Preventative maintenance through balancing vibration analysis

Turbine Technology Preventative maintenance through balancing and vibration analysis By Jerry Justice October 2000 Have you ever had a dead battery in your car? Did you discover it right when you really needed to get somewhere? Do you...

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.

Establishing a baseline
As a starting point, you must establish a baseline. This will be the measuring stick by which you compare and evaluate a change in the condition of the individual components being trended. The baseline should be collected on new or very low-time components. Ideally, this will be the best condition of the components you can expect to see during their life cycle. You should ensure all, if any, break-in times and requirements have been met before collecting the baseline data.

figure 1

In the X, Y, Plot illustration (See Figure 1, pg. 63), you can see the frequency, in RPM, along the horizontal (X) axis. This scale may also be in Hertz (Hz) or cycles per second. By moving from left to right along this axis you can easily locate the four components for our example at 10,000 RPM, 20,000 RPM, 30,000 RPM, and 40,000 RPM. By comparing the top of each of the peaks horizontally to a relative position on the vertical (Y) axis you can read the amplitude of the vibration for the component operating at that frequency. The units of measure in this case are Inches Per Second Peak (IPS Peak). For example, the N1, turning at 10,000 RPM is generating vibration with a value of approximately 0.32 IPS Peak.
If we use this data as our baseline, we now have a basis of comparison for future changes in the condition of the four components. Remember that this is a very simplistic view of how trending works. With that in mind, you should also remember that a rotating component might display various vibration characteristics according to a specific type of wear, misalignment, or imbalance condition. These characteristics include harmonics of the fundamental frequency, which would, in this case, conflict with the frequencies of one or more of the other components in our example.

figure 2

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.

In Figure 2 above, the amplitude of the generator turning at 4X drive speed or 40,000 RPM, shows an increase (depicted in red) with each subsequent data collection. The other components remain at the same amplitude for the sake of comparison in this graph.

figure 3

The amplitude of the generator baseline was 0.18 IPS. The progressive increase of that amplitude is shown in the graph above where it is shown in comparison to all the other monitored components and below as compared to time only. Above, the last three collection points at 400, 450, and 500 hours show an increase up to 0.4, 0.48, and 0.55 IPS respectively. The change in the vibration condition of the generator is progressing in a linear fashion as shown in Figure 3. While this data does not provide an indication of an impending failure, it does establish a trend that can be directly related to time and a corresponding change in vibration.

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