Machinery Vibration Analysis and Maintenance

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Maintenance Philosophies


Vibration in everyday life


Useful Vibration


Vibration Monitoring


Vibration Basics

Any external force has to overcome structure’s properties:

- Mass- Stiffness- Damping A force cause vibration!


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 the number of vibration cycles / time!


Vibration Basics


System Response

2g10N


Nature of Vibration - SHM

• X = Xo sin w t• X = Disp. at instant t• Xo = Maximum disp.• w = 2 .p. f (rad/s)• f = frequency (Hz)• t = time (seconds)


Wave Terminology


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 !


A Fast Fourier Transform

Also called the Frequency Domain or Vibration Spectrum


Harmonics


Frequency and Time Domains

• FFT is the Frequency Domain

• Time Waveform is the Time Domain


Frequency Analysis


Overall Amplitude

• It is the total vibration amplitude over a wide range of frequencies.

• Acceleration, Velocity, or Displacement.


Which to choose?


“Real world” vibration levels


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]


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.


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


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


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.


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.


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.


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.


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.

Angular Misalignment


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.


Parallel Misalignment


• The predominant peak is at 2

• Vibrations in radial direction are higher than in the axial direction.



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



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


Misalignment - Orbits


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.


Internal Looseness

• Phase is unstable.• Radial vibrations taken

at 30º. Will see different spectrums for each (directional).

• FFT will show harmonics of or even .


Looseness at Machine to Base Plate interface


Structure Looseness

• Measure each bolt, one at a time.


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.


Bump Test


Thank You For Your Interest

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Engineering

Machinery Vibration Analysis and Maintenance