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Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters

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Page 1: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Beginning Vibration Analysis

withBasic Fundamentals

By: Jack Peters

Page 2: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Beginning Vibration

2

Introduction

Understanding the basics and fundamentals of vibration analysis are very important in forming a solid background to analyze problems on rotating machinery.

Switching between time and frequency is a common tool used for analysis. Because the frequency spectrum is derived from the data in the time domain, the relationship between time and frequency is very important. Units of acceleration, velocity, and displacement are typical. Additional terms such as peak-peak, peak, and rms. are often used. Switching units correctly, and keeping terms straight is a must.

As much as possible, this training will follow the criteria as established by the Vibration Institute.

Page 3: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Mass & Stiffness

3

Mass & Stiffness

All machines can be broken down into two specific categories.

Mass & Stiffness

Mass is represented by an object that wants to move or rotate.

Stiffness is represented by springs or constraints of that movement.

Page 4: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Mass & Stiffness

4

Mass & Stiffness

fn = 1/2П k/mWhere:

fn = natural frequency (Hz)

k = stiffness (lb/in)

m = mass

mass = weight/gravity

weight (lb)

gravity (386.1 in/sec2)

Page 5: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Mass & Stiffness

5

Concept !

fn = 1/2П k/m

If k increases

Then f increases

If k decreases

Then f decreases

Page 6: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Mass & Stiffness

6

Concept !

fn = 1/2П k/m

If m increases

Then f decreases

If m decreases

Then f increases

Page 7: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Spectrum

7

What’s This ?

0.0002inch

Peak

0

Magnitude

Hz100 0 Hz

1

Page 8: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Spectrum

8

FFT, Frequency Spectrum, Power Spectrum

0.0002inch

Peak

0

Magnitude

Hz100 0 Hz

1

Page 9: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Spectrum

9

Scaling X & Y

0.0002inch

Peak

0

Magnitude

Hz100 0 Hz

1

X

Y

Page 10: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Spectrum

10

Scaling X & Y

0.0002inch

Peak

0

Magnitude

Hz100 0 Hz

1

FREQUENCY

A

M

P

L

I

T

U

D

E

Page 11: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Spectrum

11

Scaling X & Y

0.0002inch

Peak

0

Magnitude

Hz100 0 Hz

1

What is it

How

Bad

I s

I t

Page 12: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Time Waveform

12

What’s That ?

0.0004inch

-0.0004

Real

s7.996094 0 s

1

Page 13: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Time Waveform

13

Time Waveform

0.0004inch

-0.0004

Real

s7.996094 0 s

1

Page 14: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Time Waveform

14

Scaling X & Y

X

Y

0.0004inch

-0.0004

Real

s7.996094 0 s

1

Page 15: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Time Waveform

15

Scaling X & Y

TIME

A

M

P

L

I

T

U

D

E

0.0004inch

-0.0004

Real

s7.996094 0 s

1

Page 16: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Time Waveform

16

Scaling X & Y

What is it

How

Bad

I s

I t

0.0004inch

-0.0004

Real

s7.996094 0 s

1

Page 17: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Time Waveform

17

Scaling X & Y

What is it

How

Bad

I s

I t

0.0004inch

-0.0004

Real

s1 0 s

1

Page 18: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Time & Frequency

18

Double Trouble

0.0004inch

-0.0004

Real

s7.996094 0 s

1

0.0002inch

Peak

0

Magnitude

Hz100 0 Hz

1

Page 19: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

19

The X Scale

Page 20: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

20

Hertz (Hz)

One Hertz (Hz) is equal to 1 cycle / second

It is the most common term used in vibration analysis to describe the frequency of a disturbance.

Never forget the 1 cycle / second relationship !

Traditional vibration analysis quite often expresses frequency in terms of cycle / minute (cpm). This is because many pieces of process equipment have running speeds related to revolutions / minute (rpm).

60 cpm = 1 cps = 1 Hz

Page 21: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

21

Relationship with Time

The frequency domain is an expression of amplitude and individual frequencies.

A single frequency can be related to time.

F(Hz) = 1 / T(s)

The inverse of this is also true for a single frequency.

T(s) = 1 / F(Hz)

Keep in mind that the time domain is an expression of amplitude and multiple frequencies.

Page 22: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

22

Concept !

If: F = 1/T and T = 1/F

Then: FT = 1

Page 23: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

23

Concept !

FT = 1

If: F increases

Then: T decreases

If: T increases

Then: F decreases

Page 24: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

24

Single Frequency

1V

rms

0

Magnitude

Hz100 0 Hz

Pwr Spec 1

1V

-1

Real

ms62.469480 s

Time 1

X:55 Hz Y:706.8129 mV

dX:18.18848 ms dY:2.449082 mVX:27.00806 ms Y:3.579427 mV

Page 25: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

25

Multiple Frequencies

1 Hz100 0 Hz

Pwr Spec 1

1 Hz100 0 Hz

Pwr Spec 1

1 Hz100 0 Hz

Pwr Spec 1

1 Hz100 0 Hz

Pwr Spec 1

X:55 Hz Y:706.8129 mV

X:78 Hz Y:706.9236 mV

X:21 Hz Y:706.7825 mV

X:42 Hz Y:706.9266 mV

Page 26: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

26

Multiple Time

1V

ms62.469480 s

Time 78 1

1V

ms62.469480 s

Time 21 1

1V

ms62.469480 s

Time 42 1

1V

ms62.469480 s

Time 55 1

Page 27: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

27

Real Life Time

4V

-4

Real

ms62.469480 s

TIME 1

55 + 78 + 21 + 42 = Trouble !

Page 28: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

28

Frequency Spectrum

4V

-4

Real

ms62.469480 s

TIME 1

1V

rms Hz100 0 Hz

FREQUENCY 1X:78 Hz Y:706.9236 mVX:55 Hz Y:706.8129 mVX:42 Hz Y:706.9266 mVX:21 Hz Y:706.7825 mV

Page 29: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

29

The Most Copied Slide in the History of Vibration Analysis !

Am

plitu

de

Input Time Frequency

Time Waveform Spectrum

Page 30: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

30

Lines of Resolution

0.0002inch

Peak

0

Magnitude

Hz100 0 Hz

1The FFT always has a defined number of lines of resolution.

100, 200, 400, 800, 1600, and 3200 lines are common choices.

This spectrum has 800 lines, or the X scale is broken down into 800 points.

Page 31: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

31

Filter Windows

• Window filters are applied to the time waveform data to simulate data that starts and stops at zero.

• They will cause errors in the time waveform and frequency spectrum.

• We still like window filters !

Page 32: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

32

Window Comparisons

Page 33: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

33

Filter Windows

• Hanning (Frequency)• Flat Top (Amplitude)• Uniform (No Window)• Force Exponential

• Force/Expo Set-up(Frequency Response)

Hanning 16% Amplitude Error

Flat Top 1% Amplitude Error

Window functions courtesy of Agilent“The Fundamentals of Signal Analysis”Application Note #AN 243

Page 34: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

34

Filter Windows

• Use the Hanning Window for normal vibration monitoring (Frequency)

• Use the Flat Top Window for calibration and accuracy (Amplitude)

• Use the Uniform Window for bump testing and resonance checks (No Window)

Page 35: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

35

Minimum Derived Hz

The minimum derived frequency is determined by:Frequency Span / Number of Analyzer Lines (data points)

The frequency span is calculated as the ending frequency minus the starting frequency.

The number of analyzer lines depends on the analyzer and how the operator has set it up.

Example: 0 - 400 Hz using 800 lines

Answer = (400 - 0) / 800 = 0.5 Hz / Line

Page 36: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

36

Bandwidth

The Bandwidth can be defined by:

(Frequency Span / Analyzer Lines) Window Function

Uniform Window Function = 1.0

Hanning Window Function = 1.5

Flat Top Window Function = 3.5

Example: 0 - 400 Hz using 800 Lines & Hanning Window

Answer = (400 / 800) 1.5 = 0.75 Hz / Line

Note: More discussion later on window functions for the analyzer !

Page 37: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

37

Resolution

The frequency resolution is defined in the following manner:

2 (Frequency Span / Analyzer Lines) Window Function

or

Resolution = 2 (Bandwidth)

Example: 0 - 400 Hz using 800 Lines & Hanning Window

Answer = 2 (400 / 800) 1.5 = 1.5 Hz / Line

Page 38: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

38

Using Resolution

The student wishes to measure two frequency disturbances that are very close together.

Frequency #1 = 29.5 Hz.

Frequency #2 = 30 Hz.

The instructor suggests a hanning window and 800 lines.

What frequency span is required to accurately measure these two frequency disturbances ?

Page 39: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

39

Using Resolution

Resolution = 30 - 29.5 = 0.5 Hz / Line

Resolution = 2 (Bandwidth)

BW = (Frequency Span / Analyzer Lines) Window Function

Resolution = 2 (Frequency Span / 800) 1.5

0.5 = 2 (Frequency Span / 800) 1.5

0.5 = 3 (Frequency Span) / 800

400 = 3 (Frequency Span)

133 Hz = Frequency Span

Page 40: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

40

Data Sampling Time

Data sampling time is the amount of time required to take one record or sample of data. It is dependent on the frequency span and the number of analyzer lines being used.

TSample = Nlines / Fspan

Using 400 lines with a 800 Hz frequency span will require:

400 / 800 = 0.5 seconds

Page 41: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

41

Average & Overlap

• Average - On• Overlap Percent - 50%

TR#1 TR#2 TR#3FFT#1 FFT#2 FFT#3

TR#1

TR#2

TR#3FFT#1

FFT#2

FFT#3

0% Overlap

50% Overlap

How long will it take for 10 averages at 75% overlap using a 800 line analyzer and a 200 Hz frequency span?

Page 42: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The X Scale

42

75% Overlap ?

• 10 Averages• 75% Overlap• 800 Lines• 200 Hz

Average #1 = 800 / 200

Average #1 = 4 seconds

Average #2 - #10 = (4 x 0.25)

Average #2 - #10 = 1 second each

Total time = 4 + (1 x 9)

Total time = 13 seconds

Page 43: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

43

The Y Scale

Page 44: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

44

Amplitude

The “Y” scale provides the amplitude value for each signal or frequency.

Default units for the “Y” scale are volts RMS.

Volts is an Engineering Unit (EU).

RMS is one of three suffixes meant to confuse you !

The other two are:

(Peak) and (Peak - Peak)

Page 45: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

45

Pk-Pk (Peak - Peak)

1V

-1

Real

ms62.469480 s

Time 1

2V

Pk-Pk

0

Magnitude

Hz100 0 Hz

Pwr Spec 1

dX:9.094238 ms dY:1.994871 VX:22.43042 ms Y:-993.8563 mV

X:55 Hz Y:1.999169 V

The Peak - Peak value is expressed from the peak to peak amplitude.

The spectrum value uses the suffix “Pk-Pk”to denote this.

Page 46: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

46

Pk (Peak)

1V

-1

Real

ms62.469480 s

Time 1

1V

Peak

0

Magnitude

Hz100 0 Hz

Pwr Spec 1

dX:4.516602 ms dY:997.4356 mVX:27.00806 ms Y:3.579427 mV

X:55 Hz Y:999.5843 mVThe time wave has not changed. The Peak value is expressed from zero to the peak amplitude.

The spectrum value uses the suffix “Peak”to denote this.

Page 47: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

47

RMS (Root Mean Square)

1V

rms

0

Magnitude

Hz100 0 Hz

Pwr Spec 1

1V

-1

Real

ms62.469480 s

Time 1

X:55 Hz Y:706.8129 mV

dX:2.288818 ms dY:709.1976 mVX:27.00806 ms Y:3.579427 mV

The time wave has not changed. The RMS value is expressed from zero to 70.7% of the peak amplitude.

The spectrum value uses the suffix “RMS” to denote this.

Page 48: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

48

Suffix Comparison

2V

rms

0

Magnitude

Hz100 0 Hz

Pwr Spec 1

2V

Peak

0

Magnitude

Hz100 0 Hz

Pwr Spec 1

2V

Pk-Pk

0

Magnitude

Hz100 0 Hz

Pwr Spec 1

1V

-1

Real

ms62.469480 s

Time 1

1V

-1

Real

ms62.469480 s

Time 1

1V

-1

Real

ms62.469480 s

Time 1

X:55 Hz Y:706.8129 mV

X:55 Hz Y:999.5843 mV

X:55 Hz Y:1.999169 V

dX:2.288818 ms dY:709.1976 mX:27.00806 ms Y:3.579427 mV

dX:4.516602 ms dY:997.4356 mX:27.00806 ms Y:3.579427 mV

dX:9.094238 ms dY:1.994871 VX:22.43042 ms Y:-993.8563 mV

RMS

Peak

Peak - Peak

Page 49: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

49

Changing Suffixes

Many times it is necessary to change between suffixes.

Pk-Pk / 2 = Peak

Peak x 0.707 = RMS

RMS x 1.414 = Peak

Peak x 2 = Pk-Pk

Page 50: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

50

Standard Suffixes

Now that we have learned all about the three standard suffixes that might possibly confuse the “Y” scale values, what is the standard ?

Vibration Institute:

Displacement = mils Peak - Peak

Velocity = in/s Peak or rms..

Acceleration = g’s Peak or rms..

Note: 1 mil = 0.001 inches

Page 51: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

51

Engineering Units (EU)

Engineering units are used to give meaning to the amplitude of the measurement.

Instead of the default “volts”, it is possible to incorporate a unit proportional to volts that will have greater meaning to the user.

Examples: 100 mV / g 20 mV / Pa

1 V / in/s 200 mV / mil

50 mV / psi 10 mV / fpm

33 mV / % 10 mV / V

Page 52: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

52

EU’s the Hard Way

Sometimes we forget to use EU’s, or just don’t understand how to set up the analyzer.

There is no immediate need to panic if ????

You know what the EU is for the sensor you are using.

Example: An accelerometer outputs 100 mV / g and there is a 10 mV peak in the frequency spectrum.

What is the amplitude in g’s ?

Answer = 10 mV / 100 mV = 0.1 g

Page 53: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

53

The Big Three EU’s

Acceleration

Velocity

Displacement

Page 54: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

54

Converting the Big 3

In many cases we are confronted with Acceleration, Velocity, or Displacement, but are not happy with it.

Maybe we have taken the measurement in acceleration, but the model calls for displacement.

Maybe we have taken the data in displacement, but the manufacturer quoted the equipment specifications in velocity.

How do we change between these EU’s ?

Page 55: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

55

386.1 What ?

1g = 32.2 feet/second2

32.2 feet 12 inchessecond2 X foot

386.1 inches/second2

g

Page 56: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

56

Go With The Flow I

Acceleration (g’s)

Velocity (inch/s)

Displacement (inch)

x 386.1

÷ 2(Pi)f

÷ 2(Pi)fx 2(Pi)f

x 2(Pi)f

÷ 386.1

Page 57: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

57

Metric Go With The Flow I

Acceleration (g’s)

Velocity (mm/s)

Displacement (mm)

x 9807

÷ 2(Pi)f

÷ 2(Pi)fx 2(Pi)f

x 2(Pi)f

÷ 9807

Page 58: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

58

Go With The Flow II

Peak - Peak

Peak

RMS

÷ 2

x .707x 1.414

x 2

Page 59: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

59

Doing the Math Units

0.5g x 386.1 inchessecond2

g

2π x 25 cyclessecond

0.5g x 386.1 inches x 1 secondg second2 2π x 25 cycles

0.5 x 386.1 inches2π x 25 cycles second

cycle

1.23 inches/second

There is a 0.5 g vibration at 25 Hz.

What is the velocity ?

Page 60: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

60

Acceleration - Velocity

Example: Find the equivalent peak velocity for a 25 Hz vibration at 7 mg RMS ?

= (g x 386.1) / (2 Pi x F)

= (0.007 x 386.1) / (6.28 x 25)

= 0.017 inches / second RMS

Answer = 0.017 x 1.414 = 0.024 inches / second Pk

Page 61: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

61

Velocity - Displacement

Example: Find the equivalent pk-pk displacement for a 25 Hz vibration at 0.024 in/s Pk ?

= Velocity / (2 Pi x F)

= 0.024 / (6.28 x 25)

= 0.000153 inches Pk

Answer = 0.000153 x 2 = 0.000306 inches Pk-Pk

Page 62: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

The Y Scale

62

Acceleration - Displacement

Example: Find the equivalent Pk-Pk displacement for a 52 Hz vibration at 15 mg RMS ?

= (g x 386.1) / (2 Pi x F)2

= (0.015 x 386.1) / (6.28 x 52)2

= 0.000054 inches RMS

Answer = (0.000054 x 1.414) 2 = 0.000154 inches Pk-Pk

Page 63: Beginning Vibration Analysis with Basic Fundamentals Vibration... · Beginning Vibration Analysis with Basic Fundamentals By: Jack Peters. Jack D. Peters Beginning Vibration 2 Introduction

Jack D. Peters

Sensors

63

Sensors

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Accelerometers

• Integrated CircuitElectronics insideIndustrial

• Charge ModeCharge AmplifierTest & Measurement

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Accelerometer Advantages

• Measures casing vibration• Measures absolute motion• Can integrate to Velocity output• Easy to mount• Large range of frequency response• Available in many configurations

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Accelerometer Disadvantages

• Does not measure shaft vibration• Sensitive to mounting techniques and

surface conditions• Difficult to perform calibration check• Double integration to displacement often

causes low frequency noise• One accelerometer does not fit all

applications

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Mass & Charge

Mass

Ceramic/Quartz

Post

Relative movement between post & mass creates shear in ceramic producing charge.

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Accelerometer Parameters

Performance Suited for ApplicationSensitivity (mV/g)Frequency Response of target (f span)Dynamic Range of target (g level)

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Mounting the Accelerometer

500 2000 6,500 10,000 15,000 15,000Hz.

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Realistic Mounting

Stud

Bees Wax

Adhesive

Magnet

Hand Held

100 1,000 10,000Frequency, Hz

In the real world, mounting might not be as good as the manufacturer had in the lab !

What happened to the high frequency ?

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Mounting Location

Axial

Horizontal

Vertical

Load Zone

Radial

Vertical

Horizontal

Axial

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Mounting Location

Load Zone

Radial

Vertical

Horizontal

Axial

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Accelerometer Alarms

Machine Conditionrms peak

Acceptance of new or repaired equipment < 0.08 < 0.16Unrestricted operation (normal) < 0.12 < 0.24Surveillance 0.12 - 0.28 0.24 - 0.7Unsuitable for Operation > 0.28 > 0.7

Velocity Limit

Note #1: The rms velocity (in/sec) is the band power or band energy calculated in the frequency spectrum.

Note #2: The peak velocity (in/sec) is the largest positive or negative peak measured in the time waveform.

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Proximity Probes

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Proximity Probe Theory

Driver

Target

Cable

Probe

The tip of the probe broadcasts a radio frequency signal into the surrounding area as a magnetic field.

If a conductive target intercepts the magnetic field, eddy currents are generated on the surface of the target, and power is drained from the radio frequency signal.

As the power varies with target movement in the radio frequency field, the output voltage of the driver also varies.

A small dc voltage indicates that the target is close to the probe tip.

A large dc voltage indicates that the target is far away from the probe tip.

The variation of dc voltage is the dynamic signal indicating the vibration or displacement.

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Output Values

• Typical– 100 mv/mil– 200 mv/mil

• Depends on probe, cable (length), and driver.

• Target material varies output.

Calibration Examples

• Copper 380 mV/mil• Aluminum 370 mV/mil• Brass 330 mV/mil• Tungsten Carbide 290 mV/mil• Stainless Steel 250 mV/mil• Steel 4140, 4340 200 mV/mil

Based on typical output sensitivity of 200 mV/mil.

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Proximity Probes - Advantages

• Non-contact• Measure shaft dynamic motion• Measure shaft static position (gap)• Flat frequency response dc – 1KHz• Simple calibration• Suitable for harsh environments

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Proximity Probes - Disadvantages

• Probe can move (vibrate)• Doesn’t work on all metals• Plated shafts may give false measurement• Measures nicks & tool marks in shaft• Must be replaced as a unit (probe, cable &

driver)• Must have relief at sensing tip from

surrounding metal

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Typical Mounting

vertical probe horizontal probeH

Vtime

Facing Driver to Driven(independent of rotation)

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Looking at Orbits

Vertical for Amplitude

Horizontal for Time Base

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The Orbit Display

i ls

Mil s

Mils

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Unbalance

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Unbalance with Orbit

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Misalignment

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Misalignment with Orbit

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And the problem is ?

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Proximity Probe Alarms

Machine Condition< 3,600 RPM < 10,000 RPM

Normal 0.3 0.2Surveillance 0.3 - 0.5 0.2 - 0.4Planned Shutdown 0.5 0.4Unsuitable for Operation 0.7 0.6

Allowable R/C

Note #1: R is the relative displacement of the shaft measured by either probe in mils peak-peak.

Note #2: C is the diametrical clearance (difference between shaft OD and journal ID) measured in mils.

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Analyzer Input - Front End

• Coupling - AC, DC

• Antialias Filter - On, Off

AC coupling will block the DC voltage. It creates an amplitude error below 1 Hz. DC coupling has no error below 1 Hz, but the analyzer must range on the total signal amplitude.

Prevents frequencies that are greater than span from wrapping around in the spectrum.

Analyzer Input Capacitor Blocks DC

AC Coupling Capacitor in DSA

If the antialias filter is turned off, at what frequency will 175 Hz. appear using a 0 -100 Hz span, and 800 lines ?

1024/800 = 1.28

100 x 1.28 = 128 Hz

175 Hz - 128 Hz = 47 Hz.

128 Hz - 47 Hz = 81 Hz

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Low End Frequency Response

0.01V

rms

0.002

Magnitude

Hz25 0 Hz

Input 1

0.01V

rms

0

Magnitude

Hz25 0 Hz

Output 2

2

0Magnitude

Hz25 0 Hz

Freq Resp 2:1

1.1

0.9

Magnitude

Hz25 0 Hz

Coherence 2:1

X:2.8125 Hz Y:984.7542 mX:500 mHz Y:720.0918 m

X:5.34375 Hz Y:1.000001

To the right is a typical problem with frequency response at the low end of the frequency spectrum.

This low end roll off was a result of AC coupling on CH #2 of the analyzer.

Values below 2.8 Hz are in error, and values less than 0.5 Hz should not be used.

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Data Collection

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Tape Recorders “Insurance Policy”

Multi-channel digital audio tape recorders.

For the Measurement that can’t get away !

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Dynamic Signal Analyzers “Test & Measurement”

Large PC driven solutions with multiple channels and windows based software.

Smaller portable units with 2 – 4 channel inputs and firmware operating systems.

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Data Collectors “Rotating Equipment”

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Data Collectors “Rotating Equipment”

• Route Based• Frequency

Spectrum• Time Waveform• Orbits• Balancing• Alignment

• Data Analysis• History• Trending• Download to PC• Alarms• “Smart” algorithms

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Bibliography

• Eisenmann, Robert Sr. & Eisenmann, Robert Jr., Machinery Malfunction Diagnosis and Correction, ISBN 0-13-240946-1

• Eshleman, Ronald L., Basic Machinery Vibrations, ISBN 0-9669500-0-3

• LaRocque, Thomas, Vibration Analysis Design, Selection, Mounting, and Installation, Application Note, Connection Technology Center

• Agilent Technologies, The Fundamentals of Signal Analysis, Application note 243

• Agilent Technologies, Effective Machinery Measurements using Dynamic Signal Analyzers, Application note 243-1

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Thank You !

You can find technical papers onthis and other subjects atwww.ctconline.com

in the “Technical Resources” section