Page 306 - Intro Predictive Maintenance

Failure-Mode Analysis 297

true mechanical imbalance, the blade-pass and vane-pass frequency components are
broader and have more energy in the form of sideband frequencies.

In most cases, this failure mode also excites the third (3¥) harmonic frequency and
creates strong axial vibration. Depending on the severity of the instability and the
design of the machine, process instability can also create a variety of shaft-mode
shapes. In turn, this excites the 1¥, 2¥, and 3¥ radial vibration components.

14.1.7 Resonance
Resonance is defined as a large-amplitude vibration caused by a small periodic
stimulus with the same, or nearly the same, period as the system’s natural vibration.
In other words, an energy source with the same, or nearly the same, frequency
as the natural frequency of a machine-train or structure will excite that natural fre-
quency. The result is a substantial increase in the amplitude of the natural frequency
component.

The key point to remember is that a very low amplitude energy source can cause
massive amplitudes when its frequency coincides with the natural frequency of a
machine or structure. Higher levels of input energy can cause catastrophic, near instan-
taneous failure of the machine or structure. Every machine-train has one or more
natural frequencies. If one of these frequencies is excited by some component of the
normal operation of the system, the machine structure will amplify the energy, which
can cause severe damage.

An example of resonance is a tuning fork. If you activate a tuning fork by striking it
sharply, the fork vibrates rapidly. As long as it is held suspended, the vibration decays
with time; however, if you place it on a desktop, the fork could potentially excite the
natural frequency of the desk, which would dramatically amplify the vibration energy.

The same thing can occur if one or more of the running speeds of a machine excite
the natural frequency of the machine or its support structure. Resonance is a destruc-
tive vibration and, in most cases, it will cause major damage to the machine or support
structure.

Two major classifications of resonance are found in most manufacturing and process
plants: static and dynamic. Both types exhibit a broad-based, high-amplitude fre-
quency component when viewed in an FFT vibration signature. Unlike meshing or
passing frequencies, the resonance frequency component does not have modulations
or sidebands. Instead, resonance is displayed as a single, clearly defined peak.

As illustrated in Figure 14–9, a resonance peak represents a large amount of energy.
This energy is the result of both the amplitude of the peak and the broad area under
the peak. This combination of high peak amplitude and broad-based energy content
is typical of most resonance problems. The damping system associated with a reso-