Composite tension/compression strut

Abstract

A composite filament-wound shaft with an end fitting mounted on an interface region on at least one end of said shaft, wherein in said interface region filaments of the filament-wound 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 filaments in said interface region; and wherein said end fitting comprises a helical thread engaging with said interface region. The helical threaded engagement provides excellent load transmission of axial forces and is therefore well suited to tension and compression elements. The joint provides a low cost and low weight interface.

Claims

1. The A composite filament-wound shaft with an end fitting mounted on an interface region on at least one end of said shaft, wherein in said interface region, filaments of the filament-wound shaft are angled with respect to a shaft axis such that the filaments follow a path with a radial component and have been cut so as to expose ends of said filaments in said interface region; and wherein said end fitting comprises a helical thread engaging with said interface region.

2. The shaft as claimed in claim 1, wherein said end fitting further comprises grooves across the helical thread that break the helical thread into a plurality of part-helices.

3. The shaft as claimed in claim 2, wherein said grooves are axial grooves or helical grooves.

4. The shaft as claimed in claim 2, wherein said end fitting comprises four axial grooves across the helical threads.

5. The shaft as claimed in claim 1, wherein the thread is a single-start thread.

6. The shaft as claimed in claim 1, wherein said thread has a profile that comprises a cutting tooth portion arranged to cut into said interface region and a substantially flat land portion that frictionally engages with said interface region.

7. The shaft as claimed in claim 6, wherein said profile further comprises at least one channel portion adjacent to said cutting tooth portion to accommodate debris produced during a mounting process.

8. The shaft as claimed in claim 1, wherein the interface region of the shaft comprises a ramp of hoop-wound fibres 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.

9. The shaft as claimed in claim 8, wherein said helical-wound fibres over said ramp have been cut or ground parallel to the axis of the shaft to expose fibre ends and form said interface region.

Description

BRIEF DESCRIPTION OF DRAWINGS

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

(2) FIG. 1 illustrates an end fitting joined to a composite shaft;

(3) FIG. 2 shows the interface of the joint in more detail;

(4) FIG. 3 illustrates a shaft construction;

(5) FIG. 4 illustrates an interface in the process of being joined; and

(6) FIG. 5 shows an end fitting with axial grooves across the internal thread.

DETAILED DESCRIPTION

(7) 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.

(8) FIG. 1 shows a tension-compression joint 1 comprising a composite shaft 2 with a metal end-fitting 3 mounted thereon. The end-fitting 3 may be connected to other equipment via bolts 4 through flange 5. End-fitting 3 also has a cylindrical attachment part 6 which is internally threaded with a helical thread and with an internal diameter closely matched to the outer diameter of the shaft 2. It will be appreciated that closely matched may mean exactly the same size, very slightly larger (to allow movement) or very slightly smaller (to create compression upon fitting). The exact choice of internal and external diameters will depend upon the materials used, the intended use of the shaft and the forces expected to be transmitted therethrough.

(9) The detailed view of FIG. 2 shows that the thread profile is made up of teeth 7 and flat lands 8. The end-fitting 3 shown in FIG. 2 is a single-start thread so the multiple teeth 7 shown in FIG. 2 are in fact all part of the same helical thread. Similarly, all of the flat lands 8 are part of the same helical thread. The profile further comprises at least one channel portion 13 adjacent to the cutting tooth portion 7 to accommodate debris produced during a mounting process.

(10) FIG. 3 illustrates the construction of the shaft 2. FIG. 3 is somewhat schematic in nature as it shows the plies 10a-e much larger than in a real example. Also, the plies 10 in FIG. 3 do not illustrate that the individual filaments are helically wound, which would almost certainly be the case in real implementations.

(11) The shaft 2 illustrated in FIG. 3 is made up of 5 plies 10 (individually labelled 10a-10e). At the end of the shaft 2, a conical ramp 11 has been formed from hoop-wound fibres with the ramp's thickness building up towards the end of the shaft 2 so that it is thickest at the end. The helical wound plies 10 are then wound on top of the ramp 11. As can be seen in FIG. 3, as the plies 10 pass over the top of the hoop wound ramp 11, they splay radially outwardly (i.e. the hoop 11 causes them to follow a path with a radial component rather than purely axial and circumferential components). The joint interface region 12 has been formed by grinding down the thicker portion of helical plies 10 over the ramp 11 so as to conform the thickness with the rest of the shaft (although conformity of thickness is not essential). This grinding process has led to exposure of the ends of the individual plies 10a-10e such that all of those plies 10a-10e interface with the threads 7 of end-fitting 3. As the threads of end-fitting 3 interface with all of the plies 10a-10e, forces are transmitted between the end-fitting 3 and the shaft 2 via all of the plies 10a-10e rather than only through surface plies 10a and possibly 10b (which would be the case in the absence of the ramp 11).

(12) The threaded engagement of the shaft 2 and end-fitting 3 means that the threads provide excellent force transmission in the axial direction so that the joint is particularly suited to tension and compression force transmission. This means that the joint reacts the shear loading at the interface most effectively in the axial rather than the circumferential direction. The joint will still have an adequate transmission in the circumferential direction (e.g. for incidental torsional loads), but is more ideally suited to axial loads. Due to the thread, the joint will resist torsional loads in one sense (the tightening sense of the thread) more than the opposite sense (the loosening sense of the thread). The joint is also structurally efficient in that it achieves an excellent bond between the shaft 2 and end-fitting 3 with a relatively small quantity of metal, 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.

(13) Assembly of the joint is carried out by way of turning the end fitting 3 relative to the shaft 2 whilst allowing the end fitting 3 to move axially relative to the shaft 2. This may be improved further by forcing the end fitting 3 to move axially at the pitch of the thread, i.e. driving the end fitting 3 to move at an axial rate of one thread pitch per full rotation so that its driven axial movement exactly matches the rate that would be induced by the thread.

(14) FIG. 4 shows an end fitting 3 part way through mounting onto shaft 2. Arrows 20 indicate an axial driving force applied to the end fitting 3 so as to induce movement at a rate matching the natural movement rate of the thread, i.e. driving the end fitting at an axial rate of one thread pitch per full rotation around the shaft axis.

(15) FIG. 5 shows an end fitting 3. The threads 7 are interrupted by axial grooves 22 that provide passages for material cuttings to be held or even to pass along the grooves 22 and out of the joint. Any number of axial grooves 22 may be provided, although four are illustrated in FIG. 5.