Abstract
A composite shaft with an end fitting mounted on an interface region on at least one end of said shaft, and a preload structure arranged to provide a biasing force to bias the composite shaft against the end fitting; wherein the preload structure is in an interference fit with the composite shaft. The preload structure is applied to the composite shaft in a subsequent operation to the mounting of the end fitting to the shaft.
Claims
1. A composite shaft with an end fitting mounted on an interface region on at least one end of said shaft, and a preload structure arranged to provide a biasing force to bias the composite shaft against the end fitting; wherein the preload structure is in an interference fit with the composite shaft.
2. A composite shaft as claimed in claim 1, wherein the interference fit between the preload structure and the composite shaft is at least 80 microns.
3. A composite shaft as claimed in claim 1, wherein the end fitting further comprises teeth engaging with the composite shaft.
4. A composite shaft as claimed in claim 3, wherein a tooth profile taken perpendicular to the teeth comprises substantially no flat land portions in frictional contact with the shaft.
5. A composite shaft as claimed in claim 1, wherein the preload structure is arranged to increase friction between the composite shaft and the end fitting to a greater level than the friction arising from mounting of the end fitting onto the shaft.
6. A composite shaft as claimed in claim 1, 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.
7. A composite shaft as claimed in claim 6, wherein said shaft is a hollow tube, wherein said taper is formed on the outside of said shaft and wherein said preload structure is provided within the hollow tube.
8. A composite shaft as claimed in claim 6, wherein said shaft is a hollow tube, wherein said taper is formed on the inside of said shaft and wherein said preload structure is provided on the outside of said shaft.
9. A composite shaft as claimed in claim 6, wherein the taper is at an angle to the shaft axis of no more than 20 degrees.
10. A composite shaft as claimed in claim 1, wherein in said interface region fibres of said composite shaft are angled with respect to the shaft axis such that they follow a path with a radial component and have been cut so as to expose the ends of said fibres in said interface region, and wherein said shaft is a hollow tube and said preload structure is provided within the hollow tube.
11. A composite shaft as claimed in claim 10, wherein the interface region of the shaft comprises a ramp that increases in thickness in the axial direction of the shaft towards the end of the shaft, and helical-wound fibres wound over said ramp.
12. A method of mounting an end fitting to an interface region of a composite shaft comprising: mounting said end fitting to said interface region; and subsequently mounting a preload structure to the composite shaft in an interference fit with the composite shaft such that the composite shaft is sandwiched between the end fitting and the preload structure.
13. A method as claimed in claim 12, wherein the interference fit between the preload structure and the composite shaft is at least 80 microns.
14. A method as claimed in claim 12, 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.
15. A method as claimed in any of claims 12, wherein mounting the preload structure increases the frictional force between the composite shaft and the end fitting to a greater level than the frictional force that arose from mounting of the end fitting onto the shaft.
16. The method of claim 13, wherein the interference fit between the preload structure and the composite shaft is at least 100 microns.
17. The method of claim 16, wherein the interference fit between the preload structure and the composite shaft is at least 150 microns.
18. The composite shaft as claimed in claim 2, wherein the interference fit between the preload structure and the composite shaft is at least 150 microns
19. A composite shaft as claimed in claim 6, wherein the taper is at an angle to the shaft axis of no more than 10 degrees.
20. A composite shaft as claimed in claim 6, wherein the taper is at an angle to the shaft axis of no more than 7 degrees.
Description
BRIEF DESCRIPTION OF DRAWINGS
[0044] One or more non-limiting examples will now be described, by way of example only, and with reference to the accompanying figures in which:
[0045] FIG. 1 illustrates a hollow composite shaft with an end fitting mounted on the outer surface and a preload structure mounted on the inner surface;
[0046] FIG. 2 shows an enlarged detail of the teeth shown in FIG. 1;
[0047] FIG. 3 illustrates a hollow composite shaft with an end fitting mounted on the inner surface and a preload structure mounted on the outer surface;
[0048] FIG. 4 illustrates the assembly of an end fitting onto a shaft;
[0049] FIG. 5 shows in cross-section a ramp for exposing fibre ends in an interface region;
[0050] FIG. 6 shows an interference fit on an enlarged scale; and
[0051] FIG. 7 shows a cross-section of a joint for torsional loads.
[0052] FIG. 1 shows a cross-section through a composite shaft 1 with a metal end fitting 2 fitted to the end thereof. The composite shaft is a hollow cylinder, but for simplicity only one half of it is shown, along with the axis 3 of the shaft 1 indicating the centre line of the cylinder. In FIG. 1 the end fitting 2 is mounted onto the outer surface 4 of the composite shaft 1 and is attached thereto by teeth 5. The teeth 5 are formed on the end fitting 2 as a helical thread (which may be a single thread or a multi-start thread). The teeth 5 cut into the outer surface 4 of the composite shaft 1 as the end fitting 2 is screwed onto the shaft 1 in assembly. The teeth 5 compress the composite shaft in this process, increasing the frictional force between the teeth 5 and the shaft 1.
[0053] Also shown in FIG. 1 is a preload structure 6 in the form of a cylinder of relative rigid material that is sized so that its outer diameter is slightly larger than the inner diameter of the composite shaft 1 and thus when it is pressed into the shaft 1 it forms an interference fit between the two components. This interference fit causes displacement (e.g. radially outward movement or compression) of the composite shaft 1 in the region of the interference fit, which in turn presses the outer surface of the composite shaft 1 harder against the teeth 5, thus increasing the frictional force against between the composite shaft 1 and the end fitting 2. This increased engagement force reduces the axial fretting that may otherwise occur upon repeated application of axial loads across the joint between the composite shaft 1 and the end fitting 2. This reduction in fretting improves the strength of the joint and increases its lifetime.
[0054] An enlarged view of the teeth 5 of end fitting 2 engaging with the composite shaft 1 is shown in FIG. 2 (the enlarged portion being marked with the letter A in FIG. 1). As can be seen in FIG. 2, the teeth 5 are designed to engage with the shaft 1 such that a small clearance 12 is provided above the composite shaft's outer surface 4 between adjacent teeth 5. This clearance 12 provides room for material that is cut or displaced by the cutting of the shaft 1 that takes place during assembly. It can also be seen in FIG. 2 that there are no flat land portions between the adjacent teeth 5 for providing increased friction. The preload structure 6 allows for such additional friction-generating surfaces to be omitted, thus allowing the end fitting 2 to be shorter and thus lighter and less expensive. The absence of flat lands between teeth reduces the heat build up that would otherwise occur due to the increased friction as the shaft 1 and the end fitting 2 engage over a longer length towards the end of the assembly process.
[0055] FIG. 3 is similar to FIG. 1, but shows an end fitting 2 that is attached to the inner diameter 7 of the hollow composite shaft 1. Also, whereas FIG. 1 shows a cylindrical shaft 1 and a cylindrical end fitting 2, FIG. 3 shows a tapered shaft 1 and a matching tapered end fitting 2. The taper allows for engagement of the two components 1, 2 to be accomplished with a reduced number of turns and thus a reduced build up of heat from friction that might otherwise compromise the composite material, reducing its strength. In FIG. 3 as the end fitting 2 is applied to the inner surface 7 of composite shaft 1, the preload structure 6 is formed as a hollow annulus that surrounds the shaft 1, engaging with its outer surface 4. Again, the preload structure 6 is formed to engage with the shaft 1 in an interference fit, although in this arrangement the inner diameter of the preload structure 6 is formed slightly smaller than the outer diameter of the composite shaft 1 so that the composite shaft 1 is squashed or displaced inwardly by application of the preload structure.
[0056] The difference in dimensions, d between the shaft 1 and the preload structure 6 will depend on the particular joint, materials and application amongst other factors. The difference d is shown in FIG. 6 which is a highly enlarged view of an annular preload structure 6 such as that shown in FIG. 3.
[0057] FIG. 4 illustrates the assembly of an end fitting 2 onto a hollow composite shaft 1 such as is illustrated in FIG. 1. First the end fitting 2 is attached to the outer surface of the composite shaft 1, e.g. by screwing it on such that teeth 5 (not visible in FIG. 4) cut into the outer surface 4 of shaft 1. Then, after the end fitting 2 has been applied to the outer surface 4 of shaft 1, the preload structure 6 is pressed into the inside of the hollow shaft 1 so as to engage with the inner surface 7 of hollow shaft 1 in an interference fit, pressing the composite material of shaft 1 more firmly against the inner surface of end fitting 2.
[0058] FIG. 5 shows in cross-section an example of how the fibres 8 in the composite shaft 1 can be angled up to the outer surface 4 of the shaft 1 in an interface region 9 by providing a ramp structure 10 around which the fibres 8 are wound during formation of the composite shaft 1. The ramp 10 deflects the fibres 8 radially outwardly. After curing of the shaft 1, the area above the ramp 10 is ground down to the level of the rest of the shaft, thus exposing the ends of the fibres 8 in the interface region 9. As can be seen in FIG. 5, when an end fitting 2 is attached to this interface region 9, the end fitting engages with a greater number of layers of fibres 8 rather than just the surface layers, thus improving the strength and load-transmission properties of the joint.
[0059] While the joints illustrated in FIGS. 1 and 3 are optimised for axial loads (as the teeth run essentially perpendicular to such loads), FIG. 7 illustrates that the teeth 5 can equally be formed as axial splines rather than helical teeth, thus optimising the joint for torsional loads as indicated by arrows 11. FIG. 7 shows a cross-section of a joint between a composite shaft 1 and an end fitting 2 with teeth 5 in the form of axial splines. The cross-section is taken perpendicular to the axis 3 of the shaft 1.
[0060] In particular the teeth 5 shown in FIG. 7 are illustrated with a low angle tooth profile. The low angle means that the surfaces of the teeth 5 are closer to parallel with the outer surface of the shaft 1 such that the additional frictional force generated by the interference fit of the preload structure 6 has a large component parallel to the shaft surface 4 to resist movement between the shaft 1 and end fitting 2. Such low angle tooth profile can of course also be used in the examples illustrated in FIGS. 1 and 3 as well. As with FIG. 2, a clearance 12 is provided between adjacent teeth 5 to collect cut or displaced material.
