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
2g
10N

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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Machinery Vibration Analysis and Maintenance

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