Vibration and Balancing Systems: The Spectrum Plot
The Spectrum Plot – Types of Aircraft Vibration
One of the least understood features of the PBS series of vibration analyzers is the spectrum function. The spectrum functionality is a more difficult function to understand, but once mastered, technicians can quickly develop an extremely in-depth understanding on what is going on in an engine during a test.
Figure 1, below, shows a basic spectrum from the PBS system. On the x-axis is frequency, measured in Hz (per second). On the y-axis is amplitude, measured in units of velocity, inches per second pk, or IPS pk.
Reading a Vibration Spectrum
To read this spectrum chart, we look along the x-axis at frequencies where there is a spike in the vibration amplitude. In this case, we have spikes at 60hz and at 120hz. What this means is that the vibration wave detected by the vibration sensor on the aircraft can be broken down into 2 distinct waves at each of those frequencies.
We will assume that we’re working with a single shaft engine for the sake of simplicity. If we know that the engine N1 fan is rotating at 60hz, then we can safely assume that the first vibration peak is the vibration associated with the vibration from the N1 rotor. However, what gives with the 120hz signal?
Well, it could be a lot of things. The speed is a great hint, however. Since its double the speed of the N1 rotor, we call this higher spike a “harmonic” of the original N1 speed. Vibrations that occur at a harmonic of another speed are likely related to the base speed. In this case, the vibration is likely caused by something that occurs twice for every single rotation of the engine. This could be the signature of a component gear driven off the N1 shaft at a ratio of 2:1.
It is possible to get amplitude spikes at a variety of harmonics. In this case, we looked at the 2X harmonic. But it is possible to see vibration peaks in a spectrum at 3X, 4X, 5X, etc…These whole order harmonics are classified as mechanical vibrations. For example, if we are dealing with a plane that has 4 propeller blades and a 6-cylinder piston engine, we would see a 4X blade passing signature in the spectrum and a 3X signature from the engine’s firing pulses.
It is possible to half fractional harmonics, too. For example, a ½X vibration would be that which occurs every other rotation. These are called operational harmonics. These are usually caused by things such as loose fittings, misfirings, etc.
Of course, spectrums may not be as easy to read as figure 1 would suggest. They are traditionally filled with different peaks from a variety of vibration sources being present in the aircraft’s total vibration signature. By starting with the speed of the engine’s rotors, we can then work out to see if many of the different peaks fall at a harmonic speed of the main speed. With this knowledge, it makes troubleshooting the cause of a vibration spike much easier to determine.
Using Vibration Analysis as Part of a Trend Monitoring Program
So now that we know the basics of a spectrum, how do we know what is normal and what’s not normal? Well, we do not always know. Many engine manufacturers never publish a standard spectrum for what a normal engine should show. Without this information, the best that we can do is utilize good trend monitoring techniques.
When the engine is new, a baseline vibration analysis should be run to establish what the baseline spectrum is for that particular engine. As the engine ages, we continue to conduct scheduled and unscheduled vibration analysis to obtain new spectrums. We can then compare these spectrums to older ones from when the engine was newer and see how the engine is aging over time.
We expect the vibration spectrum to change progressively as the engine ages. Perhaps the change will be quicker and more pronounced in the engines early hours as compared to mid-life, but the change should be monitored closely to see if the trends are getting “out of hand.” If trends are quickly spiraling out of tolerance, its time to start looking at our spectrum plot closely to determine what is going wrong so we can quickly fix the problem and return the engine to service.