This invention relates to a screw-threaded fastener, to which torque is applied to effect a fastening action.

The reliability of machine parts subject to fluctuating loads and stresses depends upon the fatigue strength of the materials used. A screw-threaded fastener, however, relies upon an elastic interaction between mating components. The objective is for the fastener to clamp parts together with a force greater than any external force tending to separate the parts. The fastener then remains at all times under a tensile loading and is largely immune from fatigue.

The reliability of a screw-threaded fastener thus depends upon the correct initial tensile load being given to the fastener and is ensured by specifying, and then controlling, the applied torque to tighten the fastener. The applied torque is a rotational force given to the head of a fastener and can be distinguished from the tension in the fastener, which is created by the axial loading. When a rotational fastener is used to clamp parts together, the thread angle of the fastener converts the applied torque into tension in the fastener. The amount of torque applied to the fastener during the tightening thereof is thus critical, to correctly set the tensile loading.

The accurate setting of the applied torque has been achieved principally by using one of two techniques. The first is by tightening the fastener with an expensive tool such as a torque wrench. This either determines and indicates the torque applied to a fastener, or limits that torque to some pre-set value. The use of such a tool is a time consuming process and, to maintain accuracy, the tool must frequently be checked and if necessary calibrated. In turn, this requires a further cost, in both time and operator

skill. Furthermore, in complex engineering environments, different fasteners will require tightening to different torque settings and this can lead to confusion and assembly errors. The second technique is to employ a torque limiting fastener such as a shear-headed bolt. In its simplest form, such a bolt may include a relatively long head, in the axial direction, in which head an annular groove is machined. The cross-sectional area of the remaining material in the region of the groove is selected having regard to the shear strength of the material of the bolt and the required torque setting, whereby a tightening of the end portion of the head will cause that end portion to shear off when the pre-determined torque value has been reached. There have been many proposals for more complex designs of shear-headed bolts, but these all aim at achieving essentially the same end result.

A disadvantage of both of the above described techniques is that it is not possible easily to determine on a subsequent inspection the torque value to which a particular fastener has been tightened. When special tools have been used, the fastener itself gives no indication whatsoever that it has been tightened to some specified value. In the case of a shear-headed bolt, there is usually no indication on the bolt of the torque value at which the end portion of the head will shear off. Afteruεe, though it is apparent that the end portion of the head has been sheared off, it is not usually possible to determine easily at what torque that occurred. Also, in the case of a shear-headed bolt, it may be used only once, and this can lead to considerable wastage.

According to the present invention, there is provided a torque-limiting screw-threaded fastener comprising a main body having a principal head by means of which the main body may be rotated, a socket being

formed in the principal head, and torque member receivable in the socket whereby the principal head and so also the main body may be rotated by torque applied to the torque member, which fastener is characterised in that the torque member includes a sacrificial head which is arranged to shear off and leave a part of the torque member in the socket of the principal head upon a torque in excess of some pre-determined value being applied to the sacrificial head. With the fastener of the present invention, it will be appreciated that the torque to which the fastener may be tightened by torque applied to the sacrificial head is limited by the design of the torque member. A given fastener may be tightened to an appropriate torque value by employing a suitable torque member. Then, after the fastener has been tightened to the point at which the torque member shears off, the portion of the torque member remaining in the socket in the principal head will show that the fastener has been tightened to the appropriate pre-determined torque value.

A particular advantage of the present invention is that any one fastener may be tightened to a torque value appropriate for the circumstances of use of the fastener, by selecting a particular torque member designed to shear at the appropriate torque value. Also, a number of different designs of fastener may all be tightened to the same torque value, by using each fastener with the same design of torque member. The fastener of this invention thus gives considerable cost savings and practical advantages when in use.

It is highly preferred that at least the part of the torque member which remains in the socket of the principal head, following shearing of the torque member, is visually identifiable, both before and after the sacrificial head has been sheared off.

Advantageously, the entire torque member, and not just the part which remains in the socket, is identifiable in this way. Thus, the torque member may be coded to indicate its shear value, and so an operator may ensure he is using the correct torque member to tighten the fastener, and a subsequent inspection will show that the correct torque member was used. Such coding may be by way of colours, applied externally to the torque member or impregnated therein, or by way of other indicia such as alpha-numerics, shapes or other symbols applied to the member.

Preferably, the torque member together with the sacrificial head are made in one piece, from a suitable grade of material the shear strength of which is accurately predictable. The member may be made of a metal, or for light duty applications, of a plastics material. Many of these materials are easy to colour- code, to indicate the shear value thereof.

The socket in the principal head may have any profile able to receive the required torque from the torque member. Conveniently, the socket is hexagonal and in this case, the torque member may include a secondary head spaced from the sacrificial head, which secondary head is non-rotatably received in the socket, with a shaft portion extending between the sacrificial and secondary heads. That shaft portion may be conical with the smallest diameter apart adjacent the secondary head, whereby the shaft portion will shear away from the secondary head, leaving only that secondary head in the socket of the principal head. Alternatively, a groove may be formed in the shaft portion, between the sacrificial and secondary heads, the area of the material remaining in the region of the groove determining the shear strength of the torque member and shearing taking place in that region.

The principal head may have a polygonal overall

external shape, such as hexagonal, to permit the rotation thereof by conventional tools. The sacrificial head may also have a polygonal external shape but preferably differs from the principal head so that different tools have to be used to effect rotation of the sacrificial and principal heads.

The main body and principal head of the fastener may be re-used, by employing a conventional tool. By extracting the remaining part of the torque member from the socket, following a first tightening of the fastener until the sacrificial head has sheared off, a further torque member may be fitted to the socket of the principal head, to permit re-use of the fastener.

This may lead to considerable economies where components may have to be dismantled and subsequently re-assembled, as compared to having to provide an entirely new shear bolt.

By way of example only, one specific embodiment of fastener of the invention will now be described, with reference to the accompanying drawings, in which:-

Figure 1 shows, in perspective, an exploded view of the two parts of the fastener;

Figure 2 shows, in perspective, the fastener of

Figure 1 but with the two parts coupled together; Figure 3 shows, in perspective, tightening torque being applied to the sacrificial head of the fastener, using a suitable tool;

Figure 4 shows, in perspective, the detachment of the sacrificial head and part of the torque member, following the application of the pre-determined torque; and

Figure 5 shows, in perspective, the installed fastener.

Referring to the drawings, the embodiment of screw-threaded torque-limiting fastener comprises a main body 10 having at one end thereof a hexagonal

principal head 11, together resembling a conventional bolt including a plain (i.e. not-threaded) portion 12 and a screw-threaded end portion 13. The main body 10 may of course be configured to suit any particular requirements; it may thus be screw-threaded for its entire length, or it may include a location shoulder or any other variation used in the screw-threaded fastener art.

The principal head 11 has an axial hexagonal socket 14 formed therein, the flat sides of which socket are aligned with those of the principal head 11. The socket 14 has a depth of less than the axial thickness of the principal head 11, in order that the strength of the fastener is not reduced to an unacceptable limit.

The fastener also includes a torque member 15, comprising a sacrificial head 16, a secondary head 17 and a conical shaft portion 18 inter-connecting the sacrificial and secondary heads 16 and 17. The secondary head 17 is dimensioned so that it is a light press-fit in the socket 14, as shown in Figure 2. The secondary head 16 is also of hexagonal shape, but with smaller across-the-f lats dimensions than the principal head 11. The shaft portion 18 has its smallest cross- sectional area immediately adjacent the secondary head 17, the cross sectional area gradually increasing in the direction of the secondary head 16. This smallest cross-sectional area is dimensioned such that the sacrificial head 16 and the shaft portion 18 will shear away from the secondary head 17 when that secondary head is restrained against rotation and a torque is applied to the sacrificial head 16. The shearing torque value may be controlled to lie between closely defined limits, by appropriate material selection and dimensioning of the shaft portion 18.

As an alternative to the generally conical shaft portion 18, the shaft portion may be of uniform cross- sectional shape. Also, instead of relying on the shaft portion 18 shearing at its junction with the secondary head 17, a groove may be machined, or otherwise formed, in the shaft portion 18 at some point intermediate the two heads 16 and 17, the area of the material remaining in the region of the groove being selected in order to give the shaft portion a required shear torque value. Figures 3, 4 and 5 show the use of the fastener of Figure 2, to clamp together two work-pieces 20 and 21. Work-piece 20 has a clearance hole for the plain portion 12 of the fastener, and work-piece 21 has a threaded hole into which the end portion 13 of the fastener is threaded. Torque is applied to the sacrificial head 16 of the fastener, by means of a conventional spanner 22. When the principal head 11 engages work-piece 20, the torque required to tighten the fastener increases, and eventually reaches the shear value of the shaft portion 18 of the torque member 15; then, that shaft portion 18 shears off the secondary head 17, as shown in Figure 4. This leaves the main body 10 in the work-pieces 20 and 21, with the principal head 11 engaged against a face of work-piece 20 and with the secondary head 17 still in the socket 14 of the principal head 11, as shown in Figure 5.

The torque member 15 is made of a material the colour of which may be controlled in an appropriate way. This allows different colours to be selected for the torque member 15, each colour being assigned to a particular shear value. Then, the torque value at which the shaft portion 18 and sacrificial head 16 will shear off the secondary head 17 may be determined from a visual inspection, both before and after use of the torque member.

For light-duty applications, the torque member 15

may be made of an appropriate grade of a plastics material. For heavier-duty applications, the torque member 15 may be made of a brass or bronze material, the colour of which is relatively easy to control. Alternatively, the torque member 15 could be made from an aluminium alloy, which is coloured by an anodising technique. For heavy-duty applications, the torque member may be made of steel, but doped with impurities to impart a particular colour thereto. Alternatively, such a torque member could be externally coloured, or marked externally with suitable indica. It will be appreciated that even if only externally coded, the torque member may still indicate the shear value thereof after shearing, since the regions of the secondary head 17 facing the sacrificial head 16 and surrounding the shaft portion 18 will still be visible.