Figure 1-27.--Cooling comparisons between a nonair-cooled vane and an air-cooled vane.
The turbine wheel is a dynamically balanced
The turbine shaft is usually made from low-
unit consisting of blades attached to a rotating
alloy steel. It must be capable of absorbing high
disk. The disk in turn is attached to the rotor shaft
torque loads, such as exerted when a heavy axial-
of the engine. When in an unbladed form, we
flow compressor is started. The methods of
refer to this section of the unit as the turbine disk.
connecting the shaft to the turbine disk vary. One
The portion of the unit consisting of the turbine
method used is welding. The shaft is welded to
blades is called the turbine wheel. The disk acts
the disk, which has a butt or protrusion provided
as an anchoring component for the turbine blades.
for the joint. Another method is by bolting. This
This enables the blades to transmit to the rotor
method requires that the shaft have a hub that
shaft the energy they extract from the exhaust
matches a machined surface on the disk face. The
gases.
bolts then are inserted through holes in the shaft
The disk rim is exposed to the hot gases
hub. They are anchored in tapped holes in the
passing through the blades and absorbs con-
disk. Of the two methods, the latter is more
siderable heat from these gases. In addition,
common.
because the rim also absorbs heat from the
turbine blades by conduction, the disk rim
The turbine shaft must have some means for
temperatures are higher than the temperatures of
joining the compressor rotor hub. This is usually
the remote inner portion of the disk. As a result
accomplished by a splined cut on the forward end
of these temperature gradients, thermal stresses
of the shaft. The spline fits into a coupling device
are added to the stresses caused by rotation.
between the compressor and the turbine shafts.
Various means are provided to relieve these
If a coupling is not used, the splined end of the
stresses. One way is to incorporate an auxiliary
turbine shaft fits into a splined recess in the
fan, which is usually rotor-shaft driven,
compressor rotor hub. The centrifugal compressor
somewhere ahead of the disk. This will force
engines use the splined coupling arrangement
cooling air back into the face of the disk. Another
almost exclusively. Axial compressor engines may
method of relieving the thermal stresses of the disk
use either of these methods.
is by the method of blade installation. By notching
the disk rims to conform with the blade root
Various ways of attaching turbine blades are
design, the disk is made able to retain the turbine
in use today. Some ways are similar to the way
blades. This space provided by the notches allows
compressor blades are attached. The most
for thermal expansion of the disk.
satisfactory method used is the fir-tree design
1-23
