Title: Frangible coupling
Abstract: A frangible coupling for interconnecting parts comprising a first ring and a second ring coaxially arranged relative to each other and axially joined via an annular array of fuse ligaments equidistantly spaced apart from each other. The ligaments are configured to fail when an abnormal radial load of a predetermined value causes the first and second ring to move out of their coaxial relationship. When all of the fuse ligaments are severed, the communication between the first and second rings is severed. This allows the first ring to move independently of the second ring, preventing the out of balance load on the first ring being communicated to the second ring.
Patent Number: 7,025,560 Issued on 04/11/2006 to Clark
| Inventors:
|
Clark; Brynley (Bristol, GB)
|
| Assignee:
|
Rolls-Royce, plc (London, GB)
|
| Appl. No.:
|
600443 |
| Filed:
|
June 23, 2003 |
Foreign Application Priority Data
| Current U.S. Class: |
415/9; 416/2; 60/39.091; 60/223; 403/2 |
| Current Intern'l Class: |
F01B 25/16 (20060101); F01D 21/00 (20060101) |
| Field of Search: |
415/9
416/2
60/390.91,223,226.1
403/2
|
References Cited [Referenced By]
U.S. Patent Documents
| 3205024 | Sep., 1965 | Morley et al.
| |
| 3659877 | May., 1972 | Kubasta.
| |
| 4086012 | Apr., 1978 | Buckley et al.
| |
| 4375906 | Mar., 1983 | Roberts et al.
| |
| 6068452 | May., 2000 | Okada et al.
| |
| 6079200 | Jun., 2000 | Tubbs.
| |
| 6428269 | Aug., 2002 | Boratgis et al.
| |
| Foreign Patent Documents |
| 1 314 858 | Oct., 2002 | EP.
| |
| 2 752 024 | Feb., 1998 | FR.
| |
| 888116 | Jan., 1962 | GB.
| |
| 0 928 250 | Jun., 1963 | GB.
| |
| 1 199 441 | Apr., 2002 | GB.
| |
Primary Examiner: Trieu; Theresa
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
The invention claimed is:
1. A frangible coupling for the purpose of supporting a rotatable load having
a first ring, a second ring, a plurality of ligaments and a load magnification
member, said first ring and second ring interconnected by said plurality of ligaments
with the load magnification member provided on the first ring or rotatable load,
there being a small clearance maintained between said member and ligaments adjacent
thereto, configured such that, in use, when a load of a predetermined value causes
the first and second ring to move relative to one another by a predetermined amount,
thereby bringing at least one ligament into contact with said load magnification
member, at least one ligament is caused to fail.
2. A frangible coupling as claimed in claim 1 wherein the said ligaments are
substantially axially aligned.
3. A frangible coupling as claimed claim 1 wherein the first and second rings
are cylindrical.
4. A frangible coupling as claimed in claim 1 wherein the ligaments are equidistantly
spaced apart.
5. A frangible coupling as claimed in claim 1 wherein the first ring and the
second ring are coaxial.
6. A frangible coupling as claimed in claim 1 wherein the first ring and the
second ring are concentric.
7. A frangible coupling as claimed in claim 1 wherein the load magnification
member on the first ring is formed as a flange that is provided with a plurality
of semi-circular cross-section cut out portions each of which corresponds closely
to at least part of the outside diameter of a ligament part way along the ligaments,
there being a small clearance maintained between the ligaments and their corresponding
cut out portions in the flange.
8. A frangible coupling as claimed in claim 7 wherein at least one ligament is
formed with a stress raising feature in the region where, when a load of a predetermined
value causes the first and second ring to move relative to one another by a predetermined
amount, the at least one ligament is designed to contact the flange thereby increasing
the stress concentration in the at least one ligament to a level where the at least
one ligament fails.
9. A frangible coupling as claimed in claim 7 wherein each of the ligaments have
at least one waisted section.
10. A frangible coupling as claimed in claim 7 wherein the first ring is in communication
with a means for supporting a rotatable load.
11. A frangible coupling as claimed in claim 10 wherein the second ring is fixedly
joined to a fan support structure.
12. A frangible coupling as claimed in claim 1 wherein a rotatable shaft is in
communication with said first ring via a bearing support means, the load magnification
member is a rotatable member on the rotatable shaft positioned between and coaxially
with the first and second ring, thereby defining a small clearance between the
said member and the ligaments adjacent thereto, such that when a load of predetermined
value causes the first and second ring to move relative to one another by a predetermined
amount, the at least one ligament is designed to contact the member thereby increasing
the stress concentration in the at least one ligament to a level where the at least
one ligament fails.
13. A frangible coupling as claimed in claim 12 wherein the rotatable member
is a disc formed with at least one snub which extends substantially radially outward
from the rotatable member, there being a small clearance maintained between the
said snub and the ligaments adjacent thereto.
Description
BACKGROUND
The invention relates to a frangible coupling. In particular it refers to a frangible
coupling for turbo machinery.
In a conventional gas turbine engine, the fan is used for pressurising ambient
air which then passes downstream to a compressor to be further compressed. The
air is then mixed with fuel in a combustor, ignited and burned to expand the gas,
further increasing the gas pressure before exhausting via a turbine from which
energy is extracted. The engine may have a high pressure turbine which powers the
compressor, and a low pressure turbine which powers the fan.
Other engines utilise power off takes, perhaps directly from one of the turbine
stages, to drive independent fans to generate propulsive thrust remote from the
propulsion unit.
The fan typically comprises an annular array of large fan blade rotors that extend
radially outward from a supporting disc. The fan is fixedly joined to a shaft and
is rotatable about the axis of the shaft, which is rotatably supported by a number
of bearings in communication with a static fan support structure. The concentric
alignment of the fan within a surrounding fan casing is maintained by the bearings.
The bearings also act as a means to transmit aerodynamic, centrifugal and vibratory
loads into the fan support structure. During normal operation the fan is dynamically balanced.
In the rare event of the loss of a section of a blade, perhaps because of foreign
object damage or failure of the rotor blade material, there may be a substantial
rotary imbalance introduced into the fan system. This will be transmitted to the
fan support structure. If the engine is fitted to an aircraft, this may lead to
undesirable vibrations being transmitted to the airframe body. In extreme cases
the aircraft may become difficult to handle or suffer severe structural damage.
The engine may be turned off to prevent unnecessary damage to itself and the
airframe. However, whilst in flight, there may be no means to stop the aerodynamic
windmilling of the damaged engine, which may be enough to cause a substantial imbalanced
load and further damage.
Likewise, if the fan is driven remotely from the engine it may be desirable
to run the damaged fan to generate propulsion. A common requirement is to be able
to run the fan up to a predetermined imbalance load, thereby coping with a proportion
of blade loss. The fan should only be taken out of use when the imbalance load
reaches a certain unacceptable level. It may not be possible for a pilot to make
this judgement, requiring some safety feature of the fan to sever the connection
between the imbalanced load and the fan structure.
In order to accommodate the possibility of such abnormal radial loads the supporting
components for the fan may be strengthened. This may have the undesirable effect
of increasing the size, weight and expense of the fan structure. Means for the
controlled buckling of various parts of engine structure have also been utilised.
Another solution is the introduction of a coupling placed between the bearing
support structure and the fan support structure that de-couples when the imbalance
reaches a predetermined level. Such a device is frequently referred to as a structural
fuse or a frangible coupling. When decoupled the connection between the bearing
support and support structure is severed, leaving the fan supported by its shaft
and at one end by a bearing.
Conventional structural fuses are designed to de-couple above relatively
low abnormal radial loads. However, an increasingly common requirement is for the
fan to carry on rotating and generating useful thrust with a degree of out of balance loading.
The fuse has two conflicting requirements. It must withstand any fatigue or normal
operational loads but fail reliably under the increased fan blade off load. This
presents a load range within which the fuse must be designed. The extra requirement
that the fuse not fail under partial fan blade off but fail under full blade off
makes the design window prohibitively narrow.
Fuse designs exist that utilise shear bolts and spigots that fasten the supporting
components of the fan together. However, given the tolerances inherent in the design
and materials used, it is not possible to define accurately at what load the fuse
will severe the connection. Hence the fuse may sever the connection when the imbalance
is below the required lower level, resulting in premature decoupling, or above
the higher level, resulting in damage to the fan structure, engine or aircraft.
According to the present invention there is provided a frangible coupling
for the purpose of supporting a rotatable load having a first ring, a second ring,
a plurality of ligaments and a load magnification member, said first ring and second
ring interconnected by said plurality of ligaments, with the load magnification
member provided on the first ring or rotatable load, there being a small clearance
maintained between said member and ligaments adjacent thereto, configured such
that, in use, when a load of a predetermined value causes the first and second
ring to move relative to one another by a predetermined amount, thereby bringing
at least one ligament into contact with said load magnification member, at least
one ligament is caused to fail.
Preferably the first ring is formed with a flange that is provided with
a plurality of semi-circular cross-section cut out portions each of which corresponds
closely to the outside diameter of the ligaments part way along the ligaments,
thereby defining a small clearance between the ligaments and their corresponding
cut out portions in the flange.
Preferably at least one ligament is formed with a stress raising feature
in the region where it is designed to contact the flange when a load of a predetermined
value causes the first and second ring to move relative to one another by a predetermined amount.
Preferably the frangible coupling is configured such that at a predetermined
out of balance loading induced by the rotatable load at least one ligament is brought
into contact with the flange, thereby increasing the stress concentration in the
at least one ligament to a level where the at least one ligament fails.
The invention provides an internal support structure for rotatable turbo machinery
components that will fail when subjected to out of balance forces imparted to the
structure caused by a fan blade off or partial fan blade off. The stress raising
feature within the fuse accelerates the fracture process, breaking the fuse within
a narrower and predictable loading range. This allows the rotor to orbit closer
to its new centre of gravity and either transmit a reduced load by a secondary
route, or removes the load path altogether.
In commonly used aerospace metals there is a linear relationship between load
applied and stress induced. This invention employs a means whereby when the load
raises to a certain level, the rate of change of stress in the material suddenly
is increased, inducing fracture within a much smaller and predictable load range.
The stresses acting on the ligaments are magnified, causing them to fail at lower
loads than they otherwise would.
This load magnification at high loads enables the ligaments to be designed for
a long fatigue life at low loads whilst failing positively at higher loads.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and how it may be constructed and operated, will now be described
in greater detail with reference, by way of example, to an embodiment illustrated
in the accompanying drawings, in which:
FIG. 1 is a pictorial representation of a typical gas turbine engine;
FIG. 2 is a pictorial representation of a fan powered remotely from an engine;
FIG. 3 shows a schematic representation of the relevant section of a fan, illustrating
the location of the frangible coupling relative to the rotatable components;
FIG. 4 shows a first embodiment of the frangible coupling;
FIG. 5 shows a second embodiment of the frangible coupling;
FIG. 6 shows the second embodiment distorted by an out of balance force, the
distortion is exaggerated for clarity; and
FIG. 7 is a diagrammatic representation of the relationship between load applied
and stress induced in the frangible coupling.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 illustrates the main sections of a gas turbine engine
2. The overall
construction and operation of the engine
2 is of a conventional kind, well
known in the field, and will not be described in this specification beyond that
necessary to gain an understanding of the invention. For the purposes of this description
the engine is divided up into four sections—a fan section
4, a compressor
section
6, a combustor section
8 and a turbine section
10.
Air, indicated generally by arrow "A", enters the engine
2 via the fan section
4. The air is compressed and moves downstream to the compressor
6.
This further pressurises the air, a proportion of which enters the combustion section
8, the remainder of the air being employed elsewhere. Fuel is injected into
the combustor airflow, which mixes with air and ignites before exhausting out of
the rear of the engine, indicated generally by arrow "B", via the turbine section
10. A cutaway reveals the location of a frangible coupling
12.
FIG. 2 illustrates a fan unit
14 that is driven remotely from an engine.
It does not provide compressed air to the engine but is used to generate propulsive
thrust remote from the propulsion unit. In FIG. 2 the fan unit
14 is shown
mounted with its central axis vertical. This is only one embodiment, drawn here
for illustrative purposes. The fan unit may be mounted in any orientation.
For the purposes of this description the fan unit
14 is divided up into
3 sections—a fan rotor section
16, a compressor section
18
and a drive shaft and gearing arrangement
20, the latter being shown in
a cutaway view. Air, indicated generally by arrow "C", enters the fan unit
14
via the fan rotor section
16. The air is compressed and moves downstream
to compressor
18, where it is further pressurised before being exhausted
from the fan
14, indicated generally by arrow "D". A cutaway reveals the
location of the frangible coupling
12.
An enlarged view of fan assembly common to the engine
2 and fan unit
14
is presented in FIG. 3. Air, indicated generally by arrow "E", enters the fan unit
14, constrained on one side by an outer wall
22 and on the other
by a discontinuous inner wall
24. Support for the inner wall
24 is
provided by an array of support members
25 which extend radially towards,
and are in communication with, the outer wall
22. The inner wall
24
comprises several static and rotatable sections, the details of which are not required
here to appreciate the invention. The air is pressurised by an annular array of
fan rotor blades
26 and then passes downstream, as indicated generally by
arrow "F".
The fan blades
26 are fixedly joined to a shaft
28 that is rotatable
about the central axis of the fan unit
14. The shaft
28 is rotabably
supported by bearings
30 and
32 at the downstream and upstream ends
respectively. The bearing
32 is supported by the nonrotatable frangible
coupling
12 via a first static member
36. The coupling
12
is in communication with a non-rotatable section of the inner wall
24 via
a second static member
38.
FIG. 4 shows an enlarged view of the frangible coupling
12, with the
first member
36 and second member
38 removed for clarity. The frangible
coupling
12 comprises a first ring
40 axially joined via a row of
generally circular cross section fuse ligaments
42 to a second ring
44.
The first ring
40 is formed with a flange
46 that is provided with
semi-circular cross-section cut out portions
43 that correspond closely
to the outside diameter of the ligaments
42 part way along their length.
A small clearance
48 is maintained between the ligaments
42 and their
corresponding cut out portions
43 in the flange
46. The fuse ligaments
42 are equidistantly spaced apart from each other and are formed with a
stress raising feature
45, which, as shown here, may take the form of a
narrowed waist.
In normal use the primary load path from the fan shaft
28 is through the
support bearing
32, and then through the coupling
12 to the inner
wall
24, transmitted then to the support members
25 which communicate
it to the outer wall
22.
Under high out of balance loads the ligaments
42 deflect, the clearance
48 closes and the flange
46 forms part of the secondary load path.
The flange
46 acts as a load magnifacation member, and hence a consequence
of the out of balance nature of the loading is that the ligament
42 in contact
with the flange
46 will carry significantly more load than the other ligaments.
This results in the rapid failure of the ligament
42 in contact with the
flange
46. Since the applied load is rotating the adjacent ligament
42
quickly becomes loaded in a similar way and also fails. This process is repeated
until all of the ligaments
42 have failed. With the ligaments
42
severed, the first ring
40 is free to move independently of the second ring
44, allowing the out of balance shaft
28 to oscillate about a new
axis, which will result in less damage to the engine support casing than if the
out of balance force was transmitted through to the inner wall
24.
FIG. 5 presents an alternative embodiment of the frangible coupling
12.
The coupling
12 comprises a first ring
50 axially joined via a row
of fuse ligaments
52 to a second ring
54. The second ring
54
is formed with a third static member
56 (not shown) that is fixedly joined
with a non rotatable section of the inner wall
24 (not shown in this figure).
The first ring
50 is fitted with a bearing
58 that rotatably supports
the first ring
50 on a shaft
60. The shaft
60 supports the
fan blades
26 (not shown in this figure). The shaft
60 is provided
with a disc
62 positioned at about one half of the way between the first
ring
50 and the second ring
54. Extending radially outward from the
circumference of the disc
62 is a snub
64 which, in use, acts as
a load magnifacation member.
In normal in balance operation the blades
26 rotate and cause only small
deflections of the shaft
60. When subjected to abnormally high radial loads
the shaft
60 will oscillate, transmitting the oscillation to the bearing
58 and the first ring
50, causing the ligaments
52 to deflect,
as shown in FIG. 6 (exaggerated). The relative movement of the ligaments
52
and the snub
64 causes them to impact each other as the snub
64 rotates.
The impact is sufficient to cause the failure of the ligaments
52. The first
ring
50 will be forced to oscillate with the rotating out of balance load,
resulting in the snub
64 impacting on all of the fuse ligaments
52,
breaking them in turn and ultimately severing the connection between the first
ring
50 and the second ring
54. This allows the first ring
50
to move independently of the second ring
54, allowing the out of balance
shaft
60 to oscillate about its new axis, resulting in less damage to the
support casing of the fan unit
14 than if the out of balance force was transmitted
through to the inner wall
24.
During normal operation in both embodiments the fuse ligaments
52 experience
an increase in stress proportional to the load imposed by the rotating load. This
is indicated by section "G" of the graph in FIG. 7. When an abnormal radial load
is applied the stress is increased locally in at least one of the fuse ligaments
52, increasing the stress per unit force at the critical location on the
ligament
52, indicated by section "H" on the graph. Hence the overall relationship
between the load imparted to the first ring
40,
50 and stress induced
in the fuse ligaments
52 is non linear. The sudden increase in fuse ligament
stress enables a better control over the loading at which the ligament
52
will fail.
The failure of some, but not all, of the ligaments
52 may enable the coupling
12 to accommodate the out of balance load where the more rigid structure
provided by the coupling
12, when all ligaments
52 are intact, would
not sufficiently dampen the excessive oscillation.
The configurations shown in FIGS. 1,
2,
3,
4,
5 and
6 are diagrammatic. The design and positioning of the frangible coupling,
rotor blades, bearings, fan casing and other parts may vary. Likewise the combination
and configuration of these components will vary between designs. The relationship
presented in FIG. 7 is an approximation.
*
