4 vibration analysis basics

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Peru, Au gu st 2015

SIXTO SARMIENTO CHIPANA

Vibration Analyst Level III

Machinery Lubricant Analyst Level II

[email protected]

VIBRATION ANALYSIS

INTRODUCTION


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Vibratión Analysis basics


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Vibration Analysis

Why?


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Vibration Analysis provides us with early warnings ofmany potentially serious problems in rotating machinery

Worn/Damaged Bearings

Worn/Damaged Gears

Lubrication Problems

Misalignment

UnbalanceLoose Fittings/Foundation

Why use Vibration Analy

sis?


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Vibration waveforms Vibration amplitude

Time Domain

Frequency Domain

Vibration Analysis

Basic Concepts


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Examples of Vibration


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The Simple Harmonic Oscillator

What is Vibration?


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Mass-Spring Systems with Damping

What is Vibration?


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The simplest possible vibratory motion that can

exist is the movement in one direction of a masscontrolled by a single spring.

Such a mechanical system is called a single degree

of freedom spring-mass system

What is Vibration?


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What is Vibration?


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The machine turns to 1800 RPMThe machine has a large number of frequencies

For example:Line frequency: 60 Hz – 3600 CPM

Fundamental frequency: 1800CPM

The fundamental frequency coincides with the frequency of rotation

What is Vibration?


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VIBRATION CHARACTERISTICS

AMPLITUDE

Displacement

Velocity

Aceleration

FRECUENCYFASE


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wtsinAx

Displacement

Aceleration

Velocity

wtcosAwx

wtsinAw-x2

Desplacement

Aceleration

Velocity

How do we measure amplitude?


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Choose a point

to measure

C h o

o s e

a

d i r e c

t i o n

l

b


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Measurement locations - vibration

measurement


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We can measure...

DISPLACEMENT

VELOCITY

orACCELERATION

A i i i


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DISPLACEMENT = Distance

traveled (miles)

At any point in time...

VELOCITY =

Speedometer reading (mph)

ACCELERATION =

How far your foot is down.


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• Actual physical movement of a

vibrating object• Emphasizes lower frequency

components

•

Measured in mils or microns

Displacement


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• Measuring total mass trajectory.

• It is used for low frequencies (from 0 to 1.200CPM)

•

Is related to stress.

• It uses an accelerometer and the signal isintegrated doubly to obtain displacement

Displacement


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Time

A m p l i t u d e

This is a time domain recording

of vibration displacement

Displacementis the easiest to visualize


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• Measure of the speed at which the

mass is moving

• Preferred unit of measurement as iteffective over over a wide range,

from low frequency to high

frequency• Measured in inches / sec or mm / sec

Velocity


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• Measuring the rate at which the mass ismoving during its oscillations.

• It is used to frequency ranges between600 and 60,000 CPM and is related tofatigue.

• It is measured directly with a "pick up"speed or an accelerometer where thesignal is integrated to obtain velocity.

Velocity


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Velocity or speed is zero as the object

comes to a stop before moving in the

opposite directionSpeed or velocity is greatest

here


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•

Rate of change of Velocity• Emphasizes high-frequency peaks in

a spectrum

•Measured in “g’s”

Acceleration


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• It is the optimum for high frequencies, greater

than 60,000 CPM and is related to the force.

• Acceleration is more difficult to measure the

amplitude of vibration, but more often is the

parameter directly measured in the field using an

accelerometer.

Acceleration


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Acceleration is greatest where velocity is at a

minimum

Object has decelerated and stopped and is about

to begin accelerating again

Comparison of Velocity Displacement & Acceleration


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Comparison of Velocity, Displacement & Acceleration

Acceleration is usedfor measuring high

frequencies:

Bearing FrequenciesVelocity is best over a wide

frequency range

Displacement is used for

measuring low frequencies:

Low RPM


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Simple Harmonic Motion

This displacement waveform is sinusoidal.


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Simple Harmonic Motion - The Simplest Vibration

ANY vibration can be expressed as a sum of individual simple

vibrations.

=

++


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The Frequency Domain

= + + + ...

Frequency

A m p l i t u d eUse a graph to

show how the

complex signal isbroken down into

simple ones....


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Time & Frequency Domain

Time Domain waveform shown as many different frequencies

and amplitudes


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Sinusoidal time domain

Single peak in frequency

domain.


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Noise in time domain

Noise in frequency domain


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• Vibration measurement

– Point & Direction

• Displacement, velocity, acceleration

• Time domain waveforms

• Complex vibration

= sum of simple vibrations

• Frequency domain

Review


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Review:

GIRDHAR, P. (2004). Practical Machinery Vibration

Analysis and Predictive Maintenance. Third Edition. USA:

Elsevier Practical .

https://www.youtube.com/watch?v=DUznmZvSQOU
https://www.youtube.com/watch?v=DUznmZvSQOUhttps://www.youtube.com/watch?v=DUznmZvSQOU

4 vibration analysis basics

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