B. Turbine nozzle vane assembly with welded vanes A. Turbine nozzle vane assembly with loose-fitting vanes Figure 1-63. Typical turbine nozzle vane assemblies. Figure 1-65. Turbine blade with fir-tree design and lock-tab method  of blade retention.  Figure 1-64. Rotor elements of the turbine assembly.  notch to permit movement of the turbine blade when the  disk is cold.  During engine operation, expansion of the disk  decreases  the clearance. This causes the blade root to fit  tightly in the disk rim. The turbine shaft is usually fabricated from alloy steel.  [Figure 1-64 ]  It must be capable of absorbing the high torque  loads that are exerted on it. The method of connecting the shaft to the turbine disk vary.  In one method, the shaft is welded to the disk, which has a  butt or protrusion provided for the joint. Another method is  by bolting. This method requires that the shaft have a hub  that fits a machined surface on the disk face. Then, the bolts  are inserted through holes in the shaft hub and anchored in  tapped holes in the disk. Of the two connection methods,  bolting is more common. The turbine shaft must have some means for attachment to  the compressor rotor hub. This is usually accomplished by  a spline cut on the forward end of the shaft. The spline fits  into a coupling device between the compressor and turbine  shafts. If a coupling is not used, the splined end of the turbine  shaft may fit into a splined recess in the compressor rotor hub.  This splined coupling arrangement is used almost exclusively  with centrifugal compressor engines, while axial compressor  engines may use either of these described methods. There are various ways of attaching turbine blades, some  similar to compressor blade attachment. The most satisfactory  method utilizes the fir-tree design.  [Figure 1-65] The blades are retained in their respective grooves by a  variety of methods, the more common of which are peening,  welding, lock tabs, and riveting.  Figure 1-66  shows a typical  turbine wheel using rivets for blade retention. The peening method of blade retention is used frequently  in various ways. One of the most common applications of  peening requires a small notch to be ground in the edge of  the blade fir-tree root prior to the blade installation. After  the blade is inserted into the disk, the notch is filled by the  disk metal, which is “flowed” into it by a small punch-mark  made in the disk adjacent to the notch. The tool used for this  job is similar to a center punch. 1-50