BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to bolted flange
connections and, particularly, to such connections for
limiting transmitted loads during unbalance events in
turbomachinery.
DESCRIPTION OF RELATED ART
Flanges are generally held together by bolts
through bolt holes provided in each of the flanges and
that are aligned with each other. Large radial,
^p tangential, or axial loads with respect to an axial
centerline of the bolted joint can impose bending
moments or tensile forces in the flange that can cause
deformation or rupture of the bolts.
During gas turbine engine operation, a foreign
body, such as a bird, could impact the fan assembly and
cause part or all of a fan blade to become detached from
the rotor disk. Such blade loss is a certification
requirement under FAA 14 CFR part 33 rules. Fan blade
loss creates a large rotor imbalance, particularly, in
early revolutions of the imbalance causing event. This
results in the transmission of potentially damaging
imbalance forces to the bolted connection, possibly
^^ resulting in the misalignment of the flanges and bearing
^^ which they help support.
This could be a particular problem in gas
turbine engines and, more so, in aircraft gas turbine
engines that use bolted connections to support bearings
which, in turn, support rotatable rotors. Rotor blade
failures, which can be caused by foreign objects that
are drawn into the fan or compressor, can cause rotor
unbalance conditions. Such rotor unbalance conditions
can impose radial, circumferential, and possibly also
axial loads sufficient to fail supporting structure
causing loss of centerline. Such rotor unbalance
conditions can cause unintended high shear, bending, or
tensile loads, or a combination of such loads, applied
to the flange connecting bolts, leading to structural
damage, bolt deformation, and possibly to bolt rupture
and separation of the bolted casings from each other.
This can cause the centerline of the flange of the
bearing casing to shift. This, in turn, can shift the
radial location of the bearings which is an undesirable
condition even with the engine shutoff and windmilling.
Thus, it is highly desirable to maintain the centerline
of the flange of the bearing casing supporting the
bearing when there has been an unbalance load event like
fan blade out and the engine will be shutoff and the fan
qm windmilled.
SUMMARY OF THE INVENTION
A bolted flange assembly includes a first
flange bolted to a second flange, a first circular row
of first bolt holes extending axially through the first
flange, bolts disposed through the first bolt holes and
through second bolt holes extending axially at least
partially through the second flange. Each of the bolts
include a bolt head, a thread, and a shank therebetween.
Crushable spacers disposed around the shanks of a first
plurality of the bolts contact and axially extend
between the bolt heads and the first flange. Bushings
^^ disposed around a second plurality of the bolts contact
^ and axially extend between the bolt heads and the second
flange.
In an exemplary embodiment of the assembly, a
shank outer diameter of the shanks is smaller than a
thread diameter of the threads and heat shrink tubing is
disposed around the bolt shanks of at least some of the
spacers. The spacers contact the first flange on a
first flat annular surface of the first flange and the
bushings contact the second flange on a second flat
annular surface of the second flange. The spacers
3
include tubular bodies extending axially between first
and second enlarged or flanged ends.
The second bolt holes may be threaded and
extend axially partially through the second flange.
The bushings may include tubular bushing
bodies axially extending between first bushing ends and
second bushing ends. Annular bushing flanges may be on
the first bushing ends adjacent the bolt heads with gaps
between the annular bushing flanges and the first
flange.
In a more particular exemplary embodiment of
the assembly, the first bolt holes extend axially
through the first flange into an aftwardly open annular
slot on the first flange, the second bolt holes extend
^y partially through a forward extending annular rail of
the second flange, and the annular rail is received
within the annular slot.
In another more particular exemplary
embodiment of the assembly, the first bolt holes extend
axially through the first flange, the second bolt holes
extend partially through a forward extending annular
rail of the second flange, and the annular rail is
received within an inner or outer rabbet defined by an
inner or outer lip respectively extending aftwardly from
the first flange.
The bolted flange assembly may be incorporated
in a gas turbine engine forward bearing system including
A a forward bearing support structure and a fan frame.
^ The first flange is at an aft end of the forward bearing
support structure bolted to the second flange on the fan
frame.
The gas turbine engine forward bearing system
may be incorporated in an aircraft turbofan gas turbine
engine including in downstream serial flow
communication, a fan, a low pressure compressor or
booster, a high pressure compressor, a combustor, a high
pressure turbine, and a low pressure turbine
circumscribed about an engine centerline axis. The low
4
pressure turbine is joined by a low pressure drive shaft
to the fan and the low pressure compressor or booster.
The forward bearing support structure supports a forward
bearing which rotatably supports, in part, the low
pressure drive shaft.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and other features of
the invention are explained in the following
description, taken in connection with the accompanying
drawings where:
FIG. 1 is a cross-sectional side view
illustration of an exemplary embodiment of an aircraft
turbofan gas turbine engine with an exemplary embodiment
^P of a dynamic load reduction system for a bolted flange
assembly of a rotor bearing support.
FIG. 2 is an enlarged cross-sectional side
view illustration of a forward portion of the engine
with the exemplary embodiment of the dynamic load
reduction system illustrated in FIG. 1.
FIG. 3 is a perspective view illustration of
the exemplary embodiment of the bolted flange assembly
and the dynamic load reduction system illustrated in
FIG. 2.
FIG. 4 is a cross-sectional view illustration
of an exemplary bolt with a crushable spacer in the
bolted flange assembly illustrated in FIG. 3.
^k FIG. 5 is a cross-sectional view illustration
^ of an exemplary bolt with a flanged bushing in the
bolted flange assembly illustrated in FIG. 3.
FIG. 6 is a cross-sectional view illustration
of the bolt with a flanged bushing in another exemplarybolted
flange assembly.
DETAILED DESCRIPTION OF THE INVENTION
Illustrated in FIGS. 1 and 2 is an exemplary
aircraft turbofan gas turbine engine 10 circumscribed
about an engine centerline axis 12 and suitably designed
to be mounted to a wing or fuselage of an aircraft. The
engine 10 includes, in downstream serial flow
communication, a fan 14, a low pressure compressor or
booster 16, a high pressure compressor 18, a combustor
20, a high pressure turbine (HPT) 22, and a low pressure
turbine (LPT) 24. The HPT or high pressure turbine 22
is joined by a high pressure drive shaft 23 to the high
pressure compressor 18. The LPT or low pressure turbine
24 is joined by a low pressure drive shaft 25 to both
the fan 14 and the booster 16.
In typical operation, air 26 is pressurized by
the fan 14 and produces an inner air flow 15 channeled
through the booster 16 which further pressurizes the
inner air flow 15. The pressurized air is then flowed
^^ to the high pressure compressor 18 which further
pressurizes the air. The pressurized air is mixed with
fuel in the combustor 20 for generating hot combustion
gases 28 that flow downstream in turn through the HPT 22
and the LPT 24.
Referring more particularly to FIG. 2, a flow
splitter 34 surrounding the booster 16 immediately
behind the fan 14 includes a sharp leading edge 32 which
splits the fan air 26 pressurized by the fan 14 into a
radially inner stream channeled through the booster 16
and a radially outer stream channeled through the bypass
duct 36. A fan casing 30 surrounding the fan 14 is
supported by an annular fan frame 33.
^^ The fan 14 includes a fan rotor 112 having a
~ plurality of circumferentially spaced apart fan blades
116 which extend radially outwardly from a fan disk 114.
The fan disk 114 is connected to a fan shaft 118 that
is powered by the LPT 24. The fan rotor 112 is
rotatably supported on the fan frame 33 by a support
system 128. The fan frame 33 includes an annular outer
casing 130, an inner hub 132, and a plurality of
circumferentially spaced apart struts 134 extending
therebetween. The struts 134 are airfoil shaped since
bypass air passes between the adjacent ones thereof.
6
The support system 128 includes a forward
bearing system 136 including a forward bearing support
structure 138 supporting a forward bearing 139
(alternatively referred to as "No. 1 bearing"). The
forward bearing 139 rotatably supports in part the low
pressure drive shaft 25 which is connected to the fan
shaft 118. The forward bearing 139 is a thrust bearing.
The forward bearing support structure 138 illustrated
in FIG. 2 is conical and is also referred to as a
bearing casing. Other suitably shaped support
structures may also be used to support the forward
bearing. The forward bearing system 136 is disposed
between the fan shaft 118 and the forward bearing
support structure 138. The forward bearing support
^p structure 138 is secured to the inner hub 132 of the fan
frame 33.
FIGS. 2-5 illustrate a dynamic load reduction
•system 60 for a bolted flange assembly 64 of the forward
bearing support structure 138. The forward bearing
support structure 138 includes a first flange 146 at an
aft end 148 of the forward bearing support structure
138. The first flange 146 is bolted to second flange
147 which connected to the inner hub 132 of the fan
frame 33. The second flange 147 includes a forward
extending annular rail 152 received within an aftwardly
open annular slot 154 on the first flange 146.
A first circular row 156 of first bolt holes
^^ 158 extend axially through the first flange 146 into the
^^ annular slot 154. A corresponding second circular row
166 of threaded second bolt holes 168 extend axially
partially into the forward extending annular rail 152 of
the second flange 147. Bolts 160 are disposed through
the first bolt holes 158 and screwed into the threaded
second bolt holes 168. Each of the bolts 160 includes a
bolt head 170, a shank 172, and a thread 174. The
exemplary embodiment of the threaded second bolt holes
168 illustrated herein have threaded inserts 176
inserted therein to provide threads for the second bolt
1
holes. Serrated lock rings 178 are used to hold the
threaded inserts 176 in the second bolt holes 168. The
exemplary embodiment of the bolt 160 disclosed herein
has a shank outer diameter SD of the shank 172 that is
smaller than a thread diameter TD of the thread 174.
Crushable spacers 182 are disposed around the
shanks 172 of a first plurality 180 of the bolts 160.
The spacers 182 contact and axially extend between the
bolt heads 170 and a first annular surface 183,
preferably flat, of the first flange 146. Each of the
spacers 182 include a tubular body 186 extending axially
between first and second enlarged or flanged ends 188,
190. The crushable spacers 182 are crushed and act to
reduce rotating loads when the first and second flanges
U are rotated or pivoted relative to each other and to the
centerline axis 12 as illustrated by the curved arrows A
in the FIGS. This can occur as noted above during a
transient event such as a blade out event. The
crushable spacers 182 are designed to spread out the
rotating load and reduce peak loads transmitted through
the flanges into an adjacent structure.
Referring to FIGS. 3 and 5, bushings 192 are
disposed around a second plurality 194 of the bolts 160
for retaining a flange centerline (flange centerlines
coincide with the engine centerline axis 12 during
normal engine operation) when at least one of the
flanges have been loosened after the spacers have been
^^ crushed. The bushings 192 are disposed around the
shanks 172 of the second plurality 194 of the bolts 160
and axially extend entirely through the first bolt holes
158. The bushings 192 contact and axially extend
between the bolt heads 170 and a second annular flat
surface 184 of the second flange 147. Each of the
bushings 192 include a tubular bushing body 196
extending axially between a first bushing end 200 and a
second bushing end 202.
The exemplary embodiment of the bushings 192
illustrated herein includes a flanged bushing first end
3
200 having an optional annular bushing flange 204. The
annular slot 154 on the first flange 146 is defined in
part by annular radially inner and outer lips 206, 208
extending aftwardly from an annular inner slot wall 210
on the first flange 146. The inner and outer lips 206,
208 together with the inner slot wall 210 define annular I
radially inner and outer rabbets 216, 218. Annular
radially inner and outer chamfers 220, 222 are on a
forwardmost end 226 of the forward extending annular
rail 152 of the second flange 147. Though two rabbets
are illustrated herein a single rabbet may also be used
to radially center and support the forward bearing
support structure 138. The inner and outer rabbets 216,
218 have axially extending inner and outer contact
^p lengths RI, RO along which the rabbets makes metal to
metal contact and engage the forward extending annular
rail 152.
A gap L may be included between the annular
bushing flange 204 and the first annular surface 183 of
the first flange 146. The bushing flange 204, an
optional feature, is designed to limit how far, axially,
the first flange 146 can separate from second flange 147
during the unbalance event. For this aspect of the
bushing flange to be effective, the gap L should be less
than the longest engagement or contact length, which is
illustrated as being the inner contact length RI.
FIG. 6 illustrates an alternative embodiment
£i of the bolted flange assembly 64 of bolted flange
assembly in which first and second flanges 246, 247 have
first and second circular rows 256, 266 of first and
second bolt holes 258, 268 extending axially entirely
through the first and second flanges 246, 247. A nut
250 secures the bolts 160. The spacers 182 and bushings
are arranged as described above.
The bushings interspersed between the
crushable spacers on the bolts is to retain flange
centerline after the spacers crush. This is preferred
on engines having shafts supported by two bearings not
1
three. For sizing criteria, the crush area of the
spacers is sized for maximum assembly clamp as permitted
by the bolt, and bolt count is then set to meet or
exceed all normal and limit loads as set forth by design
practice requirements. As with all fuses, the spacers
must transmit normal loads reliably, and only activate
under ultimate loads. By sizing for assembly load
first, the spacers are pre-stressed to near the yield
point and ready to activate with minimum flange
separation while maximizing the flange clamp for normal
loads. Meanwhile, crush height is set to minimize
dynamic loads and tends to cap when other engine axial
clearances begin to clash.
The exemplary embodiment of the bolt 160
Q disclosed herein has a shank outer diameter SD of the
shank 172 that is smaller than a thread diameter TD of
the thread 174. Because the crushable spacers 182 must
fit over the threads 174 during installation, a loose
fit may exist between the shank outer diameter SD and a
spacer inner diameter ID. This can create
non-concentric condition at assembly between the spacer
and bolt which, in turn, drives bending into the spacer
and bolt with associated non-uniform stresses.
A method proposed herein to address this issue
provides heat shrink tubing 185 disposed around and
shrunk onto the bolt shank 172 to create an effective
shank outer diameter EOD equal to or larger than the
^ thread diameter TD (also referred to as a major
diameter) permitting a tight fit with the spacer 182 at
the spacer inner diameter ID. This effectively centers
the spacers thus improving the compressive stress
uniformity within the spacer and permits higher assembly
clamp load in order to maximize normal load capability.
With the spacer inner diameter ID of the spacer sized
to fit over and around the tubing as shrunk onto the
bolt shank, the spacer outer diameter of the spacer is
then sized for crush area.
Crushable spacer material should have a
1*
generally high tensile strength and a ratio of ultimate
tensile strength (UTS) to yield strength (YS) ultimate
strength as low as possible. The spacer should.be sized
the for yield strength YS at assembly clamp load, and
the buckling is based on an ultimate tensile strength
UTS for the load expected at an event such as blade out.
The lower the ratio of UTS/YS, the lower the flange
load (past flange separation) needed to crush the
spacer. The spacer material thermal coefficient of
expansion match with the bolt material is also a
consideration.
The present invention has been described in an
illustrative manner. It is to be understood that the
terminology which has been used is intended to be in the
^g nature of words of description rather than of
limitation. While there have been described herein,
what are considered to be preferred and exemplary
embodiments of the present invention, other
modifications of the invention shall be apparent to
those skilled in the art from the teachings herein and,
it is, therefore, desired to be secured in the appended
claims all such modifications as fall within the true
spirit and scope of the invention.
Accordingly, what is desired to be secured by
Letters Patent of the United States is the invention as
defined and differentiated in the following claims:
PARTS LIST
10. gas turbine engine
12. engine centerline axis
14. fan
15. inner air flow
16. low pressure compressor or booster
18. high pressure compressor
20. combustor
22. high pressure turbine (HPT)
23. high pressure drive shaft
24. low pressure turbine (LPT)
25. low pressure drive shaft
26. air
28. hot combustion gases
^ 30. fan casing
^ 32. sharp leading edge
33. fan frame
34. flow splitter
36. bypass duct
60. dynamic load reduction system
64. bolted flange assembly
112. fan rotor
114. fan disk
116. fan blades
118. fan shaft
12 8. support system
13 0. outer casing
132. inner hub
134. struts
136. forward bearing system
138. forward bearing support structure
13 9. forward bearing
146. first flange
147. second flange
A 14 8. aft end
^* 152. forward extending annular rail
154. aftwardly open annular slot
156. first circular row
158. first bolt holes
160. bolts
166. second circular row
168. second bolt holes
170. bolt head
172. shank
174. thread
176. threaded insert
178. serrated lock ring
180. first plurality
182. crushable spacers
183. first annular surface
184. second annular surface
185. heat shrink tubing
I'2-
186. tubular bodies
188. first enlarged or flanged ends
190. second enlarged or flanged ends
192. bushings
194. second plurality
196. tubular bushing body
200. first bushing ends
202. second bushing ends
204. bushing flanges
206. inner lip
208. outer lip
210. inner slot wall
216. inner rabbet
218. outer rabbet
22 0. inner chamfer
222. outer chamfer
•
226. forwardmost end
246. first flanges
247. second flanges
250. nut
256. first circular rows
258. first bolt holes
266. second circular rows
268. second bolt holes
A - curved arrows
L - gap
ID - spacer inner diameter
SD - shank outer diameter
TD - thread diameter
EOD - effective shank outer diameter
^^ RI - inner contact length
RO - outer contact length
IS

We Claim:
1. A bolted flange assembly (64) comprising:
a first flange (146) bolted to a second flange
(147),
a first circular row (156) of first bolt holes
(158) extending axially through the first flange (146),
bolts (160) disposed through the first bolt holes
(158) and through second holes (168) extending axially
at least partially through the second flange (147),
each of the bolts (160) including a bolt head
(170), a thread (174), and a shank.(172) therebetween,
crushable spacers (182) disposed around the shanks
^ (172) of a first plurality (180) of the bolts (160),
the spacers (182) contacting and axially extending
between the bolt heads (170) and the first flange (146),
bushings (192) disposed around a second plurality
(194) of the bolts (160), and
the bushings (192) contacting and axially extending
between the bolt heads (17 0) and the second flange
(147) .
2. The assembly (64) as claimed in Claim 1, further
comprising a shank outer diameter (SD) of the shanks
(172) smaller than a thread diameter (TD) of the threads
(174) and heat shrink tubing (185) disposed around the
^ bolt shanks (172) of at least some of the spacers (182). w
3. The assembly (64) as claimed in Claim 2, further
comprising the spacers (182) contacting the first flange
(146) on a first flat annular surface (183) of the first
flange (146) and the bushings (192) contacting the
second flange (147) on a second flat annular surface
(184) of the second flange (147).
4. The assembly (64) as claimed in Claim 2, further
comprising the spacers (182) including tubular bodies
1^
(186) extending axially between first and second
enlarged or flanged ends (188, 190).
5. The assembly (64) as claimed in Claim 1, further
comprising:
the bushings (192) including tubular bushing bodies
(196) axially extending between first bushing ends (200)
and second bushing ends (202),
annular bushing flanges (204) on the first bushing
ends (200) adjacent the bolt heads (170), and
gaps (L) between the annular bushing flanges (204)
and the first flange (146) .
6. The assembly (64) as claimed in Claim 1, further
^1^ comprising the second holes (168) being threaded and
extending axially partially through the second flange
(147) .
7. The assembly (64) as claimed in Claim 6, further
comprising:
the first bolt holes (158) extending axially
through the first flange (146) into an aftwardly open
annular slot (154) on the first flange (146),
the second holes (168) extending partially through
a forward extending annular rail (152) of the second
flange (147), and
the annular rail (152) received within the annular
^ slot (154).
8. The assembly (64) as claimed in Claim 1, further
comprising:
the first bolt holes (158) extending axially
through the first flange (146),
the second holes (168) extending partially through
a forward extending annular rail (152) of the second
flange (147), and
the annular rail (152) received within an inner or
outer rabbet (216, 218) defined by an inner or outer lip
(206, 208) respectively extending aftwardly from the
first flange (146).
9. A gas turbine engine forward bearing system (136)
comprising:
a forward bearing support structure (138) and a fan
frame (33),
a bolted flange assembly (64) including a first
flange (146) at an aft end (148) of the forward bearing
support structure (138) bolted to a second flange (147)
on the fan frame (33),
a first circular row (156) of first bolt holes
(158) extending axially through the first flange (146),
bolts (160) disposed through the first bolt holes
^^ (158) and through second holes (168) extending axially
at least partially through the second flange (147),
each of the bolts (160) including a bolt head
(170), a thread (174), and a shank (172) therebetween,
crushable spacers (182) disposed around the shanks
(172) of a first plurality (180) of the bolts (160),
the spacers (182) contacting and axially extending
between the bolt heads (170) and the first flange (146),
bushings (192) disposed around a second plurality
(194) of the bolts (160), and
the bushings (192) contacting and axially extending
between the bolt heads (170) and the second flange
(147) .
10. An aircraft turbofan gas turbine engine (10)
comprising:
in downstream serial flow communication a fan (14),
a low pressure compressor or booster (16), a high
pressure compressor (18), a combustor (20), a high
pressure turbine (22), and a low pressure turbine (24)
circumscribed about an engine centerline axis (12);
the low pressure turbine (24) joined by a low
pressure drive shaft (25) to the fan (14) and the low
pressure compressor or booster (16);
\6
a forward bearing support structure (138)
supporting a forward bearing (139);
the forward bearing (139) rotatably supporting in
part the low pressure drive shaft (25);
a bolted flange assembly (64) including a first
flange (146) at an aft end (148) of the forward bearing
support structure (138) bolted to a second flange (147)
on a fan frame (33) of the engine (10);
a first circular row (156) of first bolt holes
(158) extending axially through the first flange (146);
bolts (160) disposed through the first bolt holes
(158) and through second holes (168) extending axially
at least partially through the second flange (147);
crushable spacers (182) disposed around the shanks
^ (172) of a first plurality (180) of the bolts (160);
the spacers (182) contacting and axially extending
between the bolt heads (170) and the first flange (146);
bushings (192) disposed around a second plurality
(194) of the bolts (160); and
the bushings (192) contacting and axially extending
between the bolt heads (170) and the second flange
(147) .
11. The engine (10) as claimed in Claim 10, further
comprising:
the second holes (168) being threaded and extending
axially partially through the second flange (147),
•
the first bolt holes (158) extending axially
through the first flange (146) into an aftwardly open
annular slot (154) on the first flange (146),
the second holes (168) extending partially through
a forward extending annular rail (152) of the second
flange (147),
the annular rail (152) received within the annular
slot (154),
a shank outer diameter (SD) of the shanks (172)
smaller than a thread diameter (TD) of the threads (174)
and heat shrink tubing (185) disposed around the bolt
o
shanks (172) of at least some of the spacers (182),
the spacers (182) including tubular bodies (186)
extending axially between first and second enlarged or
flanged ends (188, 190).
12. The engine (10) as claimed in Claim 10, further
comprising:
the first bolt holes (158) extending axially
through the first flange (146),
the second holes (168) extending partially through
a forward extending annular rail (152) of the second
flange (147),
the annular rail (152) received within an inner or
outer rabbet (216, 218) defined by an inner or outer lip
^^ (206, 208) respectively extending aftwardly from the
first flange (146),
the bushings (192) including tubular bushing bodies
(196) axially extending between first bushing ends (200)
and second bushing ends (202),
annular bushing flanges (204) on the first bushing
ends (200) adjacent the bolt heads (170), and
gaps (L) between the annular bushing flanges (204)
and the first flange (146).