Turbine Technology
Preventative maintenance through balancing and vibration analysis
By Jerry Justice
October 2000
Predicting failure
The U.S. military is currently attempting
to perfect prognostic hardware and software that will predict an impending failure with greater accuracy. If adequate warning is provided, logistics and planning requirements can be completed in a timely manner, thereby reducing spares inventory, preventing unexpected failures, and providing a basis for improving design and engineering techniques for components and processes. Trending may also be expanded to include oil analysis, ultrasonic, and thermographic, methods.
Trending engine performance factors
Engine performance factors may also be trended for tracking thermodynamic characteristics and other performance factors.
Trending isn't a new science, but one that is just beginning to mature in the aviation arena. The following is a very basic synopsis of how a trending program works.
Where to start
To begin a trending program you must decide
what you want to trend. You may collect an overall survey that includes all the operating frequencies for the engine or you may wish to trend discrete spectrum data. While the overall data is good for establishing initial thresholds or the gross overall condition, many outside forces including aerodynamic loads on the airframe can affect the data quality. For this example, we will trend discrete spectrum data on three components of an engine system. The drive speed is 10,000 RPM, which is N1 at 100 percent. For the sake of simplicity, the three fictitious components are driven at whole multiples of the drive speed.
1. A hydraulic pump operating at 2X drive speed.
2. A low-pressure fuel pump operating at 3X drive speed.
3. A generator operating at 4X drive speed.
Remember that these speeds are only used as examples for this article and not indicative of a normal operating speed for the actual components.
Next, you must decide on an interval for collecting data. For this example we will collect a vibration spectrum every 50 hours. Intervals may be extended or shortened according to specific requirements. Accelerated wear in components may require shorter intervals. If no significant change is noted in any of the components, a longer collection interval may be called for. Try to collect data on or as near as possible to the scheduled interval you decide on.
Standardization is key
In order to ensure you are comparing like
data, standardize the method of collection. All parameters should be the same or as near the same as possible for each collection. The engine speed, operating temperature, and ambient conditions, especially wind, should also remain the same whenever possible. Be certain the same type sensors, cables, charge converters, and analyzers are used to collect all data. You must also ensure the data is collected using the same engineering units and modifiers (IPS, Mils, Gs and Peak, Peak-to-Peak or RMS). Remember that differences introduced into the collection process will affect the data and make meaningful analysis difficult or impossible.
Now that the baseline has been established, collect data on time, on schedule, and under the conditions as noted above. In this example, and again for the sake of simplicity, we will collect data on a schedule of every 50 hours of operation. When the individual spectra are plotted on a comparison plot, as shown in Figure 2 on page 64, changes can be measured visually with the assistance of various software tools. Analysis and evaluation can also be accomplished automatically by setting limits or flags for the target frequencies and amplitudes associated with those frequencies. A report generated when those limits are met or exceeded will then provide the information critical to the corrective action decision and the success of the trending program.
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.