Mfpt 59 Acceleration Measurements Session 4-19-05_comp

7/26/2019 Mfpt 59 Acceleration Measurements Session 4-19-05_comp

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BASICS OF ACCELERATIONBASICS OF ACCELERATIONMEASUREMENTS!MEASUREMENTS!

MECHANICAL FAILURE PREVENTION TECHNOLOGYMECHANICAL FAILURE PREVENTION TECHNOLOGY5959 thth MFPT FORUMMFPT FORUM

April 19, 2005April 19, 2005

John E. JuddJohn E. JuddDynamic Measurement Consultants, LLC Hamden, CTDynamic Measurement Consultants, LLC Hamden, CT

[email protected]@aol.com

GETTING RELIABLE AND USEFULGETTING RELIABLE AND USEFUL

INFORMATIONINFORMATION

FROM YOURFROM YOURACCELEROMETER.ACCELEROMETER.


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It is easy to get an

accelerometer tomeasure acceleration.

The problem is to keep itfrom measuringeverything else! .

Walter Kistler


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Today we will Talk About:Today we will Talk About:

Some basics of your vibration measurement.Some basics of your vibration measurement. What common industrial accelerometers are.What common industrial accelerometers are.

Their operating characteristics.Their operating characteristics.

Some of the potential problems & error sources.Some of the potential problems & error sources.

MORE IMPORTANTLY!MORE IMPORTANTLY!

Some of the information they can provide to yourSome of the information they can provide to yourPdM operation.PdM operation.

You may be surprised!You may be surprised!


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Why do you measure vibration?Why do you measure vibration?

To establish machine condition.To establish machine condition.

To extend the useful life of rotatingTo extend the useful life of rotatingmachinery.machinery.

To identify conditions that may lead toTo identify conditions that may lead toshortened life or catastrophic failure.shortened life or catastrophic failure.

Verify condition correction.Verify condition correction.


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SOME IMPORTANT THINGS TOSOME IMPORTANT THINGS TOKNOW ABOUT VIBRATIONKNOW ABOUT VIBRATION

MEASUREMENTMEASUREMENT!!

DISPLACEMENTDISPLACEMENT--VELOCITYVELOCITY--

ACCELERATION? WHAT IS THEACCELERATION? WHAT IS THE

DIFFERENCE AND WHY SHOULD YOUDIFFERENCE AND WHY SHOULD YOUCARE?CARE?


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VIBRATIONVIBRATIONS FOURS FOUR

RELATED CHARACTERISTICSRELATED CHARACTERISTICS

DISPLACEMENTDISPLACEMENT--DISTANCEDISTANCE-- D or XD or X--mil inchesmil inches VELOCITYVELOCITY--SPEEDSPEED-- D/sec =V (in/secD/sec =V (in/sec--cm/sec)cm/sec)

ACCELERATIONACCELERATION--(( V/sec)= in/secV/sec)= in/sec22

In g units = multiple of gravitational acceleration =In g units = multiple of gravitational acceleration =in/secin/sec22 /(386.1in/sec/(386.1in/sec2)2) = g= g

FREQUENCY (Hz or rpm)FREQUENCY (Hz or rpm)


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Examples of Vibration.

Period = T= Time for one cycle or revolution.

Frequency = 1/ T(seconds) = Cycles / sec = Hertz( Hz)

time


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PEAK =X/2

RMS = 0.707 X/2AVERAGE = 0.637X/2

PEAK TO PEAK =X

Periodic Sinusoidal Vibration


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LetLets begin by looking ats begin by looking at

Frequency!Frequency!CONVERT rpm to HzCONVERT rpm to Hz

where: Hz =cps = cycles perwhere: Hz =cps = cycles per

secondsecondrpm = revolutions per minuterpm = revolutions per minute

rpm/60 = Hzrpm/60 = Hz

rpm = Hz*60rpm = Hz*60


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DISPLACEMENT = XDISPLACEMENT = X

UNITSUNITS --Inch, milInch, mil--Inch, cm, mmInch, cm, mm

Usually expressed as Peak to PeakUsually expressed as Peak to Peak( mil inches = inches/1000=0.001( mil inches = inches/1000=0.001 in.(pin.(p--pp))

CONVERT one inch (CONVERT one inch (pp--pp) into mils) into mils

ANSWER = 1000 milsANSWER = 1000 mils

CONVERT .005 inch (CONVERT .005 inch (pp--pp) = to mils) = to mils ANSWER five mils!ANSWER five mils!


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Convert Displacement to VelocityConvert Displacement to Velocity

V =V = f Xf X

Velocity in/secVelocity in/sec 3.143.14 freq.(Hzfreq.(Hz)*)*DispDisp--In(ppIn(pp))

ororVelocity in/secVelocity in/sec .052*RPM*.052*RPM*DispDisp**In(ppIn(pp))

RPM = 3600 ,RPM = 3600 , DispDisp. = 1 mil = Velocity?. = 1 mil = Velocity?AnswerAnswer 0.187 in/sec0.187 in/sec


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Convert Displacement to VelocityConvert Displacement to Velocity

V =V = f Xf X

Velocity in/sec = 3.14 frequency,Velocity in/sec = 3.14 frequency, DispDisp (In, p(In, p--p)p)

ororVelocity in/sec = .052*RPM*Velocity in/sec = .052*RPM*DispDisp. (in, p. (in, p--p)p)

RPM = 3600 ,RPM = 3600 , DispDisp. = 5 mils. = 5 milsAnswer = .94 in/secAnswer = .94 in/sec


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Convert Displacement to g unitsConvert Displacement to g unitsg = .051 fg = .051 f22 XX

g = .051 fg = .051 f22

** DispDisp. (in. (in--pp)pp)or = .051(Rpm/60)or = .051(Rpm/60) 22 ** Disp.(inDisp.(in--pp)pp)

Let : Frequency = 61.45 HzLet : Frequency = 61.45 Hz&&

DisplacementDisplacement 5.18 mils5.18 milsAnswerAnswer approx 1 gapprox 1 g


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Convert Velocity into g unitsConvert Velocity into g units

gg =(2=(2f)f)22 X/(2*386.1)X/(2*386.1) (6.28 f/386.1) * Velocity(6.28 f/386.1) * Velocity

0.0163*f*Velocity0.0163*f*VelocityIf f = 61.45 HzIf f = 61.45 Hz

& Velocity = 1.0 in/sec& Velocity = 1.0 in/sec

We get gWe get g 1.01.0INTERESTING!INTERESTING!AT 61.45 HzAT 61.45 Hz

Useful Rule: 1gUseful Rule: 1g 1in/sec.1in/sec. 5.18 mils5.18 mils

Easy to estimate-@ 1800 1in/sec@ 1800 1in/sec 10mils, 0.5g, -

7200 1IPS 2.5 mils, 2g, etc Easy to estimate g orDisp.


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Frequency Relationship-

Displacement-Velocity-Acceleration

Disp-mils(p-p)

Vel-in/sec

Accel g unitsX = Disp(p-p)

Velocity = 3.14 f X (p-p)

g = .051(f)2 X (p-p) AAmpl.

mpl.

Freq.

gVel

Displacement

= 3.14

61.45 Hz

5.18 milsf f2


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Displacement = X

Velocity = fX

Acceleration(g) = .051 f2 X

Acceleration

Disp

Velocity

Frequency

Amplitude

Note: at 61.45 Hz 1g =

1 in/sec = 5.18 mils (p-p)


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Why is This Relationship

Important?

Take a look at the next few slides withTake a look at the next few slides withtime history waveforms and spectrum plotstime history waveforms and spectrum plotsshown inshown in DispDisp. Velocity and Acceleration.. Velocity and Acceleration.

Both bearing #9 and #5 are damaged.Both bearing #9 and #5 are damaged.

Displacement and Velocity suppress highDisplacement and Velocity suppress high

frequency energy containing valuable datafrequency energy containing valuable dataon bearing surface condition.on bearing surface condition.

BRG 9 IN2 0 380418f


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BRG 9, IN2, rms=0.380418Velocity time waveform.

0 20.0m 40.0m 60.0m 80.0m 100.0m 120.0m 140.0m 160.0m-1.0

-500.0m

0

500.0m

1.0

se

Re

RMS: 0.3

Live X1X: 0.0799609Y: 0.472679

IN/SEC

BD=3.6, HF=7.3, CF=6, KF=0.13, ED=3


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Acceleration Time Waveform. BRG 9, IN2, rms=1.08119

0 20.0m 40.0m 60.0m 80.0m 100.0m 120.0m 140.0m 160.0m-10.0

-5.0

0

5.0

10.0

se

Re

RMS: 1.0

Live X1X: 0.0799609Y: -0.20284

Gs

BD=3.6, HF=7.3, CF=6, KF=0.13, ED=3

Velocity Time Waveform BRG 5 OUT 0 295444


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Velocity Time Waveform BRG 5, OUT, rms=0.295444

0 20.0m 40.0m 60.0m 80.0m 100.0m 120.0m 140.0m 160.0m-1.0

-500.0m

0

500.0m

1.0

se

Re

RMS: 0.295444

Live X1X: 0.0799805Y: -0.16657

IN/SEC

BD=11, HF=12.7, CF=14, KF=10, ED=12.1

Acceleration Time Waveform BRG 5 OUT rms=2 22567


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Acceleration Time Waveform BRG 5, OUT, rms=2.22567

0 20.0m 40.0m 60.0m 80.0m 100.0m 120.0m 140.0m 160.0m-10.0

-5.0

0

5.0

10.0

se

Re

RMS: 2.2

Live X1X: 0.0799805Y: 0.209865

Gs

BD=11, HF=12.7, CF=14, KF=10, ED=12.1


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0 5.0K 10.0K 15.0K 20.0K0

500.0u

1.0m

1.5m

2.0m

2.5m

3.0m

Hz

Ma

S1X: 6600Y: 1.57844e-005

Bearing #5 - Outer Race Heavy Score

MILs

SKF 6205

52Hz

20 kHz Displacement Spectra

Low Frequency amplified

Hign Frequency suppressed!


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0 5.0K 10.0K 15.0K0

10.0m

20.0m

30.0m

40.0m

Hz

Ma

S1X: 6Y: 0.

Bearing #5 - Outer Race Heavy ScoreIN/SEC

SKF 6205

6kHz

20 kHz Velocity Spectrum

Emphasis on LowFrequency

High Frequency

Reduced


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0 5.0K 10.0K 15.0K 20.0K0

100.0m

200.0m

300.0m

400.0m

500.0m

Hz

Ma

S1X: 6600Y: 0.0703045

Bearing #5 - Outer Race Heavy Score

Gs

SKF 620552Hz

20 kHz Acceleration

Spectrum Here is the bearingcondition information!

Th NATURE OFTh NATURE OF


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The NATURE OFThe NATURE OF

ACCELEROMETERSACCELEROMETERS

THE OPERATIONAL BASICS .

IMPORTANT CHARACTERISTICS.

POSSIBLE PROBLEMS ?

HOW TO SPOT & AVOID THEM.


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AVD CONCLUSION?

For low frequency information, Balance,For low frequency information, Balance,

Alignment, Foundation, or low end bearingAlignment, Foundation, or low end bearingfault frequencies use Velocity. (in/sec)fault frequencies use Velocity. (in/sec)

For sensing early degradation in rollingFor sensing early degradation in rollingelement bearings use acceleration. (g)element bearings use acceleration. (g)


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PROVIDES AN ELECTRICAL OUTPUT

PROPORTIONAL TO THE

INSTANTANEOUS VALUE OF THEVIBRATORY MOTION.

A VIBRATION TRANSDUCER


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THE ACCELEROMETER

(Hz)

Produces voltage proportional to

Instantaneous acceleration.Acceleration = Rate of change of Velocity =

(In g units = 0.051 f2 X

Where: f = frequency (Hz)

X = displacement (PP)

Usual output voltage units = mv/g


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Voltage = Force = Mass * Acceleration= K Acceleration

SIMPLE ACCELEROMETER

VOLTAGE FOLLOWS BASE MOTION

MOTION VOLTAGE

POSITIVE NEGATIVEVOLTAGE

Down

Reaction

Force

Motion

Upward

Reaction

Force

Motion

Voltage = charge/xtal capacitance

MASS

BASE


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REACTION

FORCESHEAR STRESS

SENSOR BASE

XSTAL

Positive ChargeNegative Charge

--

-

++

MOTION


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CRYSTALS

COLLECTORS

MASSMOUNTING BASE


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Early Style Voltage Amplifier!


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Ca

Ca + Cexte o

e

Note: Sensitivity changes with cable length!

Early Style Voltage Amplifier!


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Simplified Charge Mode Amplifier!

Sensitive to triboelectric cable noise! Requires

special low noise cable!

Low Impedance Circuit!


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Low Impedance Circuit!

No Tribo noise, long cables, low coupled noise.

24v

Constant Current Signal Extraction Mode usedin most modern industrial accelerometers!

IMPORTANT ACCELEROMETERIMPORTANT ACCELEROMETER


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IMPORTANT ACCELEROMETERIMPORTANT ACCELEROMETER

CHARACTERISTICSCHARACTERISTICS SENSITIVITYSENSITIVITY

NOISE DISTRIBUTIONNOISE DISTRIBUTION LOW FREQUENCY RESPONSELOW FREQUENCY RESPONSE

HIGH FREQUENCY RESPONSEHIGH FREQUENCY RESPONSE

FILTERED OR UNFILTEREDFILTERED OR UNFILTERED

BASE STRAINBASE STRAIN

TRANSVERSE SENSITIVITYTRANSVERSE SENSITIVITY

TEMPERATURETEMPERATURE

SENSITIVITYSENSITIVITY


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SENSITIVITYSENSITIVITY

RESPONSERESPONSE

1-3mv/g

Base motion

The transducer conversionconstant along its majoraxis in Volts, or

mUsfrequency, temperatureand level ( 1 g )

Example: 100 mv/g100Hz

illivolts/g.ually at specified

@, 1g, 720 F

100 mv/g

R t t

The transducer sensitivityAlong the axis perpendicular

Transverse Sensitivity


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Maximum

Expressed as %

of basicsensitivity.

5%

View from top of accelerometer

Rotate

Motion Motion

Machine

motion

LATERAL MOTION CANOFTEN BE AS HIGH ASVERTICAL MOTION!

MINIMUM

EFFECT ON ACCURACY OF VERTICLE AXIS READING IS NEGLIGIBLE!

Along the axis perpendicularto the Sensitive axis.

Expressed as a % ofthe Basic Sensitivity.For example:3% of basic sensitivity

Triax?

Sample Change in Sensitivity


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p g y

w /Temperature

charge

voltage

5 %

Usual limit 65 0 F to 250 0F. Special units

to 400 0 F w/Electronics.


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ANSI Standard 82.11- thermal shock

gModel X

Model Y

10 SecondsAmbient to320F

Thermal shock!

Change in Sensitivity dueto temperature transient!

0.14g/C0

0.004/C0

Li it Ch i S iti it


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Linearity-Change in Sensitivity

with level

Deviation

Level g

BasicSensitivity

@ 100 Hz1 g

BASE STRAIN ERROR


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Input

motion

Induced strain

BASE STRAIN ERROR

strain= 1 inch/inch

= micro inch= 10 -6 inches

strain typical = 0.0003-0.001g/

Error signal introduced whensensor case or base mountinginterface is mechanically strained

Specification = g/ strain

If large difference at Greased


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BASE STRAIN ERROR

USUALLY OCCURS AT 1X IN LARGELOW FREQUENCY MACHINERY

USE MECHANICAL ISOLATION STUD.

1x between magnet

and hard mountedsensor check basestrain.

bushing will

minimize effect.

High Overall


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High Overall

High Discrete Signal LevelsHigh Discrete Signal LevelsHighHigh 1X1X signal levels may indicate that there is asignal levels may indicate that there is a

bending mode in the machine causing base strainbending mode in the machine causing base strain

in the sensor. The bending mode may bein the sensor. The bending mode may bedecoupled by a magnet or mountingdecoupled by a magnet or mounting

bushing placed under the sensor.bushing placed under the sensor.

Base strain=.001 g/ms X10 ms=.01g

.01g @ 300 CPM= .12 in/sec 7 mils.!

just from strain error!

1X

26


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High Overall High Discrete Signal LevelsHigh Discrete Signal Levels

HighHigh

1X1X

signal levels may indicatesignal levels may indicate

that there is a bending mode in thethat there is a bending mode in the

machine causing base strain inmachine causing base strain in

thethe sensor. The bendingsensor. The bendingmode may bemode may be decoupled by adecoupled by a

magnet or mountingmagnet or mounting bushingbushing

placed under the sensor.placed under the sensor.Base strain=.001 g/ms X10 ms=.01g

.01g @ 300 CPM= .12 in/sec

1X

26

MOUNTED RESONANTMOUNTED RESONANT


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FREQUENCYFREQUENCY

The axial resonance of the mountedThe axial resonance of the mountedaccelerometeraccelerometers sensing crystal and itss sensing crystal and its

associated mass. The frequency at whichassociated mass. The frequency at whichthe unfiltered basic sensitivity is maximum.the unfiltered basic sensitivity is maximum.

Usually expressed in kHz.Usually expressed in kHz.

VIBRATION TRANSMISSIBILITY RATIO


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A out

A input

Ratio - A out / A invs. Frequency

Ratio = forcing freq./ natural freq.Fn=1.0

K

Fn = 1/2 K/M

M XSTAL


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Frequency

Useful frequency response range

Accelerometer Sensitivity vs. Freq.

Internal

Filter

Useablerange

Typical

Specified

Range

5-10kHz

+/- 5%

Low

But

Useable

Typical

3-5 Hz

Useable for

high frequency

10 15

kHz

fn

Resonancerange 15 to25kHz

+/- 5%+/-3dB

10kHz

Mounting - High Frequency


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Effects

41

MountingMounting

AA -- Stud MountingStud MountingBB --Adhesive MountingAdhesive Mounting

CC -- Magnetic MountingMagnetic Mounting

DD -- Hand HeldHand Held

A

B

C

D

10Hz 100Hz 1kHz 10kHz

Minimum effect at low frequency!

R dif t !


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Resonances modify your spectrum!

39

Mount to solid surfaces- near load zone

id b k !


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38

avoid brackets!

Bracketresonance

can modifymachine

spectrum!

x

BEST

Better

fn


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SURFACE PREPARATION

Thin film

Silicon grease.

42

Mounting holes shall be drilled

and tapped to a depth suffic ient

to accommodate the mounting

stud.

Accelerometer mountingsurfaces shall be flat within

700 micro inches RmsSurface finish of 125 microinches

or better

Drilled and tapped holesshall be perpendicular to

the finished surfaceswithin 1 degree1 degree

A-59

ChamferRemove burrs

Suggestions regarding frequency


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response. If you have something specific to measure such as 3If you have something specific to measure such as 3rdrd

gear mesh and know the expected frequency, selectgear mesh and know the expected frequency, selectsensor with fn at least 2/3x expected frequency.sensor with fn at least 2/3x expected frequency.

For bearing measurements mount sensor close to loadFor bearing measurements mount sensor close to loadzone. This can usually be visually estimated.zone. This can usually be visually estimated.

General vibration energy up to 10/13 kHz. Select unitGeneral vibration energy up to 10/13 kHz. Select unit

with 25 kHz fn.with 25 kHz fn. If you need to measure important information above 2/5If you need to measure important information above 2/5

kHz pay special attention to method of mounting. UsekHz pay special attention to method of mounting. Useflush (not horseshoe) super magnets, rigid epoxy bonds.flush (not horseshoe) super magnets, rigid epoxy bonds.

Do not use hand held stinger probe.Do not use hand held stinger probe. Above 8kHz hard mount to flush flat surface with thin filmAbove 8kHz hard mount to flush flat surface with thin film

of silicon grease.of silicon grease.

MOUNTING & PROBESMOUNTING & PROBES


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MOUNTING & PROBESMOUNTING & PROBES

MOUNTINGMOUNTING--HARD, ADHESIVESHARD, ADHESIVES

MAGNETSMAGNETS HAND PROBESHAND PROBES

USE OF MAGNETS!TEST ACCEL


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INTERFACE #1

INTERFACE #2

HIGH FREQUECY

REFRENCE ACCEL.

WITH FLAT +/-

5% RESPONSETO 10KHz

Sweeping sinusoidalvibration Input motiongenerated from high

frequency vibrationexciter.

SUPER

MAGNET


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Thick film of grease.(+20%)

10kHz

+20%

+10% @ 6kHz


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SUPER MAGNET W/OILED SURFACE (+10%)

10kHz


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Thin film of grease-best (+5%)

The grease tends to fill thevoids and provide betterinterface coupling!


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1)HARD MOUNT

2) S-MAGNET/GREASE10 kHz


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Super magnet on drysurface @ 5 g.

Conclusion on Magnet?Conclusion on Magnet?


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Conclusion on Magnet?g

Use rare earth super magnets with machined contactUse rare earth super magnets with machined contactsurface.surface.

Use smooth machined bushing for attachment.Use smooth machined bushing for attachment. Thin film of grease or oil gives good results to 10 kHz.Thin film of grease or oil gives good results to 10 kHz.

More is not better! Do not use excessive Coating ofMore is not better! Do not use excessive Coating of

grease.grease. Thin film of silicone grease on both mating surfaces isThin film of silicone grease on both mating surfaces is

best.best.

Consider the expected g level!Consider the expected g level!

Over 5/10 g use grease on both surfaces, high strengthOver 5/10 g use grease on both surfaces, high strength25 lb magnet or hard mount if possible.25 lb magnet or hard mount if possible.

Use of a Hand Held Probe?

Be Aware!


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35

Be Aware!

Good grade 100mv/g accel!

Frequency response

10 kHz

5%

Hand Held Probe- 4 1/2 in stinger


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36

Data Amplified

500 Hz 2300 Hz

700Hz

Data attenuated

Up 5 dB

Up 4 dB

Down 8dB

SUGGESTION

Fn est. 0.16(AE/L(ms+0.3mr))


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37

10,000 Hz

SUGGESTION:

ONLY USEHANDHELD

PROBES FORCHECKING LOWFREQUENCYBALANCE ANDALIGNMENT ABOVE1200 CPM!

20 dBAttenuation!

NADA!Bearing information

Conclusion on hand probe!Conclusion on hand probe!


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pp

Be cautious in using hand probes inBe cautious in using hand probes in

general.general. Tests indicate that they are best usedTests indicate that they are best used

above 20 Hz and below 500 Hz.above 20 Hz and below 500 Hz.

For checking imbalance and misalignment.For checking imbalance and misalignment.

Not recommended for use in detectingNot recommended for use in detecting

bearing faults.bearing faults.

NOISE SOURCES!NOISE SOURCES!


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GENERALGENERAL

GROUNDING NOISEGROUNDING NOISE BASE COUPLED NOISEBASE COUPLED NOISE

EFFECTS OF INTEGRATIONEFFECTS OF INTEGRATION LOW FREQUENCY NOISE!LOW FREQUENCY NOISE!

High OutputHigh Output-- Ground Loop NoiseGround Loop Noisekeep connectors and sensors off ground!keep connectors and sensors off ground!


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keep connectors and sensors off ground!keep connectors and sensors off ground!

Machine Surface13

Case GroundedCase Grounded

Line PoweredAnalysis

Equipment

Currentflow

through

groundloop

I

noise voltageon ground

Noisecapitivelycouples

into signal!

SENSOR SYSTEM REVISITEDSENSOR SYSTEM REVISITED


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Accel.

24 Vdc

12 Vdc

Bias Level

0 Vdc

2ma constant current

>

Data System >

Vibration

Signal>

CAPACITIVELY

COUPLED

NOISE FOILS

INTERNAL

ISOLATION.

GROUND NOISE

CONTAMINATION

SHORTS OUTEXTERNALISOLATION

Internal or external isolationcould be a problem!

8

High OutputHigh Output--Ground Loop NoiseGround Loop Noiseor Base Coupled Noise!or Base Coupled Noise!


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14

pp

IsoShieldIsoShield

Thin film

Isolation

Internal

Faraday

Shield

Good solution!

TM Glass Isolated

outputs.

Trademark Vibra-Metrics Inc.

Noise isshunted toground!


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Another

Isoshield tm

Design.

High level at line frequency


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High level at line frequency

Zoom in to check actual frequency.Zoom in to check actual frequency.

Should be exactly at power line.Should be exactly at power line.

Try power turn offTry power turn off-- If it disappearsIf it disappears

instantlyinstantly --itit s electrical!s electrical!

If notIf not --itit s from other source.s from other source.

Check for single System groundCheck for single System ground ..

Remember ground noise can be fromRemember ground noise can be fromadjacent machine or sourceadjacent machine or source!!

16

High Output - Ground loop noise


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g p p

Loss of IsolationLoss of Isolation

60 Hz

Conductive materials

at the accelerometer,

cable, or connections

are conmon cause of

ground loops !

running

speed

17

Most Common Noise Problems:


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NoiseNoise --IntegrationIntegration--Low FrequencyLow Frequency ClippingClipping [[May be high frequencyMay be high frequency--above analysis range!]above analysis range!]

Bias InstabilityBias Instability--square wave FFT causes oddsquare wave FFT causes oddharmonics.harmonics.

TurnTurn--on Time, Base Strain, Cable Sensitivityon Time, Base Strain, Cable Sensitivity

Loss of IsolationLoss of Isolation--60 Hz noise60 Hz noise

Use Time Domain as a diagnostic tool.Use Time Domain as a diagnostic tool.

See strange peak? Change resolutionSee strange peak? Change resolution.. High frequencyHigh frequency--VFD, Gear noise, line coupledVFD, Gear noise, line coupled

noise from other electrical signals.noise from other electrical signals.55


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COMMON SOURCESCOMMON SOURCESOFOF

LOW FREQUENCY NOISE!LOW FREQUENCY NOISE!

High Output - Ski slopeProblem-suppresses real signal!


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00.2

0.4

0.6

0.8

1

1 10 100 1,000 10,000

M10v

M10

M10

Frequency

Ski-slope

OA= 0.8

1x =0.19

18

Understand INTEGRATION!Understand INTEGRATION!Amplifies low frequency noise.Amplifies low frequency noise.


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p q yp q y

23

FFT ofFFT ofAccelerationAcceleration

FFT integrated toFFT integrated to

VelocityVelocity

Frequency

Frequency

A

V

VelocityVelocity g/0.0163*fg/0.0163*f

or 1/for 1/f

High OverallHigh Overall--SKI SLOPESKI SLOPE


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gg

USE HIGH PASS FILTER IN DATA

COLLECTOR. SKI SLOPEW/O FILTER

Hi Pass Filterset below frequency

of interest.

Noise

W/ Filter

24

This will drop the overall component levels

due to noise or unwanted foundation motion!

High OutputHigh Output-- skiski--slopeslope((Low Frequency MeasurementsLow Frequency Measurements))


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25

Sensor / Power Supply 1/f Noise (InSensor / Power Supply 1/f Noise (In

Velocity)Velocity)Velocitynoise.

Frequency

Velocity = 61.5 x g

F

Very low frequency SIGNAL.

Note that HP filter will nothelp here! Why?

Velocity

signal

1/F

NOISE

May need higher Sensitivity

High OverallHigh Overall-- SKI SLOPESKI SLOPE


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g

Sensor TurnSensor Turn--on Timeon Time

22

Final

Bias

24 Volts

TIMET = 0

Looks like Low Frequency

Some sensors can take 8 to10 secs!Allow time before collecting data.

SKI SLOPE ?


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Real Motion?Real Motion?

Structural / support motion is usuallyStructural / support motion is usually

discrete frequency. Increasediscrete frequency. Increase

analysis resolution.analysis resolution.

Look for distinct peaks that relate toLook for distinct peaks that relate tofloor or structural resonance.floor or structural resonance.

Otherwise itOtherwise it

s probably noise!s probably noise!

Put sensor on floor or structure!Put sensor on floor or structure!

20

Other Diagnostic Tricks!


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21

Changing FFT resolution to checkChanging FFT resolution to checkfor discrete structural frequencies.for discrete structural frequencies.

5 kHz

5 kHz

Low resolution

High Resolutionsupport

noise

High Ski Slope


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Possible CausesPossible Causes::

1) Could be real motion of floor or1) Could be real motion of floor or

supporting Structure?supporting Structure?

2) Sensor turn on time?2) Sensor turn on time?

3) Sensor 1/f noise?3) Sensor 1/f noise?4) Integration processing noise?4) Integration processing noise?

6) Clipping Saturation products?6) Clipping Saturation products?

7) Aliasing signal processing errors?7) Aliasing signal processing errors?

8) Wrong high pass filter?8) Wrong high pass filter?

19

Signal Level Dropping!Trouble Level Dropping!


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Low Frequency Noise Increasing!

Random Noise=Random Noise=g(rmsg(rms) = ( g) = ( g22/ Hz */ Hz *BwBw))

= g/= g/ Hz (Bw)Hz (Bw)1/21/2Where:Where:

gg

22

/ Hz = spectral density factor/ Hz = spectral density factorand g/and g/ Hz = is the spectral noise factor.Hz = is the spectral noise factor.

Caused by the internal electronics in theCaused by the internal electronics in the

sensor. Random thermal noise and 1/f lowsensor. Random thermal noise and 1/f lowfrequency random noise.frequency random noise.

It is not constant with frequency!It is not constant with frequency!

Trouble level dropping!


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0.001g 0.01g

0,001g =100 volts!

0.1 g

Signal Level out ofsensor dropping!


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3 HzCorner

SelectSensorwith pro

lowfrequenrespons

corner!

El i i


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Electronic noiseIncreasing!

Signal to Noise RatioPeriodic signal in random noise @ 90 %.


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Ratio = signal/noise Minimum 3:1Ratio = signal/noise Minimum 3:1

Example: Signal = 3, noise = 1Example: Signal = 3, noise = 1== (3(32 +2 + 112) =2) = 3.163.16 5.3%5.3%

Signal = 2, noise = 1Signal = 2, noise = 1 errorerror 12 %12 %

At 10 DOF (5 aver.) error up to 35%At 10 DOF (5 aver.) error up to 35%

At 20(10aver.) error range up to 28%.At 20(10aver.) error range up to 28%.

At 40(20 aver.) error 25%At 40(20 aver.) error 25% Need 500 averages to approach 12%Need 500 averages to approach 12%

Reduce Noise by:


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Most vibration sources in rotating machinery areMost vibration sources in rotating machinery areperiodic. Therefore the information exists at discreteperiodic. Therefore the information exists at discretefrequencies.frequencies.

Need for bandwidth in making measurements is toNeed for bandwidth in making measurements is toaccommodate normal variation in the machinesaccommodate normal variation in the machinesrotational frequency.rotational frequency.

Unwanted random noise may be reduced whenUnwanted random noise may be reduced whennecessary by reducing the analysis bandwidth.necessary by reducing the analysis bandwidth.

Rms noise =Rms noise = gg2/Hz * BW/Hz * BW

Reducing a 5 Hz band to 1 Hz will cause aReducing a 5 Hz band to 1 Hz will cause a 5 or 2.245 or 2.24reductionreduction--increases the sample time by 5.increases the sample time by 5.

Purchasing low frequency sensor with higher sensitivity!Purchasing low frequency sensor with higher sensitivity!


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Ref: Tony Keller,Spectral Dynamics, CA.

Degrees of Freedom, Bandwidth& Averaging time?


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DOF = n = 2bT = degrees of freedom, b=DOF = n = 2bT = degrees of freedom, b=bandwidth, T = Averaging time, t = samplebandwidth, T = Averaging time, t = sampletime for one block= 1/b.time for one block= 1/b.

T = sample time * N (number of blockT = sample time * N (number of block

averages)averages) n = 2bT = 2/t*T= 2/t* N t = 2Nn = 2bT = 2/t*T= 2/t* N t = 2N

DOF = 2N( number of block averages.)DOF = 2N( number of block averages.)

A Better Way: Synchronous Averaging.

Averages out random noise signals!


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VIBRATION READING VARIATIONS

PERSON- MOUNTING-POSITION


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Some Interesting Highlights

Reference:

EPRI- Electric Power Research Institute

VIBRATION SENSOR MOUNTING GUIDE

Prepared by:

CSI, Knoxville, TN 1991

40%

EPRI GUIDE


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VARIATION WITH PERSON- FIVE PEOPLE MANUALLY

HOLDING ACCEL IN POSITION # 0

EPRI- Electric Power Research InstituteVIBRATION SENSOR MOUNTING GUIDE (CSI)

BELOW 30 HZ = 60% +/- 10% EPRI GUIDE


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PERCENT VARIATION- SEVEN PEOPLE AT SAME POINT

VS. FREQUENCY- HAND PROBE WITH 2 STINGER

Useful Range

EPRI GUIDEPeak?


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HAND PROBE 2 STINGER %PERCENT DEVIATION AT

HIGH FREQUENCY

300 %

Pressure

EPRI GUIDE


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LEVEL VS. POSITION AROUND BEARING

AT LOW FREQUENCIES

Frequency 400Hz

7:1

Negative side

ERROR SOURCE


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SIGNAL CLIPPING !

SENSOR SYSTEM REVISITEDSENSOR SYSTEM REVISITEDDynamic Range? Maximum g w/o clipping. (Not Damage!)Dynamic Range? Maximum g w/o clipping. (Not Damage!)

Power


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supply

Accel.

24 Vdc

12 +/- 2 Vdc

Bias Level

0 Vdc

2ma constant current

>

Data System >

Vibration

Signal>

8

DYNAMIC RANGEDYNAMIC RANGE

D i RD i R k lt / li ik lt / li i


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Dynamic Range=Dynamic Range= max peak voltage w/o clippingmax peak voltage w/o clipping3 x rms band limited noise floor.3 x rms band limited noise floor.

Factors:Factors:>Bias level determines +/>Bias level determines +/-- allowable voltage swing.allowable voltage swing.> Bias level varies +/> Bias level varies +/-- around 10volts.around 10volts.

> Rms noise is random with 3 sigma peaks> Rms noise is random with 3 sigma peaks> Noise varies as> Noise varies as bandwidth.bandwidth.> Power supply allows uniform +/> Power supply allows uniform +/-- voltage swing.voltage swing.

> Sensitivity 10, 100, 250, or 500 mV/g?> Sensitivity 10, 100, 250, or 500 mV/g?

w/ 24vsupply +/-

10 VOLT

15

10

10


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10 VOLTLIMIT

Mostpower now

+/- 12 volts

Bias +/- 2v

5

-15

-5

-10

-10 -5 0 +5 +10

Input Voltage Swing from Accelerometer

Crystal Assuming Power Supply at 24V

But Bias level at 14 volts.

Note:

10 volt peak =

100 g 100mv/g

10 g- 1volt/g

2 g 5volt/g

1g 10volt/g

14 voltbias

Also Remember!Also Remember!


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46

Time DomainTime Domain

Peak values canPeak values can

bebe

many times highermany times higher

than singlethan singlespectralspectral

components incomponents in

TheThe FrequencyFrequency

DomainDomain

Time

Frequency

A

A

20 G MAX

100 G100g 80

secpeak

Spectrum

w/10 gbins.

CL[P


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BIAS

VOLT

CL[P

Erratic DataErratic Data-- clippingclipping

Cli iCli i St t ?St t ?


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ClippingClipping-- Strange components?Strange components?

The FFT of a square wave consists of theThe FFT of a square wave consists of thefundamental frequency and all oddfundamental frequency and all oddharmonics.harmonics.

1 3 5 7 9 11 13

Accel

Frequency

50

SENSORSENSOR -- ClippingClipping

S S iti it !S S iti it !g LEVEL X SENS


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45

Sensor Sensitivity !Sensor Sensitivity !

10, 100, 250, or 50010, 100, 250, or 500mV/g?mV/g?

Dynamic RangeDynamic Range

12 VDC nominal

Supply voltage

normally 24 VDC

g

Bias Voltage

10-12

volts

Clippingcauses DCoffsets! Zero ref.

Clipping shifts ref upcausing DC component.


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Off set looks like low frequency andtakes RC to settle down.

1/6.28RC= fL Where:R= input zC= capacity of xstalfL = Low frequency corner

T = RC

T =1/6.28fL

Lowfrequencycorner 3 Hz

T=0.05 sec.

Cause of ski slope - low frequency noise?

C R Powersupply

Time

DC

SHIFT


Time Domain DataTime Domain Data


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Time Domain DataTime Domain Data

47

Volts

Time (seconds)

Note exponential offsets!

What effect will this produce?

Low Frequency Noise and sum/ difference frequencies.!


ClippingClipping Strange components??


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ClippingClipping-- Strange components??

The FFT of a square wave consists of theThe FFT of a square wave consists of the

fundamental frequency and all oddfundamental frequency and all oddharmonics.harmonics.

1 3 5 7 9 11 13

Accel

Frequency

50


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AMPLITUDE


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F1 + F2 F1 F2

F1+F3 F1- F3F2 +F3 F2 F3

False Spectral Components Caused

by Clipping

AMPLITUDE

FREQUENCY

Erratic DataErratic Data -- other places to look!other places to look! ClippingClipping !!


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pp gpp g

Look for g levels beyond the dynamic

range of the sensor. There may be clipping beyondthe frequency range being viewed.

Accel

Frequency F max

Normal range

of interest.

51

Most Common!Most Common! NoiseNoise --IntegrationIntegration--Low FrequencyLow Frequency

ClippingClipping--bouncybouncy--bias Instabilitybias Instability--odd harmonicsodd harmonics--


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pp gpp g yy yyclipping beyond analysis frequencyclipping beyond analysis frequency

Low frequencyLow frequency--high pass filterhigh pass filter--integrationintegration--turnturn--on time.on time.

1/rev1/rev--Base StrainBase Strain--Cable Sensitivity, increaseCable Sensitivity, increase

resolution.resolution.

60 Hz noise60 Hz noise--Loss of IsolationLoss of Isolation--ground loop!ground loop!

Signal lossSignal loss--check bias voltagecheck bias voltage Change resolutionChange resolution--If amplitude changes it isIf amplitude changes it is

Random noise and not a discrete frequencyRandom noise and not a discrete frequency..55


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ERROR SOURCEERROR SOURCE

SLEW RATE LIMITINGSLEW RATE LIMITING

THE HIGHER THE VOLTAGE SWING- THE LOWER THE

FREQUENCY AT WHICH SLEWING ERROR OCCURS.

Gain change due to


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FREQUECY RESPONSE TEST

VOLTAGESWING slew rate limiting!

Effect of slew limiting!Effect of slew limiting!


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IDEAL

Slew limited.

Slew Limiting causes reduction in highfrequency gain and peaky signal!


117/142


118/142

+ 8V

Can cause DC offset aftersignal burst!


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LINE CAPACITY LOADING!


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SLEWING RATE INCREASES WITH FREQUENCY

REQUIRED CURRENT = I = C de/dt

SR = 6.28 frequency Emax

Sl R t


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Slew Rate

Slewing Rate example


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(10 VOLTS)6.28 x 10,000 x 10

0.1 VOLTS/g

6,28 x freq. X EmaxEmax = 0.1 x 100 = 10 volts

628,000 volts/sec

0.628volts / micro- sec

100

10,000 Hz

100mv/g

Frequency

Sensitivity

Slew Rate

g

Power Supply Current Required

I = C de/dt


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Where:

I = current C= capacitance =15pf/ft

de/dt = Slew Rate .628V/ u-sec

I = 1000(15 x 10 - 12) .638 V/10 -6

I = 9ma

CABLE LENGTH @ CURRENTCABLE LENGTH @ CURRENT@ 10kHz@ 10kHz--15pfd/ft15pfd/ft--5volts5volts

Approximate cc power supply current:Approximate cc power supply current:


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pp p pp yy

0.6 ma0.6 ma 125 ft.125 ft.

1.2 ma1.2 ma 250 ft.250 ft.

2.4ma2.4ma 500 ft.500 ft.

4.8 ma4.8 ma 1000 ft.1000 ft.

9.6 ma9.6 ma 2000 ft.2000 ft.

Sensor High -Low FrequencyMeasurements?

STRANGE SIGNALS ?STRANGE SIGNALS ?


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40

BELOW 500 RPMBELOW 500 RPMLF NOISE, FLOOR/MOUNTINGLF NOISE, FLOOR/MOUNTING

AROUND 60 Hz (1x) GROUND NOISE/STRAIN!AROUND 60 Hz (1x) GROUND NOISE/STRAIN! ABOVE 5/10 kHzABOVE 5/10 kHz-- MOUNTING, CLIPPING,MOUNTING, CLIPPING,

VFD, OR GEAR MESHVFD, OR GEAR MESH !!

Usable Frequency Range

Frequency

Accel.Output

Volts

+5%

-5%

+5%

-5%

fn


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Error Sources!Error Sources!

Measuring Shock orMeasuring Shock orTransient Impulse withTransient Impulse with

accelerometer.accelerometer.

To avoidaccelerometer ringing

Impact response


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accelerometer ringing

in an impact test:fn > 10/T & flp =0.5 fn

Where: fn is mounted resonance.

T


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Shock & Vibration Handbook 3rd Edition


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Eller & Whittier

Endevco Corp. Chap. 12



130/142

Eller & Whittier



Droop Error!


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Droop error

To avoid low frequency error and zero shift:

Amplitude error

Actual impact input.

Measured impact.


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Zero shift

Low frequency response < 0.008/T

Where: T = pulse duration.



133/142

Eller & Whittier



Key Items to Observe for Impact

To minimize Amplitude Errors &To minimize Amplitude Errors &UndershootUndershoot


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UndershootUndershoot

Low Frequency Corner < .008/ T[PulseLow Frequency Corner < .008/ T[Pulseduration.]duration.]Ex. T=0.2 sec. Low corner =0.04 Hz.Ex. T=0.2 sec. Low corner =0.04 Hz.

To minimize RingingTo minimize RingingResonance Fn must be > 10/T andResonance Fn must be > 10/T and

Low Pass Filter less than 0.5 FnLow Pass Filter less than 0.5 FnEx. T= 1millisec, Fn => 10 kHz.Ex. T= 1millisec, Fn => 10 kHz.


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GETTING USEFULGETTING USEFULINFORMATIONINFORMATION

FROM YOURFROM YOUR

ACCELEROMETER?ACCELEROMETER?

[[See session onBearing Lifeguard tm

Multiple Discriminant Analysis..]]

SAMPLE


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Est. Repair Cost=$2,200

Est AvoidedCost =$22,000

MAIN CAMPUSA A PULIFE FACTORLIFE FACTOR

DISTRIBUTIONDISTRIBUTION35

MACHINES 144 MACHINESHAVE REDUCEDBEARING LIFE!


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0

5

10

15

20

25

30

100 75 50 25 FAIL

AHU

PUMPS

MOTORS

L-FACTOR

CLICK HERE FOR MEAN TREND

ILLUSTRATION

PERCENT LIFE EXPECTANCY

BEARING LIFE!

NUMBER OF MACHINES BY LIFE FACTOR

CHANGE IN EXPECTED BEARING LIFEWITH CHANGE IN MACHINE SPEED


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DROP IN LEF VS. SPEED

0

10

20

30

40

5060

70

80

90

1 2 31080 1800 3600 RPM

%L

10

WB,K2

BEARING LIFE FACTORDISTRIBUTION

50

60

BADMACHINES


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0

10

20

30

40

1 ALERT BAD

AHUPUMPS

MOTORS

BAD MOTORS

1-3 3-7 7-10L-FACTOR

MACHINES

SAMPLE

NUMBER OF MACHINES BY LIFE FACTOR

FACILITY LIFE FACTOR TREND

10

12


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0

2

4

6

8

10

JAN FEB MAR APRIL MAY JUNE

ILLUSTRATIONMEAN LIFE FACTOR TREND[100 MACHINES]

Questions?Questions?Where to get more information ?Where to get more information ?


141/142

Contact your sensor manufacturer! Contact the Vibration Institute!

Dynamic Measurement Consultants!

[email protected]

57
mailto:[email protected]:[email protected]


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Documents

Mfpt 59 Acceleration Measurements Session 4-19-05_comp