Static resonance is a function of the natural frequency of nondynamic, or stationary, machine components (e.g., casings and bearing support pedestals) and structural members (i.e., beams, piping, etc.). When one or more of the natural frequencies of a stationary structure is energized or excited, it resonates. Because static resonance is a nondynamic phenomenon, it is generally not associated with the primary running speed of any associated machinery. Rather, the source of static resonance can be any energy source that coincides with the natural frequency of any stationary component. For example, an I-beam support on a continuous annealing line may be energized by the running speed of a roll. However, it also can be made to resonate by a bearing frequency, overhead crane, or other such energy source.
The resonant frequency depends on the mass, stiffness, and span of the excited member. In general terms, the natural frequency of a structural member is inversely proportional to its mass and stiffness. In other words, a large turbocompressor's casing will have a lower natural frequency than that of a small end-suction centrifugal pump. Figure 1 illustrates a typical structural support system and the discrete natural frequency locations. Each of the arrows indicates a structural member or stationary machine component having a unique natural frequency. Note that each time a structural span is broken or attached to another structure, the stiffness changes. As a result, the natural frequency of that segment also changes. While most stationary machine components move during normal operation, they are not always resonant. Some degree of flexing or movement is common in stationary machine-trains and structural members. The amount of movement depends on the spring constant, or stiffness, of the member.