maintenance
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Vibration Monitoring
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Vibration Monitoring
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Machinery Vibration Analysis and Predictive Maintenance
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Vibration Monitoring
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Vibration Monitoring
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Machinery Vibration Analysis and Predictive Maintenance
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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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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! www.eit.edu.au
Vibration Basics
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23/08/2013
Machinery Vibration Analysis and Predictive Maintenance
System Response
10N
2g
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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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Machinery Vibration Analysis and Predictive Maintenance
23/08/2013
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 ! www.eit.edu.au
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Machinery Vibration Analysis and Predictive Maintenance
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A Fast Fourier Transform
Also called the Frequency Domain or Vibration Spectrum www.eit.edu.au
Harmonics
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Machinery Vibration Analysis and Predictive Maintenance
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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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Machinery Vibration Analysis and Predictive Maintenance
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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. www.eit.edu.au
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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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Machinery Vibration Analysis and Predictive Maintenance
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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 www.eit.edu.au
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. www.eit.edu.au
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Machinery Vibration Analysis and Predictive Maintenance
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Common Machinery Faults • Unbalance
• Electrical faults
• Bent shaft
• Oil whip / whirl
• Eccentricity
• Cavitation
• Misalignment
• Shaft cracks
• Looseness
• Rotor rubs
• Belt drive problems
• Resonance
• Gear defects
• Hydraulic + aerodynamic forces
• Bearing defects www.eit.edu.au
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 www.eit.edu.au
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Machinery Vibration Analysis and Predictive Maintenance
23/08/2013
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. www.eit.edu.au
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. www.eit.edu.au
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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. • 180º phase difference in axial plane. www.eit.edu.au
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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Machinery Vibration Analysis and Predictive Maintenance
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Angular Misalignment
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Angular Misalignment • 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. www.eit.edu.au
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Machinery Vibration Analysis and Predictive Maintenance
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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. www.eit.edu.au
Parallel Misalignment
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Machinery Vibration Analysis and Predictive Maintenance
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Parallel Misalignment
• The predominant peak is at 2 ×. • Vibrations in radial direction are higher than in the axial direction.
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Parallel Misalignment
• Phase difference in radial direction across the coupling is 180º.
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Machinery Vibration Analysis and Predictive Maintenance
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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×. www.eit.edu.au
Misalignment - Orbits
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Machinery Vibration Analysis and Predictive Maintenance
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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. www.eit.edu.au
Internal Looseness • Phase is unstable. • Radial vibrations taken at 30º. Will see different spectrums for each (directional). • FFT will show harmonics of × or even ×. www.eit.edu.au
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Machinery Vibration Analysis and Predictive Maintenance
23/08/2013
Looseness at Machine to Base Plate interface
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Structure Looseness
• Measure each bolt, one at a time. www.eit.edu.au
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Machinery Vibration Analysis and Predictive Maintenance
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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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Machinery Vibration Analysis and Predictive Maintenance
23/08/2013
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