CAT III Activity Book 170112 - A4

VIBRATION TRAINING ACTIVITY BOOK

CATEGORY III

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This manual is designed as a guide only. In practical situations there are many variables, so please use this information with care.

Copyright  1999 - 2012 Mobius Institute All Rights Reserved

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Table of Contents SIGNAL PROCESSING ................................................................................................... 4 WINDOWING ............................................................................................................... 6 AVERAGING ................................................................................................................ 7 TIME WAVEFORM ANALYSIS .......................................................................................... 7 PHASE ANALYSIS ......................................................................................................... 9 DIAGNOSING FAULTS WITH PHASE ANALYSIS ............................................................... 10 DYNAMICS and RESONANCES ...................................................................................... 14 OPERATING DEFLECTION SHAPES ................................................................................ 18 MODAL ANALYSIS ...................................................................................................... 20 CORRECTING RESONANCES AND REDUCING VIBRATION ................................................. 22 JOURNAL BEARING ANALYSIS AND ORBITS ................................................................... 25 ROLLING ELEMENT BEARING ANALYSIS ........................................................................ 28 GEARBOX ANALYSIS ................................................................................................... 29 ELECTRIC MOTORS .................................................................................................... 30 BALANCING ............................................................................................................... 31 DIAGNOSING MACHINE FAULTS ................................................................................... 34

Vibration Training Activity Booklet – CAT III

Page 4 of 35

SIGNAL PROCESSING Useful information:

. T = Time required to collect the waveform Ts = Time between each sample Fs = Sampling rate = Samples per second N = Number of samples (1024, 2048, 4096, etc.)

Window factor = 1 (no window/uniform/rectangular) or 1.5 (Hanning window) Separating frequency ≥ 2 x Bandwidth ≥ 2 x Resolution * Window Factor Required spectral lines ≥ 2 x Window factor x Fmax / Separating frequency Accuracy of frequency (at peak) = ± ½ x Resolution

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Copyright ©2005 - 2012 Mobius Institute Document ID: ABIII 120108

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Page 5 of 35 [1]


Why can the integration process create a signal‐to‐noise problem?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [2]

What is the difference between analog integration and digital integration?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [3]

What precautions should be taken when testing low speed machines (below 3 Hz or 180 RPM)?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [4]

Name three applications where one might use triggering (external signal to trigger a measurement or external tachometer signal for synchronization).

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [5]

If the record length is 2 seconds, and there are 1024 samples in the time waveform, what is the Fmax? A. B. C. D.

[6]

200 Hz 400 Hz 800 Hz 1600 Hz

If the Fmax is 10,000 Hz and a 1600 line spectrum, how long does it take to sample the vibration signal with one average only? A. B. C. D.

0.16 seconds 1.6 seconds 16 seconds 6.25 seconds

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Vibration Training Activity Booklet – CAT III [7]

If the Fmax is 160 Hz, 3200 line spectrum, how long does it take to sample the vibration signal with 10 averages (no overlap)? A. B. C. D.

[8]

0.05 seconds 0.5 seonds 20 seconds 200 seconds

If the Fmax is 160 Hz, 3200 line spectrum, how long does it take to sample the vibration signal with 10 averages and 50% overlap? A. B. C. D.

[9]

Page 6 of 35

100 seconds 115 seconds 200 seconds 20 seconds

A machine will generate key vibration signals of 100 Hz and 100.50 Hz, suggest an Fmax setting and corresponding lines of resolution setting to resolve these two frequencies (assume that a Hanning window will be used):

_________________________________________________________________________________________ _________________________________________________________________________________________ [10]

What is the difference between dynamic range and signal to noise ratio?

_________________________________________________________________________________________ _________________________________________________________________________________________

WINDOWING [1]

Name 5 common windows and their applications

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [2]

Leakage is caused by: A. B. C. D.

Noise in the time waveform A low sample rate Poorly shielded cables The signal does not begin and end at zero in the time record

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Page 7 of 35


AVERAGING [1] Explain the benefits of overlap averaging? _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [2]

What factors should you consider when selecting the number of averages (for normal testing and linear averaging)?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [3]

What are three applications for peak hold averaging?

_________________________________________________________________________________________ _________________________________________________________________________________________ [4]

What are 3 ways to deal with testing variable speed machines?

_________________________________________________________________________________________ _________________________________________________________________________________________

TIME WAVEFORM ANALYSIS [1]

Name three fault conditions that can be diagnosed via time waveform analysis.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [email protected]




Page 8 of 35

Name four phenomena that can be observed in the time waveform.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [3]

A 1500 RPM motor is driving a pump via a gearbox with 40 teeth on the input gear. What Fmax and LOR setting will allow one to see three harmonics of the gear mesh frequency in the spectrum and 6 rotations of the gear in the time waveform?

_________________________________________________________________________________________ _________________________________________________________________________________________ [4]

A 1500 RPM motor is driving a pump via a gearbox with 40 teeth on the input gear. What Fmax setting is required in order to be able to clearly see each tooth mesh in the time waveform?

_________________________________________________________________________________________ _________________________________________________________________________________________ [5]

What is the approximate frequency of the dominant event in this time waveform?

A. B. C. D.

0.63 Hz 6.7 Hz 24.7 Hz Not enough information

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Page 9 of 35 [6]


Why is time waveform analysis so important when performing vibration analysis on gearboxes?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [7]

Why is time waveform analysis useful when performing vibration analysis on rolling element bearings?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [8]

Why is time waveform analysis useful when a pump is cavitating?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

PHASE ANALYSIS [1]

Name three reasons why the peak value of the vibration signal does not coincide perfectly with the passing of the point of greatest mass in the rotor by the sensor?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Machine balancing: What would happen if we performed an ‘original run’, using an accelerometer (mounted in‐line with the tachometer), and measured a vibration reading of 0.5 Gs and a phase angle of 0° and then placed the trial weight opposite the tachometer (i.e. 180°)? (Assume a rigid rotor and a trial weight with enough mass to significantly affect the vibration reading.) A. B. C. D.

[3]

The vibration level would decrease The vibration level would increase The vibration reading would not change Not enough information

What happens to the phase angle as a machine speed is increased towards the first critical speed? A. B. C. D.

[4]

Page 10 of 35

Nothing Nothing until the machine speed reaches the critical speed; then it shifts by 180° It decreases It increases

What is the difference between relative phase and absolute phase?

_________________________________________________________________________________________ _________________________________________________________________________________________ [5]

What are two benefits of relative phase?

_________________________________________________________________________________________ _________________________________________________________________________________________

DIAGNOSING FAULTS WITH PHASE ANALYSIS For the following questions the relative vibration levels can be interpreted as follows:

High vibration:

Low vibration:

NOTE: For your own benefit, try to answer these questions before referring to the training manual

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Page 11 of 35


[1]

Which fault condition is indicated by the following bubble diagram? ___________________

[2]


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Page 12 of 35

[3]


[4]


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Page 13 of 35


[5]


[6]


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Page 14 of 35


DYNAMICS and RESONANCES [1]

Name three types of damping.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [2]

Provide examples of damping in rotating machinery

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 15 of 35 [3]


In the following equation, what does ‘

f (t )  kx  cv x  mx

’ represent?

A. Stiffness B. Damping C. Mass [4]


f (t )  kx  cv x  mx

’ represent?

A. Stiffness B. Damping C. Mass. [5]


f (t )  kx  cv x  mx

’ represent?

A. Stiffness B. Damping C. Mass. [6]


f (t )  kx  cv x  mx

’ represent?

A. Displacement B. Velocity C. Acceleration [7]


f (t )  kx  cv x  mx

’ represent?

A. Displacement B. Velocity C. Acceleration [8]


f (t )  kx  cv x  mx

’ represent?

A. Displacement B. Velocity C. Acceleration

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Page 16 of 35

If you performed a Bode plot and you noticed that amplitude was high in a certain speed range but the phase did not change substantially, what would you conclude? A. There definitely is a resonance B. There definitely is NOT a resonance C. There probably is not a resonance

[10]

If you performed a run‐down test (as shown below) and saw that the amplitude was high in a certain speed range and the phase did change by 180°, what would you conclude?

A. There definitely is a resonance B. There definitely is NOT a resonance C. There probably is not a resonance [11]

If you performed a rundown test and witnessed a Bode plot as described in question 10, and then repeated the test two weeks later to find that the natural frequency was higher than it was previously (but the amplitude at the natural frequency had not changed substantially), what would you conclude? A. B. C. D.

[12]

The machine was running at a higher speed Bracing had been added to increase the stiffness Sandbags or some other form of weight was added to the machine Someone must have added damping to the machine

Based on the test data in question 10, would you say the machine was lightly damped or heavily damped? A. Lightly damped B. Heavily damped

[13]

If you noticed that the 1x vibration in a horizontally oriented motor was much higher (>4X higher) in the horizontal direction than the vertical direction, what would you conclude? A. The machine (or the structure supporting the machine) must have a resonance that causes it to vibrate in the horizontal direction B. The structure supporting the machine must be weak or loose in the horizontal direction. C. Either A or B could be correct. Additional tests would be required to distinguish between these two possible fault conditions.

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Page 17 of 35 [14]


Is the following shape the first, second, or third bending mode?

A. First bending mode B. Second bending mode C. Third bending mode [15]

How many nodes does this mode shape have?

A. B. C. D. [16]

1 2 3 4

How many anti‐nodes does this mode shape have?

A. B. C. D.

1 2 3 4

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Page 18 of 35

Is the value of theta ζ marked by “A” more likely to be 0.05, 0.5 or 1.0?

A. 0.05 B. 0.5 C. 1.0

OPERATING DEFLECTION SHAPES (ODS) [1]

What method of data capture must you use if you wish to visualize the motion (deflection shape) at multiple frequencies (not just running speed)?

_________________________________________________________________________________________ _________________________________________________________________________________________ [2]

What is the main difference between ODS and modal analysis?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 19 of 35 [3]


What are the primary reasons to perform an ODS test?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [4]

When performing an ODS test with two accelerometers, what precautions should you take when selecting the location of the reference accelerometer?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [5]

When performing an ODS test with two accelerometers, what precautions should you take when selecting the location of the ‘roving’ accelerometer?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [6]

Can you perform an ODS test on a machine that is frequently changing speed and load? Explain your answer. A. Yes B. No

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 20 of 35

How does time‐based ODS differ from frequency‐based ODS?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

MODAL ANALYSIS [1]

What does the coherence measurement tell you?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [2]

When performing a modal test, what precautions should one take when selecting the location of the reference location?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [3]

When performing a modal test, what precautions should you take when selecting the location of the ‘roving’ accelerometer?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 21 of 35 [4]


What is the Frequency Response Function?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [5]

Name two different ways to perform a modal analysis test.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [6]

Describe the important characteristics of the hammer used in the impact test.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [7]

Describe an important precaution that must be taken when impacting a structure (beyond not damaging the structure).

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 22 of 35

When performing a modal test with a force hammer, what plots does one typically want to look at for each test point?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

CORRECTING RESONANCES AND REDUCING VIBRATION [1]

If it was determined that the speed of a machine coincided with a natural frequency, name four possible courses of action to deal with the situation.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [2]

If it was determined that a structure had a natural frequency close to the running speed of the machine, and it was not possible to change the speed of the machine, what precautions would you take when designing your structural modifications?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [3]

Two different structures were tested and their Bode plots are displayed below. Which structure has the highest ‘Q’?

A. Machine 1 B. Machine 2 [email protected]



Page 23 of 35 [4]


What is a tuned absorber?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [5]

Why aren’t tuned absorbers well suited to machines that vary in speed?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [6]

Why or when is isolation used?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 24 of 35

Are isolators designed so that their natural frequency is above, equal to, or below the running speed of the machine?

A. Below machine running speed B. Equal to the running speed C. Above the running speed [8]

When a machine is mounted on isolators or resilient mounts, what can happen when the machine starts up (or is run‐down)?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [9]

What issues have to be taken into account when specifying isolators?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 25 of 35


JOURNAL BEARING ANALYSIS AND ORBITS [1]

Does the voltage output of the non‐contact eddy current (proximity) probe increase or decrease as the shaft is moved closer to the probe? A. Increase B. Decrease C. Will not change

[2]

What is the purpose of slow‐roll subtraction (glitch removal)?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [3]

Which measurement, A or B, represents an eccentricity ratio of 0.8 (approximately)?

A. Measurement A B. Measurement B

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Page 26 of 35

Given the following centerline diagram where would the center of the shaft normally be located when the machine is at rest?

A. B. C. D. E. [5]

Position A Position B Position C Position D Position E

What is the difference between the ‘direct’ and ‘filtered’ signal?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [6]

Describe the vibration precession of an orbit.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [7]

When using a conventional data collector to perform orbit analysis, would you normally set your data collector to low resolution (e.g. 100 lines) or high resolution (e.g. 1600 lines)? A. Low resolution B. High resolution C. It is an orbit, it does not matter

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Page 27 of 35 [8]


What does a circular orbit indicate?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [9]

What does it mean if the filtered orbit is heavily elliptical?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [10]

What does it mean if the orbit is distorted or twisted?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [11]

List 5 common fault conditions in journal bearings.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 28 of 35

ROLLING ELEMENT BEARING ANALYSIS [1]

With the inner race spinning, an inner race defect will typically produce A. B. C. D.

[2]

A non‐synchronous peak A non‐synchronous peak with harmonics A non‐synchronous peak with shaft rate sidebands A sub synchronous peak

Name four root causes of rolling element bearing wear.

_________________________________________________________________________________________ _________________________________________________________________________________________ [3]

Describe the progression of rolling element bearing wear from the earliest stages up to failure in the ideal case

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [4]

Describe the four recommendations made in the course regarding how to utilize demodulated data and how this relates to making repair recommendations.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [5]

Describe the general patterns produced by the inner race, outer race and ball / roller faults.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 29 of 35


GEARBOX ANALYSIS [1]

What are the first 10 prime numbers?

_________________________________________________________________________________________ _________________________________________________________________________________________ [2]

If the input gear has 31 teeth and the output gear has 19 teeth, what are the common factors? A. B. C. D.

[3]

If the input gear has 30 teeth and the output gear has 18 teeth, what are the common factors? A. B. C. D.

[4]

All of them 9 on the 18 tooth gear and 15 on the 30 tooth gear 2 on the 18 tooth gear and 3 on the 30 tooth gear 3 on the 18 tooth gear and 5 on the 30 tooth gear

If the input gear has 30 teeth and the output gear has 18 teeth, what is the hunting tooth frequency (as a multiple of running speed)? A. B. C. D.

[7]

1X 6X 18X 30X

The input gear has 30 teeth and the output gear has 18 teeth. If one tooth is damaged on the gear with 18 teeth, how many teeth will eventually be damaged on each of the gears? A. B. C. D.

[6]

1 1, 2, 3 1, 2, 5 1, 2, 3, 5

If the input gear has 30 teeth and the output gear has 18 teeth, what is the gear assembly phase frequency (as a multiple of running speed)? A. B. C. D.

[5]

1 1, 31 and 19 1, 3, and 11 There are none – they are prime numbers

0.33X 0.11X 0.6X 30X

If the input gear has 28 teeth and the output gear has 24 teeth, what is the product of the common prime factors? A. B. C. D.

2X 4X 12X 28X

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Page 30 of 35

What are the three common configurations of a planetary gear?

_________________________________________________________________________________________ _________________________________________________________________________________________ [9]

What are some common vibration tests (include data formats) that are used for analyzing gearboxes? What technologies besides vibration analysis are useful?

_________________________________________________________________________________________ _________________________________________________________________________________________ [10]

If a worm gear has an input speed of 30 Hz and an output speed of 10 Hz and the worm wheel has 40 teeth, how many flights are there? A. B. C. D.

[11]

7.5 13.3 120 0.75

If the Fmax was 4000 Hz, and the resolution was set to 800 lines, how many time samples would be acquired per tooth mesh if the input speed of the gearbox was 30 Hz and the pinion had 33 teeth? How many revolutions of the input gear would be present in the time waveform?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

ELECTRIC MOTORS [1]

An eccentric stator (with an uneven stationary air gap) will produce... A. B. C. D.

[2]

a high 1X peak at the motor running speed increased amplitude at twice line frequency harmonics of 1X with pole‐pass sidebands sub synchronous vibration

An eccentric rotor (with a varying air gap) will produce... A. B. C. D.

a high 1X peak at the motor running speed a peak at the rotor bar frequency with twice line‐frequency sidebands increased amplitude at twice line frequency, with pole‐pass sidebands harmonics of 1X with pole‐pass sidebands

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Page 31 of 35 [3]

A cracked rotor bar will produce... A. B. C. D.

[4]


Loose rotor bars will produce... A. B. C. D.

[5]



In a motor running at 1740 RPM. What is the pole pass frequency? A. B. C. D.

60 RPM 120 RPM 120 Hz 240 RPM

BALANCING [1]

If vibration readings indicated a high displacement vibration reading at 30° should you place the trial weight at 210°? A. Yes, that will balance the machine B. It depends whether the machine has a rigid or flexible rotor, and whether the machine is being operated anywhere near the natural frequency. C. No, it should be placed at 30° D. No it should be placed at 120°

[2]

If a rotor (supported by two bearings, one at each end) that normally turns at 1800 RPM weighed 90 kg and we had to place balance weights at a radius of 350 mm, which of the following available balance weights would you use as a trial weight? (Note: W is in grams) A. B. C. D.

4 kg 40 grams 4 grams 24 grams

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[3]

Page 32 of 35

Explain what the ‘O’, ‘T’ and ‘O+T’ vectors represent.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 33 of 35 [4]


What is the magnitude of the sum of the following vectors: 4 mils at 60 degrees and 3 mils at 150 degrees

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [5]

The original run is 4 mils at 60 degrees. A 1 ounce balance weight is added at 60 degrees. The trial run results in a vector of 3 mils @ 150 degrees. Calculate the final balance weight and angle.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ [6]

You need to place a balance weight of 10 grams at 30 degrees but you only have holes available at 0 degrees and 45 degrees. How much weight must be put in each of these holes?

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 34 of 35

DIAGNOSING MACHINE FAULTS [1]

What is wrong with this machine? Please briefly explain why you have come to this conclusion. (Note that the L and R phase readings are axial readings taken on the left and the right of the shaft. The axial phase readings have been adjusted – as if they were all taken in the same direction.)

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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Page 35 of 35 [2]


What is wrong with this machine? Please briefly explain why you have come to this conclusion.

_________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________ _________________________________________________________________________________________

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CAT III Activity Book 170112 - A4

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