After a sufficient number of data samples are collected, an indication on the analyzer screen shows averaged vibration amplitude, angle, and speed. As the number of samples increases, the percent of error is steadily decreasing. The user is prompted by the analyzer to stop the data collection with a key press. If using multiple speeds, the analyzer will prompt you to set power for the next speed and the process is repeated.
For this example we will use a single speed. Using the one RPM scenario above, we determine that the amplitude of the vibration is 0.5 IPS because the 20 mV/IPS sensor sent a 10 mV signal to the analyzer. The vibration event occurred fifteen seconds AFTER the tach event. The analyzer converts fifteen seconds of a rotation to 90 degrees.
With the reflective tape in the position where it triggered the tach event, we can see that the heavy spot on the fan is located approximately 90 degrees opposite the direction of rotation from the vibration sensor. (Refer to Figures C and D above). Of course, in actuality, all this occurs at a rate from 5,000 to 20,000 times faster but the results are the same. The task now is to counter the effects of that mass.
Applying a solution
The first thing we need is an influence coefficient for the application. The coefficient influence is the amount of weight, corrected for lead or lag, required to counteract the mass causing the vibration.
Without getting too deep into theory, the lead or lag correction is done automatically by the analyzer. It is simply a correction to compensate for the transmission characteristics of the engine being balanced and a correction for the type of measurement being made.
Peak Velocity and Peak Acceleration physically occur at different points in the rotation of the unbalanced fan — 90 degrees to be exact. We may enter a known influence for the engine before acquiring data or calculate one by adding a "test weight" to the fan and collecting data from another run to see how the weight affected the previously recorded condition.
For this job, we will enter the influence of 20 gram IPS (20 grams of weight for each inch per second of measured velocity). Again, just to make things understandable, I'll use the example just as it was stated above in the previous paragraph and say that the mass is located at 90 degrees. The solution then should be to add weight at 270 degrees, in an amount sufficient to counteract the out of balance condition.
If the analyzer is programmed for the specific engine type, the location of the tach trigger and vibration sensor, as well as the class weight set are each a part of the program.
When using class weights, the limitation is the value and number of available class weights. In this job, we will use a class weight set as follows: There are five weights in the set; a -1, -2, -3, -4, and -5. Their values are 2.25, 4.52, 6.8, 8.69, and 11.44 grams respectively. For this balance job, the analyzer directs us to place a -1 and a -3 weight in hole number 3. Remember that for this job, the tape designates the number one hole. The holes are numbered in ascending order sequentially from that point, opposite the direction of rotation.
In the figure below you can see that the No. 3 hole is 180 degrees from the location of the mass. The -1 and -3 weights total 10.05 grams, which is near the 10 grams calculated as necessary to counter the 0.5 IPS. Remember that 20 gram IPS is the influence for this job.
After installing the weights, it is necessary to enter the actual amount of weight and the location back into the analyzer. In this case, we would use the class weights (-1 and -3) in the hole number (hole #3). A second run is then conducted to verify and refine the solution as necessary. This sequence of events is repeated until the fan is at an acceptable vibration limit.
The typical problems associated with noise and vibration from an unbalanced fan are decreased life of avionics and airframe components, as well as discomfort and stress on the crew and passengers. As the maintenance professional in charge of maintaining a smooth running aircraft, it's to your advantage to keep the air clear and quiet.
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