1.23. TURBINE CONSTRUCTION
The turbine rotor is one of the most highly stressed parts in the engine. It operates at a temperature of approximately 1,700° F. Because of the high rotational speeds, over 40,000 rpm for the smaller engines, the turbine rotor is under severe centrifugal loads. Consequently, the turbine disk is made of specially alloyed steel, usually containing large percentages of chromium, nickel, and cobalt. The turbine rotor assembly is made of two main parts, the disk and blades.
Nozzle vanes may be either cast or forged. Some vanes are made hollow to allow cooling air to flow through them. All nozzle assemblies are made of very high-strength steel that withstands the direct impact of the hot gases flowing from the combustion chamber.
The turbine blades are attached to the disk by using the "fir tree" design, shown in figure 1.27, to allow for expansion between the disk and the blade while holding the blade firmly to the disk against centrifugal loads. The blade is kept from moving axially either by rivets or special locking devices. Turbine rotors are of the open-tip type as shown in figure 1.27, or the shroud type as shown in figure 1.28.
The shroud acts to prevent gas losses over the blade tip and excessive blade vibrations. Distortion under severe loads tends to twist the blade toward low pitch, and the shroud helps to reduce this tendency. The shrouded blade has an aerodynamic advantage in that thinner blades can be used with the support of the shroud. Shrouding, however, requires that the turbine run cooler or at reduced rpm because of the extra mass at the tip.
Blades are forged or cast from alloy steel and machined and carefully inspected before being certified for use. Manufacturers stamp a "moment weight" number on the blade to retain rotor balance when replacement is necessary. Turbine blade maintenance and replacement are covered in a separate lesson.
|David L. Heiserman, Editor||
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Revised: June 06, 2015