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

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Another means <strong>of</strong> unloading consists <strong>of</strong> the fact that the line<br />

<strong>of</strong> the centers <strong>of</strong> gravity for the blade cross section is a specially<br />

selected curve. <strong>The</strong> fit <strong>of</strong> the blade in the web is radial in this<br />

case, without inclination.<br />

<strong>The</strong> shortcoming <strong>of</strong> these methods <strong>of</strong> bending-moment compensation<br />

is their one-nnde nature. Actually, bending moments from centrifugal<br />

forces depend upon the rotational speed <strong>of</strong> the rotor, <strong>and</strong> moments<br />

from gas forces on the density <strong>of</strong> the gas <strong>and</strong> other parameters <strong>of</strong><br />

the flow, which are different for various conditions <strong>of</strong> turbine<br />

operations.<br />

Usually turbines are designed so that the bending moment from<br />

centrifugal force is a certain fraction <strong>of</strong> gas-dynamic moment, i.e.,<br />

Mxc= -- Mx;<br />

=Mc--M,<br />

(3.16)<br />

where y is<br />

the coefficient <strong>of</strong> compensation.<br />

<strong>The</strong> value <strong>of</strong> y is selected with the operation <strong>of</strong> blades in<br />

various modes taken into account <strong>and</strong> usually comprises 0.3-0.6. A<br />

blade with linear stagger makes it possible to perform the assigned<br />

compensation only in one cross section, for which the cross section<br />

with the greatest bending stresses is naturally taken. <strong>The</strong> root<br />

section is usually such a section.<br />

<strong>The</strong>re is a more effective waý <strong>of</strong> unloading the blade by<br />

centrifugal forces - the attachment <strong>of</strong> a blade in a hinged jtint<br />

so that the total bending moment in the framing will equal zero<br />

on any mode <strong>of</strong> operation for the turbine wheel. <strong>The</strong> blade, in this<br />

case, can be lighter than with rigid framing. However, due to design<br />

complexit,. this can only be done for cold blades (long compressor<br />

blades) <strong>and</strong> only for the compensation <strong>of</strong> bending moments in the plane<br />

<strong>of</strong> rotation <strong>of</strong> the disk.<br />

256

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