This document provides an overview of orbit plot analysis, which is used to assess the condition of machine bearings and diagnose vibration issues. It explains that orbit plots visualize the motion of a machine shaft using signals from two probes, and may include a phase reference from a keyphasor. Common orbit shapes are described that indicate issues like misalignment, unbalance, oil whirl, rotor rub, and oil whip. The steps to create a spectrum plot from the vibration signal are outlined, and how spectra differ for circular and elliptical orbits. In summary, orbit plot analysis is a vibration analysis technique that visually depicts shaft motion to identify potential machine faults.
4. VIBRATION
• WHAT IS VIBRATION?
Vibration is essentially repetitive, or "cyclical", movement.
• Why do we Measure Vibration?
1. To assess the condition (primarily the bearings) of a machine
2. To diagnose the root causes of any excessive, and therefore
destructive, vibration
5. VIBRATION ANALYSIS
• Vibration analysis as a part of preventive/predictive maintenance
programs.
• Vibration analysis permit effective and reliable evaluation of the machinery
health, give early warning of impending failures, and allow determination of
the cause of malfunctions.
• Faults detected early enough can be monitored closely to ensure that
significant damage is avoided, and that the machine is operated safely
until the maintenance can be scheduled to correct the malfunctions.
there is one method of vibration analysis called “ORBIT PLOT
ANALYSIS” that will be explained in this presentation
6. WHAT IS ORBIT PLOT ANALYSIS ?
• The orbit represents the path of the shaft centerline within the bearing
clearance.
• Two orthogonal probes are required to observe the complete motion of the
shaft within.
• The dynamic motion of the shaft can be observed in real time by feeding
the output of the two orthogonal probes to the X and Y of a dual channel
oscilloscope
• If the Keyphasor output is fed to the Z axis, a phase reference mark can
be created on the orbit itself
• The orbit, with the Keyphasor mark, is probably the most powerful plot for
machinery diagnosis
7. CONSTRUCTION OF AN ORBIT
• XY transducers observe the
vibration of a rotor shaft
• A notch in the shaft (at a
different axial location) is
detected by the Keyphasor
transducer.
• The vibration transducer
signals produce two time
base plots (middle) which
combine into an orbit plot
(right)
8. PROBE ORIENTATION
• On the left side, when the probes are mounted at
0o
and 90o
R, the orbit plot and oscilloscope
display show the same view.
• On the right, when the probes are mounted at
45o
L and 45o
R, the orbit plots are automatically
rotated
• The oscilloscope, however, must be physically
rotated 45o
CCW to display the correct orbit
orientation
9. HOW ORBIT PLOT FORMED ?
• Polar plot is made up of a set of vectors at different
speeds.
• Vector arrow is omitted and the points are connected
with a line
• Zero degree is aligned with transducer location
• Phase lag increases in direction opposite to rotation
• 1x uncompensated Polar Plot shows location of rotor
high spot relative to transducer
• This is true for 1x circular orbits and approximately true
for 1x elliptical orbits
10. TYPES OF ORBIT PLOTS
• Orbits can be divided into main categories
1. Floating Orbit
Floating orbit analyses is one of method to analyse orbits that don’t use
keyphasors, hence the orbit direction is floating and starting point differ
from another.
2.Absolute Orbit
Absolute orbits have fixed starting point due to the use of keyphasors.
Nowdays, many modern vibration analysis and monitoring system uses
keyphasors.
11. TYPES OF ORBIT PLOTS
• Can also can be divided into filtered orbit dan overall orbit:
1.Filtered Orbit
The filtered orbit is the method that normally track one order and utilize the
keyphasor signal for synchronous filtering as well as reference
point. The filtered orbits are useful to track a certain order and neglecting
other orders which may cause unclear orbit plot.
• Overall Orbit
The overall orbit orbit do not use any filter and it is useful to obtain the
actual shaft movement.
12. CHARACTERISTIC AND CAUSES
• We could analyze failure of operating machine with orbital plot
analysis for several causes, such as :
1.Misalignment
2.Unbalance
3.Oil Whirl
4.Rotor Rub
5.Oil Whip
13. MISALIGNMENT
• Misalignment is the incorrect arrangement or
position of something in relation to something
else. That problem can be happen on assembly
of shaft, gear, etc.
• A bearing preload due to a cocked assembly
can also cause the orbit to have lower amplitude
in one axis that makes the ellipse look thinner.
And suddenly, if the preloading increases
further, it will result in the orbit’s shape to
resemble a number 8 character. But If the
trajectory of our imaginary point on the trace of
the orbit is continued, one can visualize that
precessions keep changing continuously.
14. UNBALANCE
• Unbalance condition is happen because the weight of the rotating part isn’t
perfectly same or balance. So that can causes unbalanced spin and make
vibration to the system. Unbalance will generally produce 1xRPM vibration
with 90° phase shift between the horizontal and vertical directions. This will
result is ellipse-shaped.
15. OIL WHIRL
• Oil whirl is present bearing clearances are
excessive. can occur when clearances
become excessive. An oil wedge is formed
that is held in place by the rotation of the
shaft.
• The friction of the shaft against the wedge
then pushes the shaft around the housing.
Fortunately (for the analyst), it occurs in a
very precise sub-synchronous frequency
range.
16. ROTOR RUB
• Rotor rub can be happen if the rotor of
motor is have a contact with. Orbit analysis
is a good tool to identify rubs.
• As mentioned earlier, partial or complete
rubs can occur when a rotating shaft
comes in contact with stationary parts like
seals or in abnormal cases of bearing
(and/or instrumentation) failures. The rub
causes the orbit to take on different random
shapes.
17. OIL WHIP
• The oil whip phenomenon occurs when the
rotor is passing through its critical speed.
Oil whip is a destructive bearing defect.
• The precession of vibration is in the
forward direction in this case, but some
reverse 1× and sub-synchronous
components are present due to anisotropy
(changes in response when operating
conditions change) of the bearing pedestal
stiffness.
18. HOW IS A "SPECTRUM" PLOT CREATED?
• The vibration is sampled (collected) over a pre-determined period of time
• sometimes a relatively simple sine wave, it will far more often be a
complex signal with a number of different frequency components
• The complex signal shown below is made up of a 1x rpm component and
a 5x rpm component being generated by the machine
• There can be, and usually are, far more influences - misalignment, bearing
problems, soft foot, looseness, frequency modulation, amplitude
modulation, and so forth
19. STEP OF CREATE SPECTRUM PLOT
1. Waveform and its half spectrum
2. Waveform and its half spectrum
3. Combined orbit and its full spectrum
20. CIRCULAR ORBITS AND THEIR FULL SPECTRA
Forward Precessi
on Spectrum on
forward side of
plot
Reverse Precession,
Spectrum on reverse
side of plot Direction
of rotation – CCW
Forward Precession,
Spectrum on forward
side of plot Direction
of rotation – CW
Reverse Precession,
Spectrum on reverse side of
plotDirection of rotation - CW
21. FULL SPECTRUM OF ELLIPTICAL ORBIT
• Orbit is generated by two counter rotating
vectors
• Forward spectrum length is twice the length
of forward rotating vector
• Reverse spectrum length is twice the length
of reverse rotating vector
• Major axis of ellipse = a +b
• Minor axis of ellipse = a - b
• Original orbit cannot be reconstructed from
full spectrum because there is no phase
information.
3 possible orbits are shown
22. CIRCULAR & ELLIPTICAL 1X ORBITS
• Direction of precession is indicated by dominant line of
“Forward” and “Reverse” components.
• Flatness of ellipse is determined by the relative size of
forward and reverse components
• When orbit is circular there is only one spectrum line
• When orbit is a line the spectrum components are
equal.
• Therefore, the smaller the difference between
components, the more elliptical the orbit.
23. REFERENCE
• Arka Sen, dkk., Polar and Orbit Plot Analysis for Unbalance Identification in
A Rotating System, Department Of Mechanical Engineering, National
Institute Of Technology Durgapur, West Bengal, India
• Agnes Muszynska, Vibrational Diagnostics of Rotating Machinery
Malfunctions, Senior Research Scientist & Research Manager, Bently Rotor
Dynamics Research Corporation, Minden, Nevada, U.S.A.
• http://freevibrationanalysis.blogspot.co.id/
• https://www.vibrationanalysis.co.uk
