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A proven method to assess gearbox condition is to collect a DYNAMIC ultrasound signal. If possible, you want to capture at least 3-5 revolutions of the gearbox. From there, analysis is straightforward. Use Ultranalysis (UAS2) software to view the signal in the time waveform and spectrum displays. Use the software’s many analysis tools to determine the exact nature of any defects. Just remember these three keys for successful ultrasonic condition monitoring.

1. Collect the best data you can, using a high quality ultrasonic data collector.
2. Consistent sensor placement must fundamentally be observed.

Figure 1 – Time waveform and Ultrasonic Enveloping Power Spectrum of a damaged gearbox from SDT270 and UAS

3. Identifying boundaries that impact data transmission is imperative.
Ultrasound is Shy… It Keeps Boundaries
Think of ultrasound as the quiet introvert. It prefers to stay in, and rarely mixes well with ultrasounds from other places. We call this “boundary behavior” and it’s another characteristic that makes ultrasound such an attractive condition monitoring technology. Ultrasound signals remain isolated to their source, making it easy to pinpoint defects without interference from other elements of the machine.
Sensor Placement
Inspectors tempted to place their ultrasound sensor directly on the gearbox cover, should reconsider. This common mistake affects data integrity. A gasket seals the cover plate to the gearbox housing. The specific acoustic impedance of the gasket material differs greatly from the cast metal of the gearbox. The change in materials a boundary barrier through which bashful ultrasound is reluctant to is pass. A better option is to place the sensor on a bolt head, which is directly connected to the gearbox housing. The result is crystal clear ultrasound signals for listening, trending, and condition assessment. HearMore: Click here to listen to Damaged Gearbox.

Figure 2 – Place sensor on bolt head

Special thanks to our partner Allan Rienstra from SDT Ultrasound Solutions for sharing his great knowledge with us!

by Allan Rienstra - SDT Ultrasound Solutions

Simon is a condition monitoring specialist from a local oil refinery. He contacted my office for advice about predicting flexible coupling failures. Currently, they perform basic vibration analysis on their pumps and motors using an overall meter. They have some success predicting bearing failures but the same cannot be said for couplings. Several unexpected failures shut them down this year.

Within the facility they identified 58 pump systems considered “A Critical”, meaning if they go down, the plant goes down. I suggested ultrasound as a fast, safe, and affordable solution. Specifically, the SDT270DU offered him best value. Not only could Simon use it to monitor couplings with ultrasound; it also takes vibration measurements, thus eliminating the need for Simon to carry two data collectors.

With SDT270 Simon trends bearings with vibration, replenishes lubrication with ultrasound, and finds worn and misaligned couplings.

By placing an airborne sensor near to the coupling Simon can quickly trend an evolving defect. The SDT270DU gives Simon the choice to either spot check for defects – good – or integrate all 58 couplings into his established bearing routes – best.

Find worn and misaligned couplings with SDT’s Flexible Sensor.

I explained to Simon how several clients already trend couplings using the Flexible Wand. The SDT270 collects a STATIC ultrasound measurement that gives four indicators of condition. The first two – Overall RMS and Max RMS – indicate the level of friction produced by the defect. When these indicators rise, maintenance may consider corrective alignment during a planned shutdown. The second two – Peak and Crest Factor – identify the emergence of impacting. Together, all four indicators establish a life cycle trend for each coupling.

Once impacting appears, the Peak indicator increases in step with Overall RMS. Crest Factor (CF) is a comparative ratio between Overall RMS and Peak. As CF trends higher it warns that the window for simple maintenance has narrowed. Inspectors may choose to collect a DYNAMIC measurement when CF alarms are triggered. The DYNAMIC measurement provides a visual representation of friction and impacting severity. For both STATIC and DYNAMIC measurements it’s important to define the signal acquisition time.

Without the ability to define user time, pulling the measurement trigger (M) misses key events
Instead, set the signal acquisition time to suit the asset. This slow speed example used 20sec to capture all defect data. Why use an ultrasound instrument that overlooks the importance of user defined acquisition time?

User defined signal acquisition time, available exclusively on SDT instruments, is a luxury that lends ultrasound technicians the highest level of precision. Without the ability to set the sample time, inspectors must guess when to pull the measurement trigger, and question the validity of their data. Simon explained that all 58 pumps turn at speeds above 1800 RPM. Accordingly, he should set his SDT270’s signal acquisition time to between one and three seconds. One to three seconds at 1800 RPM samples the coupling for 30-90 revolutions.

Shaft couplings are guarded for safety. Any ultrasound inspector working around rotating equipment must be required to demonstrate an understanding of company safety policies. Safety considerations are engineered into SDT sensors. The Flexible Wand’s 10mm diameter sensor is designed to access the coupling with the safety guard in place (see figure 2). The 21” long sensor sports a comfortable, ergonomic grip that allows an inspector to collect danger-free data.

Simon seemed convinced but wanted to Hear More. Since this solution was already working well at a nearby paper mill, I introduced Simon to the plant manager, Sunil, and invited them both to lunch. Sunil and Simon connected on so many common reliability issues that afternoon. He confirmed the affordability of this solution based on coupling failures alone but went on to explain how their mill was rolling out ultrasound for acoustic lubrication, steam trap monitoring, electrical inspection, and air leak management. Simon and Sunil continued their conversation well into the afternoon. They agreed that ultrasound, with its 8 primary applications for reliability, represented a fast, safe, and affordable technology with the potential to revolutionize reliability culture. I sat back, happily watching two impassioned specialists strategize about reliability culture. I love my job!

by Allan Rienstra - SDT Ultrasound Solutions

Condition Monitoring Expert Tip #4 by Mobius Institute

This tip is sponsored by IMVAC (International Machine Vibration Analysis Conference)

There is no doubt that technologies such as vibration analysis, oil analysis, ultrasound and infrared are very powerful. They can tell you a great deal about fault conditions in rotating machinery, electrical systems, and more. But if the criticality warrants it, you will be in a much stronger position if you have multiple technologies indicating that a fault condition exists rather than relying on just one.

For example, if vibration analysis indicates there is a problem in a gearbox, oil analysis can confirm the fault with the presence of wear particles. In the case of vibration analysis, you can utilize high frequency analysis, spectrum analysis, time waveform analysis, and phase analysis to enable you to validate your diagnosis.

There can be a great deal at stake when you make a diagnostic call on a piece of equipment. More so if it is critical equipment. At the very least, a false diagnosis may lead to equipment failure (if you miss the fault condition) or it can lead to unnecessary work and downtime. What’s more, your reputation is at stake. Sadly, people often forget when you make the right call, but it can take years for people to forget when you make the wrong call.

Thanks Mobius Institute for sharing such valuable information with us!

by Ana Maria Delgado, CRL