ArticleDynamic Vibration Absorbers
by Ray Kelm, P.E.
This paper covers the topic of vibration absorbers: what they are, what they do, when they should be considered, and how to design them properly. Formulas and plots are provided to aid in the readers understanding. Even stress calculations are provided to aid the designer in prevention of an absorber fatigue failure. For someone wanting to learn more about dynamic vibration absorbers, this brief three page paper is the ticket.
“A dynamic absorber is a simple spring/mass system relatively small in size that is mounted on a larger resonant system for the purpose of reducing its resonant response. The absorber will have sufficient weight to adequately shift the resonant frequency and a design that will not fatigue the absorber spring. The dynamic system shown in Figure 1 is a single-degree-of-freedom spring mass system with a natural frequency at the exciting frequency. After the absorber is added, the system is a two-degree-of-freedom system that includes the absorber mass Ma and the absorber stiffness Ka (Figure 2). The modified system has two resonance frequencies.
The effect of the absorber on the resonant response is given in Figure 3. The response of the system shown in Figure 1 has a peak at the resonant frequency of about 60 Hz. After the absorber is added, the system resonance splits into two resonant peaks around the initial peak. When the mass and stiffness of the dynamic absorber are properly tuned, the vibration amplitude at the initial resonant peak is greatly reduced (see Figure 3).
Design of the Absorber
The mass and stiffness of the absorber must be selected so that the natural frequency of the absorber, if mounted on a rigid base, is at the resonant frequency of the initial system. Note that the system with the absorber mounted on the machine will have two resonant frequencies on either side of the original resonant frequency. The forced response of the absorber mass should not overstress the absorber spring. After the absorber is installed, its mass will vibrate considerably more than the original mass without the absorber.
First, a dynamic absorber will not work if the main system is not resonant. Installation of a dynamic absorber on a non-resonant system will likely increase the vibration response. Second, damping of the absorber will result in high vibration levels of the main mass at the optimal tuning frequency but will reduce response at the resonant frequencies. The most effective dampers will typically not be damped for fixed-speed equipment.
Third, dynamic absorbers are effective only at one frequency. A variable-speed machine is not a good candidate for an absorber unless it is well damped to limit the vibration of the main mass at resonant peaks. Fourth, allowable stress of the spring design should be held well below the endurance limit for the spring material. Some materials are not well suited for absorber springs because they do not have an endurance limit.”
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