BUCKET AND WHEEL DOVETAIL DESIGN FOR TURBINE ROTORS
[0001] The present invention relates to turbines and
particularly to dovetail joints between the wheel of a
steam turbine rotor and steam turbine buckets.
BACKGROUND OF THE INVENTION
[0002] Dovetail attachment techniques between turbine
buckets and turbine rotor wheels for steam turbines are
well known in the art. Conventional tangential entry
dovetails on the latter stages of low-pressure rotors
operating in a contaminated steam environment have been
found to be conducive to stress corrosion cracking (SCC).
SCC is accelerated by the stress levels that are present
in the hook fillet regions of typical dovetail
configurations. Normally, these stresses are acceptable
but with contaminated steam, cracks can initiate and, if
left undetected, may grow to a depth that will cause
failure of the wheel hooks. In extreme cases, all the
hooks will fail and buckets will fly loose from the
rotor. Long experience with bucket-to-wheel dovetail
joints has indicated that the wheel hooks crack but that
the bucket hooks do not crack. This is apparently
because the NiCrMoV and similar low-alloy steels used for
low-pressure rotors are much less resistant to SCC than
are the 12Cr steels used for buckets. The steels for the
wheels give the optimum combination of properties
available for overall low-pressure rotor design
considerations. Thus, an effective means of avoiding SCC
in the typical low-pressure steam environment is to
reduce the stresses in the wheel dovetail to acceptable
levels. If the maximum stress in components operating in
a corrosive environment is below the yield strength of
the material, the resistance to SCC is greatly improved.
[0003] Bucket and wheel dovetail designs for steam
turbine rotors have been described and illustrated in
U.S. Patents Nos. 5,474,423, 5,494,408; and 5,531,569, of
common assignee. In U.S. Patent No. 5,474,423 the
dovetail joint design provides four hooks on the rotor
wheel which decrease in thickness from the radially-
outermost hooks to the innermost hooks. Additionally,
fillets are provided between neck portions of the rotor
wheel dovetails and bottom surfaces of the overlying
hooks, with multiple radii, i.e., compound fillets, in
order to decrease the stress concentrations with
increased radii of the fillets. Additional features of
that prior design include a flat surface along the
radially-outermost surface of the hook and in combination
with various forms of compound fillets. In U.S. Patent
No. 5,494,408 different fillet radii are provided between
the hooks. In U.S. Patent No. 5,531,569, compound fillet
radii are disclosed.
BRIEF SUMMARY OF THE INVENTION
[0004] In accordance with the present invention, a
rotor wheel and bucket dovetail joint design is provided
which minimizes concentrated stresses caused by the
centrifugal force of the buckets in the wheel hook
fillets, and permits larger hook fillet radii which
further reduces stress concentration. In accordance with
a principal aspect of the present invention, the rotor
wheel contact surfaces, i.e., the generally radially-
inwardly facing surfaces along the undersides of the
wheel hooks are provided with identical slant surface
angles for each hook of the dovetail at different radii
along the dovetail. It will be appreciated that the
rotor rotation causes the buckets to develop centrifugal
forces which are imposed on the dovetail through the
contact surfaces along the undersides of the wheel hooks.
These forces give rise to stresses in the dovetail with
peak stresses in the fillet, regions of the hooks. The
slant surfaces reduce the stress concentration for a
given fillet radius and permit larger hook fillet radii
that further reduce the stress concentration.
[0005] More particularly, the crush surfaces for
traditional tangential entry dovetails are on an axial-
circumferential plane with a fillet used as a transition
between the crush surface and the neck surface at the
various locations along the dovetail. These two surfaces
are 90° apart in conventional tangential entry dovetails.
In U.S. Patent No. 6,142,737, of common assignee, the two
surfaces are greater than 90° apart but vary from hook to
hook. In the present invention, these crush surfaces are
rotated such that the transition angles between the crush
surfaces, i.e., slant surfaces, and the neck surfaces (in
a radial plane) are greater than 90° and are the same at
each hook radii. The angles of rotation are called slant
angles. Concentrated stresses result when load paths are
forced to change direction. With the slanted crush
surfaces hereof, the change in direction from 90° to
larger angles is less severe and the stress concentration
is therefore lower. The slant crush surface also permits
a larger fillet radius in the same transition distance as
compared to the conventional 90° transition, with a
resulting larger fillet radius and lower concentrated
stress. It will also be appreciated that a slanted crush
surface causes a component of force in the axial
direction which gives rise to bending of the bucket leg
and an axial load on the tang of the wheel dovetail. To
minimize this effect, the slant angle is constant from
hook to hook, i.e., the same slant angle is provided at
each hook radii. Because the slant angles of the crush
surfaces are increased in angle from 90°, the fillet radii
are also increased and stress concentrations thereby
reduced.
[0006] In a further aspect, it will be appreciated
that hook thickness and length control the load sharing
between hooks, as well as the bending and shear stresses
on hooks. Consequently, the hook thickness is varied to
achieve uniform and minimum concentrated stresses, i.e.,
hook thickness increases with decreasing radial height.
[0007] The invention as described herein relates to
both three hook and four-hook dovetail designs. The
invention is also useful with other dovetails with any
number of hooks. Additionally, the invention is not
limited to rotors susceptible to SCC and the benefits and
advantages hereof can be realized for other stress-
causing conditions which initiate cracking in dovetail
hooks such as dovetail cracking in high-temperature
regions when creep is the failure mode rather than SCC.
[0008] In a preferred embodiment according to the
present invention, there is provided a dovetail joint
between a rotor wheel and a bucket rotatable about an
axis, comprising a male dovetail component on the rotor
wheel and a female dovetail component on the bucket, the
male dovetail component receiving the female dovetail
component in a direction tangential to the rotor wheel,
the male dovetail component including a plurality of
circumferentially extending hooks lying on opposite sides
of a plane normal to the axis and bisecting the male
dovetail component, each hook having a generally radially
inwardly-facing surface, the surfaces of at least a pair
of hooks on each of the opposite sides of the plane
defining angles extending away from the plane and toward
and away from the axis, the angles of the surfaces of
each pair of hooks on each of the opposite sides of the
plane being equal to one another.
[0009] In a further preferred embodiment according to
the present invention, there is provided for use in a
dovetail joint between a rotor wheel and a bucket
rotatable about an axis wherein the rotor wheel includes
a male dovetail component for receiving the female
dovetail component in a direction tangential to the rotor
wheel and the male dovetail component includes a
plurality of circumferentially extending hooks lying on
opposite sides of a plane normal to the axis and
bisecting the male dovetail component, each hook having a
generally radially inwardly-facing surface with the
surfaces of each of the hooks on opposite sides of the
plane defining angles extending away from the plane and
toward and away from the axis, the angle of each surface
being egual to the angle of every other surface, a female
dovetail component on each bucket including a plurality
of circumferentially extending hooks generally
complementary to the male dovetail hooks and having
radially outwardly directed angled surfaces generally
complementary to the angled surfaces of the male dovetail
component, the angles of the surfaces of the female
dovetail component being equal tc one another.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0010] FIGURE 1 is a schematic representation of a
typical turbine rotor wheel and bucket dovetail joint;
[0011] FIGURE 2 is a cross-sectional view of a turbine
wheel dovetail in accordance with the present invention;
[0012] FIGURE 3 is an enlarged fragmentary cross-
sectional view of the fillet and tang area of a wheel
and bucket dovetail joint in accordance with the present
invention;
[0013] FIGURE 4 is a cross-sectional view of a bucket
dovetail joint for mating with the dovetail joint of the
wheel dovetail of FIGURE 2; and
[0014] FIGURES 5 and 6 are views similar to FIGURES 2
and 4, respectively, of a further embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Referring to Figure 1, there is illustrated a
rotor body, for example, a shaft 10, mounting a rotor
wheel 12, terminating along its outer radius in a series
of male dovetail components 14. The turbine buckets 16
each include a female dovetail joint 18 along its radial
innermost portion for mating with the male dovetail joint
14, the bucket 16 including a blade 20 extending from the
female dovetail component 18. As will be appreciated,
the dovetail joint is a tangential entry-type dovetail
arrangement.
[0016] In the ensuing description, it will be
appreciated that the dovetails are symmetric in a radial
plane normal to the axis of rotation of shaft 10 and that
it is. accepted practice to refer only to half the
dovetail, i.e., the dovetail hooks along one side of the
radial plane. Thus, the present description with respect
to Figures 2-4 refers to four hooks forming the dovetail,
even though there are actually eight hooks in the
dovetail joint. In conventional practice, the hooks are
referred to sequentially as first, second, third and
fourth hooks from the radially-outermost hook to the
radially-innermost hook. Further, the contact surfaces
between the wheel hooks and the bucket hooks are known as
crush or slant surfaces. The crush or slant surface for
tangential entry dovetails lies on an axial
circumferential plane with a fillet employed as a
transition between the crush surface and the neck surface
of the dovetail. As illustrated in Figure 2, crush
surfaces 22, neck surfaces 24 and fillets 26 between
those surfaces are provided each of the hooks 28, 30, 32
and 34 of the wheel dovetail 36 which forms the joint
with the female dovetail 38 of the bucket.
[0017] As will be appreciated from a review of Figure
2, the slanted crush surfaces 22 of each of the hooks
forms an angle a with a radial plane passing through the
neck of each hook of the dovetail, the angles opening
away from the plane and both toward and away from the
rotor axis. In Figure 2, four hooks 28, 30, 32 and 34
are illustrated. Consequently, the slanted crush surface
22 of each hook 28 also forms an angle a with a radial
plane bisecting the male dovetail. Thus, it will be
appreciated that the slanted crush surfaces 22 are at. a
constant angle to the horizontal throughout the height of
the wheel dovetail. By forming slanted crush surfaces 22
at an angle to the horizontal, stress concentrations for
a given fillet radius are reduced and enable larger hook
fillet radii that further reduce the stress
concentrations. Concentrated stresses result when load
paths are forced to change direction. With the slanted
crush surface, and particularly the same crush surface
angle a for each hook, the change in direction is less
severe and the stress concentration is lower. A further
advantage of the slanted crush surface is that it permits
a larger fillet radius in the same transition distance as
compared with the prior art zero degree (0°) transition,
i.e., a crush surface parallel to the horizontal.
[0018] In a preferred embodiment, the angle a is
preferably one hundred ten degrees (110°) for each crush
slant surface 22. Further, the larger fillet radius
permitted by the slanted crush surfaces, while enabling
lower concentrated stresses, also reduces the stress in
the fillet area. In accordance with a preferred
embodiment of this invention, each of the fillet radii
transitioning between the slanted crush surface 22 and
the neck portion 24 are enlarged.
[0019] The hook thickness and length also control the
load sharing between the hooks as well as the bending and
shear stress in the hook. All of this contributes to the
degree of concentrated stress. Accordingly, the hook
thickness and lengths are varied to achieve uniform and
minimized concentrated stresses.
[0020] Referring to Figure 3, the wheel dovetail 36
also includes a wheel pocket 41 having a pocket angle p
and an axially facing surface 43 angled in a radial
inward direction away from a plane normal to the axis of
the rotor. Wheel pocket angle ß is formed at an angle to
the radial plane, preferably about five degrees (5°). The
load path is thus forced to change direction and that
change in direction produces a lower stress
concentration. Figure 3 also discloses the lower right
and left fillets. Generally, these fillets are large to
further reduce the stress concentration. For example,
the right fillet 40, i.e., the inside fillet, has a .225
inch radius. The left fillet 42, i.e., the outside
fillet, has a .140 inch radius. Traditionally, the hook
fillet 44 has a .340 inch radius, the height 46 of the
lug from the bottom of the pocket is 360 inches and the
thickness 48 of the lug is 407 inches. The tang height
and thickness controls the bending shear due to the axial
load from the bucket and are designed to minimize tang
fillet concentrated stresses.
[0021] Other significant dimensions relating to the
disclosed exemplary embodiment of the present invention
are as follows:
[0022] The radial height extends from the axially
outermost end of each top surface of the hook to the
beginning of the slant surface along its underside as
indicated by hl-h4 in Figure 2.
[0023] The neck axial length N is as follows:
N1—between hooks 28 and 30—0.980 inches
N2—between hooks 30 and 32—1.880 inches
N3—between hooks 32 and 34—2.780 inches
N4—between hook 34 and tang—3.680 inches.
[0024] Referring to Figure 4, the female dovetail 38
of the bucket 50 is illustrated and is generally
complementary to the male dovetail component illustrated
in Figure 2. The various complementary components of the
bucket dovetail are assigned like reference numerals as
the wheel dovetail, followed by the suffix B. Except for
tolerances, the dimensional characteristics of the bucket
dovetail 38 are the same as or provided in close-fitting
relation to the dimensional characteristics for the wheel
dovetail with the additional exception that the hook or
tang 52 includes an enlarged angle a of 20° with respect
to the vertical. Note that the tang 52 includes an axial
facing surface 53 angled in a radial direction away from
a plane normal to the axis of the rotor and at a greater
angle than the angled surface 43 of the male dovetail
wheel pocket 41. In an illustrative embodiment:
Dovetail height of the bucket is 4.197 inches.
The axial length between hooks are as follows:
Hook 1 (28B)—1.000 inches
Hook 2 (30B)—1.900 inches
Hook 3 (32B)—2.800 inches
Hook 4 (34B)—3.700 inches.
[0025] The neck axial lengths NB are as follows:
NB1—above hook 28B—1.900 inches
NB2—above hook 30B—2.800 inches
NB3—above hook 32B—3.700 inches
NB4—above hook 34B—4.600 inches
[0026] With the foregoing dimensions, it will be
appreciated that the dovetail shape minimizes
concentrated stresses while maintaining an overall size
compatible with existing steam paths. As compared, for
example, with the design set forth in U.S. Patent No.
6,142,737, the present invention provides a peak
concentrated stress in the wheel dovetail of 48,920 psi
for the same loading condition and this represents a 27
reduction in concentrated stress for those same
conditions.
[0027] Referring now to Figures 5 and 6, there is
illustrated a further embodiment of the present invention
wherein like reference numerals are applied to like
parts, preceded by the prefix 1. As illustrated, only
three hooks instead of four as in the preceding
embodiment are provided on each of the male dovetails 136
and the female dovetail 138. The crush surfaces 122 for
each of the hooks 128, 130 and 132, as in the prior
embodiment, have a fillet employed as a transition
between the crush surface and the neck of the dovetail.
Thus, the crush surfaces 122, neck surfaces 124 and
fillets 12 6 between those surfaces are provided each of
the hooks 128, 130 and 132. Similarly as in the
preceding embodiment, each of the crush or slant surfaces
forms an angle a with a radial plane passing through the
neck of the dovetail, the angles opening away from the
plane and both toward and away from the rotor axis. The
slanted crush surfaces 122 are at a constant angle to the
horizontal throughout the height of the wheel dovetail
136. As in the prior embodiment, these slanted crush
surfaces reduce the stress concentrations for a given
fillet and enable larger hook fillet radii that further
reduce the stress concentrations. The preferred crush
surface angle a is 110°.
[0028] Referring now to Figure 5, the wheel pockets
139 in this embodiment of the present invention have an
axially facing surface 141 angled in a radial inward
direction away from a plane normal to the axis of the
rotor. Also, the tang 152 includes an axial facing
surface 153 angled in a radial direction away from a
plane normal to the axis of the rotor and at a greater
angle than the angled surface 141 of the male dovetail
wheel pocket 139. The pockets 139 have right and left
fillets 140 and 142, respectively, having radii of .094
and 140 inches. The radius for the fillet 160
underlying hook No. 3, i.e., hook 132, is .225 inches.
[0029] Other significant dimensions in this second
embodiment of the present invention relating to the wheel
dovetail are as follows:
[0030] As in the prior embodiment, the radial height
extends from the axially outermost end of each top
surface of the hook, to the beginning of the slant surface
along its underside.
[0031] The neck axial length N is as follows:
N1 - between hooks 128 and 130 -- 1.154 inches
N2 - between hooks 130 and 132 -- 2.160 inches
N3 - between hooks 132 and the tang -- 3.193 inches
[0032] The female dovetail 138 of the bucket is
illustrated in Figure 6 as generally complementary to the
male dovetail component illustrated in Figure 5. For
example, note the tang 152 for reception in the wheel
pocket 139. The various complementary components of the
bucket dovetail of Figure 6 are assigned like reference
numerals as the wheel dovetail followed by the suffix B.
Except for the tolerances, the dimensional
characteristics of the bucket dovetail 138 are the same
as or provided in close-fitting relation to the
dimensional characteristics for the wheel dovetail 136.
For example:
Dovetail height of the bucket in 3.340 inches
[0033] The axial length between the hooks are as
follows:
Hook 1 (128B) — 1.362 inches
Hook 2 (130B) -- 2.369 inches
Hook 3 (132B) — 3.374 inches
[0034] The neck axial lengths 1NB are as follows:
INB1 — above hook 128B — 2.062 inches
INB2 — above hook 130B — 3.068 inches
INB3 — above hook 132B — 4.074 inches
[0035] While the invention has been described in
connection with what is presently considered to be the
most practical and preferred embodiment, it is to be
understood that the invention is not to be limited to the
disclosed embodiment, but on the contrary, is intended to
cover various modifications and equivalent arrangements
included within the spirit and scope of the appended
claims.
WE CLAIM
1. A dovetail joint between a rotor wheel and a bucket rotatable about an
axis, comprising:
- a male dovetail component on the rotor wheel and a female
dovetail component on the bucket, said male dovetail component
receiving said female dovetail component in a direction tangential
to the rotor wheel, said male dovetail component including a
plurality of circumferentially extending hooks lying on opposite
sides of a plane normal to the axis and bisecting said male dovetail
component, each said hook having a generally radially inwardly-
facing surface;
characterized in that
- said surfaces of at least a pair of hooks on each of the opposite
sides of the plane defining angles extending away from said plane
and toward and away from said axis, the angles of said surfaces of
each said pair of hooks on each of the opposite sides of the plane
being equal to one another.
2. A joint according to claim 1, wherein neck portions join said surfaces and
generally radially outwardly-facing portions of radially inwardly-underlying
hooks, and fillets between said neck portions and said surfaces.
3. A joint according to claim 1, wherein each hook from a radially-outermost
hook to a radially-innermost hook increases in radial thickness.
4. A joint according to claim 1, wherein said male dovetail has at least three
hooks on each of the opposite sides of said plane.
5. A joint according to claim 1, wherein said male dovetail has four hooks on
each of the opposite sides of said plane.
6. A joint according to claim 1, wherein said male dovetail has a wheel
pocket adjacent a base and on opposite sides thereof, each wheel pocket
having an axial facing surface angled in a radial inward direction away
from said plane.
7. A joint according to claim 6, wherein said female dovetail has a tang for
reception in said male dovetail wheel pocket, said tang having an axial
facing surface angled in a radial inward direction away from said plane
and at a greater angle than the angled surface of the male dovetail wheel
pocket.
8. A joint according to claim 2, wherein neck portions join said surfaces and
generally radially outwardly-facing portions of radially inwardly-underlying
hooks, and fillets between said neck portions and said surfaces, wherein
each hook from a radially-outermost hook to a radially-innermost hook
increases in radial thickness.
9. A joint according to claim 8, wherein said male dovetail has at least three
hooks on each of the opposite sides of said plane.
10. For use in a dovetail joint between a rotor wheel and a bucket rotatable
about an axis wherein the rotor wheel includes a male dovetail component
for receiving a female dovetail component in a direction tangential to the
rotor wheel and the male dovetail component includes a plurality of
circumferentially extending hooks lying on opposite sides of a plane
normal to the axis and bisecting said male dovetail component, each said
hook having a generally radially inwardly-facing surface with the surfaces
of each of the hooks on opposite sides of the plane defining angles
extending away from said plane said toward and away from said axis, the
angle of each surface being equal to the angle of every other surface, said
female dovetail component on each bucket
including a plurality of circumferentially extending hooks generally
complementary to the male dovetail hooks and having radially outwardly
directed angled surfaces generally complementary to said angled surfaces
of the male dovetail component, the angles of said surfaces of the female
dovetail component being equal to one another.

A dovetail joint between a rotor wheel and a bucket includes a male dovetail
component (14) on the rotor wheel (12) and a female dovetail component (18)
on the bucket (16). The male dovetail component has axially projecting hooks
with slanted surfaces (22) along generally radially inwardly directed surfaces
(22). The slanted surfaces (22) form included angles (α) with a plane normal to
the axis of rotation and bisecting the wheel dovetail which are larger than 90°
and remain constant for all of the hooks (28, 30, 32, 34). Single radius fillets
(26) are also provided along the transition surfaces between the slanted crush
surfaces (22) and the neck surfaces (24). The stress concentrations are
therefore minimized.