by Jim Hall 

Typically, bearings under 300 rpm’s are not tested using vibration analysis. So how do you keep an eye on them? For those technicians familiar with airborne ultrasound for bearing inspections, they know that slow speed bearings, that is bearings 300 rpm’s or less, can be tested with airborne ultrasound.

Airborne ultrasound or ultrasonic inspection for predictive and preventative maintenance has been around for 40 years. Primarily known as just a leak detector, this technology has survived by reinventing itself over and over again. Leak detection, bearing analysis, acoustic lubrication, scanning for electrical leakage and steam trap troubleshooting are just some of the many applications for which airborne ultrasound is used effectively.

The fact is, some technicians and mechanical engineers consider this technology either too basic or too subjective for predictive maintenance. Too basic? No way. The only difference between vibration and ultrasound  is the frequency of the waves being measured. Vibration analysis looks at low-frequency sound (below 20 kHz), while ultrasound measures high-frequency sound (above 20 kHz). Too subjective? I certainly don’t think so. And, since ultrasound is the earliest form of fault detection (ultrasound, vibration, then heat), it is worth your while to implement this very versatile technology.

As we listen to the bearing in the ultrasonic frequencies, we know that as the bearing degrades, we hear it in the ultrasonic range before vibration or heat is generated. 

Listening in the ultrasonic frequency range of 30-40 kHz we can hear the bearing. As this bearing degrades, the sound level increases over days, months or even years. As the sound level increases, the degradation can be heard at lower and lower frequencies. Over time, the increase will be heard in the range of 20 kHz and below. This is vibration analysis territory. Typically, when using vibration, the technology senses the bearing in the early stages of failure, and the technician can then analyze and compare readings as the bearing further degrades. The vast majority of the time, vibration analysis allows enough time to take action, either to extend bearing life or to schedule replacement, before complete or catastrophic failure. As the bearing further degrades heat builds up, and we now have temperature as a means to diagnose failure, which is when infrared thermography becomes effective. Unfortunately, when heat is present it is typically too late to save the bearing, it is in a catastrophic state and should be replaced.

One ultrasonic user, who is actually a veteran vibration technician, was at a loss when faced with numerous corporate cutbacks. However, being extremely resourceful, he decided to utilize the resources that he had to their fullest extent. He started using ultrasound as a means to listen to his motors. In fact, ultrasound became a precursor to vibration analysis! Over several years of datalogging decibels he was able to trend his motor bearings and gear boxes for wear – including when to lubricate bearings.

Over the years, ultrasound has been shown to be a very reliable tool when used to trend bearing wear. Both slow speed bearings (below 300 rpm’s) or high speed (above 300 rpm’s) can be tested using airborne ultrasound. I have had a great deal of success testing bearings as low as 7 and 8 rpm’s. The good news is that there are no tricks or difficult set-ups to perform. Simply set-up as you would any ultrasonic bearing inspection and record your reading. To record your data, you can use an audio recorder, write down the data and/or use whatever other means your instrument gives you to gauge wear of a bearing.

Recently, I logged in to review the many ultrasonic questions posted on the Maintenance Forums at the http://maintenanceforums.com/groupee website. Would you believe that in the past 12 months (September ‘06 to September ’07) questions regarding ultrasound were viewed 29,389 times?  Questions regarding bearing inspections were only viewed 5,106 times and “slow-speed bearing” questions were viewed 2,634 times.

To me, this means that overall, many maintenance professionals are seeking information about ultrasonic inspections and the capabilities of the technology. But it also indicates that many may not be familiar with inspecting bearings, let alone slow-speed bearings (under 300 rpm’s) using airborne ultrasound.

Slow speed bearing inspections require patience! To inspect slow speed bearings properly, you need to stand and listen to the bearing for at least several seconds and sometimes several minutes. Keep in mind, the lower the number of revolutions, the more time is needed to listen.

One of the tips I suggest is to use a recording device to record a wave file of the bearing noise. This is accomplished by using a patch cable from the recorder to the ultrasonic instrument’s headphone jack. As Tom Murphy of Adash DDS, says, “Having a recorder such as a mini-disc recorder that has both “gain control” and “gain crush” is essential”.”  I looked on-line and found several mini-disc recorders, which cost between $200-$300. Clipping the sound is a real problem, so take the time to research, and spend the money to purchase a good mini-disc recorder.  Sony Electronics make a good unit for this.

Figure 2 shows a heat treat oven with slow speed bearings. These bearings operate in temperatures of 750º F and turn at 7 rpm’s. Because failures are often found due to the high heat, technicians are always trying to keep a close watch on these bearings.

Figure 3 displays the decibel readings from 10 of the bearings inspected. Note how No. 7 spiked to a high of 31.2 decibels? Using an SDT 170 and DataManager Software the end-user can trend these bearings over days or weeks to predict failures. From installation to failure was a matter of just a few weeks for Bearing No. 7.

Expert acoustic engineer Tom Murphy of Adash, DDS suggests recording several revolutions/rotations over several minutes if need be. For instance, a shaft rotating at 1/9 rpm, Tom suggests 3 rotations (per inspection/recording) that’s 27 minutes total.

A magnetic base is a great tool for this inspection. I suggest you call your representative to see if a magnetic base is available for your instrument.  Before you run out the door to inspect your slow-speed bearings, look at Figure 6, which contains waveform views of two bearings. These waveforms are courtesy of Pete Marquardt, Predictive Maintenance, LLC. Pete had two slow speed bearings rotating at 42 rpm’s and he wanted to compare the two. Rather than use FFT, Pete recorded the sound of the bearings at 20 kHz.

Why 20 kHz, you ask? Pete mentioned that during this particular inspection the sound quality was better at 20 kHz instead of the normal 32-40 kHz. Pete was using the U.E. Systems’ Ultraprobe 10,000 model and associated software. Notice that while bearing number 1 has several defects, bearing no. 2 has considerably more defects and wear.

Why ultrasound? Here is a more fitting question. Why not ultrasound? It is affordable, provides datalogging capabilities, has a short learning curve and readings are quick to retrieve.

If you know your instrument and its capabilities, slow speed bearings can be diagnosed effectively using airborne ultrasound.

Photos courtesy of: Ultra-Sound Technologies Training Systems, Woodstock, GA