Figure 1-61. Turbine disk. in configuration and construction features, there is one  characteristic peculiar to all turbine inlet nozzles: the nozzle  vanes must be constructed to allow thermal expansion.  Otherwise, there would be severe distortion or warping of  the metal components because of rapid temperature changes. The thermal expansion of turbine nozzles is accomplished  by one of several methods. One method necessitates loose  assembly of the supporting inner and outer vane shrouds.  [Figure 1-63A] Each vane fits into a contoured slot in the shrouds, which  conforms to the airfoil shape of the vane. These slots are  slightly larger than the vanes to give a loose fit. For further  support, the inner and outer shrouds are encased by inner  and outer support rings, which provide increased strength  and rigidity. These support rings also facilitate removal of  the nozzle vanes as a unit. Without the rings, the vanes could  fall out as the shrouds were removed. There are three types of turbine blades: the impulse turbine  blade, reaction turbine blade, and the reaction-impulse  turbine blade. The impulse turbine blade is also referred to  as a bucket. This is because as the stream of air strikes the  center of the blade it changes the direction of the energy as it  causes the blades to rotate the disk and rotor shaft. The turbine  nozzle guide vanes can usually be adjusted during engine  overhaul and assembly in order to increase the efficiency of  the air stream striking the blades or buckets of the turbine.  [Figure 1-62] Another method of thermal expansion construction is to fit the  vanes into inner and outer shrouds; however, in this method  the vanes are welded or riveted into position.  [Figure 1-63B] Some means must be provided to allow thermal expansion;  therefore, either the inner or the outer shroud ring is cut  into segments. The saw cuts separating the segments allow  sufficient expansion to prevent stress and warping of the vanes.  The rotor element of the turbine section consists essentially  of a shaft and a wheel.  [Figure 1-64]  The turbine wheel is  a dynamically balanced unit consisting of blades attached  to a rotating disk. The disk, in turn, is attached to the main  power-transmitting shaft of the engine. The exhaust gases  leaving the turbine inlet nozzle vanes act on the blades of  the turbine wheel, causing the assembly to rotate at a very  high rate of speed. The high rotational speed imposes severe  centrifugal loads on the turbine wheel, and at the same  time the elevated temperatures result in a lowering of the  strength of the material. Consequently, the engine speed and  temperature must be controlled to keep turbine operation  within safe limits. Reaction turbine blades cause the disk to rotate by the  aerodynamic action of the airstream directed to flow past  the blade at a particular angle in order to develop the most  efficient power from the turbine engine.  [Figure 1-62] The reaction-impulse turbine blade combines the action of  both the impulse and reaction blades designs. The blade has  more of the bucket shape of the impulse blade at the blade root  and it also has more of an airfoil shape of the reaction blade on  the second half of the blade toward the outer end of the blade. The second purpose of the turbine inlet nozzle is to deflect  the gases to a specific angle in the direction of turbine wheel  rotation. Since the gas flow from the nozzle must enter the  turbine blade passageway while it is still rotating, it is essential  to aim the gas in the general direction of turbine rotation. The turbine disk is referred to as such without blades. When  the turbine blades are installed, the disk then becomes the  turbine wheel. The disk acts as an anchoring component for  the turbine blades. Since the disk is bolted or welded to the  shaft, the blades can transmit to the rotor shaft the energy  they extract from the exhaust gases. The disk rim is exposed to the hot gases passing through the  blades and absorbs considerable heat from these gases. In  addition, the rim also absorbs heat from the turbine blades  by conduction. Hence, disk rim temperatures are normally  high and well above the temperatures of the more remote  The turbine inlet nozzle assembly consists of an inner shroud  and an outer shroud between which the nozzle vanes are  fixed. The number and size of inlet vanes employed vary with  different types and sizes of engines.  Figure 1-63  illustrates  typical turbine inlet nozzles featuring loose and welded vanes. The vanes of the turbine inlet nozzle may be assembled  between the outer and inner shrouds or rings in a variety  of ways. Although the actual elements may vary slightly  1-48