Composite end connections

1. A fibre-reinforced polymer composite shaft for transmitting loads along a central axis, the composite shaft comprising:

a first interface surface extending along the central axis and comprising at least one helical groove and/or a plurality of circumferential grooves for engaging with at least one corresponding helical ridge and/or a plurality of corresponding circumferential ridges of a second interface surface of an end fitting;

wherein the at least one helical groove and/or the plurality of circumferential grooves comprises two opposing flanks each having a flank angle of between 32° and 51°, wherein the flank angle is measured between the flank and a perpendicular to the central axis in a plane that contains the central axis.

2. The fibre-reinforced polymer composite shaft as claimed in claim 1 , wherein the at least one helical groove or the plurality of circumferential grooves comprises at least one flank with a flank angle of between 34° and 48°.

3. The fibre-reinforced polymer composite shaft as claimed in claim 2 , wherein the flank angle is between 37° and 44°.

4. The fibre-reinforced polymer composite shaft as claimed in claim 3 , wherein the flank angle is between 37° and 41°.

5. The fibre-reinforced polymer composite shaft as claimed in claim 4 , wherein the flank angle is between 38° and 40°.

6. The fibre-reinforced polymer composite shaft as claimed in claim 1 , wherein the flank angle is between 32° and 36°.

7. The fibre-reinforced polymer composite shaft as claimed in claim 1 , wherein the first interface surface comprises an average radius from the central axis of between 10 and 150 mm.

8. The fibre-reinforced polymer composite shaft as claimed in claim 1 , wherein the first interface surface is approximately 30 mm long.

9. The fibre-reinforced polymer composite shaft as claimed in claim 1 , wherein the first interface surface is an inner surface of the composite shaft or an outer surface of the composite shaft.

10. The fibre-reinforced polymer composite shaft as claimed in claim 1 , wherein the first interface surface is tapered relative to the central axis or the first interface surface extends parallel to the central axis.

11. An assembly for transmitting loads along a central axis, the assembly comprising:

a fibre reinforced polymer composite shaft as claimed in claim 1 ;

an end fitting comprising a second interface surface extending along the central axis and comprising at least one corresponding helical ridge and/or a plurality of corresponding circumferential ridges that is engaged with the at least one helical groove and/or the plurality of circumferential grooves so as to mount the end fitting to the fibre reinforced polymer composite shaft; and

a preload structure arranged to provide a radial biasing force to bias the first interface surface against the second interface surface.

12. The assembly as claimed in claim 11 , wherein the end fitting is metallic.

13. The assembly as claimed in claim 11 , wherein the coefficient of friction between the first and second surfaces is between 0.01 and 0.3.

14. The assembly as claimed in claim 13 , wherein the coefficient of friction between the first and second surfaces is approximately 0.15.

15. The assembly as claimed in claim 11 , wherein the second interface surface is located radially inward of the first interface surface.

16. The assembly as claimed in claim 15 , wherein the preload structure comprises an annular member arranged to apply the radial biasing force to an outer surface of the composite shaft.

17. The assembly as claimed in claim 11 , wherein the preload structure is arranged in an interference fit with the composite shaft.

18. The assembly as claimed in claim 1 , wherein the second interface surface is located radially outward of the first interface surface.

19. The assembly as claimed in claim 18 , wherein the preload structure comprises a plug arranged to apply the radial biasing force to an inner surface of the composite shaft.