COMPOSITE SHAFT JOINT

Abstract

A composite shaft with an end fitting mounted on an interface region on at least one end of said shaft, wherein in said interface region the shaft is tapered; and wherein said end fitting comprises a surface with matching taper, the surface engaging with said interface region.

Claims

1. A composite shaft with an end fitting mounted on an interface region on at least one end of said shaft, wherein in said interface region the shaft is tapered; and wherein said end fitting comprises a surface with matching taper, the surface engaging with said interface region.

2. A shaft as claimed in claim 1, wherein the tapered surface of the end fitting is a toothed surface.

3. A shaft as claimed in claim 1, wherein the shaft is a filament-wound shaft and wherein the tapered interface region exposes a plurality of layers of filaments to the surface of the end fitting.

4. A shaft as claimed in claim 1, wherein said taper is formed on the outside of said shaft.

5. A shaft as claimed in claim 1, wherein said shaft is a hollow tube and said taper is formed on the inside of said shaft.

6. A shaft as claimed in claim 1, wherein the taper is at an angle to the shaft axis of no more than 20 degrees, preferably no more than 10 degrees, more preferably no more than 7 degrees.

7. A shaft as claimed in claim 1, wherein said end fitting surface is a toothed surface and comprises a plurality of teeth, each tooth formed as an axial spline engaging with said interface region.

8. A shaft as claimed in claim 1, wherein said end fitting surface is a toothed surface and comprises at least one tooth formed as a helical thread engaging with said interface region.

9. A shaft as claimed in claim 7, wherein said end fitting further comprises grooves that cross the teeth of the toothed surface.

10. A shaft as claimed in claim 9, wherein said grooves are axial grooves or helical grooves.

11. A method of mounting an end fitting onto a shaft, the shaft comprising an interface region that is tapered; the end fitting comprising a surface with a matching taper; the method comprising engaging said end fitting onto said shaft such that said surface engages with said interface region.

12. A method as claimed in claim 11, wherein: the end fitting comprises a toothed surface with a helical thread; and the method comprises screwing said end fitting onto said shaft while the end fitting is driven axially at a rate equal to one thread pitch per rotation.

13. A method as claimed in claim 11, wherein the shaft is a hollow shaft and the tapered interface region is formed on an internal surface of the shaft by winding filaments around the external surface of the shaft at the axial position of the interface region and then cutting or grinding the internal surface to form the taper.

14. A method as claimed in claim 11, wherein the shaft is formed by cutting a length from a longer shaft and subsequently forming said interface region adjacent to said cut.

15. An end fitting for mounting onto a shaft, said end fitting comprising a tapered surface arranged to engage with an interface region with matching taper on the end of said shaft during mounting.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0037] One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures in which:

[0038] FIG. 1 shows a convex tapered end fitting with helical toothed thread joining to a concave tapered shaft;

[0039] FIG. 2 shows a convex tapered end fitting with axial spline teeth joining to a concave tapered shaft;

[0040] FIG. 3 shows a concave tapered end fitting with a helical toothed thread joining to a convex tapered shaft;

[0041] FIG. 4 shows a concave tapered end fitting with axial spline teeth;

[0042] FIG. 5 illustrates a cross-section of a concave tapered end fitting with helical teeth joined to a convex tapered shaft; and

[0043] FIG. 6 shows an example of a tooth profile in more detail.

[0044] Composites can be made very structurally efficient (i.e. a high strength to weight ratio), however this efficiency is usually reduced in interfacing with metallic elements which may be required, e.g. for connection to other devices or equipment. A structurally efficient tension-compression joint has applications in struts, control linkages and rods, including piston rods. Such composite rods may experience any of tension/compression, bending and torque loads, depending on the application.

[0045] FIG. 1 shows a joint between a composite shaft 1 which in this example is a filament-wound rod with filaments embedded within a resin matrix (although it will be appreciated that other composite shaft constructions may also be used) and a metal end fitting 2. The metal end fitting 2 is provided with a flange 3 for attachment to other equipment (not shown), typically via bolts (also not shown).

[0046] The shaft 1 has multiple layers (plies) of filaments 4 built up to a required thickness. The composite shaft 1 could be filament wound on a parallel mandrel with a laminate suitable for the loads required (e.g. mostly low angle fibre for tension/compression, high angle fibre for torque transmission). The shaft 1 is hollow, having been formed on a mandrel, but this is not essential. Additional hoop fibre 5 (i.e. essentially circumferential windings) is provided in the end region (at the joint region) to support the joint, although in other embodiments this may not be required as discussed below. The interface region 6 (i.e. the region where the shaft 1 will contact and engage with the end fitting 2) is tapered so as to form a concave region at the end of the shaft 1. The taper is thus formed on the internal surface of the shaft 1 such that the internal diameter increases towards the end of the shaft 1. As the fibres 4 of shaft 1 form plies or layers which are essentially concentric rings, the taper cuts through many layers, exposing the ends of the fibres 4 in each layer in the interface region 6. In this way, the tapered interface region 6 ensures connection with all plies rather than just a few surface plies, thus sharing the load better.

[0047] The end fitting 2 is typically metal (although other materials may be used in some applications) and is formed with a convex tapered portion 7. The taper angle of this portion 7 matches the angle of the taper on the shaft 1 so that the two mate together neatly. The outer surface of the convex tapered portion 7 is a toothed surface, i.e. it has one or more teeth formed thereon that are arranged to cut into the interface region 6 of the shaft 1. The thread or groove form in the composite could be machined in prior to assembly but it is preferred that the threads or grooves are cut by the end fitting (i.e. self-tapping). In this example, the toothed surface takes the form of one or more helical threads 8, i.e. a tooth that spirals around the tapered portion 7 in a helical path (narrowing in diameter towards one end). Across the thread (i.e. substantially perpendicular to the helix and therefore in a substantially axial direction), are formed grooves (or flutes) 9 that break the helical tooth (or teeth in the case of a multi-start thread) into a number of partial-helices. These grooves 9 allow any debris generated as part of the cutting process to enter the grooves 9 and either remain there or be carried out of the joint area so as to prevent seizing or added friction during assembly. Flutes or grooves 9 may provide a lower channel than the thread for any composite (shaft) debris to accumulate.

[0048] To assemble the joint shown in FIG. 1 the end fitting 2 is first driven axially in the direction of arrow 10 until it is in close proximity (adjacent or even touching) the interface region 6 of shaft 1. The end fitting 2 is then rotated in the direction that will screw the helical thread 8 into the interface region 6 while at the same time being driven axially (also in the direction of arrow 10) in a controlled fashion so that it moves axially at a rate of one thread pitch per rotation. This ensures that the helical thread 8 follows the correct path and minimises damage during the engagement process. In other examples driving may not be required as the threading process will provide the axial force to draw the end fitting onto the shaft. As the taper allows a large amount of overlap to be achieved before the two parts come into contact, the amount of rotation that is required to fully engage the two tapered surfaces is significantly reduced compared with two non-tapered surfaces. However there will still be a high friction to be overcome and therefore it is preferred to apply a lubricant to facilitate the engagement. Preferably the lubricant is also an adhesive which will set (or can be cured) after engagement. The adhesive may seal the joint. The lubricant will also help to reduce and absorb heat that is generated due to friction during engagement, thus helping to prevent heat damage to the resin matrix and filaments 4 of the shaft 1. The helical thread(s) 8 provide a good connection for transmitting axial loads such as compression and tension forces. The helix angle of the threads 8 may be adjusted according to the purpose. For example a high helix angle (as close as possible to 90 degrees to the shaft axis) is best for axial loads, while a shallower angle is better for torsional loads. A zero degree angle (i.e. for spline fittings) is optimal for torque transmission. A 45 degree angle may be used for mixed torque/axial loading.

[0049] The taper of the interface region 6 and the end fitting surface 7 ensures that the engagement of the two surfaces occurs across the thickness of the shaft 1. Thus there is engagement between all (or substantially all) layers or plies of filaments 4 of shaft 1 and the end fitting 2. This distributes the force transmission amongst the filaments 4 so that the load is shared more evenly. Thus the joint does not rely on just a few surface filaments which could result in delamination if high forces are applied.

[0050] The hoop fibres 5 discussed above are not essential to the process. In some examples, they may simply not be required. In other examples, the hoop stiffness could be achieved through a press fitted additional part (e.g. a metal ring or hoop-wound composite ring) which may be left in place or removed after joint assembly.

[0051] FIG. 2 is similar to FIG. 1 in most respects and the same reference numerals are used to depict the same features. The difference is that the teeth 8 are not helical threads, but instead are splines which extend in a substantially axial direction, although with a radial component due to the taper. Each spline 8 extends straight such that it is in a plane with the axis of the shaft 1. The use of splines 8 rather than a helical thread is better for torque transmission than for axial loading (compression/tension). The engagement process will not involve any rotation of the end fitting 2, but will instead simply be an axial drive to engage the teeth 8 of the end fitting 2 with the interface region 6 of the shaft 1. Again, a lubricant (preferably an adhesive) may be used to reduce the friction of this engagement process and reduce the generated heat.

[0052] While FIGS. 1 and 2 show the interface region 6 formed on an internal surface of the shaft 1, FIG. 3 shows the interface region 6 formed on an external surface of the shaft 1. In this configuration the end of the shaft 1 is convex rather than concave. Correspondingly, the end fitting 2 is concave with the tapered toothed surface 7 formed on the inside of a cylindrical hub 11. FIG. 3 shows an end fitting with a helical thread 8, while FIG. 4 shows an end fitting with spline teeth 8. In FIG. 3, the helical thread 8 is crossed by grooves or flutes 9 which extend substantially axially. In FIG. 4, the groove(s) or flute(s) 9 are formed in a helix so as to cross the axial splines 8.

[0053] The engagement process for FIGS. 3 and 4 is much the same as for FIGS. 1 and 2 respectively, with the end fitting 2 of FIG. 3 being driven axially in the direction of arrow 12 at a rate of one thread pitch per rotation, while being rotated in the direction of arrow 13. The end fitting 2 of FIG. 4 is simply driven axially in the direction of arrow 14. Lubricant/adhesive may be applied as described above.

[0054] It can be seen that the examples of FIGS. 3 and 4 do not require hoop fibre (such as reference number 5 of FIGS. 1 and 2) to support the joint. If support is required (e.g. if the shaft 1 is hollow) a plug may be inserted (permanently or temporarily) in the hollow centre of the shaft 1 prior to engagement. The plug may be in the form of a metal or hoop-wound composite ring.

[0055] The hoop fibre 5 of FIGS. 1 and 2 may not be required after joint assembly and may thereafter by cut off (e.g. sliced off) for additional weight saving.

[0056] FIG. 5 illustrates a joint formed from an end fitting 2 mounted to a shaft 1 similar to those shown in FIG. 3, with a helical thread tooth 8 which has cut into the interface region 6.

[0057] FIG. 6 shows one example of a tooth profile that may be used for the helical thread 8 and/or the axial splines 8. The tooth profile is formed from cutting portions 15, with land portions 16 in between such that the profile alternates between cutting portions 15 and land portions 16. Either side of each cutting portion 15 there is a gully or channel 17 that allows for collection of a small amount of cut material. When grooves or flutes 9 are provided, these channels 17 feed into the larger grooves 9 for evacuating material from the cutting area.

[0058] The joints described above are structurally efficient in that they achieve an excellent bond between the shaft 1 and end-fitting 2 with a relatively small quantity of metal (or other material), thus reducing weight and cost. The joint is also mechanically simple to manufacture and join in that it can be made as a single component.

[0059] The use of a tapered interface region and tapered toothed surface allow many layers of plies to be exposed without special design of the shaft 1. Thus the shaft 1 can be manufactured using a generic process and in longer lengths and then cut to the desired size. The interface region 6 can then be cut or ground onto the shaft 1 after the shaft 1 has been cut to size. This greatly simplifies the manufacturing process as it is not necessary to make each rod individually to the right length to begin with.

[0060] The above examples may be used for tension and compression struts and rods, including piston rods. One area of particular application is the aerospace industry where weight savings are especially important, but this disclosure is not limited to aerospace applications and also encompasses other uses, e.g. automotive applications. The examples of this disclosure have the following benefits: large weight saving due to the simplicity of the interface (high strength to weight ratio), inherently corrosion resistant due to the materials involved, improved fatigue performance by ensuring load sharing across the thickness of the shaft. These designs use the load carrying properties of the fibre rather than relying on the interlaminar shear strength of the composite that is largely dependent on the resin system.