Basic Vibration Analysis.ppt

7/29/2019 Basic Vibration Analysis.ppt

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Basic Vibration Analysis

-I


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Basic Theory Of Vibration

Simple Spring Mass System

Upper

Limit

Neutral

Position

LowerPosition

Displacement

Max Acc

Mim Vel

Max Acc

Mim Vel

Max Vel

Mim Acc

It follows sine curve.


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1 revolution

Time

Amplitude

0

+

-

Time Waveform

3600 RPM = 3600 cycles per minute

60 Hz = 60 cycles per second

1 Order = One times turning speed

Heavy Spot

Rotation


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Frequency & Amplitude

Frequency:How many times oscillation is occurring

for a given time period?Units: Cycles Per Sec or Hertz, CPM

Amplitude:It is the magnitude of vibration signal.Units: Micron, MM/Sec, gs or M/Sec2


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Physical Significance Of

Vibration Characteristics

Frequency - What is vibrating?Source of the vibration.

Amplitude - How much is it vibrating?Size (severity) of the problem.

Phase Angle - How is it vibrating?Cause of the vibration.


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60 RPM

= 1 Rev / s

= 1 Hz

Frequency Measurement


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Amplitude Measurement

1. Displacement :The distance a structure moves or vibratesfrom its reference or rest position.

Unit : Microns(p-p), mils(p-p)

2. Velocity :

Rate of change of displacement. It is themeasure of the speed at which the mass isvibrating during its oscillation.Unit : MM/Sec, Inch/sec (RMS or Peak)

3. Acceleration :

It is the rate of change of velocity. Thegreater the rate of change of velocity thegreater the forces (F=ma) on the machines.Unit : M/Sec2, Inch/sec2, gs (RMS)


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B

C

A

a

b

c

t

t

t


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t

+

a

d

v


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Physical Significance Of Vibration

Amplitude

Displacement : Stress Indicator

Velocity : Fatigue Indicator

Acceleration : Force Indicator


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VIBRATION

SENSITIVITY DISPLACEMENT

VELOCITY

ACCELERATION

FREQUENCY

CPM

60 600 6000 60000 600 000

10

1

.1

.01

.001

When To Use Disp., Vel. & Acc.?


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What Is The Advantage Of Using

Velocity?

Flat frequency range compared to

displacement & acceleration.

Almost all machines generate faultfrequency between 600CPM to 60KCPM

Velocity indicates fatigue.

Velocity is the best indicator of

vibration severity.


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Vibration Transducers

Produces electrical signal of vibratory motion

Proximity Probe - Displacement

Velocity Probe - Velocity

Accelerometer - Acceleration


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Proximity Probe


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Permanently installed on large machines with sleeve

bearings.

Measures relative displacement between the bearing

housing(probe tip) and the rotating shaft.

Called Eddy Current Probe

Frequency range 0 to 60,000 CPM


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Velocity Probe


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Oldest of all.

Produces signal proportional to velocity.

Self generating and needs no conditioning electronics.

It is heavy, complex and expensive.

Frequency response from 600CPM to 60,000CPM

Temperature sensitive


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Accelerometer


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Produces signal proportional to accelerationof seismic mass.

Extremely linear amplitude sense.

Large Frequency range

Smaller in size


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1 revolution

Time

A

mplitude

0

+

-

Time Waveform

3600 RPM = 3600 cycles per minute

60 Hz = 60 cycles per second

1 Order = One times turning speed

Heavy Spot

Rotation


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1000

RPM

1 revolution

Time

Amplitude

0

+

-

4 blades = Vibration occurs 4 times per revolution

4 X 1000 RPM = Vibration occurs at 4000 cycles per minute

= 4000 CPM

Time Waveform


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Time

0

+

-

Time

Time

0

+

-

0

+

-

Time Waveform


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Time Waveform

0

-

+

Time

Time Waveform contains all the differentfrequencies mixed together.


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Time Waveform

Example of a time waveform


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Signal AcquisitionTransducer

OverallEnergy

FFT

Waveform

Spectrum

Amplitud

e

Amplitude

Time

Frequency


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Frequency

1x

4x

12x

Time0

+

-

Time

Time

0

+

-

0

+

-

Frequency

Frequency

Frequency Domain


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Phase


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The Phase Angle is the angle (in degrees) the

shaft travels from the start of data collection to

when the sensor experiences maximum positive

force.

For example, the phase angle is 90 if the sensor

experiences its maximum positive force at 90

after data collection was initiated by the

tachometer.

How Phase Angle Is Measured?


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Diagnosing Machine Faults

Unbalance

Misalignment

General looseness or wear

Bearing defects

Gear defects

Belt defects

Electrical Faults Journal Bearing Faults

Resonance


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Causes of unbalance

Improper assembly

Material Buildup

Wear

Broken or missing parts


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Freq: 60.04 Hz

Order: 1.010

Spec: .390

Unbalance

Radial vibration at 1X

shaft RPM is much

more significant thanin the axial direction

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Basic Vibration Analysis.ppt