GB2103572A - Winding a strut - Google Patents

Abstract

Intermediate bulkheads 2 having peripheral slots are arranged along a longitudinal axis with their slots in alignment. An end piece X, Y is positioned at each end of the axis and a mandrel W is positioned between each end piece and the adjacent bulkhead, the mandrels being shaped to give a smooth contour from the end piece to the bulkhead. A continuous resin-wetted fibre is wound around the end pieces and through the bulkhead slots, the resin then being cured. <IMAGE>

Description

SPECIFICATION Load bearing strut The present invention relates to open lattice struts suitable for load bearing, the structures being fabricated by systematic winding of a resin impregnated strand about a former.

It is known to fabricate load bearing components from glass fibre/polyester resin materials, e.g. in boat building. Glass fibre/polyester resin open lattice structures can also be fabricated by winding resin wetted fibres around a former and subsequently curing the resin, the former being optionally removable for example in GB patent no. 2004835A.

Thus according to the present invention there is provided a method of fabricating a load bearing strut comprising the steps of (a) arranging a plurality of intermediate bulkheads along a longitudinal axis, .(b) the bulkheads having a series of peripheral members capable of accommodating a resin wetted fibre, (c) aligning the peripheral members of each bulkhead along the longitudinal axis, (d) positioning an end piece at one or both end of the longitudinal axis having one or more members around which a resin wetted fibre may be wound, preferably (e) positioning a mandrel between the end piece and the adjacent bulkhead at each or both ends of the longitudinal axis, the mandrel being shaped to give a smooth contour from the end piece to the adjacent bulkhead, (f) winding a continuous resin wetted fibre around the end piece or end pieces and bulkheads to form the strut, and (g) curing the resin to form a rigid load bearing strut.

The preferred sequence of winding the resin wetted fibres is (a) the inner helicals (b) the axials and (c) the outer helicals. Preferably the axial windings or chords are thicker, i.e. use more passes of resin wetted fibre, than the helical windings.

The fibres are preferably glass fibre or carbon fibre or a hybrid of glass and carbon fibre.

Other fibres may be suitable such as Kevlar or boron fibre. The resin is preferably an organic resin, most preferably an epoxy resin or a polyester resin.

The invention also includes an apparatus for fabricating a load bearing strut, the apparatus comprising a pair of end pieces, a plurality of bulkheads positioned along a longitudinal axis, at least one end piece adjacent the bulkheads, and a mandrel located between the end piece and its adjacent bulkhead.

The invention further includes load bearing struts whenever fabricated by the above method or using the said apparatus.

The peripheral members on the bulkheads are preferably in the form of slots in the bulkheads.

The end pieces preferably comprise one or more pairs of parallel plates separated by a bush, the outside of the bush forming the winding point. The preferred end pieces comprise one pair of parallel plates (the spade embodiment) and two pairs of parallel plates (the fork embodiment).

Preferably there is an end piece at both ends of the longitudinal axis. The end pieces are preferably connectable to an adjacent structure by suitable means e.g. a nut and bolt connection. Most preferably the connection is through a nut a bolt passing through the bush, the outside of which form the winding point for the resin wetted fibres.

The mandrel has a tapered cross-section and is preferably cast from a fusible material (Woods metal) of melting point 100-110 C which may be eventually removed from the structure by melting when the fabrication process is complete.

The tapered area of the tube between the first or last bulkhead and the fork is the fibre direction transition area, where the axial and helical fibres in the tube are redirected to form the fork end.

In order to ensure the maximum efficiency of this comparatively highly stressed area a mandrel is used to maintain the direction of the fibre in transition and provide stability for the consolidation of the structure during the winding of the structural hoop fibre necessary in this area.

The bulkheads may be left permanently in the structure or be removed after curing. The number of bulkheads and peripheral members in the bulkhead are capable of variation dependent upon the type of structure required.

The invention will now be described by way of example only with reference figures 1(a), (b) and 2 of the accompanying drawings.

Figure 1(a) is a vertical cross sectional view through the mandrel used in fabricating a load bearing strut according to the invention and Figure 1(b) is a plan view of the same mandrel.

Figure 2 is a perspective view of the arrangement of the end pieces and bulkheads used for fabricating a load bearing strut according to the invention.

The continuous strand was produced by passing two continuous bundles of glass fibre rovings through a bath of polyester resin and then through an orifice plate to give a roughly uniform strand diameter (not shown). The glass fibre rovings used were Vetrotex (EC14, 2400 Tex) produced by St. Gobain Industries (S-22) and each bundle of rovings containing about 208 fibres of about 10 microns diameter. The polyester resin used was Cellobond A2785 CV (BP Chemicals Ltd) using MEX peroxide catalyst and 0.6 % cobalt solution as accelerator. The formulations were chosen so that easy wetting of the fibre occurred but with sufficient viscosity to reduce run-off tendency. Also it was desirable that completed winding of the structures could also be achieved before setting of the resin and so a suitable resin gelation time was chosen.

After winding, the structures were allowed to cure at room temperature for several hours, depending on the formulation.

The strut was wound on a series of formers arranged longitudinally as shown in Fig. 2. The formers used comprise end pieces X, Y between which are arranged 21 bulkheads Z. A mandrel W is positioned at each end, the first W' positioned between and abutting end piece X and bulkhead 1 and the second mandrel W11 positioned between the abutting end piece Y and bulkhead 21. The mandrels are made from Woods metal and can be melted out of the final structure subsequent to curing. The structure of the mandrel is shown in Figs. 1 (a) and (b).

The bulkheads 2-20 comprise thin discs of rigid plastics material, bulkheads 1 and 21 are load bearing and in the example were manufactured from 0.10 thin thick GRP board. The bulkheads 1-21 are longitudinally equally spaced from each other. The bulkheads carry a central hole for alignment purposes and around their periphery are a series of ten equally spaced notches for locating the resin treated fibres used in building the strut. The notches in each bulkhead are carefully aligned with those in heighbouring bulkheads.

The end pieces X, Y are fabricated from light alloy and form two end winding points around which are wound the connecting legs of the strut. The winding points are formed by the bushes and the light alloy parallel plates.

During use, the mandrel, bulkheads and end pieces were set up as in Fig. 2 and aligned. The end pieces X, Y were made co-planar and the bulkheads were arranged parallel and equally spaced. The notches or slots in the bulkheads were arranged in line and the numeral 1 row or notches were made in line with the bush centre line on end piece faces A, B.

The resin wetted fibre or rovings is wound under tension onto the tooling components starting on the end piece X. The rovings are tied around the bush on side D-C of end piece X and passed into slot 1 on bulkhead 1. The rovings are then passed i.e. to form a right hand helix, through slots 10, 9, 8....in successive bulkheads 2, 3, 4....until the roving is passed into slot 1 in bulkhead 21, the roving being further continued to wind one and half times around bush D-C at the end piece Y. The roving is then passed from bush D-C at the end piece into slot 2 on bulkhead 21. The roving is then formed into a left hand helix by passing it into slots 3, 4, 5 on successive bulkheads 20, 19, (1 8.. . eventually reaching slot 2 on bulkhead 1 from where it is continued and wound one and a half times around bush D-C at end piece X. The helical winding is then continued as in the following table: End X No. of Start End Y No. of Start Side Turns Helix Slot Side Turns Helix Slot DC 1 < RH 9 DC 1 2 LH 10 DC 1 RH 3 DC 1 2 LH 4 BA 1 RH 7 AB 1 LH 6 AB 1 + RH 5 AB 1 + LH 8 A series of longitudinal axial chords are also wound between the end pieces X, Y, the winding sequence being illustrated by the following table:: Axial Chords End X No. of End Y No. of Side Turns Slot Side Turns Slot AB 1+ 1 BA 1 < 2 AB 1+ 10 BA 17 3 AB 1+ 9 BA 13 4 CD 1+ 8 DC 13 5 CD 13 7 DC 1+ 6 CD 1+ 7 DC 1+ 6 CD 1+ 8 DC 13 5 CD 13 9 DC 13 4 AB 1+ 10 BA 11 3 AB 1+ 1 BA 1+ 2 The winding is then completed by forming a number of outer helices across the strut starting from the end pieces X, Y.The winding sequence for the rovings are as in the following table: Outer Helicals End X No. of Start End Y No. of Start Side Turns Helix Slot Side Turns Helix Slot AB 13 RH 6 AB 1 < LH 7 AB 13 RH 4 AB 13 LH 5 AB 1 RH 8 AB 13 LH 3 DC 13 RH 9 CD 1 + LH 1 DC 13 RH 10 CD 13 LH 2 The ten axial chords are built up from eight passes and the helical windings are formed from a single pass of the resin wetted fibres. Each pass is equivalent to two glass fibre roving of 2400 Tex.

After the winding of the strut is completed, it is allowed to cure at ambient temperature for several hours. The bulkheads are retained in the structure but the light alloy end pieces are dismantled and the Woods metal mandrel is removed by melting.

The completed part can then be post cured in an oven to achieve the full strength of the bonding resin. Alternatively this post cure may be combined with the oven heating required to melt out the woods metal mandrel as referred to above.

Claims (18)

1. A method of fabricating a load bearing strut comprising the steps of (a) arranging a plurality of intermediate bulkheads along a longitudinal axis (b) the bulkheads having a series of peripheral members capable of accommodating a resin wetted fibre (c) aligning the peripheral members of each bulkhead along the longitudinal axis (d) positioning an end piece at one or both ends of the longitudinal axis having one or more members around which a resin wetted fibre may be (e) winding a continuous resin wetted fibre around the end piece or end pieces and bulkheads to form the strut and (f) curing the resin to form a rigid load bearing strut.

2. A method according to claim 1 comprising positioning a mandrel between the end piece and the adjacent bulkhead at each or both ends of the longitudinal axis, the mandrel being shaped to give a smooth contour from the end piece to the adjacent bulkhead.

3. A method according to claim 2 in whih the mandrel is made from a fusible material and is removed from the strut subsequent to curing of the resin.

4. A method according to any claims 1 to 3 in which the sequence of winding the resin wetted fibres is in the order, the inner helical windings, the axial windings and the outer helical windings.

5. A method according to any of claims 1 to 4 in which the axial windings or chords are greater in cross-section than the helical windings.

6. A method according to any of claims 1 to 4 in which the bulkhead is left permanently in the structure after curing of the resin.

7. A method according to any of the preceding claims in which the fibre is glass fibre, carbon fibre or a hybrid of glass and carbon fibre.

8. A method according to any of the preceding claims in which the resin is an organic resin.

9. A method according to claim 8 in which the organic resin is an epoxy or polyester resin.

10. A method of fabricating a loac bearing strut as hereinbefore described and with reference to Figs. 1 and 2 of the accompanying drawings.

11. Apparatus for fabricating a load bearing strut according to any of claims 1 to 10 comprising a pair of end pieces, a plurality of bulkheads positioned along a longitudinal axis, the bulkheads having peripheral members capable of accommodating a resin wetted fibre, at least one end piece adjacent to the bulkheads and a mandrel located between the end piece and its adjacent bulkhead.

12. Apparatus according to claim 11 in which the peripheral members comprise slots in the bulkheads.

13. Apparatus according to claim 11 or 12 in which the end pieces comprise one or more pairs of parallel plates separated by a bus, the outside of the bush being capable of accommodating a resin wetted fibre.

14. Apparatus according to any of claims 11 to 13 comprising an end piece at both ends of the longitudinal axis.

15. Apparatus according to any of claims 11 to 14 in which the mandrel has a tapered cross section.

16. Apparatus according to any of claims 11 to 15 in which the mandrel is cast from a fusible material.

17. Apparatus for fabricating a load bearing strut as hereinbefore described and with reference to Figs. 1 and 2 of the accompanying drawings.

18. Load bearing struts whenever fabricated using a method or apparatus according to any of claims 1 to 17.