vibration sesors

7/31/2019 vibration sesors

1/46

There is no diffrence b/w these sensors thier method

of

sensing is same depend upon the EDDY currunt

losses.ACCELEROMETER & PROXIMITY are diffrent way tomeasure the vibration although they have same sensing

method as i told before

Accelerometers are a piezo-electronic (crystal)

device. A

pre- loaded crystal is charged with current and as

the

crystal is compressed or de-compressed by vibration

anoutput proportional to g's (gravity) is provided. A

"g" is

equal to 9.80 meters/second2 or one (1) standard

earth

gravity.

Accelerometers are normally used for high-frequency

bearing

cap vibration readings (Case/Bearing Cap Absolute on

machines using rolling element bearings. Usually the

outputis integrated electronically to velocity (in/sec or

mm/sec). Other applications include monitoring Gears

and

High Frequency Applications.

Eddy or Proximity Probes are a displacement device

that

measure the relative motion between the probe

mountinglocation and the target (shaft). Output is directly

proportional to displacement and is usually measured

in

mils (.001") or millimeters (mm).

Eddy Probes are used on machines with Journal

(Sleeve) type


2/46

bearings. Where the measurement of motion between the

Bearing and Shaft is critical.

Types of Vibration SensorsBy Neal Litherland, eHow Contributor

Vibration sensors are used in a number of different projects,machines and applications. Whether you're attempting to gaugethe speed of a vehicle, or to gauge the power of an impending

earthquake, the device you're likely using is considered to be a"vibration sensor." Some of them operate on their own, and othersrequire their own power source, but all of them serve the same

purpose in slightly different capacities.

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Accelerometer

o One of the most common types of vibration sensor is anaccelerometer. Accelerometers come in a variety ofdesigns, and they can detect a wide range of different

vibrations. One of the most popular versions of the

accelerometer is a pizoelectric sensor. This sort ofsensor contains a material (such as crystal quartz) thatgives off an electric charge when it detects changes inpressure. By measuring the amounts of electric chargethat pizoelectric accelerometers give off it becomespossible to determine the amount of vibration going onin the connection.
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3/46

Velocity Sensors

o A velocity sensor is mainly used to measure motion andbalancing operations on rotating machinery. These

sensors are ideal for sensing low and mid-frequencyvibrations, but not high-frequency ones. Additionally, avelocity sensor requires no electrical input in order tomeasure the force of velocity. These sensors do requireregular maintenance to be sure that they're operatingproperly however. This is especially true for sensors

who are placed on machinery that moves at a very highvelocity, since the sensors need to be firmly anchored toget accurate measurements.

o

Proximity Sensors

o Not all vibration sensors are installed directly onto thethings they're supposed to measure. A proximity sensoris a type of vibration sensor that's meant to measuredistance between an object and the probe. If the objectis vibrating that means it will be moving towards and

away from the probe, and by picking up on that motionthe sensors can detect the range of vibration takingplace. These probes may be used for small applicationssuch as detecting vibrations within machinery, or forlarger applications such as detecting vibrations in theearth as a sign of earthquakes.

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4/46

Field Application Note

Comparing Vibration Readings

Comparing vibration level readings taken by

different types of instruments and transducers

can be very confusing and can lead to mistrustof the systems involved.

Knowledge of how to properly compare

readings is required before comparing any

readings is attempted.

This application note explains the variablesinvolved in some detail and will act as a

guideline as you compare vibration readings.

Transducer Type

Three (3) basic types of vibration transducers

are available which correlate with the three (3)

types of measured physical motion,Acceleration, Velocity and Displacement.

Accelerometer


device. A pre- loaded crystal is charged withcurrent and as the crystal is compressed or de-

compressed by vibration an output proportional

to g's (gravity) is provided. A "g" is equal to

9.80 meters/second2 or one (1) standard earthgravity.

Shaft Absolute

Shaft Absolute is the measurement of the

shaft's motion relative to free space (orabsolute). Shaft Absolute can be measured

two (2) ways, the first being electronically

summing the

signals

of both a Eddy Probe measuring shaft

relative and a accelerometer measuringcase absolute, the second being using a

shaft rider which is a spring mounteddevice that physically rides on the surfaceof the shaft, normally a velocity sensor

integrated to displacement is mounted on

top of the shaft rider. Shaft Absolute is

normally used where the rotating assemblyis five (5) or more times heavier than the

case of the machine.

Engineering Units

0 to Peak (0-P)Both Velocity (in.sec, mm/sec) and

Acceleration (g's) by definition aremeasured in 0 to Peak or one/half the Peak

to Peak signal as viewed on an

oscilloscope.


5/46

Accelerometers are normally used for high-

frequency bearing cap vibration readings(Case/Bearing Cap Absolute on machines using

rolling element bearings. Usually the output is

integrated electronically to velocity (in/sec ormm/sec). Other applications include monitoring

Gears and High Frequency Applications.

Velocity Pick-up

Two (2) types of Velocity Sensors exist,mechanical and electronic. Mechanical types

are the most common and are made up of a

spring mounted coil mounted inside a magnet.Vibration causes the coil to move in relation to

the magnet which produces a voltage output

directly proportional to Velocity. Electronic

Velocity Sensors are Accelerometers with an

electronic integrator built in to the unit. Outputof a Velocity Sensor can be expressed in many

different terms, inches/second (in/sec) ormillimeters/second (mm/sec) being the

standards.

Velocity Transducers are normally used for

Bearing Cap Vibration Monitoring

(Case/Bearing Cap Absolute) on machines withrolling element bearings. They have the

advantage of high outputs and the signal is read

directly in velocity (in/sec or mm/sec).

Peak to Peak (P-P)

Displacement by definition is measured in

Peak to Peak or the actual Peak to PeakMotion of the Shaft.

Root Mean Square (RMS)

Root Mean Square (RMS) is a popular

method of measuring Case or Bearing CapVibration as many vibration engineers

have found that RMS is more indicative of

actual rolling element bearing condition.Although rarely found in vibration wave-

forms a pure sine wave RMS would be .

707 times the 0 to Peak Value.

Transducer Considerations

Frequency Response

The frequency response of a vibration

transducer is very important when

comparing readings. Transducers with awider or broader frequency response will

typically see more vibration if it is present

than a narrower bandwidth transducer.How different vibration frequencies

contribute to overall values is dependent

on their phase relationship to each other,some may add, some may subtract fromthe overall value.

Eddy Probes Displacement200

mv/mil

Velocity(Mechanical)

Velocity500mv/in/sec

Velocity

(Piezoelectric)Velocity

500-1000

mv/in/sec

Accelerometer Acceleration 100 mv/g

Mounting

How a transducer is mounted is alsocritical to comparing measurements.

Accelerometers are extremely sensitive to

the method of attachment. Differences in

bandwidth can be measured between hand-


6/46

Eddy Probes (Proximity)

Eddy or Proximity Probes are a displacement

device that measure the relative motion betweenthe probe mounting location and the target

(shaft). Output is directly proportional todisplacement and is usually measured in mils(.001") or millimeters (mm).

Eddy Probes are used on machines with Journal(Sleeve) type bearings. Where the measurementof motion between the Bearing and Shaft is

critical.

Bearing Type

Two primary types of bearings are in use today,

Rolling Element Bearings and Journal or Sleeve

Bearings.

Rolling Element Bearings are zero (0) clearancedevices. All vibration of the shaft is transmitted

directly to the bearing cap.

Journal or Sleeve Bearings are designed so thatthe oil film provides damping. The shaft is free

to vibrate within the bearing. Due to the

damping provided by the oil film very little ofthe shaft vibration is transmitted to the bearing

cap. The oil film damping provides even higher

levels of attenuation to higher frequencies.

held, magnet attached, epoxy, and stud

mounted installations.

Installation instructions must be followed

precisely to obtain the manufacturestransducer specifications. Accelerometers

not mounted perfectly perpendicular to the

surface or on a flat surface will producestress risers which will also produce false

signals.

Measurement Location

When comparing readings it is essential

that all readings are taken at the same

location and the same plane. Even smalldifferences in location can effect the

overall readings. All vibration transducers

are single plane devices and only measurein the plane in which they are held or are

mounted.

Instrument Considerations

All Instruments handle signal is different

ways. Different instruments have their own

frequency response and filtering.Knowledge must be gained on theinstruments used before the outputs can be

compared even when they use the same

transducer.

Conversion Formulas

Displacement, Velocity and Acceleration

are mathematically related to each other asa function of frequency. Electronic

integrators or differentiation are also usedto change one term to the other. Onceagain it must be understood that the

readings be of the same type or they will

not agree.

D = Displacement, P-P, Mils.

V = Velocity, 0-P, in/sec.

A = Acceleration, 0-P, g's.


7/46

Measurement Type

Only measurements of the same type can be

compared. Bearing Cap or Case Vibrationcannot be directly compared to Shaft Relative

or Shaft Absolute and visa versa.

Case Absolute

Case or Bearing Cap Absolute is themeasurement of the Case or Bearings Caps

(Location of Transducer) motion relative to free

space (or absolute motion). Case or CapAbsolute is usually used for monitoring Rolling

Element Bearings.

Shaft Relative

Shaft Relative is the measurement of motion

between the Shaft and whatever the measuringdevise is mounted to. This measurement is

normally taken with a NCPU or Proximity

Sensor. Shaft Relative measurements are usedfor Journal or Sleeve Bearing Applications.

D = 19.10 x 103 x (V/CPM)

D = 70.4 x 106 x (A/CPM2)

V = 52.36 x 10-6 x D x CPM

V = 3.87 x 103 x (A/CPM)

A = 14.2 x 10-9 x D x CPM2

A = 0.27 x 10-3 x V x CPM

Summary

In General it is difficult to get any two

readings to precisely agree with oneanother. Even when care is taken to insure

that transducers and locations are the same

and that the measurement type is the same,

agreement within +-30% depending on the

instrument is considered good.

Even though overall values will not agree

precisely spectrum Data or frequencies

will be comparable within the limits of thebandwidth of the different instruments.

Checklist

1. Is Transducer Type the same

2. Bearing Type

3. Is Measurement Type the Same4. Engineering Units the same

5. Frequency Response of Transducer6. Mounted Transducer Frequency

Response

7. Where Readings Taken at the same

location8. Where Readings Taken in the same

Plane

9. Instrument Frequency Response

Piping vibration can be an annoying problem Corrective Actions


8/46

which can consume unnecessary maintenance

activity and can affect pumping system

performance and endurance. The systemincludes the pipe, all piping supports, hangers,

snubbers, pipe to pipe interfaces, and

machinery or devices attached to the pipe. Allthese items can influence the pipe vibration

patterns.

This testing method will determine the piping

system vibration amplitudes, frequencies, nodalpoints, and the pipe modal shape. It can, also,

be used to identify defective supports,

incorrectly placed supports, and the locations ofmaximum deflection requiring additional

supports.

Analyzer/Data Collector

Many data collectors have internal circuitry

with low frequency range limitations: output

displays in acceleration units are 2 Hz andoutput displays in velocity and displacement are

5 Hz. This circuitry is an internal high pass

filter set for a 2 Hz roll-off frequency foracceleration signals and 5 Hz as the velocity

and displacement roll-off frequency. The filter

eliminates excessive noise from beingdisplayed.

This means that if an accelerometer is

connected to the data collector and the display

is setup for acceleration units, the low

frequency signals are correctly displayed downto 2 Hz. If the accelerometer signal is integrated

to velocity or displacement the low frequency

limitation is 5 Hz. Similarly, a velocitytransducer has the velocity low frequency

limitation and the integration limitation alsoapplies.

Methodology

Piping vibration analysis involves describing

how much the pipe is moving and at whatfrequency the motion exists. The piping motion

Generally, the pipe supports should be a

nodal point with little or no motion.Excessive motion at these locations

indicate that the support is faulty or

improperly installed. Vibration amplitudesshould decrease as a complex joint, such as

a tee connection, an elbow, or machine

connection, is approached.

Convert all the collected data todisplacement units using the formula:

A = 14.2 x 10-9 x D x F

where:D = Displacement (mils pk-to-pk)

A = Acceleration (G's pk)

F = Frequency (Hz).

Plot all the amplitude information which isat a common frequency on the graph to

determine the modal shape at which the

pipe is vibrating. Compare the calculatedamplitudes and frequencies with the

allowable piping vibration levels chart to

determine if corrective action is warranted.


9/46

can be further described by showing the motion

as a modal plot. Pipes can vibration in three

orthogonal directions just like a machine.Vibration data should be collected in the X, Y,

and Z axis. Since most data collectors do nothave the capability of calculating transferfunctions collected from impact/response input

signals, all data collection should be taken

while the pumping system is operating.

The vibration transducer may be attached to thepipe using a magnetic mount without affecting

the lower frequency response of the transducer.

The overall pipe length should be separated into

equal spaced lengths 3-5 feet ( 1-2 meters ) forthis test and plotted on a graph sheet. The pipe

hangers/restraints and their orientation to the

pipe should be noted on the plot.

Setup the data collector for a frequency range

for a 0-12,000 CPM (0-200 Hz) and display

units of acceleration (G's). Collect spectra at

each measurement point. Evaluate the spectrafor the components at common frequencies

noting their amplitude and frequency.

Wachel, J. C. and Bates, C. L., Techniques for Controlling Piping

Vibration and Failures, ASME Paper 76-PET-18.

The listed ASME Paper includes a

"severity chart" which could be used as a

starting point in determining the pipingsystem acceptability. This chart was

compiled from 25 years of data and may be

overly conservative for long flexible pipingsystems commonly found in power

stations.

Pipe vibration correction will involve re-tuning the pipe system to a differentfrequency. This may be accomplished by

re-locating the pipe supports, installing

different supports, isolating the pipe fromits hangers or joints, or installing

expansion joints in the pipe. Before any

modification is undertaken another pipe

analysis should be carried out to determinethat the modification does not violate other

design parameters such as machine

coupling momentums or connectionstresses.

Testing Checklist

1. Piping System Defined

2. Proper Accelerometer


10/46

3. Graph Paper

4. Analyzer Set-up

A subset of the decision of purchasinga monitoring system is the decision of

what type of system is required.Monitors are available in many

varieties; some simply display the

overall signal levels, some haveelaborate interface systems, some can

automatically collect different types of

information. The end user must decide

what is really necessary. Will themonitor be required to provide some

form of protection? Does the end userrequire that the monitor provide sometype of information? Must the

monitoring system provide diagnostic

capabilities?

PROTECTION

Protection is available in many forms.

Nearly all monitor systems availabletoday can provide machinery

protection. This means that should asensor signal exceed a predeterminedset point the monitor can initiate a

shutdown to prevent internal

machinery damage. This form of

protection is tangible and can bequantified for accounting purposes.

Additional intangible protection

provided by a basic monitoring systemare personnel and production

protection. If a machine can be

shutdown prior to catastrophicdamage, which could involve

unexpected shrapnel from the

machine, the personnel that are in thevicinity of the machine are protected.

An orderly shutdown of a machine

train can benefit the facility

INFORMATION

An information system will providedata that is useful for planning and

scheduling. This information can be

used for a "Go No-Go" decisionwhether to continue operating the

machine train or produce goods. Basic

monitoring systems are capable of

providing this type of information byalerting personnel to current

conditions.

Maintenance planning and outagescheduling requires additional

information. Information systems will

provide data as trends which give

advanced notice of elevating overallsignals.

DIAGNOSTICS

Advanced monitoring systems willprovide additional information about

the condition of the machine train

connected to the monitor. Thisinformation can be collected

automatically or manually, and upon

alarm activation or on a regular basis.

This information has many benefitswhich when properly used can

produce cost savings and downtime.

By analyzing the collectedinformation the root cause of the

elevated signals can lead to the cause

of the machine problem. This type ofinformation can lead to reducing

machine train downtime. After the

maintenance has been conducted, this

type of monitor can be used for


11/46

production and its product. Certain

production processes, such as paper

and sheet steel, are sensitive toexcessive vibration. High vibration

levels produce poor quality product.These facilities will benefit from amonitoring system that can alert

operation personnel when

unacceptable product is being

produced.

acceptance testing and machine

commissioning. Many end users have

reported correction of design flawsand incorrect operating procedures

using advanced diagnosticinformation.

Monitoring Classification Checklist

1. Protection2. Information

3. Diagnostics

Industrialmachinery with

high horsepower

and high loads,

such as steamturbines,

centrifugal

compressors,pumps and motors, utilize journal

bearings as rotor supports.

One of the basic purposes of a bearing

is to provide a frictionlessenvironment to support and guide a

rotating shaft. Properly installed and

maintained, journal bearings haveessentially infinite life.

BEARING DESIGN

A journal bearing, simply stated, is acylinder which surrounds the shaft and

is filled with some form of fluid

lubricant. In this bearing a fluid is themedium that supports the shaft

preventing metal to metal contact. The

most common fluid used is oil, withspecial applications using water or a

Plain Bearing

The plain

bearing is the

simplest and

most commondesign with a

high load

carryingcapacity and the lowest cost. This

bearing is a simple cylinder with aclearance of about 1-2 mils per inch ofjournal diameter. Due to its cylindrical

configuration it is the most susceptible

to oil whirl. It is a fairly commonpractice during installation to provide

a slight amount of "crush" to force the

bearing into a slightly elliptical

configuration.

Lemon Bore

The lemon or elliptical bore bearing isa variation on the plain bearing where

the bearing clearance is reduced onone direction. During manufacture this

bearing has shims installed at the split

line and then bored cylindrical. Whenthe shims are removed the lemon bore

pattern is results. For horizontally split


12/46

gas. This

application note

will concentrateon oil lubricated

journal bearings.

Hydrodynamic

principles, which are active as theshaft rotates, create an oil wedge that

supports the shaft and relocates it

within the bearing clearances. In ahorizontally split bearing the oil

wedge will lift and support the shaft,

relocating the centerline slightly up

and to one side into a normal attitude

position in a lower quadrant of thebearing. The normal attitude angle

will depend upon the shaft rotationdirection with a clockwise rotation

having an attitude angle in the lower

left quadrant. External influences,such as hydraulic volute pressures in

pumps or generator electrical load can

produce additional relocating forceson the shaft attitude angle and

centerline position.

An additional characteristic of journal

bearings is damping. This type ofbearing provides much more damping

than a rolling element bearing because

of the lubricant present. More viscous

and thicker lubricant films providehigher damping properties. As the

available damping increases, the

bearing stability also increases. Astable bearing design holds the rotor at

a fixed attitude angle during transientperiods such as machinestartups/shutdowns or load changes.

The damping properties of the

lubricant also provides an excellentmedium for limiting vibration

transmission. Thus, a vibration

measurement taken at the bearing

bearings, this

design creates an

increasedvertical pre-load

onto the shaft.

This bearing has

a lower load carrying capacity thatplain bearings, but are still susceptible

to oil whirl at high speeds.

Manufacturing and installation costsare considered low.

Pressure Dam

A pressure dam bearing is basically a

plain bearing which has been modifiedto incorporate a central relief groove

or scallop along the top half of the

bearing shell ending abruptly at a step.As the lubricant is carried around the

bearing it encounters the step that

causes an increased pressure at the top

of the journal inducing a stabilizingforce onto the journal which forces the

shaft into the bottom half of the

bearing.

This bearing has a high load capacityand is a common correction for

machine designs susceptible to oil

whirl. Pressure dam bearings are aunidirectional configuration.

Another unidirectional bearing

configuration is the offset bearing. It

is similar to a plain bearing, but theupper half has been shifted

horizontally. Offset bearings haveincreasing load capacities as the offsetis increased.


13/46

outer shell will not represent the actual

vibration experienced by the rotor

within its bearing clearances.

Journal bearings have many differingdesigns to compensate for differing

load requirements, machine speeds,

cost, or dynamic properties. Oneunique disadvantage which consumes

much research and experimentation is

an instability which manifests itself asoil whirl and oil whip. Left

uncorrected, this phenomenon is

catastrophic and can destroy the

bearing and rotor very quickly. Oil

whip is so disastrous because the rotorcannot form a stable oil wedge

consequently allowing metal to metalcontact between the rotor and the

bearing surface. Once surface contact

exists the rotor begins to precess, in areverse direction from rotor rotation

direction, using the entire bearing

clearance. This condition leads to highfriction levels which will overheat the

bearing babbit metal that leads to

rapid destruction of the bearing, rotorjournal, and the machine seals.

Some common designs employed are

lemon bore, pressure dam, and tilt pad

bearings. These designs were

developed to interrupt and redirect theoil flow path within the bearing to

provide higher bearing stabilities.

GEOMETRIES

Journal bearings installed in industrial

machinery today generally fall into

two categories: full bearings andpartial arc bearings. Full bearings

completely surround the shaft journal

with many differing geometries suchas elliptical, lobed, or pressure dam

Tilting Pad

Tilting pad

bearings is apartial arc

design. Thisconfigurationhas individual

bearing pads which are allowed to

pivot or tilt to conform with the

dynamic loads from the lubricant andshaft. This type of bearing is a

unidirectional design and is available

in several variations incorporatingdiffering numbers of pads with the

generated load applied on a pad or

between the pads.

VIBRATION MONITORING

A shaft supported by journal bearings

will move relative to the bearinghousing as various forces are imposed

onto the shaft. A vibration transducer

is required which can monitor therelative motion between the shaft and

the bearing. Higher vibration

frequencies are not of prime concernsince they would not be transmittedthrough the oil film reliably.

The only sensor available that can

measure relative measurements of the

shaft is the non-contacting pickup,sometimes called a displacement,

eddy current, or proximity pickup.

This type of sensor measures therelative vibration of the shaft and,

also, the relative position of the shaftwith respect to the bearing clearances.High frequencies such as blade

passage and cavitation would be

attenuated by the lubricant. Casemounted sensors would not provide an

accurate indication of the vibration

due to the inherent damping offered


14/46

configurations and usually are two

pieces, mated at a split line. Partial arc

bearings have several individual loadbearing surfaces or pads and are made

up of numerous adjustablecomponents.

The bearing inner surface is coveredwith a softer material, commonly

called babbit. Babbit, which is a tin or

lead based alloy, has a thickness thatcan vary from 1 to 100 mils depending

upon the bearing diameter. A babbit

lining provides a surface which will

not mar or gouge the shaft if contact is

made and to allow particles in thelubricant to be imbedded in the liner

without damaging the shaft.

by the lubricant between the shaft and

the bearing. For more information

about installation and theory ofoperation of NCPUs, see the STI

Application Notes: Eddy CurrentTransducer Installation, Part 1-RadialVibration

The basic purpose of a machine

bearing is to provide a nearfrictionless environment to support

and guide a rotating shaft. Twogeneral bearing styles are utilized atthis time: the journal bearing and the

rolling element bearing. For lower

horsepower and lighter loaded

machines, the rolling elementbearing is a popular choice.

Until the 1940's, the journal bearing

was the prevalent style used onmachines. As metallurgy and

machining techniques progressed,the rolling element bearing gained

greater usage. Today many of thesmaller process support machines

have rolling element bearings.

BEARING DESIGNS

FAILURE MONITORING

This style of bearing is typically

monitored using a case mountedtransducer: an accelerometer or velocitypickup. A displacement sensor

observing the shaft relative vibration

would show little, if any, vibration due

to the vibration node created by thebearing.

Using signal integration techniques,

found in many industrial datacollectors, specific frequency ranges

relating to certain defects can beemphasized. Acceleration signals,

obtained from case mounted sensors,emphasize high frequency sources,

while displacement signals emphasize

lower frequency sources, with velocitysignals falling between the extremes.

Recent innovations for determining


15/46

Rolling

Element

Bearingshave four

components:an inner

race, an outer race, a rolling element,

and a cage to support, space, and

guide the rolling elements. The

rolling elements found in today'srolling element bearings include:

balls, rollers, and tapered rollers. All

rolling element bearings have onething in common: all parts must be in

physical metal to metal contact at all

times. Installation instructionsspecify the amount of bearing pre-

load to maintain the component

contact.

Rolling element bearings have someunique concerns not found in journal

bearings. A rolling element bearing

will always force a vibration node atits location. Because of the metal to

metal contact, this bearing will

provide very little vibration damping.Although these bearings are a veryprecisely machined part they have a

limited lifetime. Each component of

the bearing will generate specificfrequencies as defects initiate and

become more prevalent.

Spherical Ball

Spherical ball bearings, as the nameimplies, utilize spherically shaped

balls as the rolling or load carryingelement. The number of balls used ina bearing varies depending on the

application. This rolling element

bearing type is designed to carryboth radial and axial loads. By

modifying the design, this bearing

can also accommodate large axial

bearing condition are Acceleration

Enveloping, Spectral Emitted Energy

(SEE), and Spike Energy. Thesemeasure high frequency resonances

generated by bearing defects. As atrended variable, in conjunction withother parameters such as displacement,

velocity or acceleration, they can give

the earliest indication of bearing

defects.

The figure depicts the overall amplitude

levels obtained from a bearing as it

progresses through continuing phases

of failure. This chart depicts overallvibration levels only. As time

progresses the earliest indication of

failure are obtained from filtered highfrequency signals because these signals

are generated by the resonance of the

bearing and by bearing componentdefects.

During the early stages of failure the

other three parameters may not generate

enough signal to be detected becausethese parameters emphasize

progressively lower frequency ranges.

As failure continues and the damagedbearing generates the individual bearing

defect frequencies, the other parameters

register signals.


16/46

loads.

Cylindrical/Spherical Roller

This type of bearing utilizes

cylindrically shaped rollers as the

load carrying element. This bearing

type is designed to carry large radialloads. This bearing, in its basic

configuration, is not well suited to

counter axial loads. The rollers mayactually be slightly barrel shaped in

certain designs. Barrel shaped rollersand their associated outer race allowfor some self alignment of the

bearing. Needle bearings are a

special adaptation of the cylindrical

roller bearing.

Tapered Roller/Land

This bearing design is a special

adaptation of the cylindrical rollerbearing. This bearing is designed to

counter axial thrust loads along withcarrying radial loads. Due to the

geometrical summation of the radialand axial loads, this bearing has a

lower radial load limit than a

similarly sized cylindrical orspherical bearing.

Certain applications may employ

tapered rollers along with tapered

races, hence the name. Special

bearings may have inner and outerraces with differing angles.

VIBRATION MONITORING

APPLICATIONS

Rolling element bearings, by their

design and installation, provide a

Viewing

the four

monitoring parameters as spectra,

additionalinformation can be obtained

about the failure modes. This figureshows the spectrum frequency content

during four stages of bearing failure. A

normal bearing or newly installedbearing will show no frequencies

except those associated with shaft

phenomenon such as balance ormisalignment.

Stage I

Stage I has some very high frequency

content in the Spike Energy region.

This zone is in the ultrasonic regionwhich requires a sensor specifically

designed to detect in this region.

Special circuitry filters pass only those

signals. Physical inspection of thebearing at this stage may not show any

identifiable defects.

Stage II


17/46

very good signal transmission path

from the vibration source to the outer

bearing housing. Also, these bearingsrequire monitoring of the unique

bearing frequencies generated by thevarious parts of the bearing, inaddition to the rotor fault

frequencies.

Bearing Frequency Calculation

Although modern rolling elementbearings are very precisely

machined, they do have micro-

defects which are potential sites for

future damage. Due to the precise

tolerances, improper installationpractices can reduce bearing life.

Extensive information has beencompiled about bearing defect

frequencies.

The figure lists the bearing defect

frequency formulas for a defect onthe balls or rollers, outer race, inner

race, and cage. The assumption for

these formulas is that the outer race

is stationary while the inner racerotates.

If the bearing dimensions are known,

the individual bearing defect

frequencies can be calculatedprecisely, or a general rule of thumb

can be applied. Using the generalized

form the inner race frequencieswould be N x RPM x 60% and the

outer race frequencies would be N x

RPM X 40%. If the bearingmanufacturer model numbers are

Stage II begins to generate signals

associated with natural resonancefrequencies of the bearing parts as

bearing defects begin to "ring" the

bearing components. A notable increasein Zones 3 and 4 region signals is

associated with this stage. Beginning

signs of defects will be found upon

inspection.

Stage III

Stage III condition has the fundamentalbearing defect frequencies present.

These frequencies are those discussed

previously in this paper. Harmonics ofthese frequencies may be present

depending upon the quantity of defectsand their dispersal around the bearingraces. The harmonic frequencies will be

modulated, or side banded, by the shaft

speed. Zone 4 signals continue to grow

throughout this stage.

Stage IV

Stage IV is the last condition before

catastrophic failure of the bearing. Thisstage is associated with numerous

modulated fundamental frequencies and

harmonics indicating that the defectsare distributed around the bearing races.


18/46

known several computer programs

are available to calculate the

necessary frequencies.

Due to the increased degradation of the

bearing the internal clearances are

greater and allow the shaft to vibratemore freely with associated increases in

the shaft frequencies associated withbalance or mis-alignment. During laterphases of stage IV, the bearing

fundamental frequencies will decline

and be replaced with random noise

floor or "hay stack" at higherfrequencies. Zone 4 signal levels will

actually decrease with a significant

increase just prior to failure

Specification of a Turbine Supervisory

Instrumentation (TSI) system can bean exhausting process when the

individual parameters must be

specified. This application note issupplied to provide a guide to be used

in selecting an appropriate TSI

system. TSI systems not only measure

bearing vibration levels, but caninclude shell expansion, differential

expansion, valve position, turbine

speed and acceleration, thrust position,phase angle, and bearing temperatures.

When an existing TSI system is being

retro-fitted the immediate indication is

that a one-for-one replacement of eachoriginal parameter is sufficient. This

approach may be adequate if the

original system was a completepackage.

Recent experience with retro-fitting

TSI systems has brought to light that

many of the existing systems could beenhanced with additional parameters.

Also, certain parameters should be

Valve Position

Correct valve positioning is required

to efficiently operate a steam turbine.

Some turbines may require severalthrottle valves be monitored and some

turbines will instrument the main stop

valve(s) to determine when they crack

from their seats.

Retro-fit valve position measurements

use DC LVDTs or DC Rotary

Potentiometers. All OEM TSI systems

include valve position measurement(s)as a startup and operation parameter.

Some OEM systems utilized AC

LVDTs while others use mechanicallinkages and scales for indication.

A retro-fitted system can be installedin the same position or at relocated to

a more accessible or protectedposition.

For more information about valve

position systems and applications see

STI Application Note, Valve


19/46

considered for complete replacement

with a different type sensor.

General

The information required under this

topic will define and describe the

turbine generator along with who willperform and/or supply the various

tasks and parts of the TSI installation.

The time frame for the systeminstallation should get consideration at

the point.

Describing the turbine generator

involves listing the number ofbearings, type of bearings,

turbine/generator manufacturer, the

number and function of each rotorsegment, etc. This information may be

obtained from the OEM operation and

maintenance manuals and is required

whether a retro-fit or an entirely newinstallation is being specified.

Documentation of the proposed TSI

should include who supplies theindividual components and service ofthe new system, along with the

number of operation and service

manuals and/or drawings required.

For more information about

installation services see the STI

Application Note, Field Service, FS.

STI Application Note, Field WiringInstallation, FWI covers many topics

of particular concern prior to andduring the electrical systeminstallation.

Monitor

Selecting the monitor follows theprocess of detailing the turbine

Position,TSI Part-2.

Eccentricity

A rotor which has been sitting idleduring overhaul or has been

inadvertently stopped during

coastdown for an extended period willdevelop a bow or bend. This condition

must be corrected by turning gear

operation and, possibly, with auxiliaryheating prior to high speed operation

to prevent internal clearance rubbing.

Eccentricity systems installed by

OEMs monitor the turbine stub shaftor a shaft collar using induction coils.

A retro-fit Eddy Probe system will

monitor the same location and manytimes use the same bracketry.


eccentricity systems and applications

see STI Application Note,Eccentricity, TSI Part-1.

Speed

Turbine speed indication supplied byOEMs come in many forms:

observing a gear wheel located inside

the front standard, electricallyconverting the generator output

frequency, or monitoring the turning

gear. A retro-fitted system using Eddy

Probe's can be specified to observeany multi-toothed gear wheel.

Applications monitoring generatoroutput frequency without an integralturning gear may require installation

of a custom gear wheel.

Speed indication may be specified as

an analog display or as a digitaldisplay and can be interfaced to a zero


20/46

generator layout. The monitor

selection generally involves deciding

what the monitor should do and howthe user will interface with it.

The monitor can be specified to be a

stand-alone output with user interface

or to interface with an another existingoutput device such as PLC or DCS.

Radial Vibration

Radial vibration is usually the heart ofthe TSI system. It gets the most

attention and generally gives the first

indication of out of specificationconditions. Most OEM TSI systems

utilized a shaft rider transducer system

to monitor vibration with a shaftabsolute output signal. An exact

replacement transducer system can be

supplied, but most customers and

OEMs are specifying a Eddy ProbeSystems. A complete vibration system

would install two sensor systems per

bearing with the sensors located 90

from each other.

For more information about Eddy

Probe Vibration Sensors and their

application see the STI ApplicationNote, Eddy Probe Transducer

Installation, Part 1-Radial Vibration.

Thrust Position

Thrust position indication includes

one or two Eddy Probe Systems toobserve the position of the thrustcollar within its bearings. This system

is an internal installation and need not

replace the existing system becausemany original installations utilize a

differential pressure system that

interfaces with the turbine hydraulic

speed system for turning gear

engagement.

Rate of Acceleration

The rate of acceleration parameter is

usually monitored during startup to

prevent over-torquing the rotors, asthe turbine approaches critical speeds,

and as the operating speed is reached

prior to line synchronization. Once thegenerator has been synchronized and

is being controlled by load dispatchers

the acceleration rate is not monitored.

Acceleration rate measurements use aspeed input to derive its output

display. Eddy Probe systems can be

installed as a replacement orsupplement an existing application.

See STI Application Note, Eddy Probe

Transducer Installation, Part-1 Radial

Vibration for relevant informationabout this type of sensor.

Phase

Phase, or phase angle, is a measure ofthe relationship of how one vibration

signal relates to another vibration

signal and is commonly used tocalculate the placement of a balance

weight. This parameter is not usually

displayed continuously but ismonitored periodically to determine

changes in the rotor balance condition,

deviations in system stiffness such as

a cracked shaft.

Phase angle measurements are

sometimes not supplied by OEMs, but

can be installed using a Eddy Probesystem. Installation involves locating

or installing a once-per-turn event

such as a key or notch that the Eddy


21/46

control system.

For more information about thrust

position sensors and their application

see the STI Application Note, EddyProbe Transducer Installation, Part 2-

Thrust Position.

Shell Expansion

Shell expansion is the measure of a

turbine case or shell moves in relation

to a fixed location usually measuredwith a Linear Variable Differential

Transformer (LVDT). Some existing

OEM systems still use spindlemicrometers or dial indicators that are

subject to mechanical damage and

human error. Although many systemsinstalled with only one LVDT are

adequate, a complete TSI system

specification should consider two

LVDTs located at each corner of theturbine shell. A second sensor will

monitor shell cocking or uneven

thermal growth which is a fairly

common occurrence during startupwhen the sliding feet may have

inadequate lubrication.

For more information about shellexpansion systems and applications

see the STI Application Note, Shell

Expansion, TSI Part-4.

Differential Expansion

Differential expansion measurementsare an important parameter receivingmuch attention during turbine startup

and warming. This parameter

measures how the turbine rotorexpands in relation to the turbine

shell, or casing.

Probe will view. An Eddy Probe

viewing a notch is easier to install and

adjust, but the installation of the notchrequires special tooling to cut the

notch. Keys are easier to apply usingglues or epoxies and are subject tocoming off due to centrifugal forces.

Temperature

Bearing temperature is a measure ofthe how hot a bearing is operating. It

may be due to overloading, mis-

alignment, improper lubricant pressure

and/or flow.

Nearly all turbine generator bearings

were originally installed or retro-fitted

with bearing temperature sensors.These sensors may be thermocouples

or RTDs. This parameter is often

overlooked possibly due to the OEM

output display located at some otherpanel not within the vicinity of the

retro-fitted TSI system. Any bearings

that was not originally equipped with

temperature sensors can be retro-fittedto accept thermocouples or RTDs.

Custom Cabinet

Congested control boards may

preclude installing the TSI rack

requiring a stand-alone cabinet. Thiscabinet can house auxiliary equipment

associated with the new TSI system,

such as power supplies, termination

strips, external relays, etc.

The cabinet can be configured to

many differing designs depending

upon the user's requirements. Cabinetsshould be sturdy enough withstand

environmental conditions, such as

moisture content, explosive


22/46

A new differential expansion system

using Eddy Probes can be retro-fitted

to any existing system. A Eddy Probeis more reliable and robust than OEM

supplied induction coil systems.


differential expansion systems andapplications see the STI Application

Note, Differential Expansion, TSI

Part-3.

atmospheres, temperature, etc.

Frequency domain measurements and

analysis have become increasinglypopular to diagnose a particular

machine fault. This measurementmode relies on processing the

transducer output signal using Fast

Fourier Transform (FFT) algorithms to

display the signal amplitudes as afunction of frequency. FFT processing

essentially separates complex signals

into individual components having asingle frequency content. This type of

display is commonly termed aspectrum.

An enhancement of spectral analysis isto define specific frequency ranges to

perform band analysis. Conceptually,

band analysis is similar to filtering asignal. The "filter" searches for

frequencies only within its frequency

range. Certain permanently installedmachine monitoring systems offer this

capability. This feature is quiteeffective, once the particular spectral

range and resolution has beendetermined, to rapidly diagnose

machine faults.

SPECTRUM

A band is essentially a band pass filter

allowing only the frequencies with theselected range to be measured. All

other frequencies are excluded fromanalysis. Many modern machine

monitor systems are capable of

monitoring specific frequency ranges

using band analysis.

RESOLUTION

Frequency resolution is an area

requiring considerable attention. If theresolution is inadequate the entire

analysis process could be meaningless

or incorrect. Some instrumentspecifications list the spectral

resolution as lines. A high resolution

would be 3200 lines per spectrum anda low resolution would be 100 lines

per spectrum.


23/46

A spectrum display is a display of

signal amplitudes on the vertical axisand the signal frequencies on the

horizontal axis. The frequency axis

units may be in hertz (hz) or in cycles

per minute (CPM). Hertz or cycles persecond may be converted into CPM by

multiplying by 60. For example: 10 hz

= 600 CPM. The horizontal axis is

scaled from 0 to some maximumfrequency (Fmax). Individual signal

frequencies will appear as peaks orspikes, each having a specific

amplitude. Properly setting the Fmax

will ensure that all of the input signalis being analyzed. This setting can be

verified mathematically by summing

the square

of eachamplitude

peak. The

square rootof this

summation

shouldapproximate

the overall amplitude level obtained

directly from the transducer's output.

BAND ANALYSIS

Band analysis involves selecting

frequency ranges of interest to allowrapid determination of a machine's

condition. Generally, each machine

fault will generate a specific, unique

frequency as the conditiondeteriorates.

Each line of

resolution

can beviewed as a

bucket orpail of aspecific

size. The signal frequencies can be

viewed as a tennis ball. If a tennis

ball's frequency matches thefrequency range of the bucket, it is

placed in the bucket. As the bucket

fills with tennis balls the peaks on thespectrum display rise. Should the

frequency range of the buckets be too

large, the tennis balls will not beadequately separated to detect

individual frequencies. This would

lead to always using the highest

resolution for spectral or bandanalysis. Higher resolutions require

greater amounts of time to display the

spectrum, thus a balance must bereached between the capture time and

the spectral resolution.

Frequency Domain Checklist

1. Overall Amplitude2. Time Base Waveform

3. Orbits

\\\


24/46

Eddy Current Transducers\

Eddy Current Transducers (Proximity

Probes) are the vibration transducer of

choice when installing vibrationmonitoring on Journal Bearing

equipped rotating machinery. Eddy

Current Transducers are the onlytransducers that provide Shaft Relative

(shaft relative to the bearing) vibration

measurement.

Several methods are usually availablefor the installation of Eddy Current

Transducers, including internal,

internal/external, and external

mounting.

Before selecting the appropriate

method of mounting Eddy Current

Transducers,special

consideration

needs to begiven toseveral

important

installation considerations that willdetermine the success of your

monitoring program.

Theory of Operation

Eddy Current Transducers work on the

proximity theory of operation. A EddyCurrent System consists of a matchedcomponent system: a Probe, an

Extension Cable and an Oscillator

/Demodulator. A high frequency RFsignal @2 mHZ is generated by the

Oscillator/Demodulator, sent through

the extension cable and radiated from

The gauge of the selected wire

depends on the length of the

instrument wire run, and should be asfollows to prevent loss of high

frequency signal:

Up to 200 feet 22 AWG



The following wiring connection

convention should be followed:

Red -24 VDC

Black Common

White Signal

Common Point Grounding

To prevent Ground Loops from

creating system noise, systemcommon, ground and instrument wire

shield must be connected to ground at

one location only. In most cases, therecommendation is to connect

commons, grounds and shields at the

Monitor location. This means that allcommons, grounds and shields must

be floated or not connected at the

machine.

Occasionally due to installationmethods instrument wire shields are

connected to ground at the machine

case and not at the monitor. In thiscase, all of the instrument wire shields

must be floated or not connected at

the monitor.

Conduit

Dedicated conduit should be provided


25/46

the Probe tip. Eddy currents are

generated in the surface of the shaft.

The Oscillator /Demodulatordemodulates the signal and provides a

modulated DC Voltage where the DCportion is directly proportional to gap(distance) and the AC portion is

directly proportional to vibration. In

this way, a Eddy Current Transducer

can be used for both Radial Vibrationand distance measurements such as

Thrust Position and Shaft Position.

Special Considerations

Number of TransducersAll vibration transducers measure

motion in their mounted plane. Inother words, shaft motion either

directly away from or towards the

mounted Eddy Current Probe will bemeasured as radial vibration.

On smaller less critical machines, one

(1) Eddy Current Transducer system

per bearing may be adequate for

machine protection.

The single Eddy Current Probe will

then measure the shaft's vibration in

that given plane. Therefore, the EddyCurrent Probe should be mounted in

the plane where the greatest vibration

is expected.

On larger more critical machines, two(2) Eddy Current Transducer systems

are normally recommended perbearing. The Probes for this type ofinstallation should be mounted 900

apart from each other. Since the

Probes will measure the vibration intheir respective planes, the shaft's total

vibration within the journal bearing is

measured. An "Orbit" or cartesian

in all installations for both mechanical

and noise protection. Flexible metal

conduit should be used from the EddyProbe to the Oscillator /Demodulator

junction box, and rigid bonded metalconduit from the junction box to themonitor.

Calibration

All Eddy Current Systems (Probe,

Cable and Oscillator Demodulator)should be calibrated prior to being

installed. This can be done by using a

SKF-CM CMSS601 Static Calibrator,

-24 VDC Power Supply and a Digital

Volt Meter. The Probe is installed inthe tester with the target set against

the Probe tip. The micrometer withtarget attached is then rotated away

from the Probe in 0.005" or 5 mil

increments. The voltage reading isrecorded and graphed at each

increment. The CMSS601 Calibrator

will produce a voltage change of 1.0VDC +-0.05 VDC for each 5 mils of

gap change while the target is within

the Systems linear range.

Gap

When installed,Eddy Current Probes

must be gapped properly. In most

Radial Vibration applications, gapping

the transducer to the center of thelinear range is adequate. For the

Model CMSS65 and 68 gap should be

set for -12.0 VDC using a Digital VoltMeter (DVM), this corresponds to an

approximate mechanical gap of 0.060"or 60 mils. The voltage method ofgapping the Probe is recommended

over mechanical gapping. In all cases,

final Probe gap voltage should bedocumented and kept in a safe place.

Internal Mounting


26/46

product of the two vibration signals

may be viewed when both Eddy

Current Transducers are connected toan SKF-CM Information System or an

Oscilloscope.

Linear Range

Several versions of Eddy CurrentTransducers are available with a

variety of Linear Ranges and body

styles. In most cases, SKF-CM'sCMSS68 with a linear range of 90

mils (0.090") is more than adequate

for Radial Vibration measurements...

Model Range Output SizeCMSS65 90 mils 200

mV/mil

1/4"x28 UNF

1" to 5" Length

CMSS68 90 mils 200

mV/mil

3/8"x24 UNF

1" to 9" Length

CMSS62 240

mils

50

mV/mil

1" x 12 UNF 1"

to 5" Length

Target Material/Target Area

Target Material

Eddy Current Transducers are

calibrated at the factory for 4140 Steelunless specified otherwise. As Eddy

Currents are sensitive to thepermeability and resistivity of the

shaft material any shaft material other

than 4000 series steels must bespecified at the time of order. In cases

of exotic shaft material a sample may

need to be supplied to the factory.

Mechanical Runout

Eddy Current Transducers are alsosensitive to the shaft smoothness for

Radial Vibration. A smooth (64 micro-

inch) area approximately 3 times thediameter of the Probe must be

provided for a viewing area. The

prepared journal area on most shaftsare wider than the bearing itself

Internal

Mounting isaccomplished

with the EddyCurrentProbes

mounted

internally to the machine or bearing

housing with a SKF-CM CMSS903Bracket or with a custom designed

and manufactured bracket. The

Transducer system must be installedand gapped properly prior to the

bearing cover being reinstalled.

Provisions must be made for thetransducer's cable exiting the bearing

housing. This can be accomplished by

using an existing plug or fitting, or by

drilling and tapping a hole above theoil line. The Transducer's cables must

also be tied down within the bearing

housing to prevent cable failure from"windage".

For added safety and reliability, all

fasteners inside the bearing housingshould be safety wired, or otherwiseprevented from working loose inside

the machine.

Advantages of Internal Mounting

Most economical installation.

Less machining required.

True bearing relative

measurement.

Usually good viewing surfacefor Eddy Probe.

Disadvantages of Internal Mounting

No access to Probe while

machine is running.

Cables must be tied down due


27/46

allowing for Probe installation

immediately adjacent to the bearing.

Electrical Runout

Since Eddy Current Transducers are

sensitive to the permeability and

resistivity of the target material andthe field of the transducer extends into

the surface area of the shaft by

approximately 15 mils (0.015"), care

must be taken to avoid non

homogeneous viewing area materialssuch as Chrome.

Another form of electrical runout canbe caused by small magnetic fields

such as those left by Magna-fluxing

without proper degaussing.

Perpendicular to shaft centerline

Care must be exercised in all

installations to insure that the Eddy

Current probes are mountedperpendicular to the shaft center-line.Deviation by more than 1-2 degrees

will effect the output sensitivity of the

system.

Orientation of Transducer(s)

As most machine casings are

horizontally split, transducers are

commonly found mounted at 450 bothsides of vertical 900 apart.

If possible transducer orientation

should be consistent along the length

of the machine train for easiermachine diagnostics. In all cases

orientation should be well

documented.

to

"windage".

Transducer cable exits must beprovided.

Care must be taken to avoid

oil leakage.

External/Internal Mounting

External/Internal mounting is

accomplished when the Eddy Probesare mounted with a Mounting Adapter

(SKF-CM CMSS911 or 904). These

adopters allow external access to theProbe yet allows the Probe tip to be

internal to the machine or bearing

housing. Care must be taken indrilling and tapping the bearing

housing or cover to insure that the

Eddy Probes will be perpendicular to

the shaft center line.

In some cases due to space limitations

External/Internal mounting is

accomplished by drilling or making

use of existing holes in the bearingitself, usually penetrating at a oil

return groove.

Advantages of External/InternalMounting

Eddy Probe replacement whilemachine is running.

Usually good viewing area forEddy Probe.

Gap may be changed while

machine is running.

Disadvantages of External/Internal


28/46

Transducer

(Probe) side

clearances

The RF Field

emitted fromthe Probe tip

of a Eddy Current Transducer in

approximately a 450 coned

shape.Clearance must be provided on

all sides of the Probe tip to preventinterference with the RF Field. As an

example, if a bearing is drilled to

permit installation, the hole must becounter bored to prevent side

clearance interference. Care must also

be taken to avoid collars or shoulderson the shaft that may thermally "grow"

under the Probe tip as the shaft grows

from heat.

Eddy Current Probe tip to tip

clearances

Although Probe tip to tip clearances

are not normally an issue on mostmachines, it should be noted that Eddy

Current Probes radiate an RF Field

larger than the Probe tip itself. As anexample, Model CMSS65 and 68probe should never be installed with

less than one (1) inch of Probe tip to

tip clearance. Larger Probes requiremore clearance. Failure to follow this

rule will allow the

Oscillator/Demodulator to create a"beat" frequency which will be the

sum and difference of the two

Oscillator/Demodulator RF

frequencies.

System Cable Length and Junction

Boxes

Eddy Current Transducer Systems area "tuned" length, and several system

lengths are available. Length is

measured from the Probe tip to the

Mounting

May

not be

truebearing relative measurement.

More machining required.

Long Probe/Stinger length(Resonance).

External Mounting

Pure external Eddy Probe mounting is

usually a last resort installation. Theonly valid reason for using this

method is inadequate space

availablewithin the bearing housingfor internal mounting. Special care

must be given to the Eddy Probe

viewing area and mechanicalprotection of the transducer and cable.

Advantages of External Mounting

Most Inexpensive Installation.

Disadvantages of External Mounting

May be subject to "Glitch" or

Electrical/Mechanical runout.

Requires mechanical

protection.

Installation Checklist

1. Mounting Type, InternalExternal/Internal External

2. Number of Transducers, X or

X&Y3. Target Material, 4140 Other4. Smooth Target Area

5. Size of Target Area

6. Junction Box Location(s)7. Metal Conduit (Junction Box

to Monitor)

8. Flexible Conduit (Junction


29/46

Oscillator/Demodulator, and is

measured electrically which can

slightly vary the physical length. Forexample, the Model CMSS65 and 68

are available in 5 and 10 meter systemlengths. Care must be taken to insurethat the proper system length is

ordered to reach the required Junction

Box.

Grounding and Noise

Electrical noise is a very serious

consideration when installing any

vibration transducer, and special care

needs to be taken to prevent

unnecessary amounts of noise. Asmost plant electrical noise is 60 HZ,

and many machines running speed isalso 60 HZ, it is difficult to separate

noise from actual vibration signal.

Therefore, noise must be kept to anabsolute minimum.

Instrument Wire

A 3-wire twisted shielded instrument

wire (ie; Belden #8770) is used to

connect each Oscillator/Demodulatorto the Signal Conditioner in the

Monitor. Where possible, a single runof wire from the

Oscillator/Demodulator (Junction

Box) to the Monitor location should

be used. Splices should be avoided.

Box to Probe)

9. Correct Instrument Wire

10. Shielding Convention,Monitor or Machine

11. Calibration

12. Gap Set

Accelerometers have been a popular

choice for rotating machinery vibrationmonitoring. They are a rugged, compact,

light weight transducer with a widefrequency response range. Accelerometers

have been used extensively in manymachinery monitoring applications. This

transducer is typically attached to the

As can be seen

in the figureabove, the

mountingmethod also has

an effect on theoperating

frequency range of an accelerometer. By


30/46

outer surface of

machinery. Generallythis machinery will

have parts thatgenerate high

frequency signals,

such as, rollingelement bearings or gear sets.

The application and installation of an

accelerometer must be carefullyconsidered for an accurate and reliable

measurement.

Accelerometers were designed to be

mounted on machine cases. This willprovide continuous or periodic sensing of

absolute case motion (vibration relative to

free space) in terms of acceleration.

Theory of Operation

Accelerometers are inertial measurement

devices that convert mechanical motion toan electrical signal. This signal is

proportional to the vibration's acceleration

using the piezoelectric principle. Inertialmeasurement devices measure motion

relative to a mass. This follows Newton'sThird Law of Motion: A body acting on

another will result in an equal action onthe first.

Accelerometers consist of a piezoelectriccrystal and mass normally enclosed in a

protective metal case. As the mass appliesforce to the crystal, the crystal creates a

charge proportional to acceleration. Thecharge output is measured in pico

Coulombs per g (pC/g) terms where g isthe force of gravity. Some sensors have

an internal charge amplifier, while othershave an external charge amplifier. The

charge amplifier converts the chargeoutput of the crystal to a proportional

voltage output in mV/g terms.

Current or Voltage Mode

This type of accelerometer has aninternal, low-output impedance amplifier

and requires an external power source.The external power source can be either a

design,accelerometers have a natural

resonance which is 3 to 5 times higherthan the advertised high end frequency

response. The frequency response rangeis limited so that a flat response is

provided over the operating range. The

SensitivityAccelerometers utilized for vibrationmonitoring are usually designed with a

sensitivity of 100 mv/g. Accelerometerscan be supplied with a wide range of

sensitivities for special applications suchas structural analysis, geophysical

measurement, or very high frequencyanalysis.

Frequency RangeAccelerometers are designed to measure

vibration over a given frequency range.Once the particular frequencies of interest

for a machine are known, anaccelerometer may be selected. Typically,

an accelerometer for measuring machine

vibration will have a frequency range from1 or 2 hertz to 8 or 10k hertz.

An accelerometer is used on machines

when high frequency measurements aredesired. In terms of energy sensed by the

transducer, acceleration will have largeramplitudes as the frequency increases. At

low frequencies, the accelerationamplitudes may be quite small giving a

false impression of an acceptablyoperating machine.

CalibrationPiezoelectric accelerometers can not be

recalibrated or adjusted. Unlike a velocity


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constant current source or a regulated

voltage source. This accelerometer is

normally a two wire transducer with onewire for power and signal, and the second

wire for common. This type of

Accelerometers have a lower temperaturerating due to the internal amplifier

circuitry. Signal cable lengths up to 500feet have negligible effect on the output

signal quality. Longer cable lengths willreduce the effective frequency response

range.

Charge Mode

Charge mode accelerometers differslightly from current or voltage mode

types. This sensor has no internal

amplifier and therefore have a highertemperature rating. An external chargeamplifier is supplied with a special adapter

cable which is matched to theaccelerometer. Field wiring is terminated

to the external charge amplifier. As withcurrent or voltage mode accelerometers,

signal cable lengths up to 500 feet have

negligible effect on the output signalquality. Longer cable lengths will reduce

the effective frequency response range.

Special Considerations

MountingThere are three mounting methods

typically used for monitoring applications:bolt mounting, glue, and magnets.

The bolt mounting method is the bestmethod available for permanent mounting

applications. this method is accomplishedvia a stud or a machined block. This

method permits the transducer tomeasure vibration according to the

manufacturer's specifications. Themounting location for the accelerometer

should be clean and paint free. The

mounting surface should be spot-faced toa surface smoothness of 32 micro-inches.

The spot-faced diameter should be 10%

larger than the accelerometer diameter.Any irregularities in the mounting surface

preparation will translate into improper

pickup, this transducer has no moving

parts subject to normal wear. Therefore,

the output sensitivity does not requireperiodic adjustments to correct for wear.

An accelerometers has internal

components which can be damaged fromshock or overheating. When an

accelerometer is suspect, a simple test ofthe transducer's bias voltage will help

determine whether it should be removedfrom service. An accelerometer's bias

voltage is the DC component of thetransducer's output signal. The bias

voltage is measured with a DC volt meteracross the transducer's signal output and

common leads with power applied. At the

same time, the power supply voltageshould also be checked to eliminate thepossibility of improper power voltage

affecting the bias voltage level.

Instrument WireThe following table is a partial list of

Belden Cables that should be used for

the instrument field wiring. These partnumbers may be cross referenced to

equivalent cables from othermanufacturers. The listed cables are

polyethylene insulated, twisted, withBeldfoil shield, drain wire, and PVC jacket.

Belden Part Numbers

P/N Nom. O.D.

18 AWG 8760 0.22"

20 AWG 8762 0.20"

22 AWG 8761 0.18"

Common Point Grounding

To prevent Ground Loops from creatingsystem noise, system common, ground

and instrument wire shield must beconnected to ground at one location only.

In most cases, the recommendation is toconnect commons, grounds and shields at

the Monitor location. This means that allcommons, grounds, and shields must be

floated or not connected at the machine.

Occasionally, due to installation methods,


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33/46

he velocity

pickup is a

very

populartransducer or sensor formonitoring the vibration of rotating

machinery. This type of vibration

transducer installs easily on machines,

and generally costs less than othersensors. For these two reasons, this

type of transducer is ideal for general

purpose machine applications.Velocity pickups have been used as

vibration transducers on rotatingmachines for a very long time, andthey are still utilized for a variety of

applications today. Velocity pickups

are available in many different

physical configurations and outputsensitivities.

Theory of OperationWhen a coil of wire is moved through

a magnetic field, a voltage is induced

across the end wires of the coil. Theinduced voltage is caused by the

transferring of energy from the flux

field of the magnet to the wire coil. As

the coil is forced through the magneticfield by vibratory motion, a voltage

signal representing the vibration is

produced.

Signal Conventions

A velocity signal produced by

vibratorymotion isnormally

sinusoidal in nature. In other words, in

Sensitivity

Some velocity pickups have the

highest output sensitivities of anyvibration pickup for rotating machine

applications. The sensitivity will vary

from manufacturer to manufacturer.The higher output sensitivity is useful

in situations where induced electrical

noise is a problem. The larger signalfor a given vibration level will be less

influenced by the noise level. Some

velocity pickups with theirsensitivities are listed below:

Sensitivity

STI LCV100

500

mv/in/sec

Frequency ResponseVelocity pickups will

have differing frequency

responses depending onthe manufacturer.

However, most pickups

have a frequencyresponse range in the

order of 10 to 1000 hz.

This is an importantconsideration when

selecting a velocitypickup for a rotatingmachine application. The

pickup's frequency

response must be within

the expected vibrationfrequencies of the

machine. Due to the

support spring for thebobbin., a natural

mechanical resonance

occurs at the low end ofthe frequency response

curve. This resonance is

either damped by the oil

contained within thesensor, or with a shunt

resistor across the coil's

leads.

Calibration


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Comparing vibration level readings taken bydifferent types of instruments and transducers

can be very confusing and can lead to mistrust

of the systems involved.

Knowledge of how to properly compare

readings is required before comparing any

readings is attempted.

This application note explains the variablesinvolved in some detail and will act as a

guideline as you compare vibration readings.

Transducer Type

Three (3) basic types of vibration transducers

are available which correlate with the three (3)

types of measured physical motion,Acceleration, Velocity and Displacement.

Accelerometer


device. A pre- loaded crystal is charged withcurrent and as the crystal is compressed or de-

compressed by vibration an output proportional

to g's (gravity) is provided. A "g" is equal to9.80 meters/second2 or one (1) standard earthgravity.

Accelerometers are normally used for high-frequency bearing cap vibration readings

(Case/Bearing Cap Absolute on machines using

rolling element bearings. Usually the output isintegrated electronically to velocity (in/sec or

mm/sec). Other applications include monitoring

Shaft Absolute

Shaft Absolute is the measurement of the

shaft's motion relative to free space (or

absolute). Shaft Absolute can be measuredtwo (2) ways, the first being electronically

summing the

signals

of both a Eddy Probe measuring shaft

relative and a accelerometer measuring

case absolute, the second being using ashaft rider which is a spring mounted

device that physically rides on the surface

of the shaft, normally a velocity sensorintegrated to displacement is mounted on

top of the shaft rider. Shaft Absolute is

normally used where the rotating assemblyis five (5) or more times heavier than the

case of the machine.

Engineering Units

0 to Peak (0-P)

Both Velocity (in.sec, mm/sec) and

Acceleration (g's) by definition aremeasured in 0 to Peak or one/half the Peak

to Peak signal as viewed on an

oscilloscope.

Peak to Peak (P-P)

Displacement by definition is measured in

Peak to Peak or the actual Peak to Peak

Motion of the Shaft.

Root Mean Square (RMS)

Root Mean Square (RMS) is a popular

method of measuring Case or Bearing Cap

Vibration as many vibration engineershave found that RMS is more indicative of

actual rolling element bearing condition.


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Gears and High Frequency Applications.

Velocity Pick-up

Two (2) types of Velocity Sensors exist,

mechanical and electronic. Mechanical typesare the most common and are made up of a

spring mounted coil mounted inside a magnet.

Vibration causes the coil to move in relation tothe magnet which produces a voltage output

directly proportional to Velocity. Electronic

Velocity Sensors are Accelerometers with anelectronic integrator built in to the unit. Output

of a Velocity Sensor can be expressed in many

different terms, inches/second (in/sec) or

millimeters/second (mm/sec) being the

standards.

Velocity Transducers are normally used for

Bearing Cap Vibration Monitoring

(Case/Bearing Cap Absolute) on machines with

rolling element bearings. They have theadvantage of high outputs and the signal is read

directly in velocity (in/sec or mm/sec).

Eddy Probes (Proximity)

Eddy or Proximity Probes are a displacement

device that measure the relative motion between

the probe mounting location and the target(shaft). Output is directly proportional to

displacement and is usually measured in mils

(.001") or millimeters (mm).

Although rarely found in vibration wave-

forms a pure sine wave RMS would be .

707 times the 0 to Peak Value.

Transducer Considerations

Frequency Response

The frequency response of a vibration

transducer is very important when

comparing readings. Transducers with awider or broader frequency response will

typically see more vibration if it is present

than a narrower bandwidth transducer.How different vibration frequencies

contribute to overall values is dependent

on their phase relationship to each other,some may add, some may subtract from

the overall value.

Eddy Probes Displacement200

mv/mil

Velocity(Mechanical)

Velocity500mv/in/sec

Velocity

(Piezoelectric)Velocity

500-1000

mv/in/sec

Accelerometer Acceleration 100 mv/g

Mounting

How a transducer is mounted is alsocritical to comparing measurements.

Accelerometers are extremely sensitive to

the method of attachment. Differences inbandwidth can be measured between hand-

held, magnet attached, epoxy, and stud

mounted installations.

Installation instructions must be followedprecisely to obtain the manufactures

transducer specifications. Accelerometers

not mounted perfectly perpendicular to thesurface or on a flat surface will produce

stress risers which will also produce false

signals.


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Eddy Probes are used on machines with Journal

(Sleeve) type bearings. Where the measurementof motion between the Bearing and Shaft is

critical.

Bearing Type

Two primary types of bearings are in use today,

Rolling Element Bearings and Journal or SleeveBearings.

Rolling Element Bearings are zero (0) clearancedevices. All vibration of the shaft is transmitted

directly to the bearing cap.

Journal or Sleeve Bearings are designed so that

the oil film provides damping. The shaft is free

to vibrate within the bearing. Due to thedamping provided by the oil film very little of

the shaft vibration is transmitted to the bearing

cap. The oil film damping provides even higher

levels of attenuation to higher frequencies.

Measurement Location

When comparing readings it is essentialthat all readings are taken at the same

location and the same plane. Even smalldifferences in location can effect theoverall readings. All vibration transducers

are single plane devices and only measure

in the plane in which they are held or are

mounted.

Instrument Considerations

All Instruments handle signal is different

ways. Different instruments have their own

frequency response and filtering.Knowledge must be gained on the

instruments used before the outputs can becompared even when they use the same

transducer.

Conversion Formulas

Displacement, Velocity and Acceleration

are mathematically related to each other as

a function of frequency. Electronic

integrators or differentiation are also usedto change one term to the other. Once

again it must be understood that thereadings be of the same type or they will

not agree.

D = Displacement, P-P, Mils.

V = Velocity, 0-P, in/sec.

A = Acceleration, 0-P, g's.

D = 19.10 x 103 x (V/CPM)

D = 70.4 x 106 x (A/CPM2)

V = 52.36 x 10-6 x D x CPM

V = 3.87 x 103 x (A/CPM)

A = 14.2 x 10-9 x D x CPM2

A = 0.27 x 10-3 x V x CPM

Summary


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Measurement Type

Only measurements of the same type can becompared. Bearing Cap or Case Vibration

cannot be directly compared to Shaft Relativeor Shaft Absolute and visa versa.

Case Absolute

Case or Bearing Cap Absolute is the

measurement of the Case or Bearings Caps

(Location of Transducer) motion relative to freespace (or absolute motion). Case or Cap

Absolute is usually used for monitoring Rolling

Element Bearings.

Shaft Relative

Shaft Relative is the measurement of motionbetween the Shaft and whatever the measuring

devise is mounted to. This measurement is

normally taken with a NCPU or Proximity

Sensor. Shaft Relative measurements are usedfor Journal or Sleeve Bearing Applications.

In General it is difficult to get any two

readings to precisely agree with one

another. Even when care is taken to insurethat transducers and locations

vibration sesors

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