elements necessary for its retention. This method
and riveting. Figure 1-35 shows a typical turbine
is shown in figure 1-36. The blade root has a stop
wheel using riveting for blade retention.
on one end of the root. The blade is inserted and
A method of blade retention used quite
moves in one direction only, while on the opposite
frequently is peening, and it applies in various
end of the blade is a tang. This tang is peened over
ways. Two of the most common applications of
to secure the blade in the disc.
peening are described in the following paragraphs.
Turbine blades may be either forged or cast,
One method of peening requires that a small
depending on the composition of the alloys. Most
notch be ground in the edge of the blade fir-tree
blades are precision cast and finish-ground to the
root before blade installation. The blade inserts
desired shape.
into the disc. The notch is filled with the disc
Most turbines in use are open at the outer
metal, which is "flowed" into it through a small
perimeter of the blades; there is a second type
punch mark made in the disc, adjacent to the
called the shrouded turbine. The shrouded
notch. The tool used for this job is similar to a
turbine blades, in effect, form a band around the
center punch, and is usually manufactured locally.
outer perimeter of the turbine wheel. This
Another method of peening is to construct the
improves efficiency and vibration characteristics
blades root in such a way as to contain all the
and permits lighter stage weights; on the other
hand, it limits turbine speed and requires more
blades (fig. 1-37).
In turbine rotor construction, it may be
necessary to use turbines of more than one stage.
A single turbine wheel often cannot absorb
enough power from the exhaust gases to drive the
parts dependent on the turbine for rotative power.
In a turbojet engine, these parts are the com-
pressor and engine-driven accessories. In the
turboprop engine, these parts are the propeller and
its reduction gearing.
A turbine stage consists of a row of stationary
vanes or nozzles, followed by a row of rotating
Figure 1-35.-Riveting method of turbine blade retention.
Figure 1-37.-Shrouded turbine blades.
Figure 1-36.-Turbine bucket, featuring peening method of
Figure 1-38.-Multi-rotor turbine.
blade retention.
1-26
