vibration measurement & analysis

7/31/2019 Vibration Measurement & Analysis

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DEFINITIONVibration is oscillating motionof a particle or body about afixed reference point. Suchmotion may be simpleharmonic (sinusoidal) or

complex (non-sinusoidal).

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QUANTIFYING VIBRATIONAMPLITUDE

Peak Level

Peak - To Peak

Average (Rectified)

RMS

Takes into account the time history of vibration.

RMS = (1/T)0T x2(t)dt

Average = (1/T)01|x|dt

PEAK TO PEAK

PEAK LEVEL RMS

AVERAGE t

PEAK LEVEL

PEAK TO PEAK

RMS

AVERAGE t

PEAK LEVEL

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UNITS OF VIBRATIONUNITS OF VIBRATIONa)a) ACCELERATION ACCELERATION , measured, measured

inin g g or or [m/s [m/s 2 2 ] ] ;;

b)b) VELOCITY VELOCITY , measured in, measured in [m/s] [m/s] ;;

c)c) DISPLACEMENT DISPLACEMENT , measured, measuredinin [m] [m] ..

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CHARACTERISTICS OFVIBRATION

Vibration may be characterized by :

a)the frequency in Hz;b) the amplitude of the measuredparameter, which may bedisplacement , velocity , oracceleration .

Normally referred to as the vibrationamplitude when expressed in units,but vibration level when expressedin decibels .

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DECIBEL NOTATION APPLIEDTO VIBRATION MEASUREMENT

Because of the wide range of vibrationamplitudes found in engineering, it isconvenient to express the measuredamplitude in DECIBELS with referenceto a fixed value.

Reference values which areinternationally accepted are as follows:

a) for velocity , the reference is 10 -3 m/s ;

b) for acceleration , the reference is 10 -5 m/s 2 .KNOWLEDGE TEAM C&I


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Contd.If Measured amplitude is A 1 andReference amplitude is A

0 , the

vibration level expressed indecibels is : 20log 10 A1/A 0 dB

Eg: If acceleration amplitude of avibrational body is 2g,acceleration vibration level= 20log 10 (2 x 9.81) m/s

2 = 125.8 dB10 -5 m/s 2

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RELATIONSHIP BETWEEN THEVIBRATION PARAMETERS

Assuming that the vibration is simple harmonicmotion, then

displacement x = A sin wt velocity v = A w cos wt acceleration a = -A w2 sin wt

Where: w = 2 f rad/s f = frequency of vibration in HzFrequencies are the same in each case, althougthere is a phase shift. The amplitudes of the

above parameters are thusdisplacement amplitude = A velocity amplitude = A wacceleration amplitude = A w2

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CHOOSING PARAMETERThe choice of the best parameterdepends on a number factors:a) the frequency and amplitudecharacteristics of the vibration,

b) the mass of the vibratingstructure, and

c) the type and size of thetransducer available.

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Contd.If the velocity, acceleration, anddisplacement amplitudes aremeasured at various frequencies,the resulting graphs of amplitude vs. frequency are referred to asthe vibration spectra , and the

shape of graphs are referred to asthe spectral shapes.

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Contd. Eg:

Each have differentaverage slopes. Their peaks occur atthe same frequencies.

The amplitude rangerequired to display the

velocity spectrum isthe smallest and thusoccupies the leastdynamic range.

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Contd.All the frequency components on the velocitycurve need a smaller relative change beforethey begin to influence the overall vibrationlevel.

The low frequency acceleration and high frequency displacement components need toexhibit much larger changes before theyinfluence the overall vibration level.Display in turn each of the three parametersand choose the one which has the flattest spectrum .This will enable one to detect machine faults ,which produce an increase in vibration level,at an early stage .KNOWLEDGE TEAM C&I


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EFFECT OF THE TRANSDUCER ONTHE VIBRATING STRUCTURE

In general, the larger the mass of thevibration transducer, the greater its

sensitivity.Unfortunately, the addition of thetransducer's mass ( m 1 ) to the mass ( m 0 ) of the vibrating structure changes the resonantfrequency of the vibrating system as follows:

where f 1 = resonant frequency of thestructure with the mass addedand f 0 = resonant frequency of the structurebefore the transducer is added.

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VIBRATION-MEASURING DEVICES

The Non Contact Type MethodEddy CurrentLaser Doppler VibrometerCapacitive

The Seismic-Mass TransducerDisplacement Pick-Up

Velocity Pick-Up

Acceleration Pick-Up

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The sensor is fixed to the bearinghousing. A gap, is produced betweenthe end of the sensor and the shaft.

A coil with cable capacitance of themeasuring lead is fed to the naturalfrequency of the tuned circuit by thematching unit.Through the alternatingelectromagnetic field, eddy current isinduced in the metal object beingmeasured,This current becomes greater, thesmaller the distance between the coil

and the object.

EDDY PROBE

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The resultant modulated signal is separatedfrom the high frequency carrier voltage, islinearized and converted by an outputamplifier into a load independent voltage

proportional to the distance.The level of the direct voltage is a measure

of the mean distance between sensor andshaft, while the superimposed alternatingvalue corresponds to the relative shaftvibration.

The connecting lead between the sensorand the matching unit affects themeasurement. It is therefore permanentlyconnected to the sensor and must not beshorted.

Contd.

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Laser Doppler Vibrometer

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SEISMIC-MASS TRANSDUCERThe seismic mass is a body of metal ,suspended from a resilient support .

Deflection of support is proportional tothe force applied to it.The inertia of the seismic mass causes it to lag behind the motion of thecasing when the casing is accelerated ,causing a deflection in the support .

This deflection forms the input to a transducer , which produces aproportional output signal.

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SEISMIC-MASS TRANSDUCER

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Contd.By choosing suitable values for the mass ,the stiffness of the support and thedamping , and by using an appropriate transducer , the same basic arrangement of seismic pickup can be designed as adisplacement pickup , a velocity pickup oran acceleration pickup (accelerometer).The seismic pickup is essentially a damped spring-mass system , and will have anatural frequency of vibration given by:

wherewn is the natural angular frequency (rad/s) is the spring stiffness (N/m)

n is the mass (kg).KNOWLEDGE TEAM C&I


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DISPLACEMENT PICKUPSHave a relatively large seismic mass and a relatively resilient support .

This gives a low value of w n to thespring-mass system.

Figure 5 shows the frequencyresponse of such a pickup withvarious values of damping ratio .

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Contd.For w/w n >> 1 , the displacement of the seismic mass relative to the casing is

practically equal to the displacement applied to the casing.

Displacements will be nearly 180 o out of phase with each other.The casing moves in one direction, theseismic mass moves in the opposite

direction relative to it it virtually stands still . = 0.707 gives the least variation of displacement ratio for values of (w/w n ) > 1).At this value of we can bring (w/w

n ) down

to about 1.75 before the error indisplacement measurement exceeds 5%.So displacement pickups are often designed to have a damping ratio of about 0.7.

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VELOCITY PICKUPSA signal proportional to velocity maybe obtained from a vibration by:

1) differentiating the signal from adisplacement pickup by passing itthrough a differentiating circuit.

2) integrating the signal from anaccelerometer by passing it throughan integrating circuit.

3) using a seismic velocity pickup.

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Contd.

Designed to have a low

value of w noperate at angularfrequencies well above

w n the motion of theseismic mass is

virtually the same asthat of the casing.

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Contd.When coupling the seismic sensor to the

vibrating structural element, a relativemovement is generated between permanentmagnet and plunger coil. A voltage isinduced in the coil which is proportional tothe vibration velocity.

e = b.l.vWheree = induced voltage.

b = magnetic induction of permanentmagnet.l = length of conductors in the plunger

coil.

v = speed of vibrationKNOWLEDGE TEAM C&I


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mV20

2

02

1 22 10 14 100 1000hZ

U rms

v rms = 1mm/s

1 2

The frequency response of the absolutevibration sensor is shown

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ACCELERATION PICKUPS(ACCELEROMETERS)

At w/w n


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Contd.

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Contd.

For < 1.0 , seismic accelerometerwill give accurate readings of

acceleration for w/w n = 0 to 0.2 .At = 0.7 accurate measurement upto w/w n = 0.5 .

Most accelerometers , however, use a piezoelectric crystal as a combined'spring' and transducer, and forcrystal 0 .

Frequency response of a piezoelectricaccelerometer can be assumed to bethat shown by the curve for = 0.01

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THE PIEZOELECTRIC ACCELEROMETER

Relatively robust and reliable sothat its characteristics remainstable over a long period of time

The piezoelectric accelerometer isself-generating, so that it doesn'tneed a power supply

No moving parts to wear out

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Contd.Material:Piezoelectric Material, i.e.,

polarized ferroelectric ceramic.Type:CompressionShearBending

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COMPRESSION ACCELEROMETER

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SHEAR ACCELEROMETER

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BENDING/TENSIONACCELEROMETER

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ENVIRONMENTAL INFLUENCE

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Contd.

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Contd.

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VIBRATION AND THE HUMAN BODY


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VIBRATION AND THE HUMAN BODY

EFFECTS:BLURRED VISION

LOSS OF BALANCE

LOSS OF

CONCENTRATION

DAMAGE TO INTERNAL

ORGANS

WHITE FINGER

SYNDROMEKNOWLEDGE TEAM C&I

VIBRATION SPECTRUM


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VIBRATION SPECTRUMANALYSIS

A

An Impulse A Wide Pulse A Wider Pulse

A A

AAA

f f

tt

f

t T T

1/T1/T

White noise

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FFT SIGNAL PROCESSING

F r e q u e

n c y A m p l i t u d e

T i m e

A m p l i t u d e

T i m e

A m p l i t u d e

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Contd.Vibration Frequency in Hz/CPS canbe expressed in CPM as:CPM = Hertz x 60 Seconds/Minute If a m/c has z RPM and vibrationfrequency is z CPM then it is called1X component , i.e.,z CPM = 1X m/c RPMSimilarly 2z, 3z CPM are 2X, 3X components respectively and so on.

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Contd.Unbalance => 1X component in horizontal andvertical directionLooseness, misalignment, resonance and reciprocating forces => 2X, 3X and highermultiples.Problems with gears usually result invibration at frequencies related to the "gear mesh" frequency or the product of the numberof teeth on the gear multiplied by the gearRPM.Aerodynamic and hydraulic problems withfans and pumps will normally show vibration

frequencies that are the product of the m/c RPM times the number of fan blades or impeller vanes .This is also called Blade Pass Frequency .

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HEALING BEARING

Fault Increase => Pulse T increase =>Decaying White Noise => No More Frequencyto Excite Accelerometer => Healing. But

Bearing approaching failure.

A

tA

t

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PRACTICAL APPLICATION


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PRACTICAL APPLICATION

0.250.400.450.64

C

D

18.02845

25.040.0

64

AA

AA

0.711.0

IVIIIIIIRMSPEAKMachine ClassesVelocity (mm/s)

BC2.84.0B1.82.5

B1.121.58

D

C

DD11.215.8

7.110.0BC4.56.4

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MACHINE CLASSES


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MACHINE CLASSESCLASS I

Small Sized Machine (Production electricalmotors up to 15 KW)

CLASS II Medium Sized Machine (Electrical motor 15-75 KW) without special foundations. Rigidlymounted engines or machines (up to 300KW) on special foundations

CLASS III Large Prime movers and other large machinewith rotating mass mounted on rigid &heavy foundations which are relatively stiff in the direction of Vibration measurement

CLASS IV Large Prime movers and other large machinewith rotating mass mounted on foundationswhich are relatively soft in the direction of Vibration measurement (Eg: Turbogenerator)

CLASS V Reciprocating machines on Soft & HardfoundationsHeavy Grinding Mills, VibratingScreen, System with unbalanceable Inertiaeffects

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THANKS

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vibration measurement & analysis

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