Multi-layer braided article

Abstract

A multi-layer braided article comprises a braid folded over itself to form a first layer and a second layer, and one or more blind stitches in the first layer. The one or more stitches define an edge of the first layer about which the braid folds from the first layer into the second layer.

Claims

1. A multi-layer braided article comprising: a braid folded over itself to form a first layer and a second layer; a set of one or more blind stitches in the first layer, the one or more stitches defining an edge of the first layer about which the braid is folded; and a core and an initial layer attached thereto; wherein the one or more blind stitches penetrate through the first layer and into the initial layer attached to the core; wherein the blind stitches attach the first layer of the braid to the initial layer; and wherein the initial layer is a second braid.

2. The multi-layer braided article according to claim 1, wherein the initial layer is attached to the core by an adhesive and/or by blind stitches in the initial layer that provide hoop tension around the core.

3. The multi-layer braided article according to claim 1, wherein the braid is formed from high strength fibres suitable for aeronautical applications.

4. The multi-layer braided article according to claim 3, wherein the fibres are one or more of: carbon fibre, glass fibre, an aramid, or a mixture thereof.

5. The multi-layer braided article according to claim 1, wherein the braid is a bi-axial braid or a tri-axial braid.

6. The multi-layer braided article according to claim 1, further comprising a second set of one or more blind stitches extending at least through the second layer; and a third layer formed on top of the second layer; wherein the second set of one or more blind stitches in the second layer is configured to maintain tension in the second layer and define a second edge about which the second layer folds over itself into the third layer.

7. The multi-layer braided article according to claim 6, wherein the second set of blind stitches penetrates through the second layer and the first layer, and optionally, into the initial layer if present.

8. The multi-layer braided article according to claim 6, wherein, for a given value of n>0: an (n+2).sup.th edge of a layer (n+2) is directly on top of an n.sup.th edge of another layer n, such that the edges are coincident; or an (n+2).sup.th edge of a layer (n+2) is not on top of the n.sup.th edge of another layer n, such that there is a stepped profile between the two layers.

9. The multi-layer braided article according to claim 1, further comprising reinforcement stitching comprising one or more blind stitches penetrating at least two layers of the multi-layer braided article at a location other than an edge about which the braid folds over itself; wherein the reinforcement stitching is configured to provide delamination resistance for the multi-layer braided article.

10. An aircraft component comprising the multi-layer braided article of claim 1; wherein the aircraft component is a blade, the blade further comprising a metal blade root configured to engage a rotor disc or hub.

Description

DESCRIPTION OF THE FIGURES

(1) Certain embodiments of the present disclosure will now be described in greater detail by way of example only and with reference to the accompanying drawings in which:

(2) FIG. 1A shows an article according to the present disclosure;

(3) FIG. 1B shows a cross-section of the article across line A-A of FIG. 1A;

(4) FIG. 2A shows a plan view of another article according to the present disclosure;

(5) FIG. 2B shows a cross-section of the article of FIG. 2A across line A-A.

(6) FIG. 3A shows the article of FIG. 1 removed from braiding machine; and

(7) FIG. 3B shows the article of FIG. 3 being cut to remove a portion of the article.

DETAILED DESCRIPTION

(8) Blind stitching is a method of stitching an article when the needle only has access to one face of the article to be stitched. This is unlike a conventional sewing machine which has two bobbins of thread, one on each side of the article to be stitched. When the needle, carrying a first thread from one side of the article, penetrates the article it picks up the second thread from the other bobbin and pulls the second thread back through the article.

(9) A machine for blind stitching is described in detail in U.S. Pat. No. 5,829,373 and a brief summary of the process is provided below.

(10) In blind stitching, the stitching machine moves a curved needle back and forth in an arcuate fashion, the arc extending through the article to be stitched. The needle has a hole near the pointed tip that carries the thread. The needle enters into a first face of the article at a first location. The needle curves through the body of the article, pulling the thread with it, and exits out through the first face of the article at a different, second, location.

(11) When the pointed tip exits the second location, a movable protrusion of the machine catches a loop of the thread that has been pulled through the article by the needle. The movable protrusion moves so as to extend the loop of thread from the second location and pull it across the first face of the article such that a loop is formed on the first face of the article. The loop surrounds a third location on the first face of the article.

(12) Meanwhile, the curved needle is pulled back through the article along the same path it originally travelled—the protrusion holding the loop of thread prevents this action from removing the thread that was originally carried through the article. The curved needle is then moved along the first face of the article and penetrates it at the third location—that is, the needle and thread enter the article inside the loop of thread.

(13) After the needle enters the article at the third location, the protrusion is moved so as to release the loop and the bobbin holding the thread is driven in reverse to pull thread back through the article from the second location and thus tighten the loop to form a stitch.

(14) The process repeats when the needle exits the first face at a fourth location and a loop of thread is again caught by the movable protrusion and a loop is formed around a fifth location on the first face. Thus, a line of stitches may be formed across the article. Further, this blind stitching process may be done automatically by a robot without human intervention.

(15) FIG. 1A shows a plan view of a multi-layer braided article 10 that is mid-way through a braiding machine 30 (also known as a braiding wheel—such devices are well known in the art). The multi-layer braided article 10 is being formed around a core 50 and a metal blade root 52. The multi-layer braided article 10 has a first set 12 of blind stitches and a second set 14 of blind stitches. Whilst multiple stitches in each set are described, in other embodiments there may be only one stitch in each set. Each set of stitches (12, 14 etc.) is schematically depicted as a pair of parallel lines in the figures.

(16) The braid of the multi-layer braided article is preferably formed from high strength fibres suitable for aeronautical applications, such as carbon fibre, glass fibre, or an aramid or a mixture thereof.

(17) FIG. 1B shows a cross-section across line A-A of the multi-layer braided article 10 of FIG. 1A. In FIG. 1B a first layer 10a of braid starts at an end 20 that sits on the blade root 52. The first layer 10a is braided along the blade root 52 and core 50 by moving the blade root 52 and core 50 in a first direction X through the braiding machine 30 such that the first layer 10a of the braid is formed across and around the blade root 52 and onto the core 50 to a first location that will become the first edge 12a.

(18) Once the braid has been braided to the first location, a first set 12 of blind stitches is stitched through the first layer 10a to clamp or “freeze” the first layer 10a of the braid at the first location.

(19) After stitching the first set 12 of blind stitches, the direction of motion of the blade root 52 and core 50 is reversed such that they move through the braiding machine 30 in a second direction X′ that is opposite to the first direction X. The first set 12 of blind stitches provides a first edge 12a around which the braid may fold and be formed into a second layer 10b. The first set 12 of blind stitches maintains tension in the first layer 10a during the braiding in the second direction X′.

(20) The second layer 10b extends over the first layer 10a. The second layer 10b is braided in the same manner as the first layer 10a, i.e. along/around the core 50 and the blade root 52, up to a second location. Thus, a multi-layer braided article is formed comprising a braid folded over itself to form a first layer 10a and a second layer 10b. One or more blind stitches are provided in the first layer 10a, the one or more stitches defining an edge of the first layer 10a about which the braid is folded.

(21) At the second location, a second set 14 of blind stitches may be stitched into the braid to clamp or freeze the second layer 10b; for example the stitches in the second layer 10b may extend into the first layer 10a.

(22) After stitching the second set 14 of blind stitches, the direction of motion of the blade root 52 and core 50 is again reversed such that they move through the braiding machine 30 in the original first direction X that is opposite to the second direction X′. The second set 14 of blind stitches provides a second edge 14a around which the braid may fold and be formed into a third layer 10c.

(23) This process may repeat for any number of layers depending on the article being formed, wherein each layer of braid is stitched by a set 12, 14, 16 of blind stitches to the layer (or layers) beneath.

(24) In the illustrated embodiments, the blind stitches in each set extend around the entire perimeter of the braid, but in other embodiments, the braid may only be stitched partly around its perimeter.

(25) It is not necessary to cut the braid at any of the folds, thus the first layer 10a is continuous with the second layer 10b and the second layer 10b is continuous with the third layer 10c etc.

(26) To finish the multi-layer braided article, the braid is braided to a second end 22 which may then be clamped to the rest of the article (or to the core 50 or blade root 52) and cut. As a result, there are only two cut ends of the fibres forming the braid.

(27) As shown in FIG. 1B and FIG. 2B, sets 18 of blind stitches may also be inserted into the article at points other than an edge 12a, 14a, 16a (i.e. not at the end where the braid is to fold over itself and become another layer). These are called reinforcement stitching 18. Reinforcement stitching 18 can provide reinforcement between the layers. In propeller blade constructed with the multi-layer braided article, this can increase the resistance to delamination of the finished blade.

(28) As shown in FIG. 2B, the reinforcement stitching 18 may optionally be added before all layers of the multi-layer braided article 10 are completed, such that the reinforcement stitches 18 do not penetrate the outermost layer of the finished article 10.

(29) FIG. 2B shows an example having an initial layer 54 formed on the core 50. The initial layer 54 may be a single layer of braid (a “second braid”) that was formed over the core 50 (and optionally blade root 52) and secured at both ends. The securing may be done by gluing the initial layer to the core. Alternatively or additionally, the securing may be effected by stitching blind stitches into the initial layer 54 in a loop around the circumference of the core. This loop of stitches can provide hoop tension in the initial layer 54 around the core 50. When this is done at both ends of the initial layer, it can anchor the initial layer 54 to the core in both directions by hoop tension provided that each end of the initial layer 54 has a loop of stitches around a region of the core having a smaller cross-sectional area whilst the region between the two ends of the core has a larger cross-sectional area. The larger cross-sectional area compared to the loop formed by the stitches at the ends prevents the loop (and thus the initial layer 54) from moving over the larger cross-sectional area and thus neither end of the initial layer can translate along the core.

(30) FIG. 1B shows an example where the edge 14a of the second layer (or, generally, the n.sup.th edge of the n.sup.th layer) and the edge 16a of the fourth layer (or, generally, the (n+2).sup.th edge of the (n+2).sup.th layer) are not formed at the same location (i.e. same longitudinal position) along the core 50. This creates a stepped profile for the braided article 10. This may be true for any value of n>0. That is, the first and third edges may be formed at different locations along the core or the fifth and seventh edges may be formed at different locations along the core 50 etc. Conversely, an (n+2).sup.th edge may be directly on top of (i.e. superposed over) an n.sup.th edge. That is, the edge n at which layer n folds into layer n+1, and the edge n+2 at which layer n+2 folds into layer n+3, may occur at the same location along the core. This may be true for any value of n>0. A braided article 10 may have a mixture of the above two options, wherein some edges are superposed on top of one another (i.e. are coincident) and other edges are staggered from one another (i.e. are at different longitudinal positions) along the length of the core 50.

(31) FIGS. 3A and 3B illustrate an embodiment where a portion 24 of the multi-layer braided article 10 is to be cut from the rest of the article 10. In FIG. 3B, the second layer 10b was originally formed to a second location and a set 14 of blind stitches was added to form an edge 14a about which the braid was folded to become the third layer 10c. The third layer 10c then extends along the blade root 52 and core 50 in the manner described hereinbefore.

(32) Reinforcement stitching 18 was added to the third layer 10c to secure it to the first and second layers 10a, 10b of the braid.

(33) As shown in FIG. 3B, a cutting plate 26 in the form of a collar has been inserted between the first and second layers 10a, 10b up to the reinforcement stitching 18. The third layer 10c is shown being cut using a cutting tool 28. The cutting tool 28 may be a laser or a cutting disc or any other suitable cutting machine. The cutting plate 26 protects the first layer 10a during cutting of the second and third layers 10b, 10c.

(34) The reinforcement stitching 18 prevents the second and third layers 10b, 10c from losing tension and/or coming undone beyond the cutting location.

(35) The cutting may be performed automatically, e.g. by a machine, thus reducing the amount of human time/labour required in producing the multi-layer braided article.

(36) As shown in FIGS. 1A and 3A, the core 50 may extend beyond the braid at one end of the multi-layer braided article 10. Similarly, the blade root 52 may extend beyond the braid at the other end of the multi-layer braided article 10.

(37) The examples shown in FIGS. 1A, 2A and 3A are in the shape of blades intended for a propeller aircraft. However, a multi-layer braided article made according to the present disclosure may take other forms and is not limited to aircraft propeller blades. For example, the multi-layer braided article may form part of a high-performance drive shaft, a connecting rod, a rod for a hydraulic actuator etc.

(38) In multi-layer braided articles 10 intended for aeronautical or aerospace uses, it is useful to use fibres having a very high tensile modulus. Suitable examples are carbon fibres, aramid fibres (e.g. Kevlar™), and/or glass fibres. Braiding machines 30 generally utilize a large number of spools (or bobbins) and thus a number of different fibres may be used for the multi-layer braided article 10 in a variety of ratios.

(39) In one example, the core 50 is a carbon fibre prepreg and the braid is primarily aramid fibres (e.g. Kevlar™) with ≤5% glass fibres.

(40) In the blade examples shown in FIGS. 1A, 2A, and 3A, the multi-layer braided article 10 is formed around a core 50 in the shape of the blade, and the braid also connects to a blade root 52. The multi-layer braided article 10 may therefore take on the general shape of the underlying core. However, other factors influence the final shape and dimensions, such as the tension of the braids, the braid angle(s) selected, the thickness of the fibres/yarns, and the number of layers of braid at each location along the core 50.

(41) It is possible to have the braid follow curves of the core 50 by altering the angle at which the core 50 enters the braiding machine 30 (specifically, as it moves through the braiding ring 31 of the braiding machine 30) as it moves in the first direction X or second direction X′. The braiding ring 31 supports the yarns close to the core just before they are braided onto the core (or onto previously braided layers). The first direction X and the second direction X′ are both defined perpendicular to the plane of the braiding ring 31.

(42) In other examples where the multi-layer braided article 10 is not for a propeller blade, the blade root may be dispensed with and the article 10 may be formed around a core of any appropriate shape.

(43) Herein, the term “first layer” may also be referred to as a “first braid layer”, the “second layer” may be referred to as a “second braid layer” etc.

(44) The present disclosure simplifies the manufacture of a multi-layer braided article. The complex shell-arrangement of U.S. Pat. No. 7,252,028 is effectively replaced by the blind stitches, thus reducing the amount of equipment required, and the stitches additionally provide inter-layer reinforcement for the article.

(45) The use of blind stitches (as opposed to conventional sewing) is advantageous since the curved needle avoids damage to the blade root surface and the core surface. It also enables the reinforcement of other areas of the braided article in the span and chordwise directions.

(46) A radial braiding machine is particularly useful as it gives more space for the stitching head of the robot.

(47) The present disclosure also reduces the number and duration of several post-braiding steps that are required by some known processes. For example, the process described in U.S. Pat. No. 8,672,636 utilises four separate braiding machines in a line to build up four layers of braid, one after another. However, after the braiding process, each layer of braided article will have two ends that need to be tidied up by hand before any curing of the finished fibre composite blade may take place. Currently, this cutting step must be performed by hand and this can take up to two-thirds of the total blade braiding time. Compared to this process, the present disclosure reduces the number of braid-ends to two, regardless of the number of braid layers, which reduces the labour time and cost of production.

(48) While the apparatus and methods of the subject disclosure have been shown and described with reference to embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the scope of the disclosure.