GB2526321A

GB2526321A - A casing arrangement

Info

Publication number: GB2526321A
Application number: GB1408986.6A
Authority: GB
Inventors: Ian Colin Deuchar Care; Daniel Clark
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2014-05-21
Filing date: 2014-05-21
Publication date: 2015-11-25
Also published as: GB201408986D0

Abstract

A fibre-reinforced composite casing for a gas turbine engine has a circumferentially extending flange or rib 116 defining a projection. A metal cap 160 is attached around the flange or rib. In use, ancillary components may be attached to the metal cap 160. The cap may be secured to the flange or rib via mechanical surface features 170 or adhesive. A layer of material may be interposed between the cap and flange or rib to provide damping or galvanic isolation. The cap provides an evenly distributed load to the flange or rib.

Description

A CASING ARRANGEMENT

This invention relates to casing arrangements for gas turbine engines, and in particular to arrangements for mounting ancillary components on composite casings.

Often ancillary components, such as cables and pipes, must be secured to the outside of the casings of gas turbine engines. On conventional metal casings, it is known to secure such components in rafts, brackets or clips attached to bosses on the casing.

The rafts, brackets or clips may also be attached to other features on the casing.

Commonly, rails, ribs or flanges are provided on such casings; sometimes only to provide mountings for ancillary components, but often also providing local stiffening and strengthening of the casing. Because of the generally isotropic and ductile behaviour of metallic materials, it is easy to provide mounting points (for example holes) in such rails, ribs or flanges without detrimental effects on the performance of the casing.

Isogrid casings are known, in which integral stiffening ribs are provided in a triangular pattern. An advantage of isogrid casings is that they behave like isotropic materials, with equal properties in all directions, although the manufacture of metal isogrids is relatively complicated as they are generally machined from solid. In metal isogrid casings, ancillary components are commonly attached to the bosses where the stiffening ribs join one another.

In an aerospace gas turbine there is a desire to reduce the weight of components whilst retaining their strength and function. Components made from OMC (Organic Matrix Composites) such as carbon-fibre composite offer potential weight reductions, but there are problems that must be overcome, and design methods appropriate for metallic components are often not applicable to composite components. In particular, this is because OMCs generally have highly orthotropic material properties. They are also much more susceptible to crushing damage and delamination in use, which can severely reduce the mechanical properties of the composite component.

Just as in metallic casings, for a composite casing the wall thickness is generally minimised (to minimise the overall weight of the casing). The casing is then strengthened and stiffened, only in the required locations, by localised thickening or by using ribs or a series of ribs, which may interact with each other.

The particular difficulty to be overcome, with composite casings, is how to secure external components, such as pipes and cables, to them without introducing problems caused by the particular mechanical properties and behaviour of composite materials.

Examples of these problems are: resisting crush loads resulting from over-tightening of fixings or bolts; providing strengthening and stiffening features to the casing; providing protection from imprecise fitting; permitting adjustment during life or repair.

Figure 1 shows a partial cut-away view of a first prior art arrangement for clamping ancillary components on to a flange of a composite casing. An annular composite casing shown generally at 12 is formed of multiple layers 14a, 14b, 14c of composite material. A flange 16, which extends around the whole circumference of the casing 12, is formed by turning or folding the layers 14a, 14b outwards from the body of the casing 12. A filler piece (or noodle) 18 supports the layers 14a, 14b and fills the space between them and layers 14c. These construction techniques are known and will be familiar to a person having ordinary skill in the art.

At any circumferential position where it is desired to attach an ancillary component, a clamp 20 is attached to the top of the flange 16 by tightening one or more bolts 22.

Faces 24, 26 engage with the corresponding faces of the flange 16 to secure the clamp to the flange. The components may then be attached, directly or indirectly, to the clamp 20.

The arrangement shown in Figure 1 has a number of problems. First, it is necessary to add load-spreading features, such as an external load-spreading layer 28 of a more resilient material such a glass-fibre composite, to distribute the clamping load into the composite material of the flange 16 without causing damage, and to provide galvanic isolation for the clamping arrangement. In some arrangements a further, usually metallic, load spreading layer is added to layer 28. Such features add weight and cost, and complicate the manufacturing processes. Second, through-thickness reinforcement, using pins 30, is needed to improve the load distribution and resistance to delamination. This adds further cost and complexity to the arrangement, and because the pins 30 operate better in tension than in compression, they are unavoidably acting in a sub-optimal manner in this application. Third, the dimensions of the flange 16 need to be accurately controlled, and the clamp 20 accurately positioned and properly engaged before tightening it, in order to achieve satisfactory clamping.

Fourth, it is necessary to make the clamp relatively "wide" in the circumferential direction, in order to reduce the load per unit area transferred into the composite.

Because of the width of the clamp it is also slightly curved (to fit the curve of the flange) which helps it to resist bending, which might cause it to exert "levering" loads on the flange.

Figure 2 shows a partial cut-away view of a second prior art arrangement for clamping ancillary components on to a flange of a composite casing. In this embodiment the casing 32 comprises two casing parts 32a and 32b. Each casing part 32a, 32b is formed of multiple layers of composite material (34a, 34b respectively) which are turned or folded outwards to form flanged ends 36a, 36b of the casing parts. Each casing part 32a, 32b has a respective filler piece 38a, 38b to support the layers 34a, 34b and to provide flat end faces for the casing parts. The casing parts 32a, 32b also comprise inner layers 40a, 40b which may provide a smooth inner surface to the casing 32.

The casing parts 32a, 32b are clamped together by a plurality of bolts 42, which pass through aligned holes in the flanged ends 36a, 36b and engage with nuts 44. At any circumferential position where it is desired to attach an ancillary component, a bracket 46 is attached in known manner (for example, by a drilled or slotted flange on one end of the bracket) to a bolt so it is supported from the casing 32. The ancillary component (not shown) may be attached directly to the bracket 46, or a further fixing, such as a pipe clip 48, may be attached to the bracket.

The arrangement of Figure 2 exhibits similar problems to that of Figure 1, particularly with respect to load distribution. Accordingly, external load-spreading layers 50a, SOb are required along with washers 52 beneath the bolts and nuts, to distribute the clamping loads so as not to cause damage. These layers 50a, SOb and washers 52 add weight and complexity to the casing arrangement. Furthermore, any misalignment of the bolts, washers or brackets 46 may cause locally high loading and consequent damage. Similar problems may arise if the dimensions of the casing parts 32a, 32b are not accurately controlled; for example, if the end faces of the flanged ends 36a, 36b are not flat and parallel. Similarly, if load spreading (usually metallic) face plates (not shown) are added on top of load spreading layers 50a & 50b, this adds further weight making the arrangement comparable to the weight of a metallic flange.

There is therefore a need to provide a more flexible and robust means of attaching ancillary components to composite casings, that is more tolerant of inaccuracies in manufacturing and assembly.

Accordingly, the invention provides a casing arrangement for a gas turbine engine as set out in the claims.

Embodiments of the invention will now be described in more detail, with reference to the attached drawings, in which Figure 1 shows a partial cut-away view of a first prior art arrangement for clamping ancillary components on to a flange of a composite casing; Figure 2 shows a partial cut-away view of a second prior art arrangement for clamping ancillary components on to a flange of a composite casing; Figure 3 shows a partial cut-away view of a first embodiment of a casing arrangement according to the invention; Figure 4 shows a partial cut-away view of a second embodiment of a casing arrangement according to the invention; Figure 5 shows a partial cut-away view of a third embodiment of a casing arrangement according to the invention; and Figure 6 shows a partial cut-away view of a fourth embodiment of a casing arrangement according to the invention.

In the drawings, like parts are denoted by like reference numbers.

Referring to Figure 3, a casing arrangement shown generally at 110 comprises an annular composite casing 112 formed in a conventional manner, for example in the same manner as the casing 12 of Figure 1. An inner layer 140 provides a smooth internal surface to the casing. The casing 112 comprises a circumferentially-extending flange or rib 116 which extends in a radially outward direction from the casing.

A metal cap 160 is attached to the rib. The cap 160 comprises two legs 162, which extend in a radially inward direction on opposite sides of the rib 116, and a top surface 164. The top surface 164 extends forwardly and rearwardly in a generally axial direction to form projecting rails 166, 168.

The metal cap 160 incorporates engagement features 170 in the form of inwardly-protruding teeth (or noggins), which engage in use with the surface of the rib 116 to secure the metal cap 160 to it and prevent it from easily being removed. The teeth engage with corresponding recesses formed into the rib 116. The recesses in the rib 116 are formed during the manufacture of the composite casing, for example by using shaped formers or pegs.

The metal cap 160 is configured so that the legs 162 exert an inward force on the rib 116, so that the cap 160 grips the rib.

In use, ancillary components (not shown) are attached to the metal cap 160 by known means. Generally the ancillary components are attached to one or both of the rails 166, 168. The ancillary components may be attached by conventional means suited to metal components, for example by clamping or by bolting through holes drilled in the rails 166, 168. Because the components are not attached directly to or through the rib 116 there are no detrimental effects on its properties. Because the metal cap 160 is configured to fit closely to the profile of the rib 116, loads are distributed effectively as they are transferred into the flange, thus avoiding the locally high loading and consequent damage that is characteristic of known methods for attaching ancillary components to composite flanges or ribs.

The rails 166, 168 need not extend along the whole (circumferential) length of the metal cap 160, but may be formed only in the positions where they are needed. Likewise, the metal cap 160 need not extend around the whole circumferential extent of the rib 116-discrete metal caps of any required size may be provided in one or more positions around the circumference. This allows the weight of the casing to be minimised, because the caps and rails are provided only in the positions where they are needed and so minimal weight is added to the casing.

A second embodiment of the invention is illustrated in Figure 4. As in Figure 3, a casing arrangement shown generally at 110 comprises an annular composite casing 112 with an inner layer 140 and a rib 116. In the embodiment of Figure 4, a metal cap 260 is attached around the rib 116 in a similar manner to that described for Figure 3. The metal cap 260 has two legs 262 and a top surface 264. The metal cap 260, in contrast to the metal cap 160 of Figure 3, does not have the projecting rails 166, 168 (Figure 3).

The principal purpose of the metal cap 260 is therefore to provide additional strength or stiffness to the rib 116 (and thereby to the casing 112) rather than to allow the attachment of ancillary components. Because the rails are not required, their omission minimises the weight of the casing arrangement. As will be appreciated, the metal cap 26 need not extend around the whole circumference of the rib 116, but may be provided only at those circumferential positions where additional strength or stiffness is required.

Although the principal use for this embodiment of the invention is to provide additional strength or stiffness, it may be feasible to use it where only light clipping of cables is required, thereby avoiding the additional weight of the rails 166, 168 of Figure 3.

Optionally, and as shown in Figures 3 and 4, the radially inner end regions 172, 272 of the legs 162, 262 are tapered. The decreasing thickness of the legs 162, 262 towards their ends results in a corresponding decrease in stiffness, which helps to avoid abrupt changes in loading across the metal/composite joint where the cap is joined to the rib.

In particular, this arrangement prevents high delamination strain at the joint face.

The metal cap 160, 260 is formed using an additive manufacture technique, such as direct laser deposition (DLD). This provides a number of advantages.

Firstly, it is easy to produce complex three-dimensional shapes, for example to fit a rib with a non-symmetrical profile. Secondly, features may be added to the cap to improve the attachment of the cap to the rib; for example, large and/or uniform surface area to assist with adhesive bonding or surface features (such as 170 in Figure 3) to provide positive engagement between the cap and the rib.

Second, if the rib 116 is measured after manufacture (or, at least, after the consolidation and initial cure) then the measurement data defining the dimensions of the rib 116 may be used to direct the additive manufacture technique that forms the metal cap 160, 260. The advantage of this is that the metal cap 160, 260 can be made to fit accurately to the actual contours of the rib 116, therefore providing simpler attachment and more effective load distribution. Because the additive manufacture technique permits the cap to be "custom-made" to fit the actual dimensions of the rib, the tolerances on the rib's dimensions need not be so tightly controlled during the manufacture of the casing, which may allow savings in time or cost and may reduce scrappage.

A further embodiment of the invention is illustrated in Figure 6. In this embodiment, the cap has a buttress to provide additional strength and stiffness in the direction perpendicular to the rib. The cap 460 has a top surface 464, which is extended in one axial direction to form a rail 466. One leg 462a of the cap 460 is essentially as in the embodiments of Figures 3, 4 and 5, but the other leg 462b is extended to form a buttress 474, which bears against the outer surface 476 of the casing 112 to provide additional support.

The invention therefore provides a casing arrangement particularly suitable for composite casings, especially where ancillary components have to be attached to the casing. Because the components are not attached directly to or through the composite material, no damage is caused by the attachment. Furthermore, incorrect alignment or fitting of the components is less critical because the metal cap, being formed of an orthotropic and relatively ductile material, is less sensitive to locally high loading than a composite casing.

Various modifications to the invention will be apparent to those skilled in the art.

The inner sides of the legs 162, 262 may be formed by electroforming direct onto the composite rib. This means that the rib can be directly used as the former for the additive manufacture, and thus provide a more precise fit. If a cold metal spray technique is used, then the whole of the metal cap can be built up in situ.

Although the invention has been described in connection with an OMC casing, it would be equally applicable to a casing made of another material or composite arrangement of materials.

The manufacture of the cap 160, 260 by an additive manufacture technique provides a number of advantages. Although DLD has been mentioned, it will be appreciated that any other additive manufacture technique (for example, electron beam melting) would also be suitable. The benefits of the invention could also be achieved using a cap formed using a different manufacturing technique, for example by casting or by conventional "subtractive" machining of a metal blank.

Particularly in embodiments where the cap is formed by techniques such as casting or subtractive machining, the cap may be formed first and the layers of composite material subsequently laid up inside the cap. The consolidation and curing steps of the casing manufacturing process may then be carried out with the cap in situ.

The cap may include integrally-formed features, such as reference marks, identifiers, diagrams or other features to help with the assembly of the casing arrangement or with the attachment of the ancillary components. In particular, such features may comprise stops or detents to aid the alignment or positioning of the ancillary components or to indicate the positional limits within which a component should be located. The cap may include integrally-formed attachment features so that particular components may be directly attached to the cap, without the need for separate mounting brackets. Additive manufacturing techniques are particularly suitable for forming such integrally-formed features.

The engagement features 170 may take any convenient form. In the embodiment described, they cooperate with corresponding features formed in the surface of the rib.

As described above, shaped formers or pegs could be used in the manufacture of the composite casing to provide holes or depressions in the surfaces of the rib, and the corresponding features on the cap engage with them in use. Such an arrangement has the advantage that machining of the composite casing (which might cause damage to the fibres) is avoided. However, it will be appreciated that the corresponding features in the rib could be formed in other ways, for example by machining of the rib after manufacture.

Alternatively, the engagement features 170 may comprise sharp teeth, which in use will dig into the composite rib 116 so as to form a mechanical engagement.

Adhesive may be used to improve the attachment of the cap to the rib. A film adhesive may be used to also to give structural damping (by permitting flexing) and to provide a galvanic barrier.

Alternatively, a layer of a different material may be interposed between the cap and the rib to provide damping or galvanic isolation, such as a glass layer, which may in turn have a paste adhesive to assist bonding with the cap.

It may not be necessary to provide engagement features 170, especially if adhesive bonding provides sufficient attachment. If engagement features are not used, it may not be necessary for the legs 162 of the cap to exert an inward force on the rib 116.

Where rails 166, 168 are provided, they need not be perpendicular to the rib 116. It may be advantageous to angle or curve the rails, for example to improve their resistance to buckling loads.

The flexibility in positioning and attachment provided by the invention makes it particularly suitable for use with composite isogrid casings, in which caps may be provided only on those stiffening ribs where additional localised stiffness is required or where ancillary components must be attached.

In the embodiment of Figure 3, the cap and rails form a I shape. It will be appreciated that the rails may take other forms. For example, in the embodiment of the invention shown in Figure 5 the rails form a wedge shape. This wedge rail shape permits the use of a self-centring clamping arrangement, as are seen in tapered or self centring chucks.

In the embodiment of Figure 6, the buttress 474 may have any required width (in the circumferential direction), so that it may extend along the whole width of the cap 460 or only along part of it. More than one buttress may be provided on a single cap. If several buttresses are provided, they may be equally or unequally spaced. This allows the additional support to be provided in the positions where it is needed, without adding unnecessary weight. The buttress may have one or more cut-outs to reduce its weight, so that it forms in effect a flying buttress.

Part or all of the rail structure may be formed from a foamed metal material, to reduce the weight of the structure. Although the foamed metal material may have lower strength and stiffness than homogeneous metal, the beam-like construction of the cap and rail will contribute to the strength and stiffness of the overall arrangement.

Claims

CLAIMS1 A casing arrangement for a gas turbine engine, the arrangement comprising an annular casing formed of fibre-reinforced composite, the casing comprising a rib or flange defining a projection, the arrangement further comprising a metallic cap attached around the projection.
2 The casing arrangement of claim 1, in which the cap has surface features to provide a mechanical attachment with the surface of the projection.
3 The casing arrangement of claim 2, in which the surface features engage in use with corresponding features on the projection.
4 The casing arrangement of claim 2 or claim 3, in which the cap is secured by adhesive to the surface of the projection.
The casing arrangement of any preceding claim, in which the cap has an attachment feature for attaching an ancillary component to it.
6 The casing arrangement of claim 5, in which the attachment feature is formed integrally with the cap.
7 The casing arrangement of any preceding claim, in which the cap comprises a buttress to provide additional strength or stiffness in a direction perpendicular to the rib.
8 The casing arrangement of any preceding claim, in which the cap is formed by an additive manufacturing process.
9 The casing arrangement of any preceding claim, in which the rib or flange is measured after manufacture and the cap is formed to correspond with the measured dimensions of the rib or flange.
The casing arrangement of any preceding claim, in which the cap is formed around the projection after the casing is manufactured.
11 The casing arrangement of any of claims 1 to 9, in which the casing is laid up and cured in the cap.
12 The casing arrangement of any preceding claim, in which the cap comprises features to help with assembly or alignment.
13 The casing arrangement of any preceding claim, in which a layer of material is interposed between the cap and the rib to provide damping or galvanic isolation.
14 The casing arrangement of claim 13, in which the material is a film adhesive.A casing arrangement substantially as described in this specification, with reference to and as shown in Figures 3 to 6 of the accompanying drawings.