Gene Vogel
EASA Pump & Vibration Specialist
When high vibration occurs on a machine at a single frequency and in only one plane, the cause is often a resonant condition. All machines have various natural frequencies. If a machine is “bumped" with sufficient force (not running), it will vibrate at those natural frequencies. Resonance occurs when an exciting force such as 1xRPM, vane pass or other similar force coincides with a natural frequency, resulting in very high vibration at that frequency. Sophisticated techniques such as operating deflection shape (ODS), modal analysis and Bode plots are often used to positively identify resonant conditions. These require dedicated vibration instruments and a well-trained technician. But there are simpler tests that can help identify resonance.
The simplest test (Figure 1) for resonance is a bump test. By recording a simple spectrum while bumping the machine (not running), the spectrum will indicate natural frequencies that might be excited by dynamic forces when the machine is running. First, it’s important to know the frequency of the high vibration when the machine is running. That can be determined again with a simple spectrum when the machine is running. If the frequency of the high vibration matches with a natural frequency indicated by the bump test, resonance is contributing to the high vibration amplitude.
When conducting the bump test, it’s important to bump the machine and measure the vibration in the same plane that the high vibration occurs. A block of wood or a soft faced mallet can be used to bump the machine. Avoid using hard faced hammers which will make a lot of noise but not excite the lower natural frequencies. And don’t use “dead blow” hammers that don’t bounce off the machine – the hammer will absorb the vibratory energy damping the response. Some vibration instruments have special setups for bump tests, but bumping the machine repeatedly while recording a standard spectrum will give satisfactory results.
Another easy test for resonance is to vary the speed of the machine and monitor the vibration amplitude. The plot below (Figure 2) illustrates the vibration amplitude response as the machine speed is varied through the resonance frequency. The amplitude will increase as the speed approaches the natural frequency and decrease after it has passed through that natural frequency. If the high vibration occurs just as the machine reaches maximum rotating speed, only the bottom half of the peak will be evident, and the vibration amplitude will decrease sharply when the speed is reduced from maximum speed.
If the speed of the machine cannot be varied in operation, it may be possible to observe the resonance characteristics when the machine is turned off and coasts down. However, there are some conditions that may mask or mimic the resonance indications. If the machine load causes the machine to reduce speed very quickly, the peak in amplitude might not be discernible. Also, an electric motor may have electrical vibration which would instantly cease once the power was disconnected, and the subsequent drop in amplitude could be mistaken for resonance.
Stiffness and mass are the two primary factors that control natural frequencies. (Damping also plays a role but to a lesser degree.) The mass of a machine typically does not change, so if a resonant condition develops it often is because deterioration of the machine base or foundation has reduced the stiffness of the mounting. Measuring the vibration amplitude at the machine mounting feet may identify the root cause of a resonant condition that has developed. Measure the vibration amplitude in the vertical direction on top of the foot, and measure the amplitude on the component to which it is bolted right next to the foot. If there is significant difference in the amplitudes, the foot is loose and may be the reason resonance has occurred. Experienced vibration technicians often perform this test with their “finger vibrometer” before even beginning a detailed analysis with a vibration instrument.
High vibration due to resonance is very common on all rotating machinery. A thorough diagnosis is needed before corrective measures are recommended. But these simple tests can be used to identify resonance before time and energy are spent attempting to “fix” the problem with balancing, alignment or more extensive repairs to the machine. If resonance is identified, fix the resonant condition first, and other repairs may not be needed.
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