Page 103 - Steam Turbines Design, Applications, and Rerating

84 Chapter Four

temperatures diminish progressively from the first stage to the ex-
haust of a multistage turbine, a situation is frequently encountered in
which stage temperatures dictate the need for integral wheels in some
of the head end stages but become sufficiently low to permit shrunk-on
wheels in the later stages. It is this situation that most commonly
results in the combination solid and built-up type of rotor construction.
This type of construction seems to be adaptable to conditions that
are most commonly encountered on large condensing generator drive
applications.

4.4 Built-Up Construction
Now that the basic categories of steam turbine rotor construction and
the uses of each have been discussed, perhaps some of the differences
in the manufacturing of the various types can be best pointed out by
attempting to trace very briefly a rough step-by-step manufacturing
sequence for each of the rotor styles.
After being received, the machining of the rough forging for a built-
up rotor shaft begins on an engine lathe where all facing and turning
operations are accomplished. In the turning of any critical shaft diam-
eters, such as journals, shaft ends, and underwheel diameters, approx-
imately 0.015 to 0.020 in (0.35 to 0.5 mm) is left for grinding to final
dimensions. Provision is made for the location of the wheels by machin-
ing circumferential grooves in the shaft. These narrow shrink ring
grooves are located axially on both sides of each intended wheel hub
location. With turning and finish grinding operations completed, the
next step in the machining sequence is normally the completion of all
necessary milling operations. These include the milling of a keyway for
each of the wheels and for any other keyed rotor components such as
couplings, thrust collars, and governor drive worm gears.
Concurrently with the machining of the rotor shaft, the turbine
wheels and blading are also in the manufacturing process. The rough
wheel forging is machined to the desired profile, and the machining of
the bore is completed. After a final trimming cut on the wheel rim, the
circumferential dovetail groove that is to receive the bucket roots is
machined in the wheel rim. Special care is necessary to achieve the
required fit of wheel dovetail to blade dovetail (Fig. 4.4). After complet-
ing the machining of the bucket airfoils and rivets, the roots on a set of
buckets are custom machined expressly to fit the groove in the partic-
ular wheel that is to receive the blades. This is necessary to achieve the
close tolerances that are required to ensure a satisfactory degree of
load sharing among the four bucket locks. The blades are assembled in
the wheel by inserting each blade individually through a radial slot
that is milled at one point in the wheel rim to provide access of bucket