Edoc-Vibration Study and Analysis

VIBRATION STUDY AND ANALYSIS
Technicians and operators every so often notice unusual vibrations or

noises on the plant or shop floor where they work daily. In order to
determine if a serious problem actually exists, they could proceed with a
vibration analysis. Vibration analysis implies, state of a
machine is identified on the basis of an analysis and study of vibration.
Successful application of vibration diagnosis requires in practice staff
with considerable degree of knowledge and
experience. Standard work of data gathering may
be performed by skilled personnel without academic educations, but
data assessment and processing of the machine condition is the task for
an engineer who has expertise in different areas like mathematics, design
of machines, dynamics, signal processing etc. and who is capable to use
this expertise in context.
Why is Vibration Study and Analysis Important?
The vibration analysis is a very important technique, in terms of

mechanical vibrations for machine diagnosis. It is based on the high
information content provided by the machine vibration signals that are an
indicator of machine condition, used for the diagnosis of faults. Vibration
analysis in a predictive maintenance program, is widely used for
monitoring and detection of initial and critical faults in machinery parts,
like shafts, bearings, rotors, couplings, motors etc. Some problems that
are usually detected by vibration analysis are: unbalance, misalignment,
bent shaft, rolling bearing faults, eccentricity, resonance, looseness,
rotor rub, fluid-film bearing instabilities, gear faults, belt/sheave
problems.
Following are the reasons that makes vibration study and analysis
important:
• Reduces equipment costs:

• Reduces labor costs
• Reduces lost production time
• Increases safety
• Increases revenue
• Increases efficiency of employee time
What is Done During Vibration Study and Analysis?
The main objectives when performing a vibration study typically fall into
one or more of three categories:
• Capacity: Map the dynamic behavior of the machine from current to

targeted speeds, yielding the most cost-effective approach by foreseeing
problems.
• Rebuild: Evaluate how rebuilds will affect the dynamic behavior of the
machine and its consequences at current or increased speeds.
• Troubleshooting: Locate and eliminate vibration sources currently having
a detrimental effect on the machinery or process.
Vibration analysis is performed by Fourier transform (by its decomposition
into Fourier series). All actions related to the study
and analysis, given below are employed in analyzers that are utilised in
vibration diagnostics.
There are different types of analyzers: operational or laboratory, with
one or more channels – but the principle of their operation is always the
same.
Fourier Transform
Periodical in time T, function x(t), can be stated as an infinite

or endless progression:
This expression means that the original function x(t) can be composed
from (infinite) number of sinusoids of different amplitudes, frequencies
of which are multiplies of the fundamental frequency ω.
Coefficients an and bn are Fourier or spectral coefficients of the function
x(t) and can be computed using expressions:
Discrete function x (t), which is defined on the set of N different instants

of time tk (k = 1, N), can be written as a finite Fourier series:
Fourier coefficients are usually depicted in the form of amplitude cn and

phase φn:
Then, the finite Fourier series can be written as:
This form of Fourier transform is called the discrete Fourier transform

(DFT). The resulting Fourier series, a set of sinusoids from which the
original waveform can be composed, is called a frequency spectrum.
There is a basic relationship between the length of the sample T, the
number of discrete values N, sampling (or capture) frequency fs,
frequency range and spectral (or frequency) resolution. Spectrum
frequency range is 0−fmax, where fmax is the Nyquist frequency and ∆f
is the frequency resolution (spacing between frequency lines).
An algorithm called Fast Fourier Transform (FFT) is used in up to date

analyzers, where N is an integer power of number 2. In fact, the upper
frequency spectral limit fmax is even more reduced in comparison with
the theoretical value (e.g. for N=211=2048, only 800 frequency lines are
used rather than 1024), which will be explained in the following chapter.
How is Vibration Study and Analysis Performed?
Raw time Waveform Analysis

Vibration study and analysis personnel
will utilize the magnitude various signals to find out the source and
nature of a problem. The most common measurement is that of vibration
– in units of acceleration, velocity or displacement. Study and analysis of
the raw time waveform gives helpful data for troubleshooting
many issues, involving those with gearboxes. Impulsive vibration is better
analyzed in the time domain.
Spectrum Analysis
This divides the total vibration into separate frequencies so that the
source of a given glitch can be easily recognized. This includes handing
out the raw time domain data through a
mathematical computation called the Fast Fourier Transform or FFT. The
FFT algorithm converts the original signal from the time domain into the
frequency domain. The outcome is that a complex signal
is divided into various contributing frequencies.
Speed Trials
Variations in vibration levels as a function of varying machine loads or
speeds are usually studied with the help of waterfall plots. Waterfalls,
also referred to as cascade plots or spectral maps, consist of a series of
spectrums placed one behind the other to generate a 3-dimensional
graph. Thee Y-axis denotes vibration amplitude, the X-
axis denotes Frequency and the Z-axis represents machine speed or time.
Waterfalls are helpful for the determination of resonances.
Impact Testing
Aim of impact testing is to recognize the natural frequencies
of usually smaller components in the machine, such as doctors, tube rolls,
drive shafts, showers, uhle boxes, etc. This accompaniment the speed
trial as the speed trial and resulting waterfalls
will denote multiple agitated natural frequencies.
The following challenge for the analyst is to find
out which module was really resonating as the waterfall depicts them all.
Synchronous Averaging
Synchronous averaging is utilised to separate the influence of vibration of
a single rotating source. This is performed by acquiring a tachometer
trigger from a prearranged felt or roll and then matching the data
acquisition time absolutely with the occurrence of the trigger. When
barring is a long-term problem, the system and history need to be
reviewed from problem inception through to present day symptoms.
Operating Deflection Shape (ODS) Analysis

Operating deflection shape analysis depicts how a machine shifts in
actual operation at particular frequencies of interest and assists find
out the reason of the motion. Frequencies utilised in the study
and analysis are usually multiples of running speed of the various rolls in
the system. If the data used in the ODS analysis is fit to natural
frequencies of the system (identified in speed trials, etc.) then the
resultant model will show mode shapes instead. Understanding which
modes are being agitated and the positions of largest movement allow for
the most effective structural modifications.
Finite Element Analysis

Finite Element Analysis (FEA) involves the mathematical modeling of a
physical system to predict the structural behavior of the system –
involving geometry, material properties, boundary conditions, etc. The
model is used to determine what the new natural frequencies of the
system will be after structural modification is applied. The model can be
as simple or extensive as required.
Following are the Different Data Acquisition Techniques:
Online Data Acquisition and Analysis

Critical machines are almost always provided with continuous online
monitoring systems. In here sensors are fit permanently on the machines
at suitable positions and linked to the online data acquisition equipment.
The vibration data are taken involuntarily for each position and the
analysis can be showed on local monitoring equipment or can
be moved to a host computer installed with database management
software.
This capability allows early identification of errors and provides protectiv
e action on critical machinery. Protective action performed by online
data acquisition and analysis equipment is in form of generating alarms
to caution the operators of an abnormal situation. In the case of critical
error, this protective action can turn off machines involuntarily to
prevent catastrophic failures. Moving the data to a host computer with
database management software improves the suitability and the power of
online data acquisition.
Advantages
• Performs continuous, online monitoring of critical machinery.

• Values are taken involuntarily with no human interference.
• Provides almost instantaneous detection of defects.
Disadvantages
• Reliability of online systems must be at the same level as the machines

they monitor.
• Failure can prove to be very expensive.
• Installation and analysis require special skills.
• These are expensive systems.
Portable Data Collectors/Analyzers

Modern data analyzers give data of any vibration characteristics in
any chosen engineering facility or unit. There are basically two types of
data collectors and analyzers, Single channel and Dual channel.
Advantages
• Can record, collect and display vibration information like a FFT spectra,
time domain waveforms and overall trend plots.
• Provides orderly collection of data.
• Automatically reports measurements out of pre-established limit
thresholds.
• Can perform field vibration analysis.
Disadvantages
• They are relatively expensive.

• Operator must be trained for use.
• Restricted memory ability therefore information must be
downloaded subsequently to collection.
Handheld Vibration Meter

A handheld vibration meter is an inexpensive and simple-to-use
instrument that is an essential part of any vibration program. Vibration
technicians and plant operators carry handheld analyzers and meters on
their periodical rounds. They provide a display of vibration levels, when
these are held in contact with machinery. The readout provides
immediate information that can be used to determine if the overall
vibration levels are normal or abnormal. These meters are
generally battery powered and utilise an accelerometer for sensing.
Advantages
• They are convenient and flexible and require very little skill to use.
• It is an inexpensive starting point for any new condition-monitoring
program.
Disadvantages
• Limited in the type of measurements that they can perform.

• No data storage capability.
Benefits of Vibration Study and Analysis
• Determination of the dynamic behavior of machine sections

• Determination of the mechanical condition of the machine
• Determination of the feasibility of a possible capacity increase
• Most cost-effective implementation of rebuilds
• Forecast future mechanical issues and reduce unintended shutdowns
• By repositioning old rolls to where they can still be utilised, we can
save on roll remakes.
• Solutions to problems such as barring and gear train failures

Edoc-Vibration Study and Analysis

Uploaded by

Copyright:

Available Formats

Edoc-Vibration Study and Analysis

Uploaded by

Document Information

Original Description:

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Edoc-Vibration Study and Analysis

Uploaded by

Copyright:

Available Formats

VIBRATION STUDY AND ANALYSIS

Technicians and operators every so often notice unusual vibrations or

The vibration analysis is a very important technique, in terms of

• Reduces equipment costs:

What is Done During Vibration Study and Analysis?

• Capacity: Map the dynamic behavior of the machine from current to

Periodical in time T, function x(t), can be stated as an infinite

Discrete function x (t), which is defined on the set of N different instants

Fourier coefficients are usually depicted in the form of amplitude cn and

This form of Fourier transform is called the discrete Fourier transform

An algorithm called Fast Fourier Transform (FFT) is used in up to date

How is Vibration Study and Analysis Performed?

Raw time Waveform Analysis

Operating Deflection Shape (ODS) Analysis

Finite Element Analysis

Following are the Different Data Acquisition Techniques:

Online Data Acquisition and Analysis

• Performs continuous, online monitoring of critical machinery.

• Reliability of online systems must be at the same level as the machines

Portable Data Collectors/Analyzers

• They are relatively expensive.

Handheld Vibration Meter

• Limited in the type of measurements that they can perform.

Benefits of Vibration Study and Analysis

• Determination of the dynamic behavior of machine sections

You might also like