Vibration

A BIT OF VIBRATION

TECHNICAL ANALYSISDIAGNOSISSOLUTION FOUND

Vibration

Measurement -- Displacement. -- Velocity.-- Acceleration.

Uses -- Displacement , Slow speed machines, f<10hz, (visible) -- Velocity , Medium speed machines, f= 10Hz - 1000Hz, (feel)-- Acceleration , High speed machines, f>10,000Hz, (neither)

Units -- Displacement = Mils, Peak to Peak -- Velocity = ins/sec, rms. -- Acceleration = g

rms = a/root 2 or (2a)/root2

Vibration

Sine(wt)

-1.5

-1

-0.5

0

0.5

1

1.5

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 101

time

ampl

itude

sine(wt))

a 2a

T• Frequency = 1/T Hz

• Amplitude = a

• Peak to Peak = 2a

LOWERLIMIT

PEAKTO

PEAK

PEAKAVG

RMS

UPPERLIMIT

NEUTRALPOSITION

SINUSODIAL MOTION

MULTIPLYNUMBER OF

BYTO OBTAIN

PEAK -TO- PEAK

PEAKRMS

AVERAGE

PEAK-TO-PEAK

PEAK RMS

1.000

0.500

0.354

0.318 0.636

0.707

1.000

2.000

0.900 1.000

1.571

3.142

AVG

1.000

1.414

2.828

1.111

Measurement Units

VibrationSum of sine(wt) and 0.5*sine(2wt)

-1.5

-1

-0.5

0

0.5

1

1.5

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77 81 85 89 93 97 101

time

ampl

itude

COMPLEX PERIODIC VIBRATION(comprising two sine waves)

-1.5

-1

-0.5

0

0.5

1

1.5

0 20 40 60 80 100 120

Time axis

Am

plitu

de

sin(wt)

0.5*sin(2wt)

sin(wt)+0.5*sin(2wt)

Summing the 2 vibrations is simply the arithmetic additionof the 2 individual Sine waves at any point

Vibration

FREQUENCY ANALYSIS OF COMPOSITE SINE WAVE

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6

NON DIMENSIONAL FREQUENCY

APL

ITU

DE

a

SPECTRAL VALUES

Frequency analysis is the cornerstone of Machine Diagnostics

Vibration Common causes

1 x rpm Out of balance.bent shaft.

2 x rpm Misalignment.Mechanical looseness.

Imp Blade number x rpm Hydraulic.

High freq. Damaged rolling element.

Vibration : Causes

Primary:- Unbalance / Hydraulic Forces / Misalignment

Secondary:- Bent Shaft / Broken Blades / Bearing damage / Causes Erosion / Eccentricity / Bad assembly / Looseness Cavitation

UNBALANCE this is important to pumps but not as critical as with turbines, motors, fans etc. The main thing is to get the way in which the rotor is balanced correct, then acceptable vibration follows as a matter of course.Generally for pumps unbalance will only be important at or near full design flow.).

HYDRAULIC FORCES these are usually more important than unbalance, particularly at low flow.

Malfunction Time base X-Y Plot Diagnosis

Misalignment

Oil Whirl

Rub

Unbalancedor bent shaft

Typical or severemisalignment

Appearance similar tounbalanceDistinction: whirl frequencyis slow: < 0.5 shaft speed

Contact produces ‘spike’ superimposed on normalshaft orbit

Oval XY Display

SHAFT DEFFECT DIAGNOSED FROM OSCILLOGRAPH DISPLAYS BY MEANS OF PROXIMITY SENSORS

Multistage boiler feed pump shaft vibration history.Shaft vibration before failure, and after proper modifications were introduced.

0.

1.

2..4

6 x

N.5

4 x

NChanges made :- Increase IMP./DIFF.clearance Change to tilt- Pad Brg. Re-align pump & Driver Proper Dynamic balancing

6 %

Rotor vibration frequency/machine speed (cps/rps0 0.5 1.0 2.0 3.0 Z x N

0.

1.

2.

3.

Sha

ft vi

brat

ion

Am

plitu

des

- mils

Oil

whi

p; R

ubbi

ng;

R

eson

ance

1 x

N =

DY

N. U

NB

AL.

2 x

N =

DY

N. M

ISA

LIG

NE

D

Z x

N =

TIP

PA

SS

ING

RA

DIA

L G

AP

: 1

%

Nature of faultFrequency

Of DominantVibration (HZ =rpm/60

Direction Remarks

Hysteresis Whirl Shaft critical speed PrimarilyRadial

Vibrations excited when passing through critical shaft speed aremaintained at a higher shaft speeds. Can sometimes be cured bychecking tightness of rotor components.

Damaged orworn gears

Tooth meshing fre-quencies (shaft rpm xnumber of teeth) andharmonics

Radial&

Axial

Side-bands around tooth meshing frequencies indicate modulation(e.g. eccentricity) at frequency corresponding to side-bandspacings. Normally only detectable with very narrow-band analysisand cepstrum.

Mechanicalloosness 2 x rpm Also sub-and interharmonics, as for loose journal bearings

Faulty belt drive 1,2,3,4, x rpm of belt Radial The precise problem can usually be identified visually with the helpof a stroboscope.

UnbalancedReciprocating

Forcesand couples

1 x rpm and/ormultiples for higherorder unbalance

PrimarilyRadial

IncreasedTurbulance

Blade & vane passsingfrequencies andharmonics.

Radial&

AxialIncreasing levels indicate increasing turbulence.

ElectricallyInduced

Vibrations

1 x rpm or 1 or 2 timessynchronous frequency

Radial&

AxialShould disappear when turning off the power.

Vibration Trouble-Shooting Chart (B)

Nature of faultFrequency

Of DominantVibration (HZ

=rpm/60Direction Remarks

Rotating membersout of balance 1 x rpm Radial A common cause of excess vibration in machinery

Misalignment &bent shaft

Usually 1 x rpmOften 2 x rpmSometimes 374 xrpm

Radial&

AxialA common fault

Damaged rollingelement Bearings(ball, Roller,etc.)

Impact rates for theindividual bearingcomponent

Also variations atfrequencies (2 to 60kHZ) often relatedto radial resonancein bearings.

Journal BearingsLoose in housing

Sub-harmonics ofshaft rpm, exactly ½or 1/3 x rpm

PrimarilyRadial

Oil Film Whirl orWhip in journal

Bearings

Slightly less thanshaft speed (42% to48%)

PrimarilyRadial Should disappear when turning off the power.

Vibration Trouble-Shooting Chart (A)

N = number of balls or rollersf = relative rev./s between inner & outer races

Contact angle

Ball Dia(BD)

PitchDia(PD)

Impact Rates 1 (Hz)For outer Race Defect f(Hz) = n fr ( 1 - BD Cos) 2 PD For Inner Race Defect f(Hz) = n fr ( 1 + BD Cos) 2 PD For ball Defect f(Hz) = BD fr [ 1 - BD Cos)² ] PD PD

Uneven vibration level’s often with shocks. * Impact-Rates :

4 6 8 1000 2 4 6 8 10,0002 4

68

10

2

4

4

68

100

2

4

600

10 2 3 4 5 6 8100 2 3 4 5 600

0.10.150.20.30.40.50.60.811.5234568101520

Pump running speed (Hz)

Pump running speed (R.P.M.)

Tran

sver

se V

ibra

tion

ampl

itude

at b

earin

g ho

usin

g 26

(pea

k to

pea

k)

Operation not permissible

Operation inadvisable

Improvement advisable

Satisfactory

Excellent

Vibration Acceptance Limits

Mils

Mic

rons Velocity 18 mm/s

11 7 2.8

Limitations shown applicable to the maximum of horizontal and vertical vibrationamplitudes as measured at pump shaft rotationalfrequency.

ExcellentEngineeringSolutions

Weir Services Division

Section 6 Vibration