Resonance, critical speed, and natural frequency are all very similar terms and refer to the same phenomenon. An increase in equipment vibration, not relating directly to a specific component, is oftentimes caused by one of the many component natural frequencies that are present on every system or object that exists in the known universe. It is a complicated topic overall, but it can be simplified by limiting the idea to how it affects us as equipment reliability specialists.
Effects of Resonance
The effect of resonance can be quite dramatic, I have seen an 11-inch shaft that had sheared in half as a result of resonance. The bending motion of this fault is often destructive, think of a paperclip being bent back and forth continuously at 1800 times per minute! That is what is happening to your machine shaft when it is operating at critical speed.
Techniques in Identifying Resonance
Using vibration analysis, we can identify resonance through a couple of techniques:
Run-up / Coast-down
This is a test where we take a series of spectral data in conjunction with a tachometer and plot it using either a waterfall or bode-type plot in which it is made quite simple to pick out where the machine’s natural frequencies are.
A bump test is where we excite the natural frequencies by “ringing” them using a special hammer or simple rubber mallet, depending on what you have on hand. Ringing the natural frequency in this method is similar to an opera singer pinging a wine glass to find the frequency in which he matches his pitch to it, prior to destroying the glass with nothing more than his voice (and the resonant frequency in the glass itself). You capture the frequency on the spectrum and identify it using the cursor.
Regardless of which test you use, you should come up with the same result.
Another important factor to consider when you have determined your equipment is running at or near its critical speed is how natural frequency physically deflects components that are affected by it. If you have a shaft operating at its 1st harmonic of natural frequency, it will have a single bending mode in which the vibration is lowest at its structural points (bearings) this is the “Nodal Point” and highest in the center “The Antinode point”. At its 2nd harmonic, on the other hand, you will have two bending modes divided evenly in between the supports. There are theoretically an infinite number of harmonics, and this pattern of bending modes goes on forever. If you make changes at the “Nodal Point”, it will have little to no effect on the natural frequency of the system, but if you make changes to the “Antinode”, you will see a drastic change occur. The Node and Antinode are easiest to find by amplitude of vibration where the “Antinode” will of course have the most vibration and the “node” will have the least.
Changing Natural Frequency
Fixing or changing a system’s natural frequency is as simple as making a change to either the stiffness or the mass of the system. By increasing the stiffness, you will increase the natural frequency, like how if you tighten a guitar string it rings at a higher tone. Decreasing stiffness will therefore lower the natural frequency. Changing the mass of the system is another option wherein increasing the mass will lower the natural frequency similar to how an upright bass has a lower tone or frequency than a violin.
Often times it is easier to move a piece of equipment out of operating at its natural frequency by changing the speed it operates in, maybe a bigger or smaller sheave if it’s a belt drive application or tuning the VFD to avoid these speeds. Other methods of changing a system’s natural frequency can include adding mass (or a big magnet) to a structure, adding support structurally to alter the stiffness of the components, it could be as simple as tightening a bolt or as intensive as rebuilding an entire base, or adding framework. Either way, you will improve the life expectancy of your equipment by making sure to never run anything at or near its critical speeds.