Design and Stress Analysis of Extraterrestrial ... - The Black Vault

Resonance modes can be shown on the frequency characteristic
of a single-disk system (Fig. 3.79). For this, along with the
angular frequency of natural vibrations, the freq'iency of the exciting
force will be plotted on the axis of the ordinates. The intersection
points of the "excitation rays" with the curves of the frequency
characteristic give the values of the resonance frequencies for the
natural vibrations of the system and the corresponding angular
velocities of the rotor.
Methods of combating critical modes
When designing, it is necessary to adopt measures for nonresonance
turbine operation, triroughout the range of working rpms.
Sometimes resonance canno. :', avoided. Then damping is used to
decrease the amplitudes oi' vibration. Let us examine measures which
are designed for Liiis.
1. A shift of critical modes to the region of high rpms. An
increase in critical rpms is possible by increasing the rigidity of
the rotor during bending, which is
achieved either by enlarging the
* shaft's cross section or by introducing additional supports.
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2. shift of critical modes to rpms below working rpms. As
is known, rotors whose critical rpm is less than the working rpm are
called flExible.
Such a shift of critical modes can be achieved
by reducing shaft rigidity during bending.
decrease in
However, ar excessive
rigidity can lead to inadmissible deflections of the
shaft and make it difficult to maintain clearances in the circulatory
part. In adaition, since, in this ,ase, the working rpms are higher
than critical, it is necessary to ensure passage through critical
velocity during acceleration and stopping of the rotor.
Rotor
operation in 'he supercritical region is tempting because, in this
region of velocities w >> p [pa6 = operating; tp = critical],
the rotor is self-centering and this leads to a decrease in the
load on the supports. Therefore, in spite of the above difficulties,
the use of flcxible rotors is advisable. For a safe transition
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