GB2294658A - Elongate stock and process for the manufacture thereof - Google Patents

Abstract

Elongate stock comprises a solid shaft in which a core (10) is bonded to a surrounding mass of reinforced resin. The resin reinforcement is typically in the form of glass fibre yarns (6) and the stock is typically manufactured by impregnating a plurality of such yarns 6 with a bonding resin and drawing the impregnated yarns around the central core 10 through a die 18 and a heating station for curing the resin. The core may be a solid body, or comprise carbon fibre or steel yarns. The stock has particular use in rockbolts and in reinforcement rods for concrete. <IMAGE>

Description

ELONGATE STOCK FOR INDUSTRIAL USE This invention relates to elongate stock for industrial use, and particularly to such stock which can be manufactured to meet specific load requirements in products made therefrom.

There are a number of industrial applications in which elongate products having broadly similar crosssectional dimensions are required to withstand different stresses in use. One such application is in mining where rockbolts are used to form secure anchors in underground tunnel walls upon which various components such as for example, safety cages, are formed. The installed rockbolt must withstand tensile stresses and also lateral or shear stresses as a consequence of movement of the substrate in which it is received.

Depending upon the environment in which the rockbolt is used, different resistances to tensile and shear stresses must be designed into the bolt.

In the mining industry rockbolts are sometimes used directly in the coal side when a working tunnel is being bored, which side is thereafter cut away as the coal is removed. In these locations, it is essential that the rockbolt is non-metallic so that the risk of sparks being generated is effectively eliminated. For this reason, rockbolts comprising plastics material have been developed. Generally, such rockbolts consist essentially of glass fibre reinforced resin. While such rockbolts have very good tensile strength, their resistance to shear forces; ie, transverse to the elongate axis of the rod, is relatively low.

It has been proposed to market glass fibre reinforced shafts of cylindrical cross-section for use as rockbolts, and reinforce the bolt against shear forces by the insertion of an elongate element in the central passage. While this can result in a rockbolt having a significantly improved resistance to lateral forces, and which also maintains its good tensile strength, the manufacture or assembly of such rockbolts is a cumbersome exercise, and although it has the advantage of being able to be completed on site, there is the substantial risk that the requisite materials may not be available when needed, resulting in the use of a rockbolt other than that specifically called for in a given situation.

The present invention is directed at elongate stock for industrial use such as in rockbolts, which stock can be designed at the manufacturing stage to have tensile and shear strengths according to a predetermined specification. According to the invention, such elongate stock comprises a solid shaft in which a core is bonded to a surrounding mass of reinforced resin.

The resin is normally reinforced by glass fibre yarns although it is of course only essential that a non metallic reinforcement is used in the surrounding mass where there is a corresponding requirement of the eventual environment in which the stock is to be used.

The relative quantities of the surrounding mass of reinforced resin and the core in a given cross-section of elongate stock according to the invention will of course vary according to the required specification for the stock. Generally though, the amount of reinforced resin in the surrounding mass will be selected with reference to the required tensile strength, and the size of the core with reference to the requisite shear resistance. It will though of course be appreciated that the core will normally also make a contribution to the tensile strength of the stock, and the surrounding mass may make a contribution to the shear resistance, although to a lesser extent. In typical stock according to the invention the diameter of the core might be substantially half the external diameter of the stock itself.

The core material in elongate stock according to the invention can be a solid body, for example of a plastics material such as polypropylene. Other suitable core materials comprise carbon fibre; steel yarns or rope; and glass fibres having a different, normally higher specification than the reinforcement used in the surrounding resin mass. Where a yarn or fibres are used, they can be braided to provide additional strength particularly against lateral or shear forces.

As noted above, stock according to the invention has one particular application in the manufacture of rockbolts for underground mines. Typically, known 22mm rockbolts are designed with a tensile strength of around 30 tonnes, but exhibit relatively low resistance to lateral forces. In many situations, and particularly those in which there is a specific need for resistance against lateral or shear forces, the tensile strength requirements are less. The present invention enables elongate stock for such rockbolts to be engineered to exhibit a lower tensile strength; for example 20 to 25 tonnes, but an increased shear strength of say, 12 to 14 tonnes. As a general guide, we have found that the sacrifice of some tensile strength can result in at least a proportional increase in the resistance to lateral or shear forces, depending on the nature of the core material used.

While rockbolts offer one preferred use for elongate stock according to the invention, there are others. One particular such application is as concrete reinforcement. Traditionally, concrete has been reinforced using steel rods, and while this is certainly successful, it does have the disadvantage of a potential for rusting, particularly if low levels of cover or facing concrete are used. An area in which reinforced concrete is being increasingly used, is in the manufacture of railway sleepers. For high speed trains, particularly high strengths are required for the railway sleepers. There is also a space problem, and a need in any event to minimise the amount of concrete used. This of course facilitates handling of the sleepers and therefore the time required for their laying.It will be understood then, that in concrete railway sleepers the use of conventional steel reinforcement has certain disadvantages. These can largely be met by using stock according to the invention which can be pre-stressed to the same levels as concrete reinforcement, but requires less cover concrete and presents fewer maintenance problems.

The invention also provides a method of producing elongate stock of the kind described above. According to the invention, such a method comprises impregnating a plurality of reinforcing fibres with a bonding resin; and drawing the impregnated yarns around a central core through a die and a heating station to bond the reinforcement yarns to each other and to the core.

Impregnation of the reinforcing fibres is conveniently accomplished by drawing the fibres through a resin bath.

Whatever impregnating technique is adopted, the reinforcing fibres are typically drawn through a guide disposed upstream of the die to locate them in a substantially annular orientation around the core material. The core material may itself be impregnated with resin, or alternatively drawn into position within the surrounding mass of impregnated fibres direct from a reel or other source. At least the core material will normally be maintained under tension throughout the process.

The method of the invention can be practised as a pultrusion, in which the cured stock is drawn from the heating station at a rate suitable to enable the other steps of the method to be properly completed. The reinforcing fibres will normally be provided from a bank of reels thereof and the core material may be drawn with these fibres as the method proceeds. It may also be drawn through a resin bath if that is the impregnating technique being used, although normally the reinforcing fibres for the surrounding resin will receive and effectively absorb sufficient resin in the impregnating step to provide a sufficient bond, and ensure an integrated mass in the final product.

The invention will now be described by way of example, and with reference to the accompanying schematic drawings wherein: Figure 1 illustrates a method of forming elongate stock according to the invention; Figure 2 shows an enlarged view in cross-section of the die at the entrance to the formation and cooling station in Figure 1; and Figures 3 and 4 are two cross-sectional views of elongate stock formed according to the invention in apparatus of the kind illustrated in Figures 1 and 2.

The apparatus shown in Figure 1 comprises a bank 2 of reels 4 upon which are mounted glass fibre reinforcing yarns 6. To the right of the bank 2 as shown is a further reel 8 of core material 10. The reinforcing yarns 6 are drawn from the bank into and through a bath 12 in which they are submerged under a guide roll 14, and impregnated with resin. From the bath 12, the yarns 6 are drawn into a heating and curing station 16 through a die 18 described in more detail below. The core material 10 is drawn directly from the reel 8 to the station 16, where it is received centrally through the die 18, again as discussed below.

In the heating and curing station 16 essentially solid elongate stock material is formed. It is discharged to a cutting station 20 where it is severed to chosen lengths. The temperature of the stock at this point can of course be determined by control of the station 16 and the speed of drawing.

The forming process is essentially one of pultrusion, and a pultruder will normally be housed to draw the stock at the downstream end of the heating and curing station 16. The core material will normally be kept under tension throughout the process, by the application of a brake to the reel 8, and the tension of the reinforcing yarns 6 can similarly be controlled by braking mechanisms on the reels 4 in the bank 2. The tension maintained in the core material 10 will normally be greater than that maintained in the reinforcing yarns 6.

As shown in Figure 2, the die 18 at the entrance to the heating and curing station defines a convergent path into which the impregnated reinforcement yarns 6 and the core material 10 are drawn. Located just upstream of the die 18 is a guide 22 having grooves or apertures defining paths for the reinforcing yarns 6 disposed around the core material 10. As the yarns and core material are drawn through the die, the resin secures a bond between the reinforcing yarn 6 and to the core material 10 resulting in a fully integrated product.

Where the core material is a solid body, then the resin will not normally penetrate very deeply thereinto.

However, if the core material itself comprises yarns or fibres, resin will penetrate through the entire core essentially resulting in an integrated product comprising a mass or resin with differential reinforcement areas. Impregnation of the core material with the resin may be accomplished substantially at the die 18 and in the curing and heating station 16; ie, without the core material 10 making intentional contact with the resin prior thereto. However, and particularly when the core material is itself porous, some impregnation thereof upstream of the die can be desirable. The path of the core material 10 to the die can readily be altered without difficulty such that it also passes through the resin bath 12.

The guide 22 fitted at the mouth of the die 18 can be selected to establish the desired pattern of reinforcement yarn 6 in the surrounding mass of resin, and of course to allow passage of the selected core material. The die 18 is also readily replaceable, enabling stock of different diameters and indeed crosssectional shapes to be formed. Figures 3 and 4 illustrate in cross-section two stock samples that can be made in the method described. In Figure 3 the core material is solid, comprising for example polypropylene, and is surrounded by a mass of resin reinforced by relatively densely packed glass fibre yarns. Figure 4 illustrates stock in which the core material is itself a mass of yarns, steel or glass fibre for example, which may be braided for extra strength.The surrounding mass of resin is reinforced by less densely packed reinforcement yarns, which are arranged in a particular pattern. By appropriate selection of a guide 22 with suitably located slots or openings it is possible to accurately predetermine the location and arrangement of the reinforcing yarns in the final product. Tension in the reinforcing yarns will of course enhance the accuracy of such location.

While the method described above does provided for the accurate location of the reinforcing yarns in the surrounding mass of resin in stock according to the invention, for many applications such accurate location is not essential. Indeed, in some circumstances it is not essential that the core is accurately located centrally in the stock. Further, it is possible to dispose two cores in the same mass of reinforced resin, and the two cores need not necessarily be of the same material. In stock material made in this manner, the cross-section will commonly be other than circular.

Stock according to the invention is of uniform cross-section, and may be adapted for its eventual application. Normally, it will be provided with a roughened surface, which can be created either during the forming method, or in a subsequent treatment. When used as the base material for a rockbolt, the end of a length of stock will be provided with a screw thread which may be moulded thereon or cut into the peripheral surface thereof. A moulded thread can provide better tensile strength at the end, but a thread cut into the surface of the bolt results in a product which is more easily handleable, both in use and in packaging and transportation. Threads may be similarly applied to the stock when it is used in other apparatus such as concrete reinforcement. Additionally though, other end units may be secured to the stock, such as locking plates to engage the end surface of a cast concrete block. The use of plastics material as the base material for the stock can significantly facilitate the attachment and bonding of end units to stock according to the invention.

Claims (20)

1. Elongate stock for industrial use comprising a solid shaft in which a core is bonded to a surrounding mass of reinforced resin.

2. Stock according to Claim 1 wherein the resin is reinforced by glass fibre yarns.

3. Stock according to Claim 1 or Claim 2 wherein the core material is a solid body.

4. Stock according to Claim 1 or Claim 2 wherein the core material comprises carbon fibre.

5. Stock according to Claim 1 or Claim 2 wherein the core material comprises steel yarns.

6. Stock according to Claim 1 or Claim 2 wherein the core material comprises glass fibre yarns.

7. Stock according to Claim 2 and Claim 6 wherein the yarns of the core are of a different specification to those of the surrounding mass.

8. Stock according to any of Claims 5 to 7 wherein the yarns of the core material are braided.

9. A rockbolt comprising a shaft formed from stock according to any preceding Claim.

10. A rockbolt according to Claim 9 with a screw thread at one end.

11. A rockbolt according to Claim 10 wherein the screw thread is moulded onto the shaft.

12. A rockbolt according to Claim 10 wherein the screw thread is cut on the shaft.

13. A reinforcement rod comprising stock according to any of Claims 1 to 8.

14. Concrete reinforced by a rod according to Claim 13.

15. A railway sleeper comprising concrete according to Claim 14.

16. A method of forming elongate stock for industrial use, which method comprises impregnating a plurality of reinforcing fibres with a bonding resin; and drawing the impregnated yarns around a central core through a die and a heating station to bond the reinforcement yarns to each other and to the core.

17. A method according to Claim 16 wherein the reinforcing fibres are drawn through a resin bath onto a guide disposed upstream of the die to locate them in an annular orientation around the core material.

18. A method according to Claim 16 or Claim 17 wherein the core is also impregnated with resin.

19. A method according to Claim 16 wherein the core merges with the reinforcing fibres after their impregnation.

20. A method according to any of Claims 16 to 19 wherein the core is maintained under tension.