What is Resonance?

Resonance is a natural phenomenon that is inherent in all things around us. Everything, from the Earth to molecules and atoms, has what is called a “Natural Frequency” or resonant frequency. When a given part or object is stimulated with enough force and at the parts natural frequency, the part will go into resonance, a condition that will magnify the vibrating frequency. For example, a pipe, base, machine, blade etc. all will have a natural frequency and can resonate if stimulated at the right frequency from a force such as imbalance, misalignment, bent shaft etc. All components in an industrial plant will have their own inherent natural frequency, ready to be stimulated into resonance from all of the plant’s sources of vibration. When this happens, parts begin to fail far faster than normal.

A range of frequencies will represent the resonant zone of an object. As the frequencies increase to the optimum resonant frequency, the magnifying amplitudes will also increase. It is important to understand that resonance is NOT a source of vibration, but it will certainly magnify a source. For example, there are many machinery vibration sources such as imbalance, misalignment, bent shaft, blade pass, gear mesh etc. Any of these sources can be significantly magnified if the source frequency is in the resonance zone of a part that “feels” that source.

Therefore a part can be partially resonant where the stimulation is just starting to get into the resonant zone. While not vibrating as much as it could if the source was at the peak resonant frequency, it can have a significant increase in its vibration amplitude. In fact, a surprisingly large amount of rotating machines in the world are currently experiencing some amount of additional destructive vibration due to the presence of some degree of resonance. That’s why is it so important to find and eliminate the resonances in your plant.

Any object actually has several resonant frequency zones. The most common one is the first resonance, the first resonance zone hit when going up in frequency. However, there are second, third, fourth etc. resonances that can also be stimulated. These are not exact multiples of the first, and will vary depending on the specific conditions of the object, such as the amount of stiffness and mass. Each resonance will actually vibrate in different, characteristic ways.

There are several ways to measure and prove if the vibration is in fact being magnified by a resonant condition or not. A part that gets excited and experiences a resonant condition will vibrate in particular ways. By measuring the amplitude in several equally spaced locations and then plotting the results, one can see the “shape” of the shaking part and compare it to known resonant plot shapes. For example, the first resonance of a straight piece of pipe will be bowed, tight on the ends and wide (larger amplitude) in the middle, like a plucked guitar string. However, the second resonance will have two sections of bowing, making the amplitudes low not only on the ends but also the middle. The third resonance will have three distinct bows etc.

Another characteristic of resonance that can be measured is its affect on phase. The reading from an accelerometer will have one phase below resonance and will shift a full 180 degrees as it go through and past a resonant zone. Therefore a sure way of determining if the vibration is in fact resonant, one can use a strobe light that is triggered by an accelerometer measuring the suspect part. By flashing the strobe on the stimulus, like Update’s Resolator, one can see if there is any shift in the angular phase position which would indicate a resonance is present at that speed or frequency.