BasicVib

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Describe Basic

VibrationConcepts

0 36090 270180

Human DevelopmenConsultants Ltd.

Training Module

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Vibrat ion conc epts

Febru ary, 2000

HDC Human Development Consultants Ltd.

All rights reserved. No part of this publication may be copied, reproduced, stored in a computer orretrieval system, published, distributed, or transmitted in any form or by any means whatsoever, including

without limitation by mechanical means, photo copying, recording, digital or electronic media, the Internet,or otherwise, without the express prior written permission of HDC Human Development Consultants Ltd.(HDC). HDC grants to the purchaser of a Single User License(as defined in the agreement with suchpurchaser) a limited license to store the electronic file(s) on one computer only and to make a singlepaper copy of this publication. HDC grants to the purchaser of a Site License (as defined in theagreement with such purchaser) a limited license to store the electronic file(s) on one local area networkserver accessible by individual users' computers at one site or location only and to make paper copies ofthis publication for a companys employees at the same site or location only. Each site or location mustpurchase a separate Site Licensefor employees at that site. HDC grants the purchaser of a CorporateLicense(as defined in the agreement with such purchaser) a limited license to store the electronic file(s)on its intranet and on computers at company sites or locations and to make paper copies for any or allemployees. Nothing in the foregoing restricts, amends or abrogates the provisions of the agreementbetween HDC and the purchaser of the applicable license. Any copying or use other than pursuant tosuch a license is illegal. For further information, please consult the applicable license agreement

This publication is designed to provide general information regarding the subject matter covered. Carehas been taken to ensure the accuracy of the information and that the instructions contained in thispublication are clear and reflect sound practice. The user understands that HDC is not providingengineering services. The user understands that any procedures (task steps) that are published orreferenced may have to be modified to comply with specific equipment, work conditions, companystandards, company policies and practices, legislation, and user qualifications. HDC does not make anyrepresentations, guarantees, or warranties of any kind whatsoever with respect to the content hereof andthe results to be achieved by implementing the procedures (task steps) herein. To the maximum extentpermitted by applicable law, in no event shall HDC be liable for any damages whatsoever (includingwithout limitation, direct or indirect damages for personal injury, damages to the environment, damages tobusiness property, loss of business profit, or any other pecuniary loss). The use of the information andprocedures (task steps) herein is undertaken at the sole risk of the user.

ISBN 1-55338-005-3

Canadian Cataloguing in Publication Data

1. MachineryVibration. I. HDC Human Development Consultants.TJ177.D48 2000 621.811 C00-900600-1

This training kit consists of the following parts:

Training Module and Self-Check Knowledge Check and Answer Key

Blank Answer Sheet Performance Check

Published by HDC Human Development Consultants Ltd.

Published in Canada

HDC Human Development Consultants Ltd.

Website: www.hdc.caE-mail: [email protected]: (780) 463-3909

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Vibrat ion con cepts

Febru ary, 2000 Page i of ii

Human DevelopmentConsultants Ltd.

Contents

Training Objectives 1

1 Introduction 1

2 Purpose and Importance of Vibration Analysis 4

3 Rotating Equipment Vibration & VibrationCharacteristics 53.1 Vibration Displacement and Frequency 5

3.2 Vibration Velocity 10

3.3 Vibration Acceleration 14

3.4 Vibration Phase 17

3.5 Random Vibrations and Shock Pulses 21

3.6 Identifying Vibration Characteristics 22

4Causes of Rotating Equipment Vibration 22

4.1 Rotating Equipment Imbalance Vibration 23

4.2 Rotating Equipment Misalignment Vibration 25

4.3 Resonance 28

5Vibration DetectionTransducers 31

5.1 Displacement Transducers (Proximity Probes) 32

5.2 Velocity Transducers 33

5.3 Piezoelectric Accelerometer Transducer 35

5.4 Mounting Transducers 36

5.5 Using Handheld Mounted Transducers 38

5.6 Transducer Location 39

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Febru ary, 2000 Page ii of ii


Contents (continued)

6Instruments for Displaying Vibration Data 40

6.1 Fixed Vibration Instruments 41

6.2 Portable Vibration Instruments 42

7 Self-Check 47

8 Self-Check Answers 53

9 Glossary 53

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Febru ary, 2000 Page 1 of 55


Training

Objectives

Upon completion of this training kit, you will be able to:

Describe the purpose and importance of vibration

detection and analysis

Describe the roles of operators and maintenance

personnel in vibration detection and analysis

Describe vibration characteristics

Identify the three most common sources of vibration

Describe the vibration characteristics for each source of

vibration

Identify the common types of vibration transducers

Describe the effectiveness of different types of

transducer mounting methods

Describe vibration measurement instruments

Complete the site vibration familiarization checklist

NOTE This training kit may contain technical terms which are new to

you. Refer to the glossary, located at the end of this module,

for an explanation of terms.

1 IntroductionEach major piece of equipment must function reliably to ensure

optimal productivity. Of all the different types of equipment,

rotating equipment tends to be among the most critical

because:

rotating equipment has moving parts that are more likely to

fail than stationary parts

failure of rotating equipment that is part of a process system

often results in a loss of system throughputs and can

potentially cause a complete shutdown of the system

All rotating equipment vibrates. The amount of vibration

indicates the condition of the equipment. Because vibration can

be seen, felt, and heard, you can determine if rotating

equipment is operating normally or if there is a change in the

equipments vibration.

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Using your senses to determine vibration levels requires

experience. You must first be able to identify the vibration

created by equipment operating normally (that is, establish a

base line for normal vibration) in order to identify abnormalvibration. When the vibration of a piece of equipment slowly

changes over a period of time (for example, six months), you

may not be able to recognize the change in vibration because:

the day-to-day change in vibration is too small to be

detected

you may forget the baseline vibration of the equipment

As rotating equipment technology improves and becomes more

reliable and energy efficient, vibration levels drop. Using your

senses to detect and measure these lower levels of vibration is

not sufficiently reliable to determine the operating condition of

the equipment. Using vibration detection and diagnostic

instruments improve the reliability of vibration measurements.

Vibration instruments also provide data on specific vibration

characteristics such as frequency and amplitude.

Vibration detection and diagnostic

instruments may either be permanently

installed or portable:

permanently installedvibration

detection instruments continuouslymonitor the vibration levels of

rotating equipment.

If the rotating equipment exceeds a

preset vibration limit, the vibration

detection instruments sound an

alarm to notify you of the operating

condition. If the rotating equipments

vibration level continues to rise and

exceeds a second preset vibrationlimit, the vibration detection

instruments will shut down the

rotating equipment.

Figure 1Fixed Vibration Detection

Instrument

DANGER ALERT

GAP

A B

BUFFERED TRANSDUCERS

OK

DANGER

ALERT

BYPASS

DUAL VIBRATIONMONITOR

4

3

2

1

0

5

4

3

2

1

0

5- 24 -

- 20 -

-16 -

- 12 -

- 8 -

- 4 -

- 0 -

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portable vibration

detectioninstruments allow

company personnel to

measure equipmentvibration on both a routine

and a needs basis. On a

needs basis, vibration

readings can help to

determine the specific

location and cause of

vibration.

Figure 2Handheld VibrationDetection Instrument

Vibration detection instruments detect vibration by converting

vibration to electrical signals. The signals are amplified and

then displayed as data on a meter, a digital display, or a chart.

Personnel then interpret the data to determine the location and

specific cause of the vibration. The vibration may be due to a

number of factors including:

imbalance

misalignment

resonance mechanical defects of bearing, couplings, etc.

Often, routine equipment vibration measurements are taken as

part of a facilitys preventative and predictive maintenance

program. When new or rebuilt rotating equipment is first

operated, a base line for vibration is established. As the

equipment wears, company personnel can compare the current

vibration data with base line vibration data to determine the

degree of change in equipment operating conditions. The

information is then used to identify the cause of the abnormalvibration and predict when a failure might occur. A more

informed decision can then be made as to the type of corrective

action required to optimize production and minimize losses.

ON OFF

PHONES

MILE

02

4 6 8 1214

AMPLITUDE

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This module, Describe Basic Vibration Concepts, describes

vibration characteristics, causes of vibration, and methods used

to detect vibration. This module also describes common types

of instruments used to measure vibration and identifies thevibration instruments specific applications. A more advanced

module titled Perform Vibration Analysisprovides directions for

using vibration data to identify sources of vibration, to

determine the severity of the vibration, and to help decide on

the best course of action.

2 Purpose and Importance of

Vibration AnalysisAll rotating equipment vibrates to some degree. Maintaining the

equipments vibration level within the manufacturers

specifications contributes to the safe and cost-effective

operation of that equipment. Excessive equipment vibration

could lead to premature equipment failure, increased

maintenance cost, and decreased productivity. To effectively

monitor equipment vibration, maintenance personnel

periodically conduct a vibration analysis. Vibration analysis is

the process by which maintenance personnel:

take vibration readings from rotating equipment

determine the severity of the vibration

identify the cause of excessive vibration in the equipment

suggest corrective action to eliminate the vibration problem

At some facilities, vibration analysis is used as part of a

comprehensive preventative and predictive maintenance

program. Maintenance personnel use vibration data to predict

the amount of time the rotating equipment can run before

breakdown. Preventative action can then be taken to avoid

costly repairs and lost productivity. If an effective preventativeand predictive maintenance program is used, the following

benefits can be realized:

safer work environment

higher productivity

increased operating time

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higher levels of rotating equipment performance

decreased maintenance cost

lower energy cost

reduced product waste

3 Rotating Equipment Vibration and

Vibration CharacteristicsVibration is defined as a back and forth motion across a neutral

position (equilibrium). The back and forth motion of a vibrating

object is complex because the vibrating motion has several

different characteristics including:

displacement

frequency

velocity

acceleration

phase

Industry uses several different methods, including numerical

values and graphs, to describe these characteristics of

vibration. Maintenance must be able to use the various forms of

vibration data to analyze equipment vibration and determine the

vibrations source.

3.1 Vibration Displacement and Frequency

Graphs of the vibrating motion are often used to describe the

characteristics of the vibration. For example, the back and forth

motion of the vibrating rod in Figure 3 can be measured and

results plotted on graph paper. The motion of the vibration is

depicted as a wave form (known mathematically as a sine

wave form).

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Figure 3Graphic Representation of a Vibrating Object

Peak

Displacement

Displacement 0

Peak toPeak

Positionof Rod

Time

Period(time for one cycle)

The graph in Figure 3 describes two vibration characteristics:

Displacementis the distance the vibrating metal rod moves

from its neutral position. The graph shows the maximum

displacement aspeak amplitude. Peak-to-peak amplitudeon

the graph indicates the maximum distance the rod moves from

one extreme position to the other extreme position.

Displacement is measured in mils (one mil is equal to 0.001

inch) or in micrometers (one micrometer is equal to 0.001

millimeters). When there is an increase in the rods

displacement, the vibration becomes more severe.

Frequencyis the number of vibration cycles that the rod

experiences in one second. The graph in Figure 3 shows one

cycle of the rods vibration and theperiodof time required to

complete that cycle. Mathematically, the frequency of the

vibration is calculated by using the following formula:

Frequency =1

= cycles per second, or hertz f =1

t =1

period t f

Frequency = number of cycles

f =cycles

time t

or

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Several important vibration analysis concepts are associated

with frequency:

All objects have a naturalvibrating frequency. If an object is

struck (e.g., with a hammer), the object will vibrate at itsnaturalfrequency. Most equipment has more than one

natural vibration frequency. The natural frequency of the

equipment changes when the equipment is weighted or

when bracing is added.

An increase in vibration frequency results in an increase in

vibration severity (the degree to which the vibration

contributes to component failure). Each time the metal

completes one cycle of vibration, the metal experiences

stress. As the frequency increases, the metal flexes more

often. If the vibration stress is above the metal fatigue limit,

the increased frequency of flexing reduces the time for the

metal to fail due to fatigue.

The severity of vibration in equipment tends to remain

constant (reaches a steady state) because the materials

making up the equipment absorb the energy created by the

vibrating forces. However, if the forced vibration is at the

same frequency as the equipments natural vibration

frequency, the vibration is amplified. Amplification of the

vibration occurs because the motion of the forced vibrationis in time with the natural vibration. An example of this

condition, known as resonance, would be pushing a child on

a swing. If timed correctly, each push adds to the amplitude

of the swings motion.

The motion of a point on a rotating shaft can be described in

terms of a sine wave, as shown in Figure 4. The graph

shows the verticalposition of the point on the shaft as the

shaft rotates. In this graph, time is measured in terms of

degrees.

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0

Po

sition

Time

90 270180 360

90

270

3600 180

The ability to describe rotation in terms of a sine wave is

fundamental to vibration analysis. For example, an

imbalanced shaft exerts a force that produces a vibration

which matches the rotational speed of the shaft.

If the rotational speed of the shaft matches the natural

frequency of the equipment, severe vibration occurs

because resonance is created. The term critical speedis

used to identify the rotational speed which matches the

equipments natural vibration. Manufacturers attempt to

design equipment so that the critical speed is outside the

operating range of the equipment. However, some turbines

have a critical speed which is below the operating speed of

the turbine. During startup, the turbine must pass through

the critical speed to reach the operating speed. When this

situation occurs, the manufacturer recommends that the

turbine be brought up to the operating speed as quickly as

possible. This startup strategy minimizes the length of time

at which the turbines rotation speed is near the critical

speed.

In vibration analysis, the fundamental frequency equals the

equipments rotational speed. The fundamental frequency

can be represented as revolutions per second:

1 Hz (frequency)= 1 rps (revolution per second)

Figure 4GraphicRepresentationof a Rotating

Point

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Some vibration instrument manufacturers refer to the

rotational speed in terms of rpm (revolutions per minute). To

convert units:

rpm = 60 rps = 60 Hz

rpm = 60 x rps rpm = 60 x Hz

rps =rpm Hz =

rpm60 60

Harmonicsare vibration frequencies which are exact

multiples of the fundamental frequency. To calculate a

harmonic frequency, multiply the fundamental frequency by

any whole number (times 1, times 2, etc.). For example, ifthe fundamental frequency is 1000 Hz, the next two

harmonic frequencies would be 2000 Hz and 3000 Hz.

Since the fundamental frequency equals rotational speed,

harmonics can then be calculated as multiples of the rps

(example, 1 x rps, 2 x rps, etc.). Some vibration instrument

manufacturers express the harmonic frequencies in terms of

rpm (example, 1 x rpm, 2 x rpm, etc.).

Often the presence of harmonic frequencies is an indication

of the source of the vibration. The training kitPerform

Vibration Analysisprovides a detailed description of the

relationships between vibration characteristics and the

source of the vibration.

When two adjacent machines are operating close to the

same rpm, a sound which alternately builds up and then falls

in amplitude can sometimes be heard. This sound is called a

beatfrequency. The beat frequency is equal to the difference

between the two rpms of the machines. For example, one

machine is operating at 600 rpm (10 Hz) and another

machine is operating at 660 rpm (11 Hz). The difference in

rotational speed between the two machines is 60 rpm (1 Hz).

In this example, the amplitude of the sound rises and falls

once per second. A beat frequency can sometimes cause

difficulties in balancing equipment because the beat

frequency can cause a continuous shift in vibration phase.

Vibration phase is explained in section 3.4 of this module.

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1 Second

600 rpm

(10 Hz)

660 rpm

(11 Hz)

1 Second Beat

Time

Loudness

3.2 Vibration Velocity

Velocity is the measure of the speed of the vibrating object.

Velocity is defined as distance divided by time. For example,

the distance between two cities is 300 kilometers. If it takes 3

hours to drive the 300 kilometers, the velocity is 100 km/hr

velocity =distance

=300 km

=100 km

time 3 h h

In Figure 6 below, the instantaneous velocity of the vibrating

rod at the extreme positions of its motion (identified by points

B and D on the graph) is zero because the rod has to stop

momentarily and then reverse direction.

Positionof Rod

Time

0 90 270180 360A

C

D

B

Zero Velocity at ExtremeDisplacement Positions

Displacement

E

Figure 5Beat Frequency

Figure 6Velocity atExtremeDisplacementPositions

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At the extreme displacement positions, B and D:

Starting at position B on the graph in Figure 7, the rod speeds

up until it reaches the neutral displacement position (point C on

the graph) and then begins to slow down until it reaches the

other extreme position (position D). At the neutral position, the

rod is traveling at its maximum velocity.

Positionof Rod

Time

0 90 270 360A

C

D

BMaximum Velocity of

Neutral Displacement Position

Displacement

180

E

Since velocity is defined as distance divided by time, the

velocity at any point on the displacement graph can be

represented by drawing a tangent (sloped line) to the curve at

that point (see Figure 8).

Figure 7Velocity at

Neutral Position

instantaneous velocity = 0 displacement

= 0 velocity

time

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Positionof Rod

Time

0 90 270180 360A

C

D

Displaceme

nt

B

Displacement

Time

X

E

The velocity at point X in Figure 8 is determined by the slope of

the tangent:

velocity = where means changedisplacement

time

The velocity of the rod can be calculated for all points on the

displacement graph and the results graphed as shown in

Figure 9.

The velocity of the vibration is directly related to displacement

and frequency. An increase in displacement or an increase in

frequency results in an increase in the velocity of the vibration.

Since both displacement and frequency are also indicators of

the severity of vibration, vibration velocity is often used as a

direct measure of vibration severity. On large rotating

equipment, the fixed vibration protective devices often use

vibration velocity as the parameter for measuring vibration

severity.

Figure 8Velocity as aTangent to the

DisplacementCurve

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Positionof Rod

Time

0 90 270180 360A

Displacem

ent

C

D

Positionof Rod

Time

0 180 360

A

90 270

MaximumVelocity

E

B C

C

Object Momentarilyat Rest

Velocity

Displacement

Time

DISPLACEMENT

VERSUS TIME

VELOCITYVERSUS TIME

B

MaximumVelocity

MaximumVelocity

E

Vibration velocity is normally measured in millimeters per

second, or inches per second. As an example, a protective

device for an electrically driven centrifugal pump may be set to

shut down the pump when the pumps vibration exceeds

5 mm/s (five millimeters per second) which is equivalent to

0.2 in/s (inches per second).

Figure 9Velocity Graph

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3.3 Vibration Acceleration

Acceleration is defined as the rate of change of velocity. As

examples:

1. To win a race, a race car accelerates rapidly to reach the

maximum velocity in the shortest time possible.

2. A passenger car accelerates slowly to reach the maximum

allowable velocity to ensure passenger comfort and fuel

economy. Vibrating objects also experience acceleration. At

the extreme positions of vibration, the object experiences

maximum acceleration because it has to change direction.

At the neutral position, the vibrating rod momentarily reaches

its maximum velocity, and no acceleration takes place.

Figure 10 uses the velocity graphic (from Figure 9) to identifythe rods acceleration at the rods extreme and neutral

positions.

Positionof Rod

0180 360

AE

B D

C

Maximum Acceleration(extreme displacement

positions)

Velocity

Displacement

Time

90 270

Zero Acceleration(neutral displacement

positions)

Time

Figure 10Acceleration atExtreme andNeutralDisplacementPositions

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Vibrat ion conc epts

F b 2000 P 15 f 55

At neutral displacement positions A, C, and E the acceleration

is zero. At extreme displacement positions B and D the

acceleration is at maximum. Since acceleration is defined as a

change in velocity divided by time, the acceleration at any pointon the velocity graphic can be represented by drawing a

tangent to the curve at that point (refer to Figure 11).

End of Sample

A full licensed copy of this kit includes:

Training Module and Self-Check

Knowledge Check and Answer Key

Blank Answer Sheet

Performance Check

BasicVib

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