Download pdf - EIT IDC Machinery Vibration Rev5

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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

IDC Technologies and EIT

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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 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.


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• Phase difference in radial direction across the coupling is 180º.




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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:

www.eit.edu.au

