Vibration PPT

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Basic Vibration Analysis

Vivek Joshi

[email protected]

Cell:91-9833475974

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Objectives

Define Vibration

� What is it, where does it come from?

Describe measurement methods

Examine amplitude measurements

� Differences between the three

Choose proper transducer

� Right probe for the application

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General DescriptionVibration : defined………..

– Movement of a body about a reference.• Response to some form of excitation•The excitation is generally referred to as a forcing function

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Sources of VibrationImbalance

Misalignment

Belts

Gears

Bearings

� Rolling Element

� Sleeve

Looseness

Electrical Anomalies

Oil Instability

Resonance

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X

Y

Y • Amplitude

X • Time

Legend

General Description

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Vibration Measurement Parameters

Vibration breaks down into two separate areas called domains.

� The time domain displays a plot called a

waveform where the amplitude is displayed

over time, such as an oscilloscope monitors

an electrical signal.

� The frequency domain displays amplitude as

a function of how often something occurs in

one cycle.

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Lets imagine that the

shaft is rotating

(The center of the shaft is really

what we are plotting, not the mass

near the edge. We just use the

mass for illustration)

Introduction to Vibration

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Am

plit

ud

e

Locate it’s peak positive

position with a red mark.

Time


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Am

plit

ud

e

Time

As the shaft rotates, time

also shifts, so we’ll locate

this new position forward

with respect to time.


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Am

plit

ud

e

Time

Continuing this process…

The mass is now 180 degrees

from it’s original position.

We could call this peak negative.


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Am

plit

ud

e

Time

As the mass returns to the“zero” reference point,notice that it is now 180degrees from the correspondinggreen marker.


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Having completed 1 revolution,

the mass returns to peak positive

position. This is one cycle.

Am

plit

ud

e

Time


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Am

plit

ud

e

Time

Connecting the reference locations with a curve...


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Am

plit

ud

e

Time


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Am

plit

ud

e

Time


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Am

plit

ud

e

Time


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Time

Am

plit

ud

eIntroduction to Vibration


Y

X

Am

pli

tude

Time

Observed in the time domain as the

amount of time to complete a cycle

The resulting waveform resembles

a sine wave.


Y

X

Am

pli

tude

Time

Every detected signal appears.

The time waveform becomes

very complicated, busy.


Each waveform signal is then

“converted” to it’s respective spectral peak.

Frequency

Am

pl it

ude

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Waveform Generation

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2 Domains : TIME and FREQUENCY

FFT

Measurement Parameters

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Amplitude

3 ways to display

� All three show severity

� Some better than others

Displacement

� Distance a body moves

� Point “A” to point “B”

Velocity

� Measures the speed

� Time from point “A” to point “B”

� rate of displacement

Acceleration

� Total force acting

� Change of direction

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Am

plit

ud

e

Frequency

Displacement

Displacement accentuates LOW frequencies,

and attenuates HIGH frequencies.

How the different units treat data...

Amplitude Relationships

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Acceleration

Am

plit

ud

e

Frequency


Acceleration accentuates HIGH frequencies,

and attenuates LOW frequencies.

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Amplitude RelationshipsA

mplit

ud

e

Frequency

Velocity

Velocity does neither. It treats all frequencies equally.

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AccelerationDisplacement

Velocity

Am

plit

ud

e

Frequency

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(D)isplacement Mils Peak-to-Peak

(V)elocity In/Sec Peak

(A)cceleration G’s RMS

The Figure below show the relationship of these three types of units.

D

VA

-

Am

plit

ud

e

Period is Time “T”

360°= One (1) Shaft Revolution

Peak to Peak

Average

R.M.S.

0 to Peak

Time

Amplitude

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Phase: is the relationship between two events. You measure phase in degrees of rotation.

CB

360°

270°

Phase

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Time

0

+

-

Tach Pulse

Peak of Vibration Signal

Phase Measured

between the two

360 degrees

Phase Measurement

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A

B

You can also use phase to describe the relationship

between two events, as shown below. Disk A is 180º

out of phase with disk B.

Phase

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Common Vibration TermsDisplacement� distance a body travels

� units of mils (pk. to pk.)

Velocity� rate at which displacement occurs

� units of inches per second in/sec (pk.) or mm/sec RMS

Acceleration� rate of change of velocity

� units of G’s (RMS)

Frequency� how often an event occurs per unit time

Phase� relationship between two events relative to a reference

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1-24

� Signal processing– time waveform or spectrum

� Problem detection– examine both sets of data

– is there a problem?

� Diagnosis– what is the source ?– Root Cause

� Choosing the transducer– the right one for the best data

An effective program...

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Amplitude and Frequency - Review

Amplitude

� Acceleration

� G’s

� Velocity

� In/sec

� Displacement

� mils

Frequency

� Cycles per minute - CPM

� Cycles per second - CPS / Hz

� Revolutions per minute -RPM

� Orders - Frequency/TS

RPM vs CPM

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1-26

TransducersFunction

� Convert mechanical signal to electrical

Three basic types

� Displacement transducer

� Velocity transducer

� Accelerometer

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shaft

non contact

displacement

transducer

bearing

Transducers

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Displacement Xducers

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Displacement Xducers

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Displacement XducersAdvantages +

� Measures relative movement

� good for sleeve brg. machines

� very good for heavy machines

Disadvantages -

� permanently installed

� frequency response : DC to 1KHz

� power required

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Connection

Case

Conductor Coil

Spring

Magnet

Damper

Velocity Xducer

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Velocity XducerAdvantages +

� best signal to severity ratio

� good signal to noise ratio

� no power required

� single dif. / int. needed

� usually pretty hardy

Disadvantages -

� usually big

� heavy

� freq. 10 to 2Kz

� temperature sensitive

� comparatively expensive

� magnetic field sensitive

� orientation sensitive

� wear and temp. changes calibration

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Settle Time

Bias Voltage10.5 Vdc

20 - 30 Vdc@

2-8mAC.C.

Supply Voltage

Amplifier

PreloadedRef. Mass

Base

Mica InsulatorConductive PlatePiezoelectric Crystal

Electrical Insulator

to signal analyzer

Accelerometers

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AccelerometersAdvantages� broad frequency range

� small, light, rugged

� ICP needs no signal conditioning

� easy to mount

Disadvantages� poor as a hand held

� limited signal to noise ratio

� reads acceleration

� power required

� double integration needed

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hand held

magnet glue wax stud

natural frequency of accelerometer

frequency

12345

5

4 3 2 1

Accelerometers

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Overall Level Criteria

The overall level is a single number calculation of the unfiltered amplitude of a vibration waveform.

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Summary of Overall Vibration Standards

An insurance agent responsible for insuring companies and their equipment established the Rathbone chart in 1939. So he could set an adequate premium, he had to know the running condition of the machinery. The agent based his chart on casing measurements made on heavy, slow-speed machines.

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Summary of Overall Vibration Standards

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ISO Vibration Chart

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DuPont Severity Chart

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Locations for Data Collection

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Frequency Regimes

Synchronous

� NxRPM where N is an integer

Sub synchronous

� <1 x RPM

Non-synchronous

� F x RPM where F is >1x RPM but not integer

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Causes – Sub-synchronous

Another component in the machine

Another machine

Belt drives

Hydraulic instability� Oil whirl, oil whip

Rubs� rotor, shaft, wheel

Cage� fundamental train - rolling element brgs.

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Causes - Synchronous

Imbalance

Pitch line run-out

Misalignment

Bent shaft

Looseness

Blade / vane pass

Recips

Gears

Slot / Rotor Bar pass

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Causes – Non-synchronous

Another machine

Belt multiples

Brgs.

Resonance

Electrical

Chains

Compressor surge

Detonation

Sliding surfaces

Lube pumps

Centrifugal clutches

U-joints