It is well known to use shearable bolts for the mechanical and electrical connection of electrical cables to electrical connectors. Such a connector commonly comprises a socket into which an end of the cable is inserted. The wall of the socket contains one or more threaded bores in which threaded bolts are received. Rotation of the bolt brings it into engagement with the cable, thereby clamping the cable to the opposite wall of the socket. By providing that the fastener (bolt) shears at a predetermined applied torque the exertion of excessive force on the cable is prevented.

A potential problem with the use of shearable fasteners is that when the predetermined applied torque is reached the fastener yields suddenly. This results in the user's arm moving in an uncontrolled and sudden manner, with a risk of damage to the muscles or other tissues of the upper arm, and/or a risk of impact of the lower arm or hand with surrounding obstructions. These dangers may be particularly acute where, as is often the case, the electrical connector is fitted in a confined space such as an excavated pit in a pavement or roadway or an underground chamber.

There has now been devised an improved form of shearable fastener that overcomes or substantially mitigates the above-mentioned disadvantages.

According to the invention, there is provided a shearable fastener comprising an operative part capable of clamping engagement with an object to be fastened and a shearable part adapted to shear from the operative part upon the application of a predetermined torque, wherein the operative part and the shearable part are joined by a restraining component effective to retard movement of the shearable part when the shearable part shears from the operative part.

The fastener according to the invention is advantageous primarily in that by retarding the motion of the shearable part when it shears from the operative part of the fastener, the restraining component reduces the risk of injury to the user of the fastener.

The operative part of the fastener is most commonly an externally-threaded shank, and the shearable part is a head configured for engagement with a suitable drive tool such as a socket wrench. The fastener thus most commonly has the form of a bolt. The head may have any suitable form, a square or hexagonal external shape being most preferred, though any non-circular form may be utilised for engagement of a drive tool with the exterior of the head.

Alternatively, the head part of the fastener may be provided with an axial bore of non-circular (eg square or hexagonal) cross-section such that a drive tool such as an Allen key may be inserted into the bore to apply the necessary torque to the head part.

The shearable part is generally joined to the operative part at a shear plane defined by a local weakening in the fastener. Such a weakening most commonly takes the form of a narrowed neck or similar formation. There may be just one weakening or there may be a plurality of such weakenings, eg a plurality of shear planes arranged as described in British Patent No 2299640, the teaching of which is incorporated herein by reference.

The restraining component may extend internally of the fastener, eg in a bore (which may be circular or shaped in section) extending from the shearable part into the operative part, or may be located externally of the fastener, eg as a collar or the like surrounding the point at which the shearable part shears, in use, from the operative part. In any event, the engagement of the restraining component with both shearable part and operative part is preferably sufficient to inhibit movement of the shearable part away from the operative part when shearing takes place. This engagement may simply be a frictional engagement, eg where

the restraining component fits closely in a bore extending between the shearable part and the operative part, provided that the engagement of the restraining component with both shearable part and operative part is sufficient to inhibit movement of the shearable part away from the. operative part when shearing takes place. Alternatively, the restraining component may be bonded to one or both of the shearable part and the operative part, eg by means of adhesive.

The restraining component is preferably of a material which is capable of considerable angular deformation without shearing. This is in contrast to the shearable and operative parts of the fastener which will generally be formed integrally in a material (most commonly metal such as brass or aluminium) which yields without substantial angular deformation. In other applications (eg where electrical conductivity is not required), relatively brittle plastics material may be used for the shearable and operative parts of the fastener. Suitable materials for the restraining component include suitable plastics and elastomeric materials such as rubbers, as well as metals such as mild steel and annealed aluminium.

Such materials as are capable of twisting without shearing may be particularly suitable, the torsion of the motion-damping component retarding the angular acceleration of the shearable part immediately after shearing has occurred.

According to another aspect of the invention, there is thus provided a shearable fastener comprising an operative part capable of clamping engagement with an object to be fastened and a shearable part adapted to shear from the operative part upon the application of a predetermined torque, the operative part and the shearable part being formed integrally and connected so as to shear with relatively low angular deformation, wherein the operative part and the shearable part are joined by a restraining component capable of relatively high angular deformation without shearing, said restraining component being effective to retard movement of the shearable part when the shearable part shears from the operative part.

The extent of angular deformation between the operative part and the shearable part of the fastener prior to shearing may be very low, eg less than 10°. The restraining component may be capable of twisting through a much greater angle, eg of the order of 10-20° or even more.

The restraining component may itself shear, or it may retain its integrity. In either case, however, shearing of the fastener takes place in a more controlled and smoother movement, as compared with the prior art, reducing the risk of injury to the user, as described above.

Embodiments of the invention will now be described in greater detail, by way of example only, with reference to the accompanying drawings, in which Figure 1 is a side view in section of a first embodiment of a shearable bolt according to the invention; Figure 2 is a plan view of the head part of the bolt of Figure 1; Figure 3 is a view similar to Figure 1 of a second embodiment of a shearable bolt according to the invention; Figure 4 is a view similar to Figure 1 of a third embodiment of a shearable bolt according to the invention; Figure 5 is a plan view of the head part of the bolt of Figure 4; Figure 6 is a view similar to Figure 1 of a fourth embodiment of a shearable bolt according to the invention; and Figure 7 is a sectional view on the line VII-VII in Figure 6.

Referring first to Figure 1, a first embodiment of a shearable bolt according to the invention is generally designated 10. The bolt 10 comprises an externally threaded shank 11 which is connected by a narrowed neck 12 to a head part 13.

As can be seen from Figure 2, the head part 13 is of hexagonal form.

The bolt 10 is used in conventional fashion. The shank 11 is engaged in a threaded bore in an electrical connector (not shown). Rotation of the bolt 10 brings its tip into clamping engagement with an electrical cable introduced into the connector. Continued rotation of the bolt 10 (eg by means of a suitable socket wrench or other tool applied to the hexagonal head part 13) increases the clamping force until a predetermined torque is reached. At that point, the bolt 10 shears at the neck 12, releasing the head part 13 from the shank 11.

As thus far described, the bolt 10 is identical to a conventional shearable bolt.

Where the bolt 10 differs from a conventional bolt is in the provision of a central blind bore 14 that extends through the head part 13, past the neck 12 and a short distance into the threaded shank 11. The bore 14 is of hexagonal cross-section and is filled with a plug 15 of tough, elastomeric or soft metallic material.

The plug 15 is capable of considerable angular deformation under twisting or torsion. Thus, when the torque applied to the head part 13 of the bolt 10 exceeds the predetermined shearing torque, the neck 12 yields quite suddenly, but torsion of the plug 15 retards the consequential acceleration of the head part 13 and constrains its movement. The plug 15 thus acts as a restraining component.

This leads to a considerably reduced risk of injury to the user.

After shearing, the head part 13 and the plug 15 can simply be pulled away from the shank 11.

The embodiment shown in Figure 3 (generally designated 30) differs from that of Figures 1 and 2 in that, instead of shearing at a neck between the threaded shank 31 and the head part 32, the bolt 30 shears at any one of several shear

planes defined by four circumferential slits 33 which are axially spaced along the shank 31. The slits 33 define a series of shear planes, the torque necessary to cause the shank 31 to shear at each plane increasing from the plane nearest the tip of the shank 31 to that nearest the head part 32 of the bolt 30. In use, the shank 31 shears preferentially at the shear plane in the exposed part of the shank 31 that is closest to the external surface of the connector in which the bolt 30 is engaged (as described in British Patent No 2299640, the teaching of which is incorporated herein by reference).

Once again, an axial bore 34 of hexagonal section is formed in the bolt 30 and this is filled with a plug 35 of elastomeric or soft metallic material which exerts a braking effect on the sheared part of the bolt 30 when the applied torque exceeds that at which shearing takes place. In this case, for the plug 35 to have its desired effect irrespective of the shear plane at which the shank 31 shears, the plug 35 must extend from the head part 32 to a point beyond the shear plane furthest from the head part 32. In practice, for reasons of manufacturing simplicity, the bore 34 extends right through the bolt 30, from the head part 32 to the tip of the shank 31.

As for the embodiment illustrated in Figures 1 and 2, when the shank 31 shears (irrespective of the particular shear plane at which shearing takes place), the plug 35 counteracts the acceleration of the sheared part of the bolt 30 (ie the head part 31 and the part of the shank 32 that extends as far as the shear plane), and reduces the movement of that sheared part. After shearing the plug 35 can be withdrawn from the bore 34 in the shank 31.

Turning now to Figures 4 and 5, a third embodiment of a bolt according to the invention is generally designated 40. This bolt 40 is broadly similar to the bolt 10 of Figures 1 and 2. The difference lies in the form of the plug 45 which links the threaded shank 41 and the head part 42. In this embodiment, the part of the plug 45 which extends through the neck 43 between the head part 42 and the shank 41 is of reduced diameter so that in the region at which shearing takes place the

plug 45 is spaced from the internal wall of the bore 44. This reduces any tendency of the plug 45 to inhibit shearing, whilst still permitting the plug 45 to exert its restraining action on movement of the sheared head part 42. It also allows a wider choice of materials for the restraining component since its resilience can be tailored by adjusting its geometry.

A further difference from the embodiment of Figures 1 and 2 is that the plug 45 is formed with a hexagonal lip 46 around its upper periphery, this lip 46 being received within a correspondingly shaped recess formed in the upper surface of the head part 42. The provision of the lip 46 facilitates removal of the plug 45 from the bore 44 after the head part 42 has been sheared off, in that pulling the head part 42 away from the shank 41 necessarily causes the plug 45 to be withdrawn from the bore 44. Such a lip may be unnecessary if the plug 45 is itself designed to shear, or if suitable arrangements are made (eg roughening or knurling of the mating surfaces) to ensure tighter frictional fit of the plug in the head 42 than in the shank 41.

In the embodiment 60 of Figures 6 and 7 a threaded shank 61 is once again linked to a head part 62 by a narrowed neck 63. The neck is arranged to shear when a certain predetermined torque is applied to it. This embodiment 60 differs from those described above in that the restraining component is a collar 64 that surrounds the point at which the neck 63 shears, rather than extending through a bore within the bolt 60.

The collar 64 is of hexagonal cross-section and locates in correspondingly shaped recesses in the juxtaposed upper surface of the shank 61 and under surface of the head part 62. The collar 64 is most conveniently moulded around the neck 63, its engagement with the head part 62 and shank 61 being sufficiently close to limit movement of the head part 62 when the head part 62 shears off.