Ultrasound for Slow Speed Bearing Monitoring, Part 2

July 1st, 2016

Slow speed bearings play an important role in a wide variety of industrial plants – they are common components in equipment, machinery, conveyor belts and tools. If bearings fail or malfunction, it could hold up operations and cause a major loss in productivity. Bearing monitoring is a crucial part of any strong plant maintenance program.

Part 1 of this 2-part series based on CMRP Ron Tangen's presentation at Ultrasound World XI focused on the logistics of a reliability centered maintenance plan for bearing monitoring. Tangen used the Dakota Gasification Company (DGC) as an example of an organization that realized its facility maintenance was inadequate and took steps to implement condition based monitoring with the help of tools like ultrasound probes. Part 2 will go into greater depth on how to measure and understand the health of these bearings.

Part 2: Bearing Analysis

Understanding slow speed bearing condition requires a different type of software and set of tools than regular bearings. To be more specific, FFT will not work on slow speed bearings. Instead, a Spectralyzer chart – measuring frictional force over a short timeframe – can reveal the bearing's health at a specific moment. A DMS chart provides decibel reading over a longer timeframe and shows past, present and future health. Both are valuable tools in assessing slow speed bearing condition.

The best ultrasound tools help maintenance professional collect and trend data.The best ultrasound tools help maintenance professional collect and trend data.

Analysis tools
The Spectralyzer is adept at converting frictional forces into meaningful, readable information. These forces are separated into two categories:

  • Impacting: Short duration frictional forces like contamination, splitting, balling, fretting or broken parts.
  • Whitenoise: Constant frictional forces like a tight or new bearing, a loose or old bearing, heavy lubrication load or light lubrication load.

With the proper training, maintenance technicians will be able to examine the Spectralyzer chart and identify each type of friction based on telltale signs like deviation from the baseline measured over a period of time. Minor impacting registers around 8 dBs, while moderate impacting reaches closer to 13 dBs, for example. Severe impacting can get as high as 30 dBs or more.

By trending the data points gathered by the Spectralyzer in a DMS chart, maintenance professionals can begin to project bearing failures and act preemptively. The DMS chart provides the current dB reading, which provides the risk of failure at that time. Meanwhile, historical values indicate changes in bearing health, where the slope is the rate of failure. Extend that slope past the current reading and you're left with a projected failure rate and predicted risk. It's also important to mark bearing replacements on the chart and allow for a brief wear-in period. The chart should display a 24-to-36 month reading.

Bearing life cycle trend
With an ultrasound program and the above analysis tools, maintenance teams can track the entire life cycle of their bearings from installation through to failure. With a consistent program, technicians should be able to see a clear difference between a brand new, well-lubricated bearing and an old, over- or under-lubricated one.

"Bearings can recover from certain levels of failure."

The DGC found that bearings can recover from certain levels of failure if that malfunction is identified and remedied early enough. For example, over-lubrication can cause a slow speed bearing to reach failure faster, but if maintenance professionals uncover that error and correct it, the bearing may resume its expected performance and failure rate.

Bearings will fail – that is a reality of plant maintenance. But there is a difference between end-of-life failures and premature failures caused by human error or negligence. Ultrasound and its analysis tools are both designed to give maintenance professionals the insight they need to prevent early failure.

The process for DGC was not without its challenges. Ultrasound proved much more sensitive than they initially anticipated – a single fault in a bearing does not necessarily equate to failure. Additionally, bearing failure is a slow process, but thermal failure can be quick, so it's important to be able to trend and analyze data to determine the specific cause and failure rate. Is that rate normal, or is there some issue at play? Finally, ultrasound is not a 100 percent predictive tool, but rather it works best as an integral part of a complete plant maintenance plan. A successful program must also include:

  • Management support
  • A committed individual to champion the plan
  • Communication between all those involved
  • A commitment to continuous improvement

All in all, DGC found ultrasound to be an effective tool for monitoring slow speed bearing health. It helped the organization reduce the incidence of catastrophic failure and also to evaluate the future risk of failure. Alongside the best analysis tools and with management support behind them, Tangen and his team were able to implement a successful bearing condition monitoring system and improve DCG's plant reliability.

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