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WorksheetsVibration Analysis Quiz
Total questions: 120
Worksheet time: 3600secs
The RMS level is...
Half the peak level
Twice the peak level
0.707 times the peak level (for a pure sine wave)
Half the peak-to-peak level
Identify the RMS level on this chart (choose between A, B, C, and D as indicated on the graph).
A
B
C
D
The relationship between frequency and period is:
They are the same
Frequency (Hz) = 1/period (seconds)
Frequency (Hz) = period 2
Period (minutes) = 1/frequency (CPM) 2
Is the RMS level of this vibration signal equal to 0.707 times the peak level?
Yes
No
The measurement that is proportional to the rate of change of velocity is...
Displacement
Velocity
Acceleration
Jerk
The measurement that is proportional to the fatigue experienced by the shaft and bearing is...
Jerk
Displacement
Velocity
Acceleration
The measurement unit that is most sensitive to low frequency vibration is...
Displacement
Velocity
Acceleration
Hertz
When integrating from acceleration to velocity what happens to the phase?
It shifts by 90 degrees
It shifts by 180 degrees
It does not change
It depends on the location of the phase reference
Phase is measured in units of...
Seconds
RPM
Degrees
Orders
What is the phase relationship between these two signals?
In phase
180 degrees out of phase
90 degrees out of phase
270 degrees out of phase
Which statement is true?
Orders of running speed are reported in units of ‘X’
CPM and Orders are the same when dealing with 2-pole motors
Orders are only used for variable speed machines
None of the above
If the 10X peak is at 2500 CPM, the running speed of the machine must be _____ CPM
250
2500
25,000
41.6
A sine wave in the time waveform will produce what in the spectrum?
Harmonics that are not evenly spaced
Harmonics that are evenly spaced
A single peak
Sidebands
A periodic (repetitive) but non sinusoidal wave will produce what kind of spectrum?
An attractive spectrum
A single peak
Sidebands
Harmonics
A single impulse (impact) in the time waveform will produce what in the spectrum?
Half-order harmonics
A raised noise floor
Odd harmonics
Even harmonics
Two sine waves that are very close to the same frequency (say, 0.5 Hz apart) will...
cause beating
cancel each other out
create harmonics
create sidebands
Sub-harmonics can best be described as...
same as normal harmonics, but smaller
½x, 1x, 1½x, 2x etc.
1x, 2x, 3x, 4x, 5x etc.
19x, 20x, 21x
Amplitude Modulation can best be described as...
the periodic change in the frequency of a signal
the sound a radio makes when it is not correctly tuned
the periodic change in the amplitude of a signal
harmonics that don't look quite right
When sampling data, how does the sample rate relate to Fmax?
The sample rate is 2.56 x the Fmax
They are the same
The sample rate is the Fmax divided by 2.56
None of the above
For the same Fmax, if you increase the number of Lines of Resolution...
You cannot change the number of lines without also changing Fmax
The measurement will take longer to complete
The test time will not change
The measurement will take less time to acquire
Why not collect data with the highest Fmax setting supported by your analyzer (and just 800 lines)?
Because the test will take longer
Because with a higher Fmax setting the resolution is reduced
Because you will need a special transducer
All of the above
Why not collect data with the highest Lines of Resolution setting supported by your analyzer?
The test takes longer
The data takes more memory in the database
The data takes longer to unload from the data collector
All of the above
When route testing rotating machinery in a condition monitoring program, which window setting (of the options below) is most commonly used?
Hanning
Flat top
Exponential
Uniform/rectangular/no window
The window most commonly used for bump tests is:
Hanning
Flat top
Hamming
Uniform/rectangular/no window
The primary reason for using the Hanning window is:
To speed up the measurement
To reduce the impact of “leakage”
To increase frequency accuracy
To increase amplitude accuracy
When route testing rotating machinery in a condition monitoring program, how many averages are normally taken?
Between 1 and 10
10 - 20
20 - 30
100+
When route testing rotating machinery in a condition monitoring program, what type of averaging (of the options below) is typically used?
Linear
Time synchronous
Peak hold
Exponential
True or false: Linear averaging works by averaging the time waveform
True
False
How many revolutions of the shaft would you normally like to see in a time waveform (if the fault condition causes a change in vibration multiple times per revolution)?
4 - 10
10 - 50
50 - 100
100+
If you increase the Lines Of Resolution setting, will you see more or fewer revolutions of the shaft in the time waveform?
More revolutions
Fewer revolutions
The same number
If you wish to capture a short-duration event in greater detail (i.e. capture more samples during the event), which of the following settings should you change?
Fmax (frequency range)
Lines Of Resolution
Number of averages
Settling time
If you increase the Fmax (frequency range) setting, will you see more or fewer revolutions of the shaft in the time waveform?
More revolutions
Fewer revolutions
The same number
Which of the following fault conditions is easier to diagnose if you use time waveform analysis instead of using spectrum analysis alone (which one benefits the most)?
Unbalance
Misalignment
Broken gear teeth
Cavitation
Is it possible to detect bearing wear with time waveform analysis?
No
Yes
What type of sensors are typically used to monitor the movement of a shaft inside a journal bearing?
Accelerometers
Proximity probes
Velocity probes
Charge mode accelerometers
When monitoring a large journal bearing with two proximity probes, which graph is most commonly analysed?
Overall trend
Time waveform
Orbit
Spectrum
Which sensor involves a magnet and a coil moving relative to each other?
Shaft rider
Proximity probe
Accelerometer
Velocity sensor
Which of these accelerometers sensitivities would best measure very low amplitude vibration?
10 mV/g
50 mV/g
100 mV/g
500 mV/g
Select the best accelerometer mounting point from the available options:
A
B
C
D
Which of these are important factors for selecting a test point?
Safe access to the test point
Good mechanical transmission path
As close to the bearing as possible
All of the above
Which is the best method of mounting an accelerometer (the best frequency response)?
Stinger, or probe tip
Flat magnetic mount
Two-pole magnetic mount
Stud mount
What are the benefits of mounting (or target) pads?
They show exactly where the sensor should be mounted
They provide a flat surface for the sensor
They are easy to clean (to remove debris)
All of the above
In the context of a condition monitoring program, what is repeatability?
The ability to test machines in the same order every time
The ability to perform a test in exactly the same way every time
The ability to perform measurements very quickly
All of the above
Insufficient settling time, a sensor heating up or cooling down during the test or thumping the sensor against the machine will result in...
a typical reading
a damaged sensor
an error message on the data collector
a “ski-slope” in the spectrum
If the vibration at 1X increases dramatically what should you do?
Schedule the rotor for balancing
Schedule the rotor for cleaning
Perform additional analysis to ensure that it requires balancing
Wait to see if the vibration gets any worse
Which peaks in the spectrum are associated with unbalance in a machine with a supported rotor (bearings at each end)?
1X, 2X, and 3X
1X in the axial direction
1X in the vertical and horizontal direction
Wait to see if the vibration gets any worse
The forces generated by an unbalance mass are proportional to...
the amount of lubricant in the bearings
the load on the machine
the speed of the machine (linearly)
the square of the speed of the machine
If a rotor has static unbalance...
the two ends of the machine in the vertical direction will be in phase
the two ends of the machine in the vertical direction will be out of phase
If a rotor has a couple imbalance...
the two ends of the machine in the vertical direction will be in phase
the two ends of the machine in the vertical direction will be 180 deg out of phase
How will the vibration change?
The 1X amplitude will increase in the vertical and horizontal direction without a change in the axial direction
The 1X amplitude will increase in the vertical, horizontal and axial directions
What vibration pattern will likely be measured if a vertical machine goes out of balance?
1X higher at the top of the machine than the bottom and higher in one radial axis
1X the same amplitude in all radial directions
Primarily a high 1X Axial
1X higher at the bottom of the machine than the top with harmonics
A ship sailing west and being blown by a wind from the north will travel which direction?
South
North-west
North-east
South-west
When performing a single-plane balance on a flexible rotor, the trial weight should produce a force equal to ____ of the rotor weight.
50%
20%
10%
15%
The desired effect of adding a balance weight is to...
have the opposite effect of the original unbalance
get close to the accepted tolerance level
stop the machine from shaking
replace any mass lost from drilling
You may be unsuccessful in balancing a machine due to:
Looseness
Calculation errors
Resonance
Any of these (and more)
True or false: You can achieve a balance solution by adding weight or by removing an equivalent mass on the opposite side of the rotor
True
False
True or false: If the residual (existing) unbalance is less than the permissible (minimum acceptable) unbalance, the rotor is out of balance
True
False
When looking for misalignment, one should analyse vibration data:
In all axes on both sides of the coupling
In the axial direction only
In the radial directions only
At the coupling
Is it possible to detect misalignment using a laser alignment system?
No
No, unless it wasn’t available when the machine was last aligned
Yes
It depends upon the brand of alignment system
To confirm the existence of angular misalignment one would look for what phase relationship across the coupling in the axial direction?
180 degrees out of phase
90 degrees out of phase
In-phase
There is no clear phase relationship
True or false: One typically has angular or offset misalignment, but rarely both types at the same time.
True
False
Which of the following are common causes of misalignment?
Inaccurate assembly of components
Relative position of components changing after assembly
Distortion due to forces exerted by piping
All of the above
It is important to correct a misalignment condition because...
the high vibration level can create resonant conditions
the coupling warranty will be voided
misaligned machines can create pipe-strain
high vibration levels can damage bearings and cause premature failure
If the vibration does not indicate that misalignment exists, does it mean that the machine is correctly aligned?
Yes
No
If a shaft is bent, what will be the phase relationship when measured at either end of the machine?
In-phase in the vertical direction, but out-of-phase in the horizontal direction
In-phase when measured axially
Out-of-phase when measured axially
Phase is not a useful tool for diagnosing a bent shaft
When aligning a machine, always make sure you:
Use dial indicators
Use a laser system
Lock out the machine so it cannot be started accidentally
Verify the alignment with a straight edge
The bottom line is that precision aligned machines will be more:
Noisy
Expensive to maintain
Susceptible to bearing wear
Reliable
Are alignment tolerances normally tighter on high speed machines or low speed machines?
Low speed machines
High speed machines
What would happen if you did not correct the soft foot condition?
It would be difficult to align the machine
The case of the motor would be distorted and there would be an uneven air gap between the rotor and stator
The bearings and seals would be under stress
All of these reasons and others are correct
True or false: A soft foot check should be completed before and after vertical angular misalignment has been corrected.
True
False
What is “bar sag”?
When the circumference of the coupling is not perfectly round
Where the bars used to support the dial indicators bend or sag due to gravity
The amount the shaft sags due to gravity
The stem sticking as it moves in or out of the dial
When moving a machine, horizontal moves are often done with ______ and vertical moves by adding ______.
Shims, spacers
Spacers, shims
Jacking bolts, shims
Shims, jacking bolts
Which of the following is a cause of rotating looseness?
Motor not bolted down securely
Loose cowling or other metallic structure
Wear within a rolling element bearing
Pedestal bearing not bolted down sufficiently
Rotating looseness typically results in what pattern in the vibration spectrum?
1X harmonics (and in some cases 2X, 3X, 4X...)
Non synchronous peaks
Sidebands
All of the above
A more serious form of rotating looseness or rotating looseness in a journal bearing creates which pattern in the vibration spectrum?
Larger 1X harmonics
Sub-harmonics
A high 1XH
A symmetrical time waveform
Can phase be used to detect rotating looseness?
Yes, the lack of a phase relationship points to looseness
Yes, there is 180 degree phase shift
Yes, all radial points are in phase
No
Foundation flexibility (structural weakness) most commonly results in what pattern in the vibration spectrum?
1X harmonics
1XV much higher than 1XH
1XH much higher than 1XV
1X sidebands
Rotating looseness is associated with which pattern in the time waveform?
Clipping or asymmetry
Modulation
Sinusoidal
Random impacting
If the sheaves are eccentric, how will the vibration change?
The vibration at 1X will increase
The vibration at 2X will increase
You will witness peak below running speed with harmonics
The vibration will not change
Will the 1X vibration reduce to normal levels if you balance an eccentric sheave?
No
Yes
If the sheaves are misaligned, how will the vibration change?
The vibration at 1X will increase and there will be harmonics
The vibration at 2X will increase
You will witness peak below running speed with harmonics
The vibration will increase in the axial direction at 1X
Why would you use a strobe on a belt drive?
It freezes the motion so repairs can be made
It enables you to inspect the sheaves and belts for wear and other damage
It enables you to test the natural frequency of the belts
Is the vibration known as the “belt rate” higher or lower in frequency than the running speed (of either pulley/sheave)?
Higher
Lower
The same
It depends on the diameters of the sheaves
At what frequency do belts resonate?
At the running speed of the sheave in question
It depends on the belts and their tension
Belts do not resonate, they just flap when loose
Does anyone really know?
What percentage of bearings reach their design lifetime?
No more than 10%
No more than 50%
No more than 90%
100%
Order normalizing a graph is useful because...
it helps to identify the source of the vibration that damaged the bearings
it highlights which peaks are synchronous and non-synchronous
forcing frequencies are always orders
order normalizing is not useful
A decibel (dB) or logarithmic graph scale helps with bearing analysis because...
it is easier to determine relative amplitude
it is easier to see higher frequencies
one can see low amplitude peaks in the presence of high amplitude peaks
it is easier to determine which peaks are non-synchronous
Why is a high pass filter used in a demodulation/enveloping system?
To amplify the useful low frequency signals
To amplify all of the high amplitude high frequency signals
To remove the high amplitude, low frequency signals
To remove the high frequency noise
If you see low amplitude, non-synchronous peaks in an envelope spectrum, what should your response be?
Check the machine for rotating looseness
Check the machine for gear faults
Schedule the bearings for replacement
Continue to monitor the machine and check spectra and time waveforms
What should you do if you detect non-synchronous harmonics with 1X sidebands however the frequency does not match the bearing frequencies in your database?
Suspect that it is a different fault condition
Assume that it is a bearing from a nearby machine
Assume that the bearing does actually have a fault condition and proceed accordingly
Report a bug to your data collector vendor
In countries with a 50 Hz line frequency, if the machine turns at just under 3000 RPM, is it a two-pole motor or a four-pole motor?
Two-pole motor
Four-pole motor
The slip frequency is...
the actual speed of the motor
another term for the synchronous speed
the difference between the actual speed and the synchronous speed
the number of times per second that the squirrel slips inside the cage
Select a cause for an eccentric stator.
Purchasing from “Jim-Bob's Used Stators” instead of your normal source
Over-lubrication, which increases the slip
Soft foot
Loose holding-down bolts
pole-pass frequency is...
the number of poles times the motor running speed
the difference between the number of poles and the synchronous speed
the same as the line frequency (50 or 60 Hz)
the slip frequency times the number of poles
What would you suspect if you saw that the 1X through 4X peaks were surrounded by sidebands (with very little separation)?
There is rotating looseness
There may be broken rotor bars or a cracked end-ring
The rotor was loose and the air-gap changes periodically
The resolution of the analyzer is too high
What would you suspect if you saw that the twice-line-frequency peak was high?
There could be soft foot
The stator may be eccentric
There could be loose windings or laminations
Any of the above could be correct
The gear mesh frequency is calculated by...
adding the number of teeth on each gear
dividing the number of teeth on the input by the teeth on the output
multiplying the number of teeth on the gear by the speed of that gear
multiplying the number of teeth on each gear
Apart from the gear mesh frequency peak, what other pattern is typically found in the vibration spectrum from a gearbox?
Beating
Non synchronous peaks
Sub synchronous peaks
Shaft rate sidebands around gear mesh
What other condition monitoring technology is especially useful in gearbox monitoring?
InfraRed Thermography
Ultrasound
Wear particle analysis (ferrography)
Vibration is the only useful technology
What is the best tool for detecting cracked or broken gear teeth?
Motor current analysis
Perform thermographic analysis on the lubricant
Spectrum analysis
Time waveform analysis
If a gearbox was quoted as having a speed ratio 1:8, which statement is true?
If the output speed was 8000 RPM, the input speed must be 1000 RPM
If the intermediate shaft turned at 1000 RPM, the motor turns at 3000 RPM
If the input shaft turns at 8000 RPM, the output shaft turns at 1000 RPM
It is a double reduction gearbox
Which one (or more) of the following will results in an increase in amplitude at the pump vane rate frequency?
Non-uniform variable pitch blades
Blade or vane wear (abrasion or cavitation)
Operation (improper performance parameters)
Inlet or discharge line restrictions
All of the above
How would the spectrum change if the pumping system was experiencing cavitation?
An increase in both the vane pass frequency and the noise floor
An increase in amplitude at the pump running speed
An increase in amplitude in an area below the running speed
A series of harmonics of pump vane rate
How would the spectrum change if the pumping system was experiencing turbulence?
An increase in amplitude at the pump vane rate frequency
An increase in amplitude at the pump running speed
A hump of noise in an area below the running speed
An increase in high frequency vibration
Under what conditions might you observe running speed sidebands around the pump vane rate peak?
Rotating looseness in the pump bearings
Damage on the inner race of the bearing
Impeller loose on the shaft
Flow turbulence
Resonance occurs when...
there is rotating looseness
the machine speed varies
vibration levels increase
a forcing frequency coincides with a natural frequency
In order to avoid resonance, one should operate a machine…
at a natural frequency
at any speed that is not within +/-20% of the natural frequency
above the natural frequency
below the natural frequency
If the running speed of the machine was changed so that it coincided with a natural frequency, how would the vibration amplitude change?
It will remain unchanged
It will increase if the machine was out-of-balance and the machine speed was decreased
It will decrease in amplitude due to the resonance
It will increase in amplitude due to the resonance
A bump test is…
a way to seek revenge on annoying machines
a way to test whether your suspicion of a natural frequency could be correct
a way to tell if your colleague is awake
a way to change the dynamics of the machine
Is it possible to perform a bump test on a machine when it is running?
No
Yes
Why would you perform an ODS test?
To visualize vibration, which may include mode shapes due to resonance
To model the structure of the machine mathematically to understand its modal parameters
To generate an animation of the structure for your records
To test how the machine vibration changes as speed changes
How would the natural frequency, and the amplitude of vibration at resonance, change if the damping is increased?
Natural frequency will increase and amplitude will decrease
Natural frequency will decrease and amplitude will increase
Natural frequency won’t change and amplitude will decrease
Natural frequency will decrease and amplitude won’t change
How would the natural frequency, and the amplitude of vibration at resonance, change if the mass of the machine is increased?
Natural frequency will increase and amplitude will decrease
Natural frequency will decrease and amplitude will increase
Natural frequency won’t change and amplitude will increase
Natural frequency will decrease and amplitude won’t change
Which series of standards is used to check measurements taken from (non-contact displacement) proximity probes?
ISO 10816
ISO 7919
ISO 18436
ISO 1940
True or false: Alarms can be set on overall level readings but not on spectra or time waveforms.
True
False
It is important to set alarms on vibration spectra because…
you will detect changes in the data and begin analysing the right data more quickly
most software can use the alarms to diagnose fault conditions
the exception reports generated can be automatically sent to plant management
ISO 10816 will advise of the fault severity
In a condition monitoring program, the most important reason to set alarms is to:
Be sure you are in compliance with ISO standards
Be in compliance with insurance company standards
Make the analysis process more efficient
Detect bearing defects
Why is it important to perform acceptance testing?
Because not all machines that you purchase will actually be in perfect condition
Because the vendors are more likely to manufacture high quality equipment
Because it will provide you with baseline “as new” vibration readings
Because more than 60% of machines are defective
How do you set the vibration limits to be used in acceptance testing?
Use the same limits you use in your normal vibration program
The vendor/supplier of the equipment will often provide the limits
Use ISO standards and other industry standards
All of the above
What sort of fault conditions would normally be tested for during acceptance testing?
Bearing defects, resonance, and unbalance
Cavitation and turbulence within the pump
Rated machine speed and current
Air flow through cooling fans
