Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 2, Mechanical Resonance
Section 2, Considerations for the Probability of Resonance
Every machine part, every span of steel, aluminum and even concrete has resonant frequencies. Theoretically, every segment of pipe, concrete base, rotor, etc. could be suspected as the possible resonant part. Yet, certainly not all possibly resonant parts have an equal chance of actually resonating. For example, rotor resonant frequencies are almost always designed to not coincide with rotor speeds. Almost never will rotors such as armatures, rotary pump assemblies, fans, blowers, or new papermachine rolls be resonant to their own running speeds. However, variable speed rotors such as in turbines, centrifugal compressors and papermaking machines that have their speeds increased over when new, do have more chance of being resonant to their own running speeds. Yet, so few rotors are actually resonant that the writer estimates that for over 95 percent of all rotating machines, there is less than 1 percent possibility that rotor resonance is part of the problem.
Update's experience is that the largest exception to the above observation is with smaller diameter papermachine rolls that are run at higher speeds than when new. Such rolls have a considerably higher chance of being resonant and have almost equal possibility of exacerbating the vibration problem as the possibility for resonance in the non-rotating structural support member. The same could be true for some fan or blower rotors, especially those with relatively long shafts. The manufacturer attempts to keep rotor resonance frequency away from operating rpm. However, the same model fan or blower is used for different output, depending on process requirements. As a result, the various applications require different speeds which could result in a specific model not being resonant for most applications and resonant in one. (For more on rotor resonance, see chapters on "Resonant Whirl" and "Removing Whip From a Previously Balanced Rotor or Roll.")
Apart from the considerations above, machine vibration that is magnified by resonance is almost always due to a non-rotating part such as a segment of steel base, pedestal, a support column, beam, brace or span of pipe. A non-rotating part may be the source for amplitude magnifying resonance in approximately 20 percent of all machines. However, many specialists may be surprised at this number as their own experience does not reveal resonance magnification in one out of five machines. The reason might be that they too often don't recognize that they are working with a resonant machine unless the amplitude is magnified considerably, such as when the source frequency and a part's resonant frequency are very closely tuned to each other (e.g. within 5 percent of the actual resonance frequency.) With very large magnifications, resonance is easily suspected. However, when the source vibration is, for example, about 15 percent away from the peak of the resonance curve, the magnification is not as large, and resonance may not be suspected. Yet, such partial resonance could easily double or triple the vibration amplitude. Using the various methods given in this textbook to detect resonance, will reveal complete or partial resonance.
Recognizing that even partial resonance decreases machinery bearing and seal life (as well as increasing the possibility for creating fatigue cracks), it is important to know that resonance is much more common than suspected. The writer has never conducted or seen a survey on how often resonance occurs, but with over 40 year's experience with the subject, estimates at least 20 percent of all running machines have some magnification due to partial or to full resonance of such parts as pipe lengths, pedestals, portions of bases (especially steel), covers, beams, columns, bearing housings, etc. Or, sometimes a resonant part may not be one that is easily suspected.
For example, it may be difficult to imagine that a solid concrete base is actually a "spring system" with several resonance frequencies. Equally difficult to visualize as a spring system, are relatively rigid cast steel bearing housings and supports. Yet, they too are spring systems with resonant frequencies. Bearing housings are usually relatively rigid resulting in resonance frequencies that are higher than moderate running speeds, such as 1800 rpm or under in the Western hemisphere and 1500 rpm and under in most of the rest of the world. In Europe, for example, Update has experienced relatively rigid bearing housing support resonances at 3000 rpm. However, it probably is more common for a bearing housing to have one of its resonant frequencies resonated by a relatively high frequency source, such as an impeller's rpm x number of vanes or a gearmesh frequency of rpm x number of teeth. It is also possible for a bearing that has a relatively high failure rate to be mounted in a housing that is resonant to one of the bearing's defect frequencies. It is suspected that such relatively rigid part resonances occur in only about one machine in 100. They are noticed most often when bearing life is exceptionally short, even when the usual corrections for unbalance, misalignment, etc. have been performed properly.
Then there are relatively large and rigid parts that can also resonate a machine's vibration, such as large diameter pipe, concrete decks, columns or beams with cross-sections of large depth. Even relatively thick concrete bases are subject to resonance. Although larger and more rigid parts have resonant frequencies in the same range as most machinery vibration sources, they do not actually resonate unless the source vibration energy is so strong that they can resonate such a large or rigid part. As a result, resonance magnification of such parts magnifies vibration in less than 1 percent of all machines. (For more on this subject, see "Detuning Resonant Part vs Reducing Vibration at the Source" and "Using Shaker to Determine if Fault is Due to Weak Structure or Vibration Course.")
This textbook contains only part of the information in our Practical Vibration Analysis seminar.