pressure across its blades, as in an airfoil. See
same time, space is provided by the notches for
figure 1-33.
thermal expansion of the disc.
The impulse-reaction turbine combines the
The turbine shaft, shown in figure 1-31, is
rotational forces described in the two previous
made from low-alloy steel. It must be capable of
turbines. It derives its rotation from the weight
absorbing high torque loads, such as when a heavy
of air striking the turbine blades and the airfoil
axial-flow compressor is started.
reaction of air passing over the blade's surface.
The turbine wheel is a dynamically balanced
The methods of connecting the shaft to the
unit with blades attached to a rotating disc. The
turbine disc vary. One method used is welding.
disc is attached to the main power-transmitting
The shaft is welded to the disc, which has a butt
shaft of the engine. The jet gases leaving the
or protrusion provided for the joint. Another
turbine nozzle vanes act on the blades of the
method is by bolting. This method requires that
turbine wheel, causing the assembly to rotate at
the shaft have a hub that matches a machined
a very high speed. The high rotational speed
surface on the disc face. The bolts are then
causes heavy centrifugal loads on the turbine
inserted through holes in the shaft hub and
wheel. The elevated temperatures result in a lower-
anchored in tapped holes in the disc. Of the two
ing of the strength of the material. The engine
methods, bolting is more common.
speed and temperature must be controlled to keep
To join the turbine shaft to the compressor
turbine operation within safe limits.
rotor hub, make a splined cut on the forward end
The turbine wheel, without blades, is known
of the shaft. The spline fits into a coupling device
as a turbine disc. The disc acts as an anchoring
between the compressor and turbine shafts. If a
part for the turbine blades. Since the disc is
coupling is not used, the splined end of the
attached to the rotor shaft, the exhaust gas energy
turbine shaft may fit into a splined recess in the
extracted by the blades is imparted to the shaft.
compressor rotor hub. The axial compressor
The disc rim exposes the hot gases passing
engine may use either of these methods.
through the blades and absorbs considerable heat
from these gases. In addition, the rim also absorbs
There are various ways of attaching turbine
heat from the turbine buckets by conduction.
blades or buckets, some similar to compressor
Hence, disc rim temperature slopes are quite high
blade attachment. The most satisfactory method
and well above the temperatures of the more
used is the fir-tree design, shown in figure 1-34.
remote inner portion of the disc. As a result of
The blades are retained in their respective
these temperature slopes, thermal stresses are
grooves by a variety of methods; some of the more
added to the stresses due to rotation.
There are various methods provided to relieve,
common ones are peening, welding, locking tabs,
at least partially, these stresses. One such method
is the incorporation of an auxiliary fan somewhere
ahead of the disc. Usually rotor-shaft driven, it
forces cooling air back into the face of the disc.
Another method of relieving the thermal
stresses of the disc follows as incidental to blade
installation. The disc rims are notched to conform
with the blade root design. The disc is made
adaptable for retaining the turbine blades. At the
Figure 1-34.-Turbine blade with fir-tree design root and
tab lock method retention.
Figure 1-33 .-Impulse and reaction blades.
1-25
