Patent Application
20190120283
This application claims priority to European Patent Application No. 17275169.5 filed Oct. 20, 2017, the entire contents of which is incorporated herein by reference.
The present disclosure relates to splined end fittings, in particular metallic end fittings for tubular components made of fibre reinforced polymer matrix composite material. This disclosure is concerned with splined end fittings that can transmit torsional forces to/from a composite material structural component.
Composite structural components are typically made of a polymer matrix composite material, often a fibre-reinforced polymer matrix composite material using glass and/or carbon fibre reinforcement, e.g. carbon fibre reinforced polymer (CFRP). Composite structural components offer the opportunity for lightweight and cost effective load transmission solutions. The largest benefits are often achieved when the load path and geometry are simple. Load transmission components, such as drive shafts, rods and struts, are ideal candidates for polymer matrix composite material and such composite structural components are increasingly being used on commercial aircraft in addition to automotive and construction industries. These composite structural components generally require an end fitting having a complex form to interface with other components. Metals are efficient in terms of weight and cost for forming an end fitting having a complex geometry. However, joining a composite structural component to a metallic end fitting in a robust way poses significant challenges, especially in the aerospace industry where the joint must be formed in a certifiable manner.
It is known to form a load bearing end connection for a fibre-reinforced composite material component using a metal insert embedded inside a cylindrical structure made of a polymer matrix composite material. To manufacture such an end connection, the metal insert may be located onto a mandrel and the composite material structure then filament wound over the assembly. Upon curing, the mandrel is removed leaving the metal insert embedded inside the composite material structure. The metal insert may include a threaded bore that provides a mounting point for attaching the end connection to other components for the transmission of generally axial loads. Some examples of such an end connection are seen in EP2229540 and EP3133298.
Many aerospace applications require composite structural components that can transmit rotational movements, i.e. torque, between moving parts (e.g. control surfaces of an aircraft wing). In order to reliably transmit torque, a metallic end fitting must not slip under rotation and should not introduce stresses resulting in gross failure of the composite material component.
U.S. Pat. No. 7,874,925 provides an internally splined metallic end fitting which is press fitted onto the end of a composite torque transmission shaft. The primary load transfer mechanism is made through an interference fit with the external surface of the composite tube and the serrated internal splines that form teeth cutting into the external surface of the composite tube. The splined internal surface is designed to accommodate the resulting composite material debris generated during the press fitting process. To support the torque shaft during the press fitting process, the tube is reinforced by an internal plug of the same composite material that has been wound to have a high hoop strength. The internal plug encourages spline cutting and the correct interference fit at the flat sections between the cutting teeth.
The present disclosure seeks to provide alternative and improved end fittings.
According to the present disclosure there is provided a metallic end fitting for a given tubular component made of fibre reinforced polymer matrix composite material, the end fitting comprising: first and second concentric surfaces extending longitudinally along a central axis to form a socket for receiving an end of the tubular component; the socket having an outer diameter defined by the first surface, and an inner diameter defined by the second surface, wherein the outer diameter and inner diameter are chosen to provide an interference fit with respective outer and inner surfaces of the given tubular component when the component is press fitted into the socket; wherein each of the first and second concentric surfaces comprises one or more longitudinal cutting teeth.
When the given tubular component is press fitted into the socket, the one or more longitudinal cutting teeth are designed to cut one or more longitudinal grooves into respective outer and inner surfaces of the composite material tubular component to form a mechanical locking interface. Thus the installed composite tubular component is cut on both its outer and inner surfaces and an interference fit is generated at both its outer and inner surfaces.
Preferably the one or more cutting teeth comprise a plurality of axial or helical splines.
In some examples the end fitting comprises: the first surface facing radially inwardly and comprising a plurality of splines extending along the central axis, each spline flanked by a pair of troughs extending along either side of the spline and projecting radially outwardly to a diameter greater than the outer diameter, and each spline projecting radially inwardly from the outer diameter, to form a corresponding plurality of longitudinal cutting teeth, adjacent cutting teeth being spaced apart by lands extending circumferentially at the outer diameter; and/or the second surface facing radially outwardly and comprising a plurality of splines extending along the central axis, each spline flanked by a pair of troughs extending along either side of the spline and projecting radially inwardly to a diameter less than the inner diameter, and each spline projecting radially outwardly from the inner diameter, to form a corresponding plurality of longitudinal cutting teeth, adjacent cutting teeth being spaced apart by lands extending circumferentially at the inner diameter.
In such examples, the troughs either side of each spline allow for redistribution of the composite section and for the containment of debris composite material formed during the press fitting process. It will be appreciated that the splines of this type of end fitting act to cut grooves into the surfaces of a given tubular component by removing composite material rather than thermoplastically and/or mechanically deforming the composite material.
The double splined arrangement of first and second concentric surfaces removes the need for a separate hoop plug and halves the overall length of the splines, resulting in a significant size reduction in the end fitting. Cylindricity and straightness during assembly of the end fitting may be improved by a single component having the first and second concentric surfaces of the socket, ensuring correct pressure to counteract the pressure exerted by the cutting teeth and interference fit. The mechanical engagement of the first and second surfaces with both outer and inner surfaces of the composite material tubular component means there is engagement with a larger number of fibres, e.g. twice as many, in contrast to a standard splined end fitting where only a single surface forms a mechanical interface. Single splined end fittings are weaker due to shear stresses that develop in the bend at the outmost region of the tube.
Some advantages of an end fitting according to the present disclosure include lighter weight, stronger, easier to manufacture, better material utilization, reduced component count, simplified assembly, and improvements to geometrical alignment. The end fitting may be especially useful in torque applications requiring improved load transfer, for example tubular components used as torque drive shafts. One or more of these advantages may become more apparent from the examples disclosed below.
In a first set of examples, the first and second concentric surfaces are parallel with the central axis. The socket is therefore straight.
In a second set of examples, at least one of the first and second concentric surfaces is angled relative to the central axis. The socket is therefore tapered on one or both of the first and second concentric surfaces. In preferred examples both the first and second concentric surfaces are angled relative to the central axis, preferably at the same angle e.g. to form a symmetrically tapered socket. In such examples the first and second surfaces may both be angled to converge towards the central axis from an open end of the socket that receives the end of the tubular component. The first and/or second concentric surface may be angled relative to the central axis at an angle θ, wherein, in the most general case, 0<θ<90°, and in preferable examples 0<θ≤60°. For example a suitably tapered socket may be defined by 0<θ≤45° or 0<θ≤30°. The angle θ may be chosen depending on parameters such as the wall thickness of the given tubular component and/or the axial length of the end fitting. Shallower angles (i.e. lower values of θ) are preferred for improved tensile strength.
In examples wherein the socket is tapered, the given tubular component will also have respectively tapered outer and/or inner surfaces so to provide the interference fit. Typically a tubular component made of fibre reinforced polymer matrix composite material will comprise a high proportion of fibres that are axially aligned or have a low helical wrap angle. To form the tapered outer and/or inner surfaces, the walls of the tubular component may be cut at an angle and this will expose a large proportion of the axial fibre reinforcement for engagement with the first and second surfaces of the socket. A tapered socket can thereby achieve a much higher degree of engagement with the fibres of the composite material than a straight socket. Furthermore, it is counter-intuitive for the socket to be tapered as this would be expected to make it easier for the given tubular component to pull out of engagement with the end fitting.
In one or more examples of the present disclosure, the socket may comprise an open end arranged to receive an end of the given tubular component, an end portion providing the open end, and splined surface portions providing the first and/or second concentric surfaces, wherein the end portion comprises further first and second lead-in surfaces that are angled to extend radially away from the outer and inner diameters. This means that the end portion does not provide an interference fit with respective outer and inner surfaces of the given tubular component when the component is press fitted into the socket. The end portion may therefore provide an optional flared lead-in to assist insertion of the given tubular component into the socket.
In one or more examples of the present disclosure, the socket may comprise an open end arranged to receive an end of the given tubular component and a base at an opposite end of the socket, wherein the base comprises an undercut having a diameter different to the outer and/or inner diameter defined by the first and second splined surfaces. Such an undercut at the base of the socket can aid assembly, improve the stresses within the end fitting assembly, and provide a place for the collection of debris (produced by tooth self-cutting), and optionally for adhesive. The undercut may take the form of a cavity having non-splined surfaces.
In one or more examples of the present disclosure, the end fitting may be a multi-piece component. For example, the end fitting may be a two-piece component comprising a first component providing the first surface of the socket and a second component providing the second surface of the socket. This may simplify manufacture of the end fitting. In one or more other examples of the present disclosure, the end fitting is a one-piece component. The socket may therefore be formed in a single piece, reducing the part count.
It will be appreciated that the number of splines, the length of the splines, the spacing of the splines, etc. for the first and second surfaces can be chosen depending on the size of the socket and/or the given tubular component. While there is in some examples disclosed a plurality of splines extending along the central axis, this does not mean that the splines are necessarily parallel to the central axis. The splines on the first and/or second surface may, for example, be angled across the central axis, or undulate as they extend along the central axis, or may even extend at least partially helically. A helical arrangement of splines may be used to transmit both torsional and axial load. The longitudinal cutting teeth may therefore be designed such that they are not purely longitudinal.
It will be appreciated that the wall thickness of a given tubular component dictates the custom outer and inner diameters of the socket. In one or more examples, the outer diameter defined by the first surface and the inner diameter defined by the second surface are chosen to provide a predetermined interference fit with respective outer and inner surfaces of the given tubular component when the component is press fitted into the socket. For example, the predetermined interference fit may be set at 0.15-0.2 mm for a given tubular component having a typical wall thickness of 5-6 mm.
The relative angular position, or timing, of the first and second surfaces may be chosen to avoid stress concentrations and optimise weight. Providing the end fitting as a single component allows the timing to be precisely controlled and fixed. For example, the end fitting may be manufactured such that splines on the first surface are not exactly radially opposite splines on the second surface, such that the splines of the first and second surfaces are to some extent interleaved. When assembled with a thin-walled tubular component, this prevents the presence of angularly coincident splines that could reduce the wall thickness of the tubular component below an acceptable level. In some examples at least one spline on the first surface coincides with a flat on the second surface.
There is further provided by the present disclosure an end fitting assembly comprising a tubular component made of fibre reinforced polymer matrix material inserted into the socket of the end fitting.
As is discussed above, in examples wherein the socket is tapered on at least one of the first and second concentric surfaces, the tubular component comprises respectively tapered outer and/or inner surfaces so to provide the interference fit.
The fibre reinforcement may comprise metallic, glass and/or carbon fibres. In one or more examples the tubular component comprises a carbon fibre reinforced polymer (CFRP) composite material. Such materials are inherently corrosion resistant and provide a large weight saving and improved fatigue performance.
Since the end fitting provides a socket with a second surface to support the inner surface of the tubular component, a plug of hoop reinforcement is no longer required. The tubular component may therefore mainly comprise axial fibre reinforcement (at least in an end region that interfaces with the end fitting).
The composite material tubular component may be made using any suitable manufacturing technique. A fibre-reinforced polymer matrix composite material may be formed by braiding, automated fibre placement (AFP), prepreg wrap techniques or pultrusion methods. However in preferred examples the composite material tubular component is a filament wound structure. Filament winding techniques are particularly well-suited for making tubular components from carbon-fibre reinforced polymer (CFRP).
In any of these examples, preferably the composite material is a thermoset material. This means the composite material cannot be reshaped by deformation when press fitted against a splined surface.
In one or more examples of the present disclosure, the end fitting assembly may optionally comprise an adhesive bond between the first surface of the socket and an outer surface of the tubular component, and/or an adhesive bond between the second surface of the socket and an inner surface of the tubular component. It has been found that the debris resulting from the action of the one or more cutting teeth reinforces the adhesive around the mechanical interface formed by the splines to further enhance the stability of the joint. This optional adhesive bond also acts as a sealant to encase the exposed fibres in the cut groove(s) (and potentially any exposed fibres on the end face of the tubular component), providing protection from the external environment. Further details of suitable adhesive bonding materials and techniques maybe found in U.S. Pat. No. 7,874,925, the disclosure of which is hereby incorporated by reference.
There is further provided by the present disclosure a torque transmission/drive shaft comprising a tubular component made of fibre reinforced polymer matrix material and an end fitting as disclosed herein press fitted onto each end. Torque drive shafts are commonly called “torsion shafts” or “torsion tubes”
There is further provided by the present disclosure a method of making a metallic end fitting, as disclosed herein, for a given tubular component.
There is further provided by the present disclosure a method of assembling a torque transmission/drive shaft, comprising: providing a tubular component made of fibre reinforced polymer matrix material; and press fitting a metallic end fitting, as disclosed herein, onto each end of the tubular component.
During assembly, the two internally splined metallic end fittings are simultaneously pushed and press fitted onto opposite ends of the tubular component e.g. through the application of a compressive load of 10-200 kN. The load required for such assembly may depend, for example, upon the outer and inner diameters of the socket, the wall thickness of a given tubular component and/or properties of the splines (e.g. spline length or spacing).
In one or more examples of the present disclosure, the end fitting is made from any suitable metallic material. The end fitting is preferably machined out of a hard, stiff and strong metallic alloy such as corrosion resistant steel alloy.
The present disclosure extends to a metallic end fitting for a given tubular component made of fibre reinforced polymer matrix composite material, the end fitting comprising: first and second concentric surfaces extending longitudinally along a central axis to form a socket for receiving an end of the tubular component; the socket having an outer diameter defined by the first surface, and an inner diameter defined by the second surface, wherein the outer diameter and inner diameter are chosen to provide an interference fit with respective outer and inner surfaces of the given tubular component when the component is press fitted into the socket; the first surface facing radially inwardly and comprising a plurality of splines extending along the central axis, each spline flanked by a pair of troughs extending along either side of the spline and projecting radially outwardly to a diameter greater than the outer diameter, and each spline projecting radially inwardly from the outer diameter, to form a corresponding plurality of longitudinal cutting teeth, adjacent cutting teeth being spaced apart by lands extending circumferentially at the outer diameter; and the second surface facing radially outwardly and comprising a plurality of splines extending along the central axis, each spline flanked by a pair of troughs extending along either side of the spline and projecting radially inwardly to a diameter less than the inner diameter, and each spline projecting radially outwardly from the inner diameter, to form a corresponding plurality of longitudinal cutting teeth, adjacent cutting teeth being spaced apart by lands extending circumferentially at the inner diameter.
One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures, in which:
There is seen in
The outer surface 12 is splined, wherein it comprises a plurality of radially and longitudinally extending cutting teeth spaced by longitudinally and circumferentially extending lands. The inner surface 14 is similarly splined. The cutting teeth are shown in in more detail in
As is seen most clearly from
The base portion 4 further comprises connecting holes 20, so that the end fitting 2 may be connected to another component (not shown). The end fitting 2 is formed of a metallic material as a one-piece component.
The splines shown in this figure extend longitudinally, which gives a connection with a tubular component the best torsional strength, although the splines may extend helically (akin to a screw thread) to give improved axial load transmission. Helically extending splines may extend at an angle to the longitudinal direction of less than or equal to 15° although angles up to 89° may be beneficial in some examples.
As shown in
The undercut 18 shown in
Adhesive may be disposed in the socket 13 before the composite material tube 22 is inserted to increase the strength of the mechanical locking interface. In this case the undercut 18 also serves to collect excess adhesive that may be displaced as the tube 22 is inserted.
In another example, illustrated in
A wall of a fibre reinforced polymer matrix composite material tube 122 is also shown in
During assembly, the composite material tube 122 is inserted into the socket 113. As the tapered surfaces 124, 126 match the socket 113, the composite material tube 122 is press fitted into an interference fit with the outer and inner surfaces 112, 114. The cutting teeth of the outer and inner surfaces 112, 114 cut longitudinal grooves into the tube 122 to form a mechanical locking interface which prevents relative rotation of the end fitting 102 and the composite tube 122.
As in the previous example, the undercut 118 at the base of the socket 113 serves to reduce stress in the end fitting 102 and to collect debris created by the cutting teeth as the composite material tube 122 is inserted into the socket 113. Also as in the previous example, adhesive may be disposed in the socket 113 before the composite tube 122 is inserted and the undercut 118 may serve to collect any excess adhesive.
As described above, in examples of the present disclosure the concentric surfaces of the socket provided by the end fitting are splined.
In the illustrated example, the end fittings 502 are identical, but in similar examples where connections are required on both ends of a composite material tube 522, dissimilar end fittings may be employed, or one metallic end fitting according to the present disclosure may be used on a first end of the composite tube, with a different connection means used at the other end.
| Number | Date | Country | Kind |
|---|---|---|---|
| 17275169.5 | Oct 2017 | EP | regional |
