Vibration monitoring

LOHITKUMAR VASTRAD
1
BMS College of Engineering
Department of Mechanical
Engineering
VIBRATION MONITORING AND ANALYSIS
presentation
by
Mechanical department,

MACHINE MAINTENANCE TECHNIQUES
Three types of maintenance schemes are be used in practice
1.Breakdown maintenance: In this technique the machine is allowed to fail
and after failure it will be replaced by another machine.
2. Preventive maintenance: Maintenance is performed at fixed intervals such
as every 3000 operating hours or once a year.
3. Condition-based maintenance: Measurements are carried out between
fixed intervals such that the changes are observed in running conditions
regularly.
2

LITERATURE REVIEW
1. Joëlle Courrech and Ronald L. Eshleman in their paper titled
“Condition monitoring of Machinery” have reviewed the basic concept
of Vibration based condition monitoring and discussed some of its
application.
2. J. Mitchell in his work “An Introduction to Machinery Analysis and
Monitoring” has explained the concept of vibration analysis technique
to achieve condition monitoring.
3

 Several methods can be used to monitor the condition of a machine, as
indicated below.
METHODS OF CONDITION MONITORING TECHNIQUE
4

VIBRATION MONITORING
 Vibration monitoring is the most common method employed for achieving
condition monitoring of a system.
 Because any mechanical reciprocating or rotating machines generate their
own vibration patterns (signatures) during operation.
 So, in this method the vibration levels of the machinery are initially
measured with the help of suitable transducers.
 These vibration levels measured can be used to predict the catastrophic
failure of the machinery and planning for subsequent scheduling of
repairing works.
5

Warning signs of machine failure
Figure 2 : The warning signs of machine failure
( Courtesy: Mechanical Vibrations- SS Rao)6

VIBRATION MONITORING (Contd..)
 Raw signals obtained contain a lot of background noise which makes it
difficult or even impossible to extract useful information by simply
measuring the overall signal.
 It becomes necessary to develop an appropriate filter to remove the
unwanted signals.
 Vibration should be measured at carefully chosen points and directions in
order to capture useful monitoring data,.
7

Types of Vibration Monitoring systems
Vibration monitoring systems are broadly classified into two types
1. Periodic monitoring system or off-line monitoring:
 In this type of monitoring system machine vibration is measured or
recorded initially and later it will be analyzed in the field.
It is usually used when
 Very early warning of faults is required.
 Measurements are made at many locations on a machine.
 Machines are complex.
8

2. Permanent monitoring system or online monitoring:
 In this type of monitoring system machine vibrations are measured
continuously at selected points of the machine.
 These measurements obtained will be constantly compared with
acceptable levels of vibrations as per the vibration severity charts, given
by standards such as ISO 2372.
 The main function of this system is to protect the equipment by providing
a warning and subsequently shut the machine down when a preset safety
limit is exceeded.
 Here transducers are mounted permanently at the selected measurement
points on the machine.
9

VIBRATION ANALYSIS
 Vibration Analysis is a two step process involving the ACQUISITION and
INTERPRETATION of machinery vibration data.
 Its purpose is to determine the mechanical condition of a machine and
specific mechanical or operational defects.
 The Data Acquisition procedure is a means of systematic measuring and
recording of the vibration characteristics needed to analyze a problem.
 The Data Interpretation involves comparing the recorded data with
characteristic vibrations of various standard causes.
10

Vibration Analysis Techniques
a) Time-Domain Analysis:
 Time-domain analysis uses the time history of the signal.
 The signal is stored in an oscilloscope or a real time analyzer and
transient impulses are noted.
 For e.g. discrete damages such as broken teeth in gears and cracks in
races of bearings can be identified easily from the waveform of the casing
of a gearbox.
Figure 4: Time-domain waveform of a faulty gearbox
(Courtesy: Mechanical Vibrations- Singiresu S. Rao,
University of Miami)
11

b) Frequency-Domain Analysis:
 The frequency-domain signal or frequency spectrum is a plot of the amplitude
of vibrations versus the frequency.
 As the machine starts developing faults, its vibration level and the shape of the
frequency spectrum changes.
 By comparing the frequency spectrum of the machine in damaged condition
with the reference frequency spectrum of the machine in good condition, the
nature and location of the fault can be detected.
12

 Another important characteristic of this spectrum is that each rotating element in a
machine generates frequency which can be easily identified, as illustrated in Fig.6
Figure 6: Relationship between machine
components and the vibration spectrum.
(Courtesy: Mechanical Vibrations-
Singiresu S. Rao, University of Miami)
13

DATAACQUISITION
 Data acquisition is the essential first step in vibration analysis.
 The right data must be acquired under the right conditions to completely
interpret a machine’s condition.
 Data acquisition can be done in several ways depending on the available
instruments.
 Displacement, Velocity and Acceleration are the measurement parameters
used in Data acquisition.
14

Common types of measurements used in Data
acquisition
i. Overall vibration amplitude measurements: These measurements provide a
quick check of general machinery condition.
ii. Amplitude Vs Frequency measurements: Amplitude Vs Frequency
measurements provide frequency spectrum which is used to pinpoint the
problem to a specific frequency or range of frequencies.
iii. Amplitude Vs Time measurements: These measurements can be made
during machine operation to detect vibrations that would not be apparent from
Amplitude Vs Frequency analysis.
iv. Phase measurements: Phase measurements are important when analyzing
mechanical problems in machinery.
Phase measurements offer a convenient way to determine how one part is
vibrating relative to another part.
15

DATA INTERPRETATION
 Once the necessary information have been collected by any means
(manual, or semi-automatic or automatic) the next step is to review
and compare the readings with the standard vibration pattern of
various defects.
 If a machine part has some defect, the frequency of vibration
resulting from this defect will be a multiple of the RPM.
 This multiplying factor will be different for different defects.
16

Some causes Of Vibration and its identification through data
interpretation
i) Unbalance: It is found that the vibration caused due to unbalance will be identified by
observing the Vibration amplitude vs frequency data curves generated by the recorder.
ii) Mechanical looseness: The vibration due to looseness can be detected from Amplitude
vs Frequency when taking the reading in vertical direction.
iii) Misalignment : A comparative axial vibration is the best indication of misalignment
or a bent shaft.
17

USE OF COMPUTERS IN CONDITION MONITORING
PROGRAMS
 Computers can be of great help in handling, filing, storing data and in performing tedious
computations such as spectrum comparison and trend analysis.
 A condition monitoring system which incorporates computer must include.
i. A recording device for storing the analog or digital time signals or frequency spectra.
ii. An analyzer with both fast Fourier transform (FFT) narrowband analysis and advanced
diagnostic techniques.
iii. A computer and appropriate software which provides
a. Management of the measurement program, storage of reference spectra.
b. A comparison of spectra and a printout of significant changes.
c. Trend analysis of any chosen parameter (individual component or overall level in a
given frequency range).
18

CONCLUSION
 The technique of vibration based condition monitoring helps in
finding the defects in system and to predict the catastrophic failure
of the system.
 The applications of vibration monitoring have been highlighted and
recent developments in the use of computers in condition monitoring
programs have also been presented.
19

REFERENCES
[1] Eshleman, R. L., “Machinery Vibration Analysis II Notes,” Vibration Institute,
Willowbrook, Ill., 2000.
[2] Eshleman, R. L.: “Basic Machinery Vibrations,” VI Press, Clarendon Hills, Ill., 1999.
[3] Mitchell, J. S.: “An Introduction to Machinery Analysis and Monitoring,” Penwell
Publishing Company, Tulsa, Okla., 1981
[4] Joëlle Courrech and Ronald L. Eshleman,: "Condition monitoring of machinery", A paper
on Condition monitoring and analysis.
[5] A book on "Mechanical Vibrations" by Singiresu S. Rao, University of Miami.(ISBN 978-
0-13-212819-3) Page no: 870-928.
20

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