Page 307 - Intro Predictive Maintenance

298 An Introduction to Predictive Maintenance

Figure 14–9 Resonance response.

nance frequency is indicated by the sharpness or width of the response curve, w n, when
2 is
measured at the half-power point. R MAX is the maximum resonance and R MAX
the half-power point for a typical resonance-response curve.

Static
When the natural frequency of a stationary, or nondynamic, structure is energized, it
will resonate. This type of resonance is classified as static resonance and is considered
a nondynamic phenomenon. Nondynamic structures in a machine-train include casings,
bearing-support pedestals, and structural members such as beams, piping, and the like.

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 anneal-
ing line may be energized by the running speed of a roll; however, it can also be made
to resonate by a bearing frequency, overhead crane, or any of a multitude of other
energy sources.

The actual 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 the mass and stiffness of the member. In other words, a large turbo-
compressor’s casing will have a lower natural frequency than that of a small end-
suction centrifugal pump.

Figure 14–10 illustrates a typical structural-support system. The natural frequencies
of all support structures, piping, and other components are functions of mass, span,
and stiffness. Each of the arrows on Figure 14–10 indicates a structural member or
stationary machine component with 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.