If decibels increase over a period of time or continue upwards from a previous read, you should lubricate and if no significant difference is noted, then you must assume the bearing is headed to failure. Watching the decibel levels over time will assist you in determining first or early failure to a catastrophic failure. Ultrasound is the earliest indication of bearing failures, before heat and before vibration.
It's important to remember that the ultrasound instrument is a high-frequency receiver that receives a high frequency shortwave signal measuring from 1/8-inch to 5/8-inch long, is low energy and mostly travels in straight lines, making it directional. If you take a reading at the base of the Zerk fitting, for instance, return to that location for repeatability (Figure 1).
Figure 1: Taking ultrasonic readings of pump/motor 4.
Photo Courtesy of: Jim Hall, Ultrasound Technologies Training Systems
You want consistent readings when trending bearings on a motor. You want to measure "apples to apples" and "oranges to oranges," or non-drive end to non-drive end, etc. Most ultrasound instruments allow the user to set the instrument at the previous sensitivity and frequency settings so the end-user can quickly see any differences from the previous reading.
Are you using multiple instruments from two or more manufacturers or two or more models from the same manufacturer? If so, know your instrument! Be familiar with the instrument's sensitivity or amplitude level. Is the instrument linear across all ranges of sensitivity or amplitude?
Recently, I visited a waterpark where the maintenance superintendent and I are good friends. He explained that they had nothing but hands, fingers and ears to diagnose bearing failures. I told him that ultrasound is the most complete instrument a mechanic can have. It's inexpensive, it has a multitude of applications and the return on investment, in most cases, is immediate.
I've been serving this waterpark for a couple of years now, providing underground leak locating services (Figure 2).
Figure 2: Locating underground waterline leaks in kiddie pool area of waterpark.
Photo Courtesy of: Jim Hall, All Leak Detection & Locate
Occasionally, I would watch as the technicians listened for bearing sounds and touched the motor with their hand, purposely raising their index finger as if to feel for vibrations, thinking this alone would aid them in determining whether or not they had a bad motor bearing.
Out of the eight pumps and motors of the waterpark's filtration system, five 60 hp motors and three 50 hp motors were coupled by a flex coupling to a pump.
With no real predictive maintenance program and no predictive maintenance equipment, my maintenance superintendent friend was facing the "perfect storm" for failures. Flex couplings between the motor and the pumps are fine, especially when no alignment tools other than a straightedge were used to align the motor and pump. A flex coupling may correct some misalignment, but when the ability of the flex coupling to correct is exceeded, the motor, pump, or both may be headed for failure.
We took several readings of the 60 hp motor bearings (outboard/inboard) using an ultrasonic pistol. Readings were in the mid-range of 35 to 45 decibels. However, one pump was trending at 53 decibels. It also had a loud pitch, an overall loudness that really didn't sound like bearings.
Puzzled by this, we took a reading with a vibration data collector. The vibration analysis indicated an imbalance problem, but not a significant bearing problem as of yet.
Fast forward a few months. All eight of the filtration motor and pumps were removed, the old motor stands replaced and the flex couplings (motor/pump) replaced with a direct drive setup. Still wanting to engage the waterpark in the idea of trending its pumps, I called once more and proposed taking readings and preparing a database for future readings. This time I was joined by Adrian Messer, Manager of U.S. Operations for UE Systems, Inc. (Figure 3).
Figure 3: Adrian Messer, UE Systems, Inc., ultrasonic inspection of bearings.
Adrian brought with him a new state-of-the-art ultrasound instrument. The fairly new instrument has tons of onscreen applications at your fingertips.
With the UE Systems' Ultraprobe 15000 Touch instrument set for dual display of fast fourier transform (FFT) and the time waveform chosen (Figure 4a, top), we watched and listened. What we saw clearly was significant harmonics. Was it harmonics from the inner, outer, cage, ball pass frequencies? We didn't know right away. Changing the display to just FFT (Figure 4b, bottom), we noticed a cursor highlighting a fault frequency of 175.8.
Figures 4a & 4b: Dual display of the Ultraprobe 15000® instrument used above, FFT (top) and time waveform (bottom). FFT only below. Users have their choice of views.
The latest version of advance spectra analysis software has a new on screen bearing calculator. The end-user simply enters the RPM and how many balls or roller elements are in the bearing. You can also select outer race, inner race, ball pass, or cage frequencies (see Figures 5a and 5b). On pump #4, our harmonic cursor indicated the inner race bearing frequencies as the problem.
Figure 5a: Pump motor No. 4, played through software identifies inner race problem.
Photo Courtesy of: Jim Hall, Ultrasound Technologies Training Systems
Figure 5b: All five pump motors overlay. Notice No. 4 with inner race in "white" coloring.
Photo Courtesy of: Adrian Messer, UE Systems, Inc.
Still not satisfied with the results, I forwarded the findings to Peter Marquardt of Predictive Maintenance LLC in West Point, Virginia. Pete is certified Ultrasound Inspector Level II and Vibration Analyst Level II. He offers services in vibration analysis, thermal imaging, ultrasonic leak detection, ultrasonic bearing analysis, precision laser alignment, field balancing and consulting.
Pete wrote in his reply to me, "Utilizing the ultrasound data collected at the filtration pump/motor number four 'outboard' bearing, from a vibration analysis point of view and without actually having been there to collect this data using my own vibration analyzer, I was able to determine the 6212 bearing has an inner race defect. A normal 6212 bearing has 10 rolling elements. At 1750 rpm, this bearing would produce a vibration at inner race frequency of around 175Hz. The FFT data shown on the image sent has a dominant harmonic of 172Hz. This, along with the running speed sidebands and running speed harmonics, would indicate the most reasonable conclusion for this analysis. Inner race defects will usually produce these running speed harmonics as the race goes in and out the load zone once per revolution. The ultrasonic waveform data in live mode also shows this impacting with a slight ring down."
Trending bearings with decibels alone may sometimes not be enough. Recording a wav file, either with an ultrasound instrument or a simpler digital recorder, and then analyzing the wav file further in a FFT or time waveform diagnostic software may be your next great move. However, I must caution anyone wanting to utilize FFT or time waveform to make a diagnosis that you should first understand how to properly capture repeatable signals for analysis.
Randy Stiver, the Ultrasound Level II Certification Program instructor at Ultrasound Technologies Training Systems (USTTS), demonstrates to Ultrasound Level II (USTTS-II) attendees how to properly capture ultrasonic detector output signals using digital recorders, vibration spectrum analyzers, oscilloscopes and personal computers. The use and understanding of the captured signals are then used for analysis. This better equips even the novice in the fundamentals of sound analysis when using FFTs and time waveform along with ultrasound frequencies to determine faults.
Can ultrasound determine every fault each time? No. Therefore, you should still utilize, if available, vibration analyzers, oil analysis, infrared and ultrasound instruments. Their usage is especially important when you're in doubt or mission critical depends on a motors operation.
Click Here to Read Part 2 - The Update, after another bearing survey of the eight motors and pumps was performed in Summer 2012.
