ArticleLow Speed Bearing Analysis
by Bruce Weathersby
If you want to know how to collect vibration data properly on low-speed equipment, this paper is a good choice. This paper starts out describing the nuances and issues of collecting low frequency (low speed) vibration data (below 300 cpm) including instrumentation limitations and ends with a discussion of an actual data collection application. It would be good for vibration analysts of all skill levels that have not had a significant amount of experience with low speed data collection. There are generous use of illustrations, graphs, and charts to ensure that the subject material is thoroughly explained. The paper is descriptive in nature and shows the practical side of the field work necessary to ensure success with low speed data collection. The paper ends by using actual field data similar to a case history to show the development of successful data collection tools and techniques.
In 1998 several of our large slow speed reactors were experiencing bearing problems that resulted in unplanned outages. From historical data we were averaging 4 unplanned shutdowns per year because of it. We knew that as many as 4 of these reactors were in some phase of bearing degradation at any given time. Each unplanned outage caused between 5 and 7 days of lost production.
Although we routinely analyzed and predicted bearing distress in higher speed equipment, we had not been able to do so at the 3 to 12 rpm turning speeds of these reactor shafts. In 1998 we began an investigation into techniques that would help us predict bearing condition in these reactors. A technique called PeakVue® was examined and did predict some of the early failures. Ultimately we settled on conventional techniques as being the most informative for our particular application. This was true only after we identified several pitfalls (associated with instrumentation) that exist in this slow speed area and learned to work our way around them.
By 2000 we were able to more accurately assess the condition of each bearing and plan repairs to avoid production losses. In 1999 we also identified the design flaw causing these bearing failures. We have subsequently changed the equipment design. There have been no unplanned outages (due to bearing failure) since we learned to predict the beginnings of bearing degradation.
The large physical size of the reactor and bearings involved may be of importance so a thorough description is given for comparative purposes.
In today’s world the data collector far outnumbers any other type of FFT instrument used to collect vibration data. It is most commonly used with an accelerometer. These devices have become so portable and powerful they have largely displaced the dedicated “real time analyzer” of just a few years ago. Actually the terms “data collector” and “real time analyzer” are rather vague and misleading. Later in the paper they are more accurately described. The important point here is to recognize that some data collectors/FFI’ analyzers have moderate to severe limitations in this low speed area, especially the closer the speed gets to 0 rpm. A discussion of analyzer performance is given while analyzing signals down to 3 cpm.
The nature of diminishing amplitude levels that describe bearing faults (in inches per second) as you approach 0 rpm is very important. Bearing defects, at low speeds, produce smaller amplitudes (in inches per second) than would be normal at higher speeds. A discussion with accompanying graphs is presented.”
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