CAT III Worked Examples PDF

Worked examples Worked examples CAT III Worked examples Page 71 UNDERSTANDING SIGNALS 1. A sine wave with a peak amplitude of 3 mm/secRMS is added to another sine wave with the same frequency, amplitude and phas...

Worked examples Worked examples CAT III Worked examples Page 71 UNDERSTANDING SIGNALS 1. A sine wave with a peak amplitude of 3 mm/secRMS is added to another sine wave with the same frequency, amplitude and phase. What will the resultant vibration look like? What if the second wave is 180 degrees out of phase with the first? How is the principle in part 2 of the question used in real life? 2. A sine wave of 100 Hz, 0.8 in/secpk amplitude and another of 100.5 Hz, 0.2 in/secpk amplitude are added together. Please describe the resultant waveform: a. What is its maximum amplitude? b. What is its minimum amplitude? c. What is this effect called? d. If you could hear the resultant vibration, what would it sound like (be specific)? 3. Please describe the spectrum that would result from the time waveform below: 4. Please describe the spectrum that would result from the time waveform below which is the result of amplitude modulation: [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 72 CAT III Worked examples 5. There are two different causes of a raised noise floor in the spectrum (not including bad data or instrumentation problems). They are: ____________________ and _____________________. Please describe common causes (in machines) of these two patterns of vibration. Because these two might have very different causes but look the same in the spectrum it is a good idea to also analyze the _______________________. 6. Amplitude modulation results in a pattern of _______________________ in the spectrum. It is often associated with these machine components: _______________, _______________, _______________. In the time waveform, amplitude modulation can be confused with ________________, however _________________ will result in ___________________ in the vibration spectrum. SIGNAL PROCESSING 1. Please identify the filter in the image below (low pass, high pass band pass) Explain the transition band, pass band and stop bands Explain “fc” on the plot Which filter would be used for which of these applications - ISO RMS overall measurement, removing very low frequencies, Fmax setting? 2. What is integration? What are two ways to perform integration on a vibration signal? What happens to the amplitudes of high frequency signals when you integrate? What happens to very low frequency signals? What are the pros and cons of the two methods of doing integration? In each case, what would the default units of the time waveform be? 3. Describe the gain setting and “auto ranging” settings on a data collector. When do you manually set the gain and when do you use auto ranging? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 73 4. Describe the difference between using analog anti aliasing filters and the “delta-sigma” method. Are anti aliasing filters high pass or low pass? 5. Describe the difference between dynamic range and signal to noise (S/N) ratio. What is the dynamic range of a 12 bit A/D converter? 6. Fmax = 2000 Hz, Lines of Resolution (LOR) = 1600, Window = Hanning (WF = 1.5), 10 averages, overlap = 0%. How long does it take to collect data? How long is the time waveform? How long does it take to collect data if there is 50% overlap? What is another benefit of overlap averaging apart from saving time? 7. Select an Fmax and Lines of Resolution (LOR) setting to capture pole pass frequency (PPF) sidebands around shaft rate harmonics in a four pole AC Induction motor in a 60 Hz line frequency environment, with a slip frequency of 30 RPM. The Fmax should be fifty orders of run speed. A Hanning window is used. How long will the measurement take with 4 averages, no overlap? 8. Please match the average type to the application: Bump test Linear averaging Normal route test Peak hold averaging Special gearbox test Negative averaging Bump test on a machine that is running Exponential averaging Give more weight to newer data in averaging process Time synchronous averaging Find the highest vibration amplitudes on a machine when tested over a period of time 9. Describe the trigger setting, gain setting and window on the hammer channel for a calibrated hammer test. Describe in some detail why you make these selections and how they work. 10. Describe auto correlation, what it is, what it is used for and briefly how it works. 11. Describe order tracking, what it is, when it is used and how it works. Describe other, possibly more practical options for solving the problems order tracking is often used to solve. 12. Describe Time Synchronous Averaging. Is the time data averaged or are the FFTs averaged? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 74 CAT III Worked examples How can you synchronize a test to an intermediate shaft of a gearbox if you cannot get a tachometer on that shaft? 13. Match the filter type to the following data collector settings: Fmax Low pass ISO RMS overall reading (10 - 1000 Hz) High pass 10 Hz low cutoff frequency Band pass Filtering out data below 5000 Hz for demodulation reading Band stop 14. What is the (10 Hz) low cutoff frequency used for? 15. Match the window type to the application: Hanning Sensor calibration Flat top Bump test Rectangular Normal route test 16. The (spectrum/time waveform) is measured and the (spectrum/time waveform) is calculated using an algorithm called the _______________. 17. If the Fmax is 1000 Hz then the sample rate is ________________ and the time between samples in the time waveform is ________________. 18. If Fmax = 1000 Hz and there are 800 lines of resolution: How many samples will be in the time waveform? How long (in seconds) will the time waveform be? If this measurement is taken on a shaft rotating at 1200 RPM, how many revolutions of the shaft will appear in the time waveform? What is the resolution of the spectrum? If a Hanning window is used (WF = 1.5), what is the bandwidth of the measurement? 19. The Hanning window is used to solve a problem called __________________. 20. How many averages are typically used with linear averaging on typical machines? What overlap %? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 75 21. What is the approximate frequency (in Hz and CPM) of the impacting in the waveform below? What could cause this? What else is interesting about this time waveform? TIME WAVEFORM ANALYSIS 1. Is the waveform below amplitude modulation or beating? Assuming it is amplitude modulation, please draw or describe the spectrum you will see is a specific way - in other words, calculate the applicable frequencies. Next assume it is beating. What are the frequencies? What will the spectrum look like? What is the relative amplitude of the two frequencies? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 76 CAT III Worked examples 2. Please describe the spectrum associated with this modulated waveform. Where will we see this effect in a machine? 3. The plot below is a circle plot from a gear box collecting using time synchronous averaging. The circle represents the input shaft. How many teeth are on the input gear? How many of those teeth are potentially damaged? 4. Please explain or interpret the phase readings below. They were taken with a tachometer as shown by the arrow. [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 77 SYSTEM DYNAMICS 1. In a simple mass spring system: If the stiffness of the spring increases, the natural frequency goes (up/down). If the stiffness decreases it goes (up / down). If the mass increases it goes (up/down) and if the mass decreases it goes (up/down). The three types of damping are _____________________, ______________________, _____________________. A bicycle pump produces which type of damping when you press down on it? When a mass bounces up and down on a spring, the spring heats up - this is the result of which type of damping? True or false, the magnetic field in a motor provides damping? Damping is proportional to (acceleration, velocity, displacement). Please provide an example of this. Force is proportional to (acceleration, velocity, displacement) Stiffness is proportional to (acceleration, velocity, displacement) 2. A mass on a spring is pulled down and let go. Match the descriptions to how it behaves if the system is (A) critically damped, (B) over damped, (C) under damped or (D) has no damping. It will very slowly go back to rest as if moving through honey critically damped It will bounce up and down forever over damped It will go back to rest at “normal” speed and then it will stop under damped It will bounce up and down a number of times before coming to rest has no damping What will the value of ζ be in each case? Damping has a (small/large) effect on the natural frequency. [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 78 CAT III Worked examples 3. Imagine you are holding the spring mass system in your hand as in the figure. You slowly move your hand up and down. Describe what happens as you gradually increase the speed of your up and down movement by filling in the blanks below. The spring mass system has a single frequency that it likes to vibrate at called its ___________________. When you move your hand slower than that frequency, the mass moves (in/out of) phase with your hand, and the spring behaves as if it is (rigid/flexible). The amplitude of displacement of the mass is (less than, equal to, greater than) that of your hand. As you speed up the movement of your hand, the (amplitude/phase) of the mass begins to (lead/lag) that of your hand and the spring becomes more (rigid/flexible). When you move your hand at exactly the same frequency the mass and spring likes to vibrate, the system is said to be in ________________ and the phase ________________ by _______________ degrees and the ____________ increases, perhaps significantly, although it depends on the amount of _______________. As you continue to speed up past this frequency, the phase ________________ until it reaches ________________ and the __________________ goes down. This entire process can be described in a _________________ or _________________ plot. [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 79 4. This graph below is called a __________ or ___________ plot. The numbers 5445, 6035, 7927 refer to what? How would you describe what has happened in this curve between 500 and 7927? What is the big picture of what is happening? How have the amplitude and phase changed? (please be specific) 5. What is the name of the plot below? Between the curves A and B, which has more damping? How can you tell? What is the approximate natural frequency? How can you tell? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 80 CAT III Worked examples 6. Please define the terms “high spot” and “heavy spot’ When balancing a rotor, does one attempt to place a trial weight opposite the high spot or heavy spot? When using a proximity probe to measure vibration displacement and another proximity probe as a Keyphasor, is the phase reading relative to the high spot or heavy spot? Which one lags as one approaches a critical speed? In the plot below, ignoring other sources of lag, how far is the heavy spot from the high spot at 5922 RPM? Please name three other sources of phase lag. 7. Describe the meaning of an MDOF system and how it relates to natural frequencies and SDOF systems. In a complex structure like a bell, each natural frequency coincides with a form of movement called a _____________ that includes points called ______________ that do not move and other points called _____________ that move the most. Each natural frequency can be defined in terms of _______________, _______________ and ________________. If structure like a diving board (below) was in resonance, would you expect it to vibrate the same amount in the vertical and horizontal directions? Explain your answer. [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 81 ODS, MODAL ANALYSIS AND RESONANCE 1. True or false: An ODS is primarily performed to understand the natural frequencies of a structure? True or false: An ODS is performed with the machine running or the structure vibrating. True or false: A two channel frequency based ODS can be performed on a machine that is changing speed during the test. In the image below, RMS overall values were taken as shown. What can you assume about the movement of this beam? 2. What test would you perform to animate the deformation of an airplane hull as the plane takes off? What sort of test equipment would you need? What specific data would you collect? How could you predict the natural frequencies and modes of an airplane hull prior to building the airplane? If you wanted to animate the movement of machine with an angular misalignment, what tests would you take to do so? What equipment would you need? 3. True or false: Modal analysis can be performed with the machine running? When you hit a drum with a soft mallet it makes a different sound than when you hit it with a wooden drum stick. Please explain why this is and what is happening. What is a common cause of bad data in a calibrated hammer test and what data plots does one look at to see if the problem occurred? Please explain the calibrated hammer test methodology including data collector set up options (windows, averaging, triggers, gain settings etc), graph displays etc. [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 82 CAT III Worked examples 4. You have done a simple ODS on the beam in the image below measuring RMS overall levels as shown in the image. Does this look like resonance problem? If so, which mode is excited? What is one way to modify the structure to solve the problem (be specific)? 5. When a centrifugal pump driven by a motor on a VFD is operating at 1370 RPM it produces considerable vibration. When you look at the spectrum you notice the vane pass frequency peak is very high in amplitude and it has a small haystack beneath it. The same peak is much lower when operating at different speeds. Is this resonance? If so, what are two possible solutions? What is another possible and common explanation for this behaviour? 6. The graph below was supplied with a resilient isolation mount. There was also a note stating the natural frequency of this isolator is 10 Hz and ζ = 0.2. If I place these beneath my machine, what effect will they have if my machine is running at: a. 1 in/sec @ 2 Hz b. 1 in/sec @ 10 Hz c. 1 in/sec @ 15 Hz d. 1 in/sec @ 30 Hz At which frequency does the mount provide the best isolation ? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 83 RESONANCE TESTING 1. Does one typically look for resonance problems when conducting one’s monthly routes? List three or more indicators of a possible resonance problem that do not require special tests or measurements to detect. Please describe in detail how to do a bump test. Include details about how to configure the data collector: Fmax, LOR, average type, window, gain settings etc. Also describe how to bump the structure and what to bump it with. What is negative averaging? Describe 3 ways to do a run up or coast down test and the pros and cons of each. 2. What are the differences between cross channel phase measurements and phase measurements using a tachometer or keyphasor? What is the main difference between the FRF used in a modal analysis test and the transmissibility measurement used in an ODS? Coherence is measured from ______________ to ______________ and the reading should be above _______________ at frequencies of interest. Causes of low coherence include: (name at least 3) Is averaging used when doing a calibrated hammer test and or when measuring coherence? Please explain. What is “linearity” and “non linearity”? 3. The plot below is an accelerometer calibration curve. Please describe what this curve means, how it was measured, what the axes are and what is happening on the left side where the curve ramps down and on the right side where it gets higher. What is the name of this measurement? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 84 CAT III Worked examples 4. The plot below is a coherence measurement on a steel beam. What might be the cause of the low value marked with the arrow? How is the coherence at the other frequencies? ROLLING ELEMENT BEARINGS 1. If the BPFI is 4.9x, what is a good Fmax setting (in orders) for a demodulation or PeakVue plot? 2. What are some benefits of using a high frequency bearing test like PeakVue or demodulation? What are some things to be careful about? 3. I look up a bearing in a database and it says BPFI = 4.9. The bearing is installed in a belt driven fan. The fan shaft turns at 1.3X. What pattern will a fault on the inner race create in the spectrum (draw it)? 4. I see the following peaks in an AC motor with rolling element bearings: 4.56x, 4.9x, 5.24x, 9.8x. What fault might this be? 5. You are looking at a spectrum in acceleration from a centrifugal pump with rolling element bearings being directly driven by a 4 pole AC induction motor. You notice a hump in the spectrum around 90,000 CPM with peaks on it separated by about 3.2x. You place your harmonic cursor at 3.2x to see if these peaks are harmonics of 3.2x but they do not seem to be. Please describe the most likely cause of this pattern in the spectrum and describe these peaks and why they appear where they do. What would this look like if you demodulated it using a high pass filter of 70,000 CPM? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 85 JOURNAL BEARINGS 1. Based on the image below, what is the direction of shaft rotation? What is the precession of the orbit? Is this a direct or filtered orbit? What is the phase of the X sensor compared to the Y sensor? What is the approximate shaft rate? Which probe has a higher amplitude? Does this machine have an obvious fault? 2. Please describe “oil whirl” in a journal bearing. What are some of the causes of oil whirl? What frequency does it occur at? What does it look like in the spectrum? What does it look like in an orbit or a live orbit? 3. A 200 mV/mil prox probe with a set point of -10V has moved 5 mils closer to the shaft. How much voltage will the probe put out? If it remains in this position will the probe put out an AC or DC voltage? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 86 CAT III Worked examples 4. What is slow roll glitch removal? Do proximity probes work differently with different shaft materials? Explain. Is the image below a filtered or unfiltered orbit? 5. The plot below is measured using the (AC/DC?) signals from (how many?)_________ proximity probe(s). The shaft is rotating (clockwise / counter clockwise). The outer edge of the plot is given a value of _________ and the center of the plot is given a value of _________ ? The numbers along the curve running up from the bottom are _________________. This plot is used to show ____________________________. [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 87 6. Match the orbit patterns to the faults: 1x filtered orbit - the orbit gets bigger Misalignment 1x filtered orbit - the orbit gets flatter, more elliptical 1x filtered orbit - the orbit is a big circle filling the Oil whip bearing clearance with lots of keyphasor dots 1x filtered orbit - the orbit has a flat spot or Unbalance looks like a tear drop 2x filtered orbit - figure 8 or banana shape Oil whirl Live orbit - 2 Keyphasor dots that don’t rotate Live orbit -2 Keyphasor dots that rotate in reverse precession Rub ELECTRIC MOTORS 1. A four-pole motor is driven by a VFD set to 39 Hz. The motor has a slip frequency of 20 RPM. There are 42 motor bars. What is the motor shaft speed in RPM? At what frequency in Hz and orders will the 2xLF appear? Where will the motor bar frequency and its sidebands appear (in Hz and orders)? 2. A four pole AC induction motor, not on a VFD, has been observed to slow down a bit when under heavy load and to speed up a bit when unloaded. Please explain why this is happening from the point of view of electromagnetism. 3. A six pole AC induction motor is being operated with a 50 Hz line frequency. It has a slip frequency of 20 RPM. What is its actual speed? What is the pole pass frequency (PPF)? If I have an Fmax of 1000 Hz, 800 LOR and a Hanning window, will I be able to see pole pass frequency sidebands around1x? If there is not enough resolution what motor faults might you miss? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 88 CAT III Worked examples GEARBOX ANALYSIS 1. A two-stage gearbox is driven by a 4-pole AC Induction motor. It has 73/57 teeth on the first stage and 43/33 on the second stage. Please compute the following in orders: GMF1, intermediate shaft speed (S2), GMF2, output shaft speed (S3) The gear is driving a pump with a 22312 bearing in it with an inner race fault. How will this fault present itself (specifically)? 2. Time synchronous averaging (TSA) is useful in complex gearboxes because? If my gearbox has an intermediate shaft that rotates at 3.7x but I can only place a tach on the input shaft, can I do TSA? Explain. If I have a defective rolling element bearing how will it show up in a TSA test? 3. A 1500 RPM motor is driving a centrifugal pump with 7 vanes via a gearbox. The input gear has 42 teeth and the output gear has 27 teeth. What is the gear assembly phase frequency? If the gear has this wear pattern what pattern will appear in the spectrum? How many teeth will eventually be damaged on each gear? What is the hunting tooth frequency? If given the choice would you buy this gearbox or one with 43 and 29 teeth if they cost the same? Please explain your answer. 4. Please describe the pattern labelled Gnf in the graph below. Explain the sidebands around Gnf. What other vibration tests (besides the FFT) could detect this pattern? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 89 5. A 1500 RPM motor is driving a centrifugal pump with 7 vanes via a gearbox. The input gear has 42 teeth and the output gear has 27 teeth. Select a setting for Fmax and LOR that will give you three harmonics of the gear mesh in the spectrum and 5 rotations of the input shaft in the TWF. How many rotations of the output shaft will be visible in the time waveform? With these settings, how many harmonics of the vane pass frequency will be present in the spectrum? If the motor runs at 1440 RPM, will there be enough resolution (with a Hanning window) to separate 4x and 2xLF? Will this be good test setup for finding fundamental bearing tones? 6. The circular plot below is from the input gear of a gearbox. The input shaft rate is 1x. How many teeth are damaged in this gear? Apart from the gearmesh frequency peak, what other pattern might this fault produce in the spectrum? What may have caused this wear pattern? BALANCING 1. You have to place a balance weight of 10 grams @ 15 degrees but you only have fan blades at 0 and 45 degrees. How much weight should go on each blade? 2. If you placed a trial weight at a radius of 100 cm and you calculated a balance solution of 10 grams @ 45 degrees, how much weight would you place at 45 degrees if you had to place it at a radius of 10 cm? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 90 CAT III Worked examples 3. A vector describes an _____________ and a _____________. 4. Please draw the vector 4 mils @ 90 degrees 5. (You will not be asked to add or subtract vectors on the exam but it is important to understand this if you will do balancing). Please add 3 mils @ 0 degrees to the vector above and calculate the solution graphically and mathematically. 6. Please describe the significance of the “O”, “T” and “O+T” vectors in the diagram below. Which of these are measured and which are calculated? Describe what a “trim” run is. [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 91 7. When balancing a rotor in-situ (in place), if the trial weight is too small (what happens?)________________, if it is too large (what happens?) _________________. The trial weight should generate a force equal to approximately ______% of the rotor weight and it should change the original phase and amplitude readings by ______%. 8. Wide rotors often suffer from ____________ or ____________ unbalance and they therefore require a __________ plane balance solution. 9. The amount of unbalance remaining in a rotor after it has been balanced is called its ________________ unbalance. 10. Describe the two general types of balance standards. What are they based on? What are the benefits of each type of standard and why do both types exist? 11. The chart below is being used as balance criteria for a fan on resilient mounts. It is classified as BV-3. After you attach a trial weight it has a 1x amplitude of 9 mm/s RMS. What is your recommendation? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 92 CAT III Worked examples 12. An inexpensive HVAC fan is operating in a clean room near some very high precision process equipment that is used to print computer chips. You have been asked to choose a balance standard for this fan. Would you select one based on residual unbalance or vibration amplitude? Please explain your answer. [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 93 SETTING ALARMS 1. An acceptance test is being performed on a fan classified as BV-3. The fan is mounted on isolators and the 1x amplitude at normal operating speed is 5.5 mm/secRMS. Is this acceptable? 2. Use the table from ISO 10816-7 to answer the following questions. A new 1800 RPM centrifugal pump with a power rating of 100 KW is running at 50% of its Best Efficiency Point (BEP). It is a non critical pump (Category 2). It has a measured overall level of 3.5 mm/secRMS. What should you report? When operating at 90% of BEP, the same pump as above has an overall level of 3.0 mm/secRMS. If you want to use this value as a baseline and trend the readings from here. How will you set the alarm level? Note that in this standard, a change from baseline should not exceed 25% of the upper limit of zone B. Following from the previous question, what would you report if the pump was tested again at 70% BEP and the reading was 3 mm/secRMS. What if it was tested at 90% BEP and the reading was 4.3 mm/secRMS? 3. Using the ISO 10816-8 Acceptance criteria for a reciprocating compressor chart. Make a recommendation based on the following measurements on a new horizontal compressor. The compressor has been installed in your plant and is being tested in place under normal operating conditions. If there is a problem, please suggest a recommendation or follow up test. Foundation readings on each corner 1.5, 2.2, 3.1, 2.7 mm/secRMS Frame readings (top): 4.3, 4.7, 5.5 mm/secRMS Cylinder lateral: 5.5, 6.2, 4.9, 7.3 mm/secRMS Cylinder rod: 8.3, 11.3, 9.2, 12.4 Piping max level 33.7 mm/secRMS [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 94 CAT III Worked examples 4. What is the name of the alarm displayed below? What is a limitation of this type of alarm (please describe a limitation based on this actual data). What other common type of alarm would solve this problem? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 95 FAULT DETECTION 1. The plot below is from a pump with sleeve bearings being driven by a motor via a flexible coupling. The pump shaft rate is marked with the arrow. Please describe the pattern shown in the plot and the most likely cause of it (i.e. mechanical fault condition) 2. In the order normalized plot below from an 1800 RPM induction motor, which peak is two times the electric line frequency (2xLF)? Assuming the motor is tested at the same speed and load, what might it mean if the amplitude of this peak goes up? 3. In the plot below from a 1500 RPM centrifugal pump with rolling element bearings driven by a motor via a flexible coupling, what is the most likely source of the peaks labeled “A” and “B”? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 96 CAT III Worked examples 4. What is the most likely source of the peaks marked with the arrows “A” and “B”in the order normalized plot below, from a 3000 RPM induction motor with rolling element bearings? 5. What is the most likely source of the peaks marked with the arrows in the order normalized plot below, from a 3600 RPM axial flow fan with rolling element bearings driven by an induction motor via a flexible coupling? 6. Structural looseness and structural weakness produce the same pattern in the vibration spectra. Please describe this pattern. What is the difference between structural looseness and structural weakness? How would one resolve each of these problems? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 97 7. The order normalized plot below is from an 1800 RPM induction motor driving a centrifugal pump via a flexible coupling. The pump has 13 and 19 vanes on the first and second stages respectively. Identify the pump vane related peaks in the spectrum. Identify the motor bar related peak(s) in the spectrum. How many poles does this motor have? If the motor is running at 1760 RPM, what is the slip frequency? What is the pole pass frequency? 8. Describe the benefits of using a Log or dB amplitude scale on a vibration spectrum. DATA ACQUISITION 1. A 200 mV/mil proximity probe has a set point of -10 V. If the shaft moves 5 mils closer to the probe, how much will the voltage reading change? 2. The DC voltage output from a proximity probe relates to _______________ and the AC voltage relates to ________________? 3. A proximity probe pointed at a keyway in the shaft can be used to measure ________________ and _________________? When used in this way it is called a _____________________. 4. When analyzing data from a pair of proximity probes mounted 90 degrees apart from each other on a turbine journal bearing, the graph one typically analyses is called ____________________. [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 98 CAT III Worked examples 5. The plot below is produced from (AC or DC?) voltages from the proximity probes? The outer edge of the plot has a value of __________ ? How does this relate to the shaft’s position on the bearing? The arrow in the top left corner of the plot indicates ___________________. How is this plot is measured? What do the numbers along the line towards the bottom of the plot refer to? 6. Please list at least 4 common faults or conditions that are common in large journal bearings that can be detected using vibration analysis. 7. Name another technology that can be used to find early indications of wear in the bearing and another technology that can be used to detect late stages of wear. 8. A large journal bearing can go from perfect health to catastrophic failure in approximately how much time? Therefore one typically employs a _____________________ system to monitor these bearings. 9. A 100 mv/g sensor is being subjected to 5 g’s of vibration within its linear range of operation. What voltage output will be read from the sensor? 10. Please match the accelerometer sensitivity (10 mV/g, 100 mV/g, 500 mV/g ) to the application: A high speed compressor with high amplitude high frequency vibration 10 mV/g A low speed bearing 100 mV/g A typical 1800 RPM centrifugal pump and motor 500 mV/g 11. What is a piezovelocity sensor and why might someone use one? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 99 12. Describe what is meant by the “frequency response” of an accelerometer or a sensor mounting technique. 13. Which sensor mounting technique would have a better high frequency response: 2 pole magnet on a painted surface or flat magnet on an installed target pad? Apart from frequency response issues what is another important benefit of using test target pads? 14. Please select the bearings to monitor in the machine below as well as the measurement axes. There is more than one correct answer, so please explain why you have chosen the test points you have chosen. 15. Please describe the benefits of creating a machine test guide for each machine. What key pieces of information should be documented on the test form and why? [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 100 CAT III Worked examples Charts, tables and formulae N = Number of samples, Ts = Sample period, T = Length of time waveform, Fs = Sample rate, Fmax = Maximum frequency measured, LOR = Lines of resolution, R = Spectrum resolution [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com CAT III Worked examples Page 101 [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com Page 102 CAT III Worked examples [email protected] Copyright ©2020 Mobius Institute www.mobiusinstitute.com