Apparatus and method for reducing blade flop in steam turbine

A steam turbine of the type employing a plurality of axial entry, integral shroud blades comprises spring means resident in a slot machined in at least one face of each shroud. The spring means urge against the adjacent shroud and reduce relative motion between adjacent blades.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to steam turbines, and more 
specifically to an apparatus and method for reducing blade "flop" that may 
occur during turning gear operation. The present invention has particular 
application to steam turbines of the type employing "axial entry, integral 
shroud" blades, but is not limited thereto. 
2. Description of the Prior Art 
Steam turbines of the type employing "axial entry, integral shroud" blades 
comprise a rotor with a disc (i.e., the portion of the rotor that holds 
the blades) having a plurality of generally fir tree-shaped, generally 
axially extending grooves, with the blades circularly disposed 
therearound. Each blade has a generally fir tree-shaped root at a proximal 
end thereof in registration with one of the grooves and a shroud integral 
with the blade at a distal end thereof. As is known, blade "flop" may 
occur during turning gear operation of the turbine, i.e., the blade root 
may rock circumferentially, and even axially, in the groove with which it 
is registers when the centrifugal force is insufficient to urge the root 
radially outward, thereby causing root/groove fretting and undesired 
noise. Additionally, gaps between adjacent shrouds may open under hot disc 
conditions and contribute to blade flop. Moreover, the mating faces of 
adjacent shrouds may wear from snubbing therebetween. Obviously, these are 
undesirable conditions. 
SUMMARY OF THE INVENTION 
An apparatus for reducing blade flop comprises spring means resident in a 
generally axially extending slot disposed in at least one face of each 
shroud. The spring means urges against the shroud adjacent the slot 
whereby the force exerted by the spring means reduces relative motion 
between adjacent blades. According to one embodiment of the invention, 
there is only a single slot disposed in each shroud. According to another 
embodiment of the invention, there is a generally axially extending slot 
disposed in both faces of each shroud, and the slots in opposing faces of 
adjacent shrouds are substantially aligned so that each spring means 
resides in the aligned slots. An access slot disposed in the top of each 
shroud that communicates with the slot in the face of the shroud may be 
provided to aid in the insertion and removal of the spring means. 
According to the preferred embodiment of the invention, each spring means 
comprises a leaf spring. According to another embodiment of the invention, 
each spring means comprises a washer such as a Belleville washer. 
A method of assembling the turbine to reduce blade flop comprises the steps 
of machining a generally axially extending slot in at least one face of 
each shroud and inserting a spring means in each slot so that each spring 
means urges against the adjacent shroud. 
Reference is made to commonly assigned copending application Ser. No. 
18,322, filed Feb. 24, 1987, now U.S. Pat. No. 4,767,273 for a related but 
alternate solution to the problem addressed by this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, wherein like numerals represent like 
elements, there is illustrated in FIG. 1 a portion of a turbine labelled 
generally 10 comprising a disc 20 and a plurality of blades 12. Each blade 
12 comprises a platform 14, a root 16 disposed at the proximal end thereof 
and an integral shroud 26 disposed at the distal end thereof. The disc 20 
comprises a plurality of generally axially extending grooves 18 disposed 
therearound. As illustrated, each root 16 and each groove 18 have a 
generally fir tree shape and each root 16 is in registration with one of 
the grooves 18. 
Although it is often desirable to minimize the gap between adjacent 
platforms 14, there may be a small clearance 22 between adjacent platforms 
14 which may open further under hot disc conditions. Additionally there 
may be a small clearance 24 between each blade root 16 and the edges of 
the groove 18 with which the root 16 registers. Still further, there may 
be a small clearance 28 between adjacent shrouds 26, 26' which may also 
open further under hot disc conditions. The existence of the gaps 22, 24, 
and 28 may result in blade "flop" during turning gear operation. Still 
further, the mating faces, e.g., 40, 46 (FIG. 2) of adjacent shrouds may 
wear from snubbing. 
As illustrated in FIG. 2, shroud 26 has a pair of faces 40, 42. Shroud 26' 
has a pair faces 44, 46 and shroud 26" has a pair of faces 48, 50. Face 40 
of shroud 26 opposes face 46 of shroud 26'. (Shroud 26 is shown as being 
only partially inserted, but it should be understood that when the shroud 
26 is fully inserted by sliding in the direction of arrow 36, the faces 40 
and 46 will fully oppose each other). Similarly, face 44 of shroud 26' 
opposes face 50 of shroud 26'. Thus, the shroud of each blade has a pair 
of faces that oppose faces of adjacent shrouds. 
As also shown in FIG. 2, there is at least one generally axially extending 
slot disposed in at least one of the faces of each shroud. Thus, there is 
a generally axially extending slot 30 disposed in the face 42 of shroud 
26, a generally axially extending slot 31 disposed in the face 46 of slot 
26', and so on. See FIGS. 2 and 3. If desired, a generally axially 
extending slot may be disposed in both faces of each shroud. Thus, shroud 
26 may be provided with a second generally axially extending slot 32 in 
the face 40 thereof. Shrouds 26', 26", etc. may also be provided with 
similar second slots as shown. Preferably, the slots in opposing faces of 
adjacent shrouds are substantially aligned in the axial direction, as 
shown in FIG. 2. Thus, when shroud 26 is fully inserted, the slot 32 will 
be aligned with the slot 31, and so on. 
As shown in FIG. 3, there may be an access slot disposed in the top of each 
shroud that communicates with the slot in the face of each shroud for 
reasons described below. 
Resident in each of the generally axially extending slots is a spring means 
34. As will be appreciated, when installed, each spring means 34 will urge 
against the adjacent shroud. It will also be appreciated that the force 
exerted by the spring means 34 will reduce relative motion between 
adjacent blades. 
The spring means 34 may be a leaf spring, such as illustrated in FIG. 5. 
Alternatively, the spring means 34 may be a washer spring, such as a 
Belleville washer. If a Belleville washer is used, the shape of the slot 
should be modified accordingly. 
FIG. 4 illustrates in further detail the cooperation between the spring 
means 34 and adjacent shrouds, 26', 26" and the substantial radial 
alignment of slots 31, 32. 
According to the present invention, a method of assembling a turbine of the 
type hereinbefore described comprises the following steps. First, a 
generally axially extending slot is machined in at least one face of each 
shroud. A first blade 12 is inserted in the disc 20 by registering its 
root 16 with a groove 18. A spring means 34 is placed in the slot and held 
therein while the root 16 of another blade 12 is inserted, as shown by 
arrow 38, into the next groove 18 (see FIG. 2). As will be appreciated, 
the spring means 34 compresses against the faces of adjacent shrouds to 
prevent movement therebetween. Assembly in this manner is continued in the 
direction of the arrow 38, around the disc, until all blades have been 
installed. 
The access slots provided in the top of the shrouds permit easy insertion 
and removal of the spring means, e.g., by the use of a screwdriver. 
The present invention provides the following advantages: 
(1) Blade flop during turning gear operation is minimized. This is 
particularly true in designs where a nominal gap is intentionally provided 
between adjacent shrouds to avoid shroud compression and/or buckling. 
(2) Assembly of the turbine is simplified since the assembler no longer 
need be concerned if there are gaps between adjacent shrouds. 
(3) Wear between opposing faces of adjacent shrouds will be reduced. 
(4) Blades whose shroud faces are already excessively worn can be saved by 
machining the slot and providing the spring means of the present 
invention. 
(5) The spring means may serve as an additional means of locking the 
"closing blade", i.e., the last blade inserted in a row. As is known, a 
special locking device is currently required to lock the "closing blade". 
See, e.g., commonly assigned copending patent application Ser. No. 844,496 
filed Mar. 26, 1986. 
(6) The spring means may help to increase shroud interference (snubbing) in 
the second mode of vibration, which is predominantly in the axial 
direction. 
(7) As illustrated in FIG. 6, implementation of the present invention 
improves the blades' vibratory characteristics. Increasing shroud 
interference may help to raise system frequency which, in turn, results in 
lower blade stresses. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof and, 
accordingly, reference should be made to the appended claims, rather than 
to the foregoing specification, as indicating the scope of the invention.