EMAIL TIPS A Learning Publication from Full Spectrum Diagnostics Vol. 88 November 2012

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EMAIL TIPS Volume 88 November 2012 PDF Format Available by Request

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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

4

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• Concentrated Time Waveform • Concentrated Spectrum Analysis • Concentrated Phase Analysis • Concentrated Rolling Element Bearings

& Gear Analysis

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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.

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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.

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 band-pass 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 4.0 Force Window for Impulse Testing (Input)

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.

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 7.0 Locus of Orbit Measurement Points

Figure 8.0 Filtered Orbit at 1x RPM

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? 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. 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 10.0 Transducer Frequency Range Summary Table

Figure 11.0 Potential Integration Noise Problems in Low Speed Applications

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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INSTRUCTORS:

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.

Dan can be contacted by phone at (612) 875-9959, or via Email at ModalGuy@aol.com

ModalGuy@aol.com

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.

LpagL@aol.com

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