04ª apostila 13 sources of vib

7/27/2019 04 Apostila 13 Sources of vib

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Sources of Vibration1

Sources of Vibration


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Forces and Vibration

Input Forces x System Response = Vibration

Autospectrum(Response) - Input (Magnitude)

Mobility : Input : Input : FFT Analyzer

0 100 200 300 400 500

1m

10m

100m

1

[Hz]

[m/s]



0 100 200 300 400 500

1m

10m

100m

1

[Hz]

[m/s]



0 100 200 300 400 500

1m

10m

100m

1

[Hz]

[m/s]



0 100 200 300 400 500

1m

10m

100m

1

[Hz]

[m/s]

x =

Forces caused by

Imbalance

Shock

Friction Acoustic

Structural

Parameters:

Mass

Stiffness Damping

Vibration

Parameters:

Acceleration

Velocity Displacement

Frequency Response H1 (response, excitation)ili l

0 100 200 300 400 500

-20

0

20

[Hz]

ili l

0 100 200 300 400 500

-20

0

20

[Hz]


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Fault Diagnosis

Frequency analysis: The tool for diagnosis of faults


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Faults in Rotating Shafts

Frequency:

Mainly radial

Frequency:

Radial and axial

Vibration:

Misalignment and bent shaft

Unbalance:

Vibration:

Cracks in shafts

The components at the rotation speed andthe second harmonic will increase.(B. Grabovski: VDI berichte n. 320 1978)

Rotation speed

Rotation speedOften 2nd harmonicsometimes 3rd and 4th harmonic


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Misalignment


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1

0.1

0.01

100 200 300 4000 500

10

1

0.1

0.01

100 200 300 4000 500

10

Misalignment in Gearbox85 Hz

Measurement using accelerometer

50 Hz

100 Hz

170 Hz

85 Hz

50 Hz

100 Hz, Mains 2nd order

170 Hz

mm/s

mm/s

Hz

Hz

Measurement using velocity transducer

Before repair

After repair

50 Hz

3000 rpm85 Hz

5100 rpm

Gear


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Rotating Shaft


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Fault in Bearings


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Oil Whirl and Oil Whip

Low Eccentricity Natural Frequency

Bearing (oil) stiffnessHigh Eccentricity Natural Frequency

Shaft stiffness

(Hz)(Hz)(Hz)(Hz)


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Oil Whirl and Oil Whip


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Loose Journal Bearing


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Journal Bearing Looseness (after repair)


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Vibration from Gears


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Faults in the Vibration Spectrum

Toothmesh Frq.

T

F = 1/T


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Tooth Deflection

Varying numbers no. of teeth

carry the load!New Gears also.


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Tooth Wear


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Tooth Meshing Frequency and Harmonics


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Ghost Component


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Ghost independent of Load


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Cement Mill Drive


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Ghost before and after Repair


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Gearbox and Zoom


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Electrical Machines


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Electric Motor Vibrations

10090 110

0 600 1200


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Faults in Rolling Element Bearing


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Frequency of DominantVibration (Hz = RPM/60)

Nature of Fault Direction Remarks

1 RPM

Usually 1 RPM

Often 2 RPMSometimes 3 and 4 RPM

Rotating Membersout of Balance

Radial A common cause of excessvibration in machinery

Misalignment and

Bent Shaft

A common faultRadial

andAxial

Vibration Trouble-shooting Chart (1/4)

Sub-harmonics ofshaft RPM, exactly1/2 or 1/3 RPM

PrimarilyRadial

Journal BearingsLoose in Housing

Looseness may only develop atoperating speed and temperature(eg. turbomachines).

Slightly less than halfshaft speed(42% to 48%)

PrimarilyRadial

Oil Film Whirl orWhip in JournalBearings

Applicable to high-speed(eg. turbo) machines.

Shaft critical speed PrimarilyRadial

Hysteresis Whirl Vibrations excited when passingthrough critical shaft speed are

maintained at higher shaft speeds.Can sometimes be cured bychecking tightness of rotorcomponents.


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Impact rates for theindividual bearing

component*Also vibrations at highfrequencies (2 to 60 kHz)often related to radialresonances in bearings

Radial

andAxial

Damaged RollingElement Bearings(Ball, Roller, etc.)

Uneven vibration levels, often

with shocks.


Contact Angle

Ball Diameter (BD)

n = number of balls of rollers

fr = relative rev./sec betweeninner and outer races

Pitch Diameter (PD)

* Impact rates

For an outer race defect: [ ]

= cosPD

BD

1f2

n

Hzf r

For an inner race defect: [ ]

+= cos

PD

BD1f

2

nHzf r

For a ball defect: [ ]

=

2

r cosPD

BD1fBD

PDHzf

[ ]

= cos

PD

BD1f

2

1Hzf rFor a cage defect:

(assuming pure rolling motion)


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Tooth meshingfrequencies (shaftRPM number of teeth)and harmonics

RadialandAxial

Damaged or worngears

Sidebands around toothmeshing frequencies indicatemodulation (eg. eccentricity) atfrequency corresponding to

sideband spacings. Normallyonly detectable with verynarrow-band analysis

2 RPMMechanicalLooseness

Also sub and interharmonics,as for loose Journal bearings

1, 2, 3 and 4 RPM of belt RadialFaulty Belt Drive

1 RPM and/or multiples forhigher order unbalance

PrimarilyRadial

UnbalancedReciprocating

Forcesand Couples



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Blade and vanepassing frequenciesand harmonics

RadialandAxial

IncreasedTurbulence

Increasing levels indicateincreasing turbulence

1 RPM or 1 or 2times synchronousfrequency

RadialandAxial

ElectricallyInduced Vibrations

Should disappear whenturning off the power

Vibration Trouble Shooting Chart (4/4)

04ª apostila 13 sources of vib

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