U.S. patent application number 12/162865 was filed with the patent office on 2009-01-22 for fan blade retaining structure.
This patent application is currently assigned to IHI CORPORATION. Invention is credited to Takashi Oka.
Application Number | 20090022593 12/162865 |
Document ID | / |
Family ID | 38509521 |
Filed Date | 2009-01-22 |
United States Patent
Application |
20090022593 |
Kind Code |
A1 |
Oka; Takashi |
January 22, 2009 |
FAN BLADE RETAINING STRUCTURE
Abstract
A disk 10 includes a plurality of dovetail grooves 12 which are
spaced at a predetermined angular interval in a circumferential
direction and extend in an axial direction. A fan blade 20 includes
a dovetail part 22 which is fitted to each of the dovetail grooves
in an axial direction and capable of transmitting a centrifugal
force produced during rotation to the disk. The disk further
includes a flange part 14 which is positioned in front of the
dovetail grooves and extends outward in a radial direction. A
ring-shape retainer member 30 is fitted between a front surface 22a
of the dovetail part 22 and a rear surface 16a of the flange part
14 and transmits an axial forward load acting on the fan blade 20
to the disk 10 via the rear surface 16a of the flange part.
Inventors: |
Oka; Takashi; (Tokyo,
JP) |
Correspondence
Address: |
GRIFFIN & SZIPL, PC
SUITE PH-1, 2300 NINTH STREET, SOUTH
ARLINGTON
VA
22204
US
|
Assignee: |
IHI CORPORATION
Tokyo
JP
|
Family ID: |
38509521 |
Appl. No.: |
12/162865 |
Filed: |
March 13, 2007 |
PCT Filed: |
March 13, 2007 |
PCT NO: |
PCT/JP2007/054871 |
371 Date: |
July 31, 2008 |
Current U.S.
Class: |
416/220R |
Current CPC
Class: |
F04D 29/322 20130101;
Y02T 50/60 20130101; F05D 2220/36 20130101; F01D 5/323 20130101;
F01D 5/3015 20130101; Y02T 50/671 20130101 |
Class at
Publication: |
416/220.R |
International
Class: |
F01D 5/30 20060101
F01D005/30 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 2006 |
JP |
2006-067620 |
Claims
1. A fan blade retaining structure in which a plurality of fan
blades is configured to be attached to an outer periphery of a disk
rotationally driven by a turbine, wherein the disk includes a
plurality of dovetail grooves configured to be spaced at a
predetermined angular interval in a circumferential direction and
to extend in an axial direction, wherein each of the fan blades
includes a dovetail part configured to be fitted to each of the
dovetail grooves in an axial direction and capable of transmitting
a centrifugal force produced during rotation to the disk, wherein
the disk includes a flange part configured to be positioned in
front of the dovetail grooves and to extend outward in a radial
direction, and wherein the fan blade retaining structure further
comprises a ring-shape retainer member configured to be fitted
between a front surface of the dovetail part and a rear surface of
the flange part so as to transmit an axial forward load acting on
the fan blade to the disk via the rear surface of the flange
part.
2. The fan blade retaining structure according to claim 1, wherein
the flange part of the disk includes disk tooth parts configured to
be spaced at a predetermined angular interval in a circumferential
direction and to project outward in a radial direction, wherein the
retainer member includes a ring part configured to have a rear
surface coming into contact with the front surface of the dovetail
part and retainer tooth parts configured to be formed inside the
ring part so as to be spaced at a predetermined angular interval in
a circumferential direction and to project inward in a radial
direction, and wherein each of the retainer tooth parts is capable
of passing between the adjacent disk tooth parts in an axial
direction and comes close to the rear surface of the disk tooth
part to overlap therewith at the same position in a circumferential
direction.
3. The fan blade retaining structure according to claim 2, further
comprising: a fastener configured to fix the retainer tooth part to
the disk tooth part in a state where the retainer tooth part comes
close to the disk tooth part to overlap with each other.
4. The fan blade retaining structure according to claim 2, wherein
the disk tooth part is formed at a position between the adjacent
dovetail grooves in a circumferential direction.
5. The fan blade retaining structure according to claim 2, wherein
the disk tooth part is formed at the same position as that of the
dovetail groove in a circumferential direction.
Description
TECHNICAL FIELD
[0001] The present invention relates to a fan blade retaining
structure for preventing a fan blade from coming off from a disk
even when an axial forward load acts on the fan blade upon breaking
the fan blade.
BACKGROUND ART
[0002] FIG. 1 is a schematic view illustrating a turbofan engine
110 for an airplane. The engine 110 includes a fan assembly 112
which is coaxially disposed with respect to a center shaft 116 and
is driven by a core engine 114. During the operation of the engine
110, external air 118 is sucked by the fan assembly 112 so as to
produce a thrust for enabling an airplane to fly.
[0003] When the airplane having the above-described engine 110
takes off or lands, for example, a comparatively large bird 120 may
be sucked into the fan assembly 112. In this case, the bird 120
impacts on fan blades 122 extending outward from a rotor disk 124
in a radial direction, so that a part of the fan blades 122 is
broken and scattered. Additionally, alien materials except for the
bird may be sucked into the fan assembly to thereby damage
components. In addition to the supposition, the fan blades 122 or a
part thereof may be broken and scattered due to a large load or the
like. Hereinafter, such a phenomenon will be referred to as FBO
(Fan Blade Off).
[0004] In the event of the FBO, a part of the fan blades 122 may be
scattered to collide against the adjacent other fan blades 122, so
that an axial forward (upstream) shock power acts on the fan blades
122.
[0005] Since the axial forward shock power acting on the fan blades
122 acts in a direction in which the fan blades 122 come off from
the rotor disk 124, the fan assembly 112 needs to be provided with
a structure capable of retaining the fan blades 122 in the rotor
disk 124 even when the axial forward shock power acts on the fan
blades 122.
[0006] Patent Document 1 has already proposed a fan blade retainer
for satisfying the above-described requirements.
[0007] Patent Document 1 discloses a retainer assembly 126 for
retaining the fan blades 122 in the rotor disk 124. As shown in
FIG. 2, the retainer assembly 126 includes the rotor disk 124, a
first blade retainer 138, and a second blade retainer 144.
[0008] The rotor disk 124 includes a plurality of dovetail posts
128 which are spaced at a predetermined angular interval in a
circumferential direction, and a dovetail groove 130 is formed
therebetween. Each fan blade 122 includes a dovetail 132 fitted to
the dovetail groove 130 in an axial direction, and the dovetail 132
is immovably retained in a radial direction by the dovetail post
128. With the above-described configuration, a centrifugal force
produced during rotation of the fan blades 122 is transmitted to
the rotor disk 124 via the dovetail post 128.
[0009] The first blade retainer 138 is provided so as to prevent
the fan blade 122 from moving in an axial forward direction
(upstream direction). The first blade retainer 138 includes a fixed
plate 140 fixed to the rotor disk 124 so as to immovably retain the
fan blade 122 in the dovetail groove 130 in an axial direction. The
fixed plate 140 is inserted in an outward radial direction into a
pair of grooves 142 formed in the adjacent dovetail posts 128.
[0010] The second blade retainer 144 is provided so as to prevent
the axial movement of the fan blades 122 by serving as a chock
after the fan blades 122 move by a predetermined distance in a case
where the axial movement of the fan blades 122 cannot be prevented
by the first blade retainer 138. With the above-described
configuration, both the first blade retainer 138 and the second
blade retainer 144 disperse the axial forward shock energy acting
on the fan blades 122 so that each fan blade 122 is retained in the
dovetail grooves 130.
[0011] [Patent Document 1]
[0012] U.S. Pat. No. 5,282,720 `Fan blade retainer`
[0013] FIG. 3 is a schematic view illustrating a dovetail structure
according to a conventional art. As shown in the drawing, it is
necessary to attach the fan blades of the turbofan engine to a
peripheral portion of a circular disk (called a fan disk or a
spinner) rotationally driven by a turbine. For this reason, in the
past, there was adopted a conventional dovetail structure in which
a dovetail part is formed in a root portion of the fan blade so as
to extend in a longitudinal direction and the dovetail part is
fitted to a dovetail groove formed in the periphery of a disk.
[0014] In addition, in order to prevent the fan blade from coming
off from the dovetail groove due to the axial forward power, in the
past, a member called a retainer was fastened to a front portion of
the blade by fastening bolts, nuts, and the like.
[0015] Examples of the retainer include a circular-ring type for
functioning with respect to a plurality of blades and a plate type
for functioning with respect to each of the blades.
[0016] However, in case of the circular-ring type retainer, since a
gap between an action portion of a load and a fastening portion
such as a bolt and a nut is long, a problem arises in that the
stress acting on the bolt due to the load is difficult to be
estimated and the retainer is difficult to be designed. Meanwhile,
in case of the plate type retainer, since the retainer is mounted
to each blade, a problem arises in that operability is poor. For
this reason, in both cases of the circular-ring type retainer and
the plate type retainer, a problem arises in that a decrease in
weight is difficult to be achieved and a cost is high.
[0017] In addition, in case of the retainer assembly described in
Patent Document 1, although both the first blade retainer 138 and
the second blade retainer 144 can disperse the axial forward shock
energy acting on the fan blades 122 so that each fan blade 122 is
retained in the dovetail groove 130, a problem arises in that the
structure is complex and a manufacturing cost is high.
[0018] In the retainer assembly, since it is necessary to provide a
plurality of first blade retainers 138 and the second blade
retainers 144, a problem arises in that the operability during an
assembling operation is poor.
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0019] The present invention is contrived in consideration of the
above-described problems. That is, an object of the invention is to
provide a fan blade retaining structure capable of surely
preventing a fan blade from coming off from a disk even when an
axial forward load acts on the fan blade upon breaking the fan
blade, of being easily designed with high estimation precision of
stress, and of enabling a fastening bolt to be lighter in weight,
smaller in size, and fewer in number than a conventional fastening
bolt, thereby reducing the number of components. Accordingly, it is
possible to provide the fan blade retaining structure capable of
realizing a decrease in weight and cost.
[0020] According to the invention, there is provided a fan blade
retaining structure in which a plurality of fan blades is
configured to be attached to an outer periphery of a disk
rotationally driven by a turbine, wherein the disk includes a
plurality of dovetail grooves configured to be spaced at a
predetermined angular interval in a circumferential direction and
to extend in an axial direction, wherein each of the fan blades
includes a dovetail part configured to be fitted to each of the
dovetail grooves in an axial direction and capable of transmitting
a centrifugal force produced during rotation to the disk, wherein
the disk further includes a flange part configured to be positioned
in front of the dovetail grooves and to extend outward in a radial
direction, and wherein the fan blade retaining structure further
includes a ring-shape retainer member configured to be fitted
between a front surface of the dovetail part and a rear surface of
the flange part so as to transmit an axial forward load acting on
the fan blade to the disk via the rear surface of the flange
part.
[0021] According to a preferred embodiment of the invention, the
flange part of the disk includes disk tooth parts configured to be
spaced at a predetermined angular interval in a circumferential
direction and to project outward in a radial direction.
[0022] The retainer member includes a ring part configured to have
a rear surface coming into contact with the front surface of the
dovetail part and retainer tooth parts configured to be formed
inside the ring part so as to be spaced at a predetermined angular
interval in a circumferential direction and to project inward in a
radial direction.
[0023] Each of the retainer tooth parts is capable of passing
between the adjacent disk tooth parts in an axial direction and
comes close to the rear surface of the disk tooth part to overlap
therewith at the same position in a circumferential direction.
[0024] The fan blade retaining structure further includes a
fastener configured to fix the retainer tooth part to the disk
tooth part in a state where the retainer tooth part comes close to
the disk tooth part to overlap with each other.
[0025] The disk tooth part is desirably formed at a position
between the adjacent dovetail grooves in a circumferential
direction.
[0026] The disk tooth part is desirably formed at the same position
as that of the dovetail groove in a circumferential direction.
[0027] According to the configuration of the invention, since the
retainer member is provided so as to be fitted between the front
surface of the dovetail part and the rear surface of the flange
part, even when the axial forward load acts on the fan blades upon
breaking the fan blades, it is possible to transmit the axial
forward load from the front surface of the dovetail part to the
disk via the rear surface of the flange part and the retainer
member.
[0028] Since a stress occurring in the retainer member by the axial
forward load mainly corresponds to an axial compressive stress and
a stress occurring in the flange part by the axial forward load
mainly corresponds to an axial stretching stress, it is possible to
accurately estimate an internal stress occurring in the retainer
member and the flange part in accordance with the axial forward
load.
[0029] Since the retainer member and the flange part are prevented
from being broken by setting the internal stress to a sufficiently
small stress, it is possible to surely prevent the fan blade from
coming off from the disk.
[0030] Since the ring-shape retainer member is used, it is possible
to reduce the number of components.
[0031] Since the axial forward load does not act on the fastener
(bolt, nut, and the like) for fastening the retainer member to the
flange part by transmitting the axial forward load to the retainer
member and the flange part, it is possible to more reduce the
weight, the size, and the number of the fastening bolt than a
conventional fastening bolt, and thus to reduce the number of
components. Accordingly, it is possible to realize a decrease in
weight and cost.
[0032] In addition, the flange part includes the disk tooth parts
which project outward in a radial direction and the retainer member
includes the ring part which comes into contact with the front
surfaces of the dovetail parts and the retainer tooth parts which
project inward in a radial direction. Also, each of the retainer
tooth parts can pass between the adjacent disk tooth parts in an
axial direction and is disposed adjacent to each rear surface of
the disk tooth parts.
[0033] Accordingly, the retainer member can be easily fitted
between the font surface of the dovetail parts and the rear surface
of the flange part in such a manner that the retainer tooth parts
of the retainer member pass between the adjacent disk tooth parts
in an axial direction and are rotated up to the same positions in a
circumferential direction so as to be adjacent to the rear surfaces
of the disk tooth part.
[0034] Since the axial forward load does not act on the fastener by
fixing the retainer tooth parts to the disk tooth parts at the same
positions in a circumferential direction by using the fastener
(bolt, nut, and the like), it is possible to more reduce the
weight, the size, and the number of the fastener than the
conventional fastener, and thus to reduce the number of
components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a schematic view illustrating a turbofan engine
for an airplane.
[0036] FIG. 2 is a configuration view illustrating a fan blade
retainer disclosed in Patent Document 1.
[0037] FIG. 3 is a schematic view illustrating a dovetail structure
according to a conventional art.
[0038] FIG. 4 is an overall perspective view illustrating a state
where a fan blade retaining structure according to the invention is
disassembled.
[0039] FIG. 5A is a transverse sectional view illustrating an
engine provided with the fan blade retaining structure according to
the invention.
[0040] FIG. 5B is an enlarged view illustrating a B part shown in
FIG. 5A.
[0041] FIG. 6A is a view taken along the line C-C shown in FIG. 5B
in a state where a retainer member is being mounted.
[0042] FIG. 6B is a view taken along the line C-C shown in FIG. 5B
in a state where the retainer member is completely mounted.
BEST MODE FOR CARRYING OUT THE INVENTION
[0043] Hereinafter, preferred embodiments of the invention will be
described in detail with reference to the accompanying drawings. In
addition, in the respective drawings, the same reference numerals
are given to the same components and the repetitive description
thereof will be omitted.
[0044] FIG. 4 is an overall perspective view illustrating a state
where a fan blade retaining structure according to the invention is
disassembled. The fan blade retaining structure according to the
invention is configured such that a plurality of fan blades 20 are
attached to the outer periphery of a disk 10 which is rotationally
driven by a turbine (not shown).
[0045] As shown in the drawing, the fan blade retaining structure
according to the invention includes the disk 10, the fan blades 20,
a retainer member 30, and a fastener 40.
[0046] The disk 10 includes a plurality of dovetail grooves 12
which are spaced at a predetermined angular interval in a
circumferential direction and extends in an axial direction.
Although it is desirable that the number of the dovetail grooves 12
is the same as that of the fan blades 20, a plurality (two or more)
of fan blades 20 may be attached to one dovetail groove 12.
[0047] In this example, the dovetail grooves 12 are parallel to a
shaft center 1 of a rotary shaft, but may be inclined with respect
to the shaft center 1. In addition, each of the dovetail grooves 12
may be formed into a linear shape or a circular arc shape so long
as each of the fan blades 20 is inserted and fitted in an axial
direction from the front side.
[0048] The fan blade 20 includes a dovetail part 22 which is formed
in the inner end in a radial direction so as to be fitted to the
dovetail groove 12. The dovetail part 22 is configured to be fitted
to the dovetail groove 12 in an axial direction from the front side
and to transmit a centrifugal force acting on the fan blade 20
produced during rotation to the disk 10.
[0049] FIG. 5A is a transverse sectional view illustrating a fan
part of an engine provided with the fan blade retaining structure
according to the invention, in which only the upper side of the
shaft center 1 of the rotary shaft is shown. FIG. 5B is an enlarged
view illustrating a B part shown in FIG. 5A.
[0050] In addition, in the drawing, Reference numeral 1 denotes the
shaft center of the rotary shaft of the disk 10 and the fan blade
20, Reference numeral 2 denotes an air flow passage, Reference
numeral 3 denotes the inner peripheral surface of the air flow
passage, Reference numeral 4 denotes an intake air flow, and
Reference numeral 5 denotes an axial forward power acting on the
fan blade 20 in the event of FBO.
[0051] As shown in FIGS. 4, 5A, and 5B, the disk 10 further
includes a flange part 14 which is positioned in front of the
dovetail groove 12 and extends in a radial outer direction. The
flange part 14 is integrally connected to a hollow cylindrical part
13 which extends forward from the inner end of the disk 10.
[0052] The flange part 14 further includes disk tooth parts 16
which are spaced at a predetermined angular interval in a
circumferential direction and project outward in a radial
direction.
[0053] The retainer member 30 includes a hollow disk-shape ring
part 32 and retainer tooth parts 34 which are integrally formed
with the inner portion of the ring part 32.
[0054] The ring part 32 includes a rear surface 32a, and the rear
surface 32a comes into contact with a front surface 22a of the
dovetail part 22 inserted into the dovetail groove 12.
[0055] In addition, the retainer tooth parts 34 are spaced at a
predetermined angular interval in a circumferential direction and
project inward in a radial direction.
[0056] As shown in FIG. 4, in this example, the disk tooth parts 16
are formed at centers in a circumferential direction of the
dovetail grooves 12 which are adjacent to each other in a
circumferential direction and are not positioned on extension lines
of the dovetail grooves 12. Accordingly, in this example, the
retainer tooth parts 34 are formed in a circumferential direction
so as to have the same pitches as those of the dovetail grooves 12
and have the same number as that of the dovetail grooves 12.
[0057] With the above-described configuration, the disk tooth parts
16 can be formed at the radial positions so as to have the
substantially same heights as those of the dovetail parts 22, and
the forward axial load produced from the dovetail parts 22 can be
surely supported by two disk tooth parts 16 in a circumferential
direction.
[0058] In addition, the invention is not limited to this
configuration, but the disk tooth parts may be formed at the same
positions as those of the dovetail grooves in a circumferential
direction.
[0059] In this case, although the disk tooth parts 16 need to be
formed at the radial positions lower than the bottom portions of
the dovetail parts 22 so as not to be positioned on the extension
lines of the dovetail grooves 12, the axial forward load produced
from the dovetail parts 22 can be surely supported by the disk
tooth parts 16 disposed at the same positions in a circumferential
direction.
[0060] In addition, the number of the disk tooth parts 16 and the
retainer tooth parts 34 needs not to be the same as that of the
dovetail grooves 12, but the number may be two or more (for
example, four, eight, sixteen or the like) so long as a balance is
ensured during a high-speed rotation.
[0061] FIG. 6A is a view taken along the line C-C shown in FIG. 5B
in a state where the retainer member is being mounted. FIG. 6B is a
view taken along the line C-C shown in FIG. 5B in a state where the
retainer member is completely mounted.
[0062] In FIGS. 4 to 6A and 6B, an axial distance (thickness) from
the rear surface 32a of the retainer member 30 to each front
surface 34a of the retainer tooth parts 34 is set to be a smaller
value than an axial distance from the front surface 22a of the
dovetail part 22 to each rear surface 16a of the disk tooth parts
16 in a mounting state. It is desirable that the gap is set so that
the retainer member 30 smoothly rotates about the shaft center 1 in
a state where the retainer member is mounted.
[0063] As shown in FIG. 6A, each of the retainer tooth parts 34 can
pass between the adjacent disk tooth parts 16 in an axial
direction. In addition, as shown in FIG. 6B, the retainer tooth
parts come close to the rear surfaces of the disk tooth parts 16 to
overlap therewith at the same positions in a circumferential
direction by rotating the retainer member 30 about the shaft center
1 by a predetermined angle (in this example, 30 degree or so). It
is desirable that the close gap therebetween is set to be small so
long as the retainer tooth parts do not interfere with the disk
tooth parts within an operation temperature range of the
engine.
[0064] In FIGS. 4, 5A, and 5B, the fastener 40 includes a bolt 41
and a nut 42 which are screw-connected to each other while passing
through a through hole 35 formed in the retainer member 30 and a
through hole 17 formed in the flange part 14 of the disk 10 so as
to fix the retainer tooth parts 34 and the disk tooth parts 16 at
the same positions in a circumferential direction.
[0065] In addition, in this example, the flange part 14 extends
inward in a radial direction, and the through hole 17 is formed in
the extending portion. An inverse L-shape portion is formed in the
retainer member 30 so as to come into contact with the extending
portion, and the through hole 35 is formed in the inverse L-shape
portion.
[0066] However, the invention is not limited to this configuration,
but may be configured such that the retainer member 30 is formed
into, for example, a ring-shape flat plate instead of the inverse
L-shape portion and the ring-shape flat plate is directly fixed to
the disk 10.
[0067] With the above-described configuration, the retainer tooth
parts 34 and the disk tooth parts 16 are fixed at the same
positions in a circumferential direction during the time when the
ring-shape retainer member 30 shown in FIG. 6B is completely
mounted and the engine is operated.
[0068] In addition, the retainer member 30 can transmit the axial
forward load acting on the fan blade 20 to the disk 10 via the rear
surfaces 16a of the disk tooth parts 16 while being fitted between
the front surfaces 22a of the dovetail parts 22 and the rear
surfaces 16a of the disk tooth parts 16.
[0069] With the above-described configuration, since there is
provided the retainer member 30 fitted between the front surfaces
22a of the dovetail parts 22 and the rear surfaces 16a of the
flange part 14 (the disk tooth parts 16), even when the axial
forward load occurs acts on the fan blade upon breaking the fan
blade 20, the axial forward load can be transmitted from the front
surfaces 22a of the dovetail parts 22 to the disk 10 via the
retainer member 30 and the rear surfaces 16a of the flange
part.
[0070] Since a stress occurring in the retainer member 30 by the
axial forward load mainly corresponds to an axial compressive
stress and a stress occurring in the flange part 14 by the axial
forward load mainly corresponds to an axial stretching stress, it
is possible to accurately estimate an internal stress occurring in
the retainer member 30 and the flange part 14 in accordance with
the axial forward load.
[0071] Since the retainer member 30 and the flange part 14 are
prevented from being broken by setting the internal stress to a
sufficiently small stress, it is possible to surely prevent the fan
blade 20 from coming off from the disk 10.
[0072] Since the ring-shape retainer member 30 is used, it is
possible to reduce the number of components.
[0073] Since the axial forward load does not act on the fastener 40
(bolt, nut, and the like) for fastening the retainer member 30 to
the flange part 14 by transmitting the axial forward load to the
retainer member 30 and the flange part 14, it is possible to more
reduce the weight, the size, and the number of the fastener 40 than
a conventional fastener, and thus to reduce the number of
components.
[0074] In addition, the flange part 14 of the disk 10 includes the
disk tooth parts 16 which project outward in a radial direction and
the retainer member 30 includes the ring part 32 which comes into
contact with the front surfaces 22a of the dovetail parts 22 and
the retainer tooth parts 34 which project inward in a radial
direction. Also, each of the retainer tooth parts 34 can pass
between the adjacent disk tooth parts 16 in an axial direction and
is disposed adjacent to each rear surface 16a of the disk tooth
parts 16. Accordingly, the retainer member 30 can be easily fitted
between the font surface 22a and the rear surface 16a of the flange
part in such a manner that the retainer tooth parts 34 of the
retainer member 30 pass between the adjacent disk tooth parts 16 in
an axial direction and are rotated up to the same positions in a
circumferential direction so as to be adjacent to the rear surfaces
of the disk tooth part 16.
[0075] Since the axial forward load does not act on the fastener 40
by fixing the retainer tooth parts 34 to the disk tooth parts 16 at
the same positions in a circumferential direction by using the
fastener 40 (bolt, nut, and the like), it is possible to more
reduce the weight, the size, and the number of the fastener than
the conventional fastener, and thus to reduce the number of
components.
[0076] The invention is not limited to the preferred embodiments,
but may be, of course, modified variously in the scope without
departing from the spirit of the invention.
* * * * *