When we talk about Condition Based Maintenance (CBM) or, Predictive Maintenance (PdM) as it is referred to in most circles, vibration monitoring is generally what immediately pops into everyone’s mind. There are other PdM technologies out there; there is thermographic imaging, temperature monitoring, oil analysis, partial discharge, impedance testing………..…..and just about anything where we can collect data on a routine basis and compare that data against similar components or trend over time, that activity can be considered a predictive technique. But all in all, when I begin discussions on PdM, it almost always ends up as a vibration monitoring discussion.
So in this blog we will discuss the technology of vibration monitoring. Why? It is my experience that vibration monitoring is the least understood PdM technology out there. It is a complicated science made even more complicated by the monitoring equipment manufacturers, vendors that sell the monitoring equipment and vendors that sell the training. There’s a lot of ‘Bro’ science out there as far as what the technology can and cannot provide and there are a number of autonomous (smart) systems out there where the designer(s) were a bit on the….…shall I say..….ignorant side. And the training, that’s yet another subject that’s reserved for future blog(s). What I generally find is that a majority of my clients (and of course, potential clients) have swallowed the Kool-Aid, hoping for miracles that only modern technology can bring. Yet astonishingly, wasting a ton of dollars on technology and training that just isn’t necessary for the equipment types found at their facilities. Yes indeed, the only one making money in most instances is the sales guy; most vibration monitoring implementations that are dependent on technologies just don’t pan out as expected.
Understanding The Technology……….
Whenever I interview a new client, I will always start out by presenting the following questions;
What is your motivation for a vibration monitoring program? What do you want this PdM technology to do for you?
The answer I expect but rarely receive is;
I want to monitor the dynamic condition of my rotating equipment and improve overall machine condition over time. This will improve my bottom line by minimizing the probability of failure. This is an investment in good maintenance practices.
The answer I generally receive and is a general disappointment is;
I want to find bad bearings.
If I were to receive the first answer then I can assume my client has a management style of ‘Preventing or Mitigating Failure(s)’ whereas when I hear the second answer, the client is in a reactive mode. At this point, he’s not looking at preventing failure or extending service life, he’s just looking at building a ‘schedule’ around equipment failure. And believe me, this (the second answer) attitude is predominant on an international scale, crossing all disciplines in all industries.
When CBM (PdM) monitoring goals are failure based, you only receive a partial benefit from the technology. With the limited goal of just detecting failure, you are only maintaining profitability through the management of production outages, e.g. you still have the same number of outages with the only benefit of fewer unplanned events. However, if you were to embrace preventing failure as a business goal, extending service life, then you can plan on a reducing the total number of outages. This goal improves profitability.
What I find interesting is that the technology sales guys, looking for that next major sale, are specifically looking for (preying on) those customers that are failure detection driven. Why? These are the customers that are generally………..technically challenged and…………lack experience in program implementation. WOW!!!! It is rare that I find a maintenance manager, maintenance superintendent or maintenance supervisor, or even a competent technician (a super technician), that is knowledgeable of AC signal processing, digital sound processing, can remember anything from 1st year calculus or Newtonian physics and be a ‘gear head’ that understands compliance based Quality Assurance practices at the same time. And the sales guys know this……………and can impress you into buying technology(s) that you probably will never understand, much less use.
The Technology……………
Once past the initial discussion of program motivation, I like to direct a client toward a general discussion of the technology. No matter what your facility goals are, failure prevention or failure detection, if you understand the following categorizations of the technology you can save a bundle on monitoring equipment and training. Then you can focus your resources on the program. Structure and discipline (the program) are required no matter what the goal; failure prevention or failure detection.
Before we get too far ahead of ourselves, please note that the following discussions are associated with equipment that utilize ‘rolling element bearings’ and where vibration is measured at the bearing cap. This discussion is not specific to machines that utilize ‘sleeve’ type bearings. With sleeve type bearings, we generally measure shaft vibration directly using a proximity probe or shaft rider attachment and accelerometer. However, even though the bearing type and measurement locations differ, discussions about the measurement technology still apply; most of the time anyway.
Vibration Monitoring Technology is broken down into two basic frequency ranges, Low Frequency and High Frequency.
Low Frequency Monitoring; 0 to 2 KHz - This range of the frequency spectrum is used to determine overall equipment condition. Specifically alignment, balance and structural deficiencies (including resonance) are identified in this range. These are the conditions that shorten bearing life, crack housings and cause ergonomic issues. Generally, however NOT necessarily, a thorough spectrum analysis within this frequency range is completed early in the machines life (e.g. start-up or acceptance) and any deficiencies corrected are a one-time effort. A quick check of overalls (summation of frequency components originally analyzed; commonly called broadband, filter out) should be completed semi-annually, after any major repairs or after any alterations are made to the machine or its associated system. This is to ensure any material changes do not inadvertently cause vibration problems. The thorough or full spectrum analysis need only be repeated when a large jump in the overalls are detected.
High Frequency Monitoring; 5 KHz to 40 KHz - There are several technologies specific to this frequency range with the two predominant commercial technologies being Shock Pulse Monitoring (SPM) and High Frequency Enveloping (signal demodulation). Both of these technologies are based on Pulse Theory or white noise detection. Monitoring this range of the frequency spectrum using these technologies is specific to identifying rolling element bearing defects and in some cases, may be applicable to gear train faults. Depending on the technology used, bearing faults can be detected as early as 12 months in advance of an actual audible indication. Once audible, shutdown should be immediate because continued operation is sure to cause secondary damage. Shock Pulse and simple Envelope Amplitude monitoring are very easy to train on, work well with formal lubrication programs and fit within most autonomous maintenance efforts. As a minimum, bearing condition should be evaluated by one of these technologies on a monthly basis.
So what kinds of devices are on the market than can monitor in these frequency ranges?
On the Low Frequency side there are two basic choices, a digital box or an analog device. There is a substantial difference in cost, in training and the experience required to use each. And one should also note that many, many, many of the existing standards that we use today for factory and field acceptance (our performance criteria) are based on the use of an analog device. They were never written or intended for the digital world.
Low Frequency Meters
Analog Vibration Meter; A Low Frequency vibration measurement device is basically a simple voltage meter. The meter uses op-amp(s) to integrate acceleration to velocity (1st integration) and displacement (2nd integration), e.g. (divide by 2πf). It also can provide Peak, Peak-To-Peak and RMS values. Generally the frequency range of the meter is limited to 1 to 2 Khz. Some of these analog based meters may include AC signal conditioning for High Frequency monitoring. This meter, based on its circuitry simplicity and limited display options, is very inexpensive and easy to train on.
Digital Vibration Analyzer; A digital (computational) device that digitizes an analog AC signal and then through software and firmware, provides a visual reproduction of that AC signal (time domain). And through Fast Fourier Transform (FFT), the device provides a visual representation of frequencies within a selected bandwidth (frequency domain). The device can be programed to provide Peak, Peak-To-Peak and RMS values and calculate the conversion between acceleration, displacement and velocity. The device can be used with various input devices (velocity, acceleration, displacement, microphone…..), virtually any AC voltage input. Some analyzers include signal demodulation (enveloping) circuitry, e.g. the device may include analog high pass or band pass filters plus a rectifier circuit as an AC signal conditioner for High Frequency monitoring. This meter, based on the sound boards that digitize the signal, processors, memory, display and generally proprietary software, firmware are very expensive (10X the price of analog devices). Also, any associated software is licensed to ‘each’ user. They also require a dedicated training effort (at least 40 hours) and many hours of experience before reliable understanding of the data can be made.
High Frequency Meters
Bearing Checkers; Two variations of Pulse Theory devices are Shock Pulse and High Frequency Demodulating (enveloping) devices.
• Shock Pulse; Again, a basic analog voltage meter that measures broadband background noise. And through a mechanically tuned accelerometer assembly (spring mounted accelerometer), the meter measures maximum voltages spikes caused by ball to race impacts (pulses) in a bearing. Noise (white noise, infinite vibration spectrum) from the ball impacts cause the accelerometer assembly to go into resonance, giving a momentary higher voltage indication. The resonance frequency of the accelerometer assembly (spring mass assembly) is approximately 32 Khz, well above the noise (vibration) floor of general equipment operation. (Natural Frequency (fn) = 1 / 2π √ K / M). This meter, based on its circuitry simplicity and limited display options, is very inexpensive (similar in price to an analog vibration meter) and easy to train on.
• Enveloping Device; High Frequency demodulating device; AM radio technology. Again, a basic analog voltage meter that measures the AC signal from a standard accelerometer. The analog signal is processed through a high pass filter and a rectifier. Generally the high pass filters on these devices only allow frequencies above 5 Khz to be monitored. Again, based on pulse theory, the high frequency noise is a result of ball to race impacts (pulses) in a bearing. Noise (white noise, infinite vibration spectrum) from the ball impacts is filtered, amplified and measured as voltage amplitude. This filtered frequency range is well above the noise (vibration) floor of general equipment operation. As the signal amplitude increases, a correlation between general bearing condition and noise (voltage) can be assumed, e.g. bearing failure. This meter, based on its circuitry simplicity and limited display options, is very inexpensive (similar in price to an analog vibration meter) and easy to train on.
Maintenance Concepts
Vibration Technology Summary Table
| |
Term
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Meaning
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Low Frequency
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Machine Condition
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High Frequency
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Rolling Element Bearing Condition
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Low Frequency Meter
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Analog and Digital
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High Frequency Meter
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Analog, Pulse Theory detectors.
|
Analog
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Less Expensive
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Digital
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More Expensive
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Analog
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Easy Training and Open Program Roll Out
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Digital
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Complex Training and Limited Program Roll Out (Licensing; User Restriction)
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Vibration Consensus Standards
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Written to Analog and Low Frequency, need interpretation for Digital. No standards available for High Frequency.
|
Now there are exceptions to these generalities but ‘mostly’ the devices listed are what’s offered in the world of handheld and installed monitoring today. There are software variations and many, many colors and flavors to choose from. You are only limited by your knowledge of the technology or……...whatever the salesman can convince you is needed to manage the reliability of your plant. Yes,…..…whatever the salesman can convince you is needed……....to be world class; maybe?
Decisions, Decisions…………..
If I can get my customer to this point, I can generally convince him that maybe, just maybe the salesman that presented him with a do it all chrome plated digital analyzer, with a single user license (one person, not one company) listed at $17,500, may not be the best place to start.
Maybe we should start out with a $1700 Low Frequency analog meter and maybe spend an additional $1700 on a High Frequency bearing checker. To kick off the program we’ll select business critical equipment to monitor and use an Excel Workbook to trend data. Before we even get into training and program development costs, I can show a 5X cost differential.
So at less than 5X the cost of a very basic Digital Analyzer, you can build a program that addresses both preventing failure and detecting failure. Although, if you still insist on a program just to find bad bearings then buy a two or three High Frequency bearing checkers, invest in four hours of training and an Excel Workbook to archive the data, and you're there; an open roll out (multiple users) bearing failure detection PdM program for under $5000. WOW!!! Less than a third of the cost of just one Digital Analyzer.
So with a just a minimal knowledge of the technology available, you can make decisions that benefit the bottom line. And with just few successes, documented successes, I guarantee you will want to grow into a failure prevention PdM program.
Gosh Darn; Maintenance, what a Concept!!!!
MMJennings
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