EMAIL TIPS A Learning Publication from Full Spectrum Diagnostics
Vol. 88 November 2012
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This Month’s Features: Signal Processing: Windowing Functions The Hanning Compromise Self-Windowing Functions The Amplitude Advantage The Transducer: Where the Measurement Starts
2012 Training Schedules CORE TRAINING SERIES: ASNT: Intro (IVA) – VAI – VAII – VAIII ISO: Intro (Cat I) – Cat II – Cat III – Cat IV CONCENTRATED TRAINING TRACK: • Concentrated Time Waveform • Concentrated Spectrum Analysis • Concentrated Phase Analysis • Concentrated Rolling Element Bearings & Gear Analysis Concentrated Courses Earn ASNT VAIII & ISO Cat IV Hours
SPECIALTY & ADVANCED TRAINING • ODS/Modal – Precision Balancing • On-site Mentoring
Another Look at the Cover: The Signal Processing theme of this month’s Tips focuses on Window Functions, thus the window images. The signal processing Window Function actually pass-band shape is better represented as a door or archway (below). The shape determines how the data is represented in the frequency spectrum, more specifically if the emphasis is attaining better frequency accuracy, better amplitude accuracy, or a compromise between the two.
SIGNA AL PROC CESSING G TIP 1 Most commercial c instrument setups forr monthly conditio on monitorin ng will includ de several Windowing W Functio ons that are used to en nhance or modify m the raw tim me waveform m. The use of the “Han nning” or “Hann”” window re educes mea asurement le eakage in the FF FT spectrum m by forcin ng non-perio odic time sample es (blocks) to o “appear” pe eriodic. A “periodic” time domain d bloc ck will begin n and end with a relatively co omplete sinu usoid. The waveform will be essentially e z zero amplitud de at the beginning of the time block and d the end of o the time block. In real-life e applications, this is a rare r event. When the time bllock is term minated and d usually in ncludes a fraction n of the next waveform m, resulting in some signal distortion. This distorttion is crea ated when perform ming the FFT T on the wa aveform, and d is found in the re esulting freq quency spectrum. Visuallyy, the disto ortion on th he dominantt periodic peaks in the data will w include a broader prrofile often describ bed as a “w wide skirt” in nstead of the cleaner distinct peaks in th he spectrum.. When the distortion (also known k as “leakage”) is significant, the skirt can ma ask modulation signature es in the datta such as closely spaced side ebands.
Figure 1.0 1 The Hanninng Window “Taaper” affect on the t TWF
wing is an n essentiall signal processing p Window method d for steadyy-state vibra ation trending. Proper implem mentation of the Hannin ng Window creates a much cleaner freq quency spe ectrum with reduced leakage e distortion. It is known k as the best compro omise window w function. Addition nal window options can be selected d for other and vibration steady state non-steady measurrements. Th he Rectangu ular (Uniform m) window is recom mmended fo or transient data d collectio on, where amplitu udes are nott as important as good frequency f responsse. The Flat Top window w is recomm mended for Proximity Probe dissplacement measureme ents. Here the freq quency is kn nown, but mo ore precise amplitude is required. These e windows are a selected d for their specificc “Frequen ncy” or “A Amplitude” emphasis capabilities, respecctively.
Figure 2.0 2 The Hanninng Window “Taaper” affect on the t FFT
aveform is The images to the right show how the wa modifie ed for the Hanning Window, W the resulting frequen ncy spectrum m as comp pared to the e Uniform and Fla attop Windo ows, and the e Pass Ban nd shapes for each h. The Passs Band is the e effect of th he chosen window w weighting function f on each e line of resolution in the FFT specttrum. The e Pass Ban nd shape determines the frequency and/or amplitude accuraccy of each “w window” function. Figure 3.0 3 Rectangulaar / Hanning / Flattop F Pass Baand Shapes
SIGNAL PROCESSING TIP 2 Windowing is not limited to Condition Monitoring (PdM) programs performed on steady state operating machinery, but can also be used for transient type analysis. Natural Frequency testing via impact methods is traditionally performed using a “Uniform”, “Rectangular”, “Boxcar” or “None” type window function. All of these trade names are synonymous with what is known as a “Self Windowing Function”. If a window is self windowing the entire transient “event” is contained or captured within a single time block and generates no leakage distortions in the resulting frequency spectrum. This type of window is not the first choice when the analyst is concerned with amplitude accuracy in the measurement. Since a natural frequency test is all about frequency, we need a window function emphasizing this attribute. The self windowing function is superior for determining precision frequency accuracy.
Figure 4.0 Force Window for Impulse Testing (Input)
Natural frequency testing requires a couple of other windowing functions that are helpful in data collection, especially in noisy (background) conditions. The Force Window is applied to the Hammer Input (Impact, Impulse) to limit background noise. The size of the impulse time block is considerably larger than the actual duration of the input pulse. A bandpass window addresses this by applying a smaller window around the impulse and attenuating the noise both before and after the transient event. A Force Window of +/- 10% is commonly set to reduce these noise sources. On the output side, an Exponential Window is applied to the accelerometer response to ensure that the “event” is fully contained within the time block. Remember, a “self-windowing effect” is ensured if the entire event is contained within the time block specified for the analysis. The exponential window attenuates the amplitude of the acceleration response to artificially force the waveform to zero. The drawback is the addition of a false “damping” effect in the data sample. Again, the amplitude is distorted, which is of little interest, while allowing an unaltered frequency response to pass.
Figure 5.0 Exponential Window for Impulse Testing (Output)
Figure 6.0 Input / Output Time Waveform Profiles
SIGNAL PROCESSING TIP 3 The Flattop Window is a pass band shape that is applied to the time block samples that functions to reduce the potential for amplitude error in the resulting frequency spectrum. The drawback with the use of this window is that the frequency accuracy is significantly limited. This seems a little backward in the world of vibration condition monitoring and machinery diagnostics. The niche here centers on the special machine application and particular transducer used to detect these faults. The special application is in diagnosing problems in fluid film bearings where there is a “thin” line between what is normal and a potentially unstable operating condition. The stable operation of a fluid film bearing requires the establishment of a thin oil film wedge to support the rotor. When we say “thin” here we are talking about detecting extremely small displacements in the bearing on the order of several mils (thousandth of an inch). To accurately detect displacements at this level, our measurement capability must be an order of magnitude better. An accelerometer attached to the bearing housing at relatively large distances from the shaft is not going to cut it! These conditions call for a specialized transducer also known as a Displacement Probe, Proximity Probe, or Eddy Current Probe that is installed through the bearing housing and targeted at the rotating shaft.
Figure 7.0 Locus of Orbit Measurement Points
Figure 8.0 Filtered Orbit at 1x RPM
The Flattop Window de-emphasizes what is “known” (i.e., the rotating speed of the shaft) and emphasizes the “unknown”, in this case the very small amplitude shaft displacements as it rotates on the oil film. The flat pass band filter shape ensures an amplitude accuracy variation of -0.1 dB, or approximately 1%. Critical displacement levels are closely associated with bearing internal clearances. An operating displacement level in excess of 50% of the bearing diametrical clearance indicates a fast approaching instability threshold. The correct transducer combined with appropriate signal processing is the key to successful monitoring of fluid film bearing applications. Figure 9.0 Proximity Probe Setup for Orbit Analysis
SIGNAL PROCESSING TIP 4 Signal Processing begins with the transducer. Proper selection of the appropriate transducer for your application requires knowledge of the type of machine, bearing design, and the rotating speeds involved. Some low-frequency transducer limitations should be reviewed before analysis begins. Let’s start with the accelerometer. All accelerometers used for vibration analysis, regardless of sensitivity, will have a low-frequency end “roll-off” where potential low-speed machinery faults can lie undetected. When the transducer signal is integrated to produce a “velocity” amplitude parameter, the low frequency end can become more distorted due to integration “noise”. Exactly how good are various sensitivities of commercially available accelerometers?
Figure 10.0 Transducer Frequency Range Summary Table
The “standard” 100 mv/g accelerometer is good to 0.5–3.0 Hz (30-180 CPM). Increased sensitivity of the 500 mv/g and 1000 mv/g units can push the low end to 0.3-1.0 Hz and 0.2-0.6 Hz, respectively. The super sensitive 10,000 mv/g accelerometers can get you to 0.07-0.1 Hz (4.2-6.0 CPM), but can be saturated by higher frequency sources. See Figure 10.0. Accelerometer choices cover a pretty broad range. Velocity transducers can eliminate any integration noise, but they have similar low frequency end limitations. Velocity transducers are mechanical instruments and will include a higher end response limitation that is well below the accelerometer maximum response range.
Figure 11.0 Potential Integration Noise Problems in Low Speed Applications
The selection of a non-contact Proximity or Eddy Current Probe can drop the frequency response all the way to DC (0.0 Hz), but may include some amplitude sensitivity problems. They also require access to the shaft and include setup and calibration for best results. Another transducer alternative is a surface mounted Strain Gage. These units will allow measurement to DC (0.0 Hz) and produce enough sensitivity to detect the faintest of signals. Obviously, not the transducer of choice for any application; but can make all the difference in very low-speed applications with minimal vibration response. There is always a transducer for the job. Sometimes the analyst must think outside the box for creative answers. Figure 12.0 Low Speed Enhancement with 10 v/g Transducer
REAL DATA: The Odd and Uncommon www.fullspec.net Much of Vibration training and certification centers on vibration spectra and data that is “canned” or “cartooned” to make a point or convey a message. Full Spectrum Diagnostics shares this simplification of data in many presentations and publications. Let’s face it this generalized formatting is essential to understanding the basics of vibration response. We also must face that your data may not look like my data. Variations abound. This new column is dedicated to the presentation of the odd and unusual in machinery vibration. Some of these examples cannot be explained. It seems sometimes that the more you know, you realize how little you know. Our first item is a vibration signature from a 2-pole AC Induction motor that I have never seen before. The data came from an unloaded test on a motor manufacturer test bed. The motor particulars are
found in the signature below and included the 1x rotational speed at 3,578 RPM, the 2x response at 7,156 CPM. The 2x Electrical Line Frequency is present as well. All amplitudes are very low, thus considered acceptable. What was unusual were the undefined 4,588 CPM and 6,146 CPM peaks. Rotor Speed: 3,578 RPM 2x LF: 7,200 CPM Rotor Bar Count: 40 Stator Slot Count: 48 Rotor Slip: 21.04 CPM FPOLE: 42.08 CPM FSLOT: 1,010 CPM
The "Stator Slot" Frequency (FSLOT) is defined as the number of stator slots times the rotor slip. This was not a found as a "stand-alone" frequency. It did not directly match either of the "unknown" frequencies, but it could be added to 1x RPM and subtracted from 2x RPM (as a sideband). The frequencies produced were found to be an exact match for the non-synchronous faults, 4,588 & 6,146 CPM! FSLOT: 1,010 CPM 4,588 CPM = [ Rotor Speed + FSLOT ] 6,146 CPM = [ 2x Rotor Speed - FSLOT ]
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THE VIBRATION FAULT GUIDE The Vibration Fault Guide pocket reference was first introduced in January of 2005. Since 2005 weâ€™ve averaged sales of nearly 1000 units per year! The guide can be purchased in singles or bulk sets, as requested. Many customers order units in quantities of 100-1000 and opt to personalize the front and back covers with their logo and company contact information. They are used as training aids and distributed with their products as a part of marketing campaigns. No matter whose name & logo appears on the cover, itâ€™s all Full Spectrum Diagnostics know how on the inside! The 110 pages define over 40 vibration related faults, signal processing formulas, severity charts, and alarm band profiles. A handy glossary defines 50 common vibration terms Order now at: http://www.fullspec.net/store.html Or by Email @ firstname.lastname@example.org Or by Phone @ (763) 577-9959
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THE VIBRATION TECHNIQUES GUIDE The Vibration Analysis Techniques Guide is a 108-page pocket sized information treasure trove. Information on dozens of analysis techniques, specifications and data presentation formats are included. If you liked the Vibration Fault Guide, your next educational step should be the Vibration Analysis Techniques Guide get yours now at:
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THE VIBRATION ANALYSIS WALL CHART The Vibration Analysis Wall Chart is a 46” x 36” Full-Color Laminated Reference for your drab office wall. The overall Alarm charts in the center of the chart are surrounded by groupings of over 40 dominant rotating machinery faults. The fault groupings include frequency content and dominant directional response that effectively allow the analyst to “narrow-down” the potential sources and zero-in on current vibration problem. When combined with the Vibration Fault Guide, the Vibration Analysis Wall Chart completes the diagnostic analysis loop. Weather you require one chart or a need to wall-paper your office, we can help!
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THE VIBRAT TION ANA ALYSIS PERIODIC P C TABLE Thiss Full Color Laminated L 13 x 17 inch card-stock table provide es a “quick-lo ook” method d of distinguishing one machinerry fault from another and d suggests Diagnostic D T Tests or forrmula that may be use ed to build a case and make m the call! Thiss Chart is a “Logical” “ ana alysis tool th hat classifiess vibration prroblems by Frequency F C Content and Directional Response. The potentia al vibration sources s are instantly red duced based d on the analyst’s a current measu urement data a. The Table e “forces” the e user to think logically and a classify faults acccordingly. Indivvidual Faultss are Foot Note Referen nced to the Vibration V Fa ault Guide fo or a more de etailed revie ew. If you th hink the Vibrration Faultt Guide is va aluable, you ur next educa ational step should be th he Vibration n Analysis Periodic P Ta able! Get yours now at: a www.fulllspec.net, or o by phone at a 763-577-9 9959
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Full Spectrum Diagnostics’ Lead Instructor & Seminar Author is Dan Ambre. Dan is a graduate of The University of Iowa with a Bachelor’s degree in Mechanical Engineering, and has completed additional graduate level course work in Engineering Dynamics from The University of Illinois at Chicago, and Florida Atlantic University. Dan is a Certified Vibration Analysis Level III Instructor (Category IV) with over 16 years of Vibration Training and Certification Experience. Dan’s 25+ years of vibration experience in the Aviation & Aerospace Industries comes from positions at Sundstrand Aviation Corporation and Pratt & Whitney (United Technologies Corporation). This fieldwork includes Vibration & Acoustic testing, Rotor Dynamics analysis of high speed Rotor Systems, Experimental Modal, and Finite Element Analysis. His consulting experience base comes from positions at Technical Associates and Full Spectrum Diagnostics, which he founded in June of 2000. He is a Registered Professional Engineer in the States of North Carolina and Minnesota.
In 2004, after years of close association, Louis G. Pagliaro joined Full Spectrum Diagnostics. Lou fulfills multiple roles as our Seminar Sales Coordinator, Senior Instructor and course content co-author. Lou is a certified Level III Vibration Analyst (since 1996) and was recently re-certified by American Society of Nondestructive Testing as an ASNT PdM Level III in Vibration Analysis (Category IV). He has over 30 years of varied industrial experience, including Maintenance Management, Vibration Analysis, Vibration Training, Certification, and course development in areas of Noise Control, Precision Maintenance, Precision Alignment, Preventive Maintenance, and Maintenance Skills Enhancement. His worldwide teaching credentials include instruction of TAC, Update International, CSI, Entek and SKF customers in eight countries. Louis is a graduate of Niagara College in Welland, Canada. Lou can be contacted by phone at (704) 577-3953, or via Email at Lpagl@aol.com.
Dan can be contacted by phone at (612) 875-9959, or via Email at ModalGuy@aol.com
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Refinery Compresso C or & Founda ation
Tips include Signal Processing and Transducer topics