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EIT IDC Machinery Vibration Rev5

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  • 23/08/2013

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    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

    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

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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

    Machinery Vibration Analysis and Predictive Maintenance

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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 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)

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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.

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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 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.

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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.

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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 .

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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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