Preventative maintenance through balancing vibration analysis

If the generator is replaced because of other non-vibration-associated defects, the baseline for the generator may remain the same and the data for the life of the generator used as a comparison in the life of the new generator. However, an exception to the rule arises when the initial data indicates a significant difference, either above or below, the established baseline. If this occurs, you must investigate the situation to determine which generator is closer to the norm. If the vibration data from several generators of the same operating time are available for comparison, it becomes a matter of elimination as to which generator falls outside the norm.
It is easy to see that the more data you gather, the more accurate your trending program becomes. In Figure 4, page 65; four generators, A, B, C, and D are plotted to the 500 hour point. Notice that generator C failed prior to the 500-hour point. The sharp increase in the vibration amplitude just prior to failure is typical. With this knowledge in hand, you may choose to remove and replace the generator when a similar trend is noted in the future. While you cannot accurately predict the failure down to an hour of operation, you can heed the warning signs that were produced by other generators in your database prior to their failure.
You must also realize that some failures will produce little, if any, warning prior to a failure. If this proves to be the rule rather than the exception you should reduce the time between data collection intervals. If, in the case of Generator C, a failure mode is clearly in its advanced stages, you should set the limit for the generator at a point prior to the anticipated failure. For example, in this case you should set the limit at 1.2 IPS. Remember that these vibration levels are only used for this example and do not reflect actual values or recommended limits for any component.

When a sufficient quantity of data is collected to establish a limit, you can install a limit line in your software and/or analyzer if it is capable of this function. Some analyzers are capable of storing a database of frequency ratios associated with the components you wish to monitor. These ratios are synchronized with a tachometer input of the driving component. The database is then queried for the turning ratio of the requested component. A key press can then identify individual components. The database may also contain additional display information such as vibration limits of the component set by manufacturer standards or by the user from established trending limits. (See the example below). This analyzer also has the ability of allowing the user to install a limit line in a Setup, which then shows the established limits of the displayed frequencies on screen in the form of an encapsulating line as you collect and view the spectrum. (See Figure 5). If a limit is exceeded, you are aware of it instantly.
The idea behind any trending program is to take corrective action for an impending failure before it gets expensive. Although still in its infancy, trending in aviation applications is already saving untold thousands of dollars in overhaul cost for operators of corporate fleets and regional airlines. Having been in the vibration analysis and balancing business for a few years I still think the largest challenge to establishing vibration analysis as a viable aviation tool is the education of A&P technicians in its use. Reluctance to use something you don't understand is human nature. Several vibration equipment manufacturers offer on site training as well as scheduled courses in understanding and using vibration analysis. With the current advancements being made, those who are ready to meet the coming challenge of their use will be the recognized leaders in this little understood but invaluable area of aircraft maintenance.