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 highstrength 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 opentip type as shown in figure 1.27, or the shroud type as
shown in figure 1.28.
Figure 1.28. Turbine Blade
"Fir Tree Root"
Shroud.
Figure 1.27.
Turbine Wheel
Open Tip.
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
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