“History:

In the late 20th Century (early 1970s), before vibration data collectors were invented, overall vibration amplitudes were measured with vibration meters. The vibration amplitudes at key locations on the equipment were noted on paper forms and plotted on graph paper as trend charts. The recommended allowable vibration amplitudes were determined from general machinery vibration severity charts similar to Figure 1. These charts had been constructed by early researchers in the field based on their measurements, observations and experiences. If the vibration amplitude exceeded the level determined from the chart, a sample of the vibration time waveform was recorded on magnetic tape and played into a spectrum (FFT) analyzer. The resulting spectrum was “analyzed” using the analyst’s experience, published information, and knowledge gained by attending training courses and seminars. Sometimes, the maximum allowable amplitude was drawn on the vibration amplitude trend chart as a horizontal line.

At our location, we noticed that, although the levels determined form the severity chart were a fairly reliable guide, we sometimes had equipment failures before the overall vibration exceed the allowable vibration. We also noticed the vibration amplitude trending up from amplitudes below the vibration limit suggesting that damage was occurring at levels well below the limits suggested by the severity charts. 

We stopped drawing the “allowable vibration amplitude” line on out tread charts and used the slope of the trend line itself as an indicator of changes in the “condition” of the machine. The faster the amplitude was increasing, the more concerned we became, and more effort was put into trying to determine the cause of the increased vibration. We stopped relying on a predetermined value of the vibration amplitude to take action and tried to understand why the vibration was changing. With a new machine, we compared the initial vibration amplitudes to the vibration severity chart and our own experience. After, we had obtained some vibration history; we watched the vibration trend charts for changes that could indicate trouble.

The purchase of a data collector and software allowed us to set alarms for peaks in our spectra as well as for overall readings. We could also set alarms at levels below our “action” levels to give us an early warning of modest changes in vibration amplitude. The addition by the vendors of rate of change alarms as well as band alarms and alarm envelopes for spectral data allowed us much more flexibility to customize our alarms. However, our alarms were still set based on industry recommendations, published information, and our own experience with our equipment.”