"Metal-to-metal contact sets off a ripple effect: A stress wave races through the metal components, causing the components to vibrate due to resonance".
How Much of a Risk Are You Willing to Take?
What are your goals? Do you want to know that a bearing may fail just days before it is likely to fail, with no prior warning? Or would you like to know that a bearing has been poorly lubricated, or has a minor defect that will develop into a major fault? With the techniques described in this article you could learn these things months (certainly weeks) before the bearing is likely to fail. With that extra time you could change the lubrication, order parts, organize the labor, and look for the best opportunity to perform the bearing replacement. The result is a safer plant with less downtime, less stress, and higher profits.
Brief Recap
In the previous article, a few important points were made that are pertinent to this article:
1. As bearings begin to fail, the vibration is very low in amplitude, and the frequency is very high (beyond your ability to hear, even with the best screw driver).
2. Simple spectrum analysis will not reveal the fault until it has developed to stage three, unless you take special precautions (listed later in the article).
3. To measure high frequency vibration you must mount the sensor correctly.
Beyond Your Hearing: Ultrasound
The ultrasound technique is very easy to implement. The measurement tool listens for very high frequency vibration and provides an indication of amplitude. It also amplifies the vibration and shifts (heterodynes) the frequency so that you can hear it through headphones. Therefore, you can listen for the telltale sounds of poor lubrication and bearing distress. When used correctly and appropriately, ultrasound instruments can be very complimentary to the other techniques described in this article.
What Are "Stress Waves," and Why Should I Care?
Before describing the next two techniques it is important to briefly introduce the concept of the stress wave (also known as shock pulse). Metal-to-metal contact sets off a ripple effect: a stress wave races through the metal components, causing the components to vibrate due to resonance. The stress wave is a very short-duration, low-amplitude, high-frequency wave. Every time the rolling elements roll over the damaged area on the inner and/or outer race (or as the damaged areas on the rolling elements contact the raceways), a stress wave will be generated. We can seek to detect that wave with techniques such as Shock Pulse, PeakVue, and SWAN (Stress Wave ANalysis, not discussed further in this brief article). The vibration that results can also be detected via the envelope method, and as the fault develops further, via the time waveform and spectrum.
There is one very key point you must be aware of: we are talking about very high frequencies, and as such the vibration sensor must be mounted correctly. Unless specifically designed for the purpose (e.g., Shock Pulse), a handheld probe is horribly inadequate. Even a two-pole magnet mounted directly to the machine surface is not adequate! All of the analyzer vendors will tell you, you must properly prepare the surface and use an attachment pad (or stud mount) in order to achieve the best results.
It is also important to note that there are other defects that will generate stress waves and high-frequency vibration, including looseness, gear wear, and cavitation. That can help us to detect those conditions, but it can confuse our attempts to detect bearing and lubrication faults.
Shock Pulse
The vibration sensors provided by SPM and PRÜFTECHNIK are designed to amplify (through resonance) high-frequency vibration (at approximately 35 kHz). As noted earlier, lubrication and physical defects (including wear/spalls) will generate vibration around this frequency. The vibration can be displayed as an amplitude to be trended, or a spectrum can be displayed in order to better understand the specifics of the defect: inner race, outer race, etc.
Spike Energy
The Spike Energy (units of gSE) technique aims to utilize the accelerometer's mounted resonance to amplify the high frequency vibration. However, in more recent years, the accelerometers provided have not been manufactured to have a repeatable resonance characteristic. What that means is that when you change your accelerometer, the amplitudes will change.
PeakVue
The PeakVue technique, developed by Emerson Process Management (CSi Division), is also designed to detect the stress wave; however, it is performed in a different way. The signal from the accelerometer is digitally sampled (converted from analog voltages to digital numbers) at a very high rate so that the very short duration stress waves can be detected and quantified. The PeakVue waveform and spectrum provide an indication of the bearing defect. As with all of the techniques, the accelerometer must be mounted correctly, and the filter settings (used to "tune in" to the bearing vibration) must be set correctly.
Enveloping
Also known as "demodulation," the enveloping technique, which is used by a large number of vibration analyzer vendors, has been optimized to measure the low-amplitude, high-frequency bearing vibration. See Figure 1.
The envelope spectrum is then checked for signs of the fault condition. Similar to the spectrum that results in the Shock Pulse, Spike Energy, and PeakVue systems, we are looking for peaks, sidebands, and harmonics that are related to the four characteristic bearing frequencies: Ball Pass Frequency Outer race (BPFO), Ball Pass Frequency Inner race (BPFI), Ball (or roller) Spin Frequency (BSF), and Fundamental Train (or cage) Frequency (FTF). See Figure 2 for a summary of the progression we expect to see.
The Spike Energy (units of gSE) technique aims to utilize the accelerometer's mounted resonance to amplify the high frequency vibration. However, in more recent years, the accelerometers provided have not been manufactured to have a repeatable resonance characteristic. What that means is that when you change your accelerometer, the amplitudes will change. See Figure 1.
The envelope spectrum is then checked for signs of the fault condition. Similar to the spectrum that results in the Shock Pulse, Spike Energy, and PeakVue systems, we are looking for peaks, sidebands, and harmonics that are related to the four characteristic bearing frequencies: Ball Pass Frequency Outer race (BPFO), Ball Pass Frequency Inner race (BPFI), Ball (or roller) Spin Frequency (BSF), and Fundamental Train (or cage) Frequency (FTF). See Figure 2 for a summary of the progression we expect to see.
Spectrum Analysis
If we do not use one of these techniques and simply view a spectrum, then we may have limited success unless we take precautions:
- Acceleration is most sensitive to high-frequency vibration, so if we view the spectrum in units of acceleration (Gs or mm/s2) and have a high Fmax (70X or higher) and, better yet, we view the spectrum in logarithmic format, then we will achieve the best results (with a spectrum alone).
- If we view the spectrum in units of velocity (in/sec or mm/s), then we may need to wait until the bearing is at stage three until we see positive signs of the fault. Increasing the Fmax and viewing the spectrum in logarithmic format will help significantly.
When viewing the velocity or acceleration spectrum (or any spectrum from PeakVue, enveloping, etc.) there are a few techniques that help to achieve the best results:
- Look for peaks at frequencies that are non-integer multiples of the shaft speed (e.g., 3.09X, 4.65X, 7.89X, etc.).
- There should be harmonics of those frequencies (e.g., peaks at 3.09X, 6.18X, 9.27X, etc.).
- Check for sidebands of the turning speed of the shaft. If they exist, then suspect a fault on the inner race. If there are no sidebands, suspect an outer race fault.
- Check for sidebands of the fundamental train frequency (slightly less than half the turning speed of the shaft). If they exist, then suspect a fault on the rollers/balls.
Time waveform analysis
It is typically possible to view the time waveform from the Shock Pulse, PeakVue, and envelope process, but I'll focus on the raw waveform from the accelerometer. In the early stages of the fault condition it will be very difficult to detect the fault with a time waveform. However, as the fault develops, an acceleration waveform can reveal the fault, especially when taken from low-speed machinery. As the fault develops, the waveform will have characteristic "pulses" and patterns that indicate the condition of the bearing fault. In the later stages of the fault, a waveform in velocity units can display the defect quite clearly.
Characteristic "modulated" pattern in the acceleration waveform (often called the "angel fish" pattern).
"Spikes" in the velocity waveform indicate the presence of a severe fault.
Conclusion
I hope this article has helped to provide a basic understanding of these techniques. They have all been used for many years to successfully detect bearing faults at a very early stage. The key is to mount the sensor correctly, choose the correct settings, and analyze the data correctly.
Jason Tranter is the founder of Mobius Institute and author of iLearnVibration and other training materials and products. Jason has been involved in vibration analysis in the USA and his native Australia since 1984. Before starting Mobius Institute Jason was involved in vibration consulting and the development of vibration monitoring systems. www.MobiusInstitute.com