DRIVING TOOL FOR SHEARABLE FASTENERS

Abstract: A drive tool (10) for use in association with a shearable fastener comprising an operative part and a head part shearable therefrom upon application of a predetermined torque, said drive tool (10) comprising a first assembly adapted to be engaged with the head part of the fastener so as to apply rotational drive thereto, and a second assembly which engages the operative part of the fastener when said first assembly is engaged with the head part of the fastener,wherein said first assembly and said second assembly are coupled such when the operative part and the head part of the fastener are joined the first assembly and the second assembly rotate together, and when the head part shears from the operative part the first assembly is capable of restricted rotational movement relative to the second assembly, whereby sudden movement of the drive tool upon shearing of the fastener is inhibited or prevented.

Description: DRIVING TOOL FOR SHEARABLE FASTENERS This invention relates to improvements in the use of shearable fasteners, and in particular to improvements in the use of shearable bolts of utility in electrical connectors. 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 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 a drive tool for use in association with a shearable fastener which overcomes or substantially mitigates the above-mentioned or other disadvantages of the prior art. According to the invention, there is provided a drive tool for use in association with a shearable fastener comprising an operative part and a head part shearable therefrom upon application of a predetermined torque, said drive tool comprising a first assembly adapted to be engaged with the head part of the fastener so as to apply rotational drive thereto, and a second assembly which engages the operative part of the fastener when said first assembly is engaged with the head part of the fastener, wherein said first assembly and said second assembly are coupled such when the operative part and the head part of the fastener are joined the first assembly and the second assembly rotate together, and when the head part shears from the operative part the first assembly is capable of restricted rotational movement relative to the second assembly, whereby sudden movement of the drive tool upon shearing of the fastener is inhibited or prevented. The drive tool according to the invention is advantageous primarily in that by retarding the motion of the drive tool when the fastener shears, it reduces the risk of injury to the user. The drive tool as defined above may be formed integrally with a handle or the like enabling manual operation of the tool. Alternatively, the drive tool may be adapted for connection to a separate tool such as a socket wrench or the like. The coupling between the first assembly and the second assembly is such that in the absence of an applied force of sufficient magnitude to prevent it, the two assemblies can rotate as one. When such a force is applied, however, rotation of one assembly relative to the other takes place. However, such rotation is not free rotation, but is instead restricted by the coupling of the two assemblies. Most preferably, the coupling is frictional, the degree of frictional force between the first and second assemblies being sufficient to inhibit relative movement of the two assemblies. Alternatively, the two assemblies may be coupled by means of resiliently or elastically deformable components. In another alternative, the two assemblies may be coupled by a damped motion, eg using a hydraulic shock absorber or the like. Engagement of the second assembly with the operative part of the fastener is most preferably by means of a rod or the like depending from the second assembly and engaging in a bore or recess in the operative part. Most preferably, such a bore extends from the head part into the operative part. Most preferably, the rod or the like is of non-circular, eg square or hexagonal, cross- section, and the lowermost extent of the bore into which the rod or the like extends is of similar dimension and cross-section so that the rod or the like is non-rotatably received within the operative part. The upper extent of the bore, on the other hand, is of larger dimension and/or circular cross-section so that the head part of the fastener, once sheared from the operative part, can rotate freely about the rod or the like. The rod or the like is preferably aligned coaxially with the axis of rotation of the fastener so that the axis of rotation of the drive tool is preserved, even after the fastener has sheared. Thus, a fastener used in association with a preferred embodiment of the drive tool of the invention comprises an operative part and a head part shearable therefrom upon application of a predetermined torque, the operative part and the head part being co-axially aligned and having a bore extending from the head part into the operative part, at least a portion of the bore within the operative part being of non-circular cross-section so as to be capable of positive engagement with a correspondingly-shaped rod or the like inserted into it, and the portion of the bore extending through the head part being of enlarged dimension such that the head part, when sheared from the operative part, is freely rotatable about the rod or the like. The operative part of the fastener is most commonly an externally-threaded shank, and the head part is configured for engagement with the drive tool. 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. The head 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. A currently preferred embodiment 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 of a shearable bolt according to the invention, showing hidden detail; Figure 2 is a view of the bolt of Figure 1 from above; Figure 3 is a side view of a drive unit according to the invention for use in association with the bolt of Figures 1 and 2; Figure 4 is a view of the drive unit of Figure 3 from above; Figure 5 is a view of the drive unit of Figure 3 from below; and Figure 6 is a sectional view on the line A - A in Figure 4. Referring first to Figure 1 , a shearable bolt is generally designated 1 and is intended for use in securing electrical conductors within electrical connectors. The bolt 1 comprises a threaded shank 2 and a hexagonal head 3. The head 3 is connected to the top of the shank 2 by a tapered neck 4. The lowermost extremity of the neck 4, at which the neck 4 joins the shank 2, represents a plane of weakness, at which the bolt 1 shears when the torque applied to the head 3 exceeds a predetermined value. A blind axial bore 5 is formed within the bolt 1 , extending from the head 3, through the neck 4 and into the shank 2. Within the shank 2, the lowermost extent of the bore 5 is of regular hexagonal cross-section. Within the head 3 and neck 4, however, the bore 5 is of circular cross-section, the diameter of the circular part of the bore 5 being equal to (as in the illustrated embodiment) or greater than the maximum cross-sectional dimension of the lower hexagonal part of the bore 5. Turning now to the drive unit illustrated in Figures 3 to 6, this is generally designated 10 and comprises a hollow, generally cylindrical housing formed from upper and lower parts 11,12. Fixed axially to the end face of the upper part 11 is an upper socket 13 for a square drive tool, eg a "54" drive. The upper socket 13 may be fixed to the upper part 11 by being formed integrally therewith or, as illustrated, by being welded. Fixed to the lower end face of the lower part 12 is a lower socket 14 which may, as illustrated, be of the "Metrinch" type (see Figure 5) capable of receiving bolt heads having either metric or imperial dimensions. Again, the lower socket 14 is fixed by welding, but could alternatively be fixed by virtue of being formed integrally with the lower part 12. The lower end face of the lower part 12, and also the base of the lower socket 14, are formed with centrally circular openings 15,16 (see Figure 6). The juxtaposed edges of the upper and lower parts 11 , 12 are formed with annular flanges 17, 18, each of which has six equiangularly spaced bores 19,20. The bores 20 in the lower part 12 are plain, while those 19 in the upper part 11 are threaded. Socket head cap screws 21 are used to join the upper and lower parts 11 , 12 to form the complete housing, a friction disc 22 being captivated between the upper and lower parts 11 , 12. The friction disc 22 has a diameter equal to that of the flanges 17, 18. Obviously, the friction disc 22 has six bores which are brought into registration with those 19, 20 in the juxtaposed flanges 17, 18. The friction disc 22 is annular, having a central opening, and a restraining assembly is mounted upon the disc. The restraining assembly comprises a cylindrical body 23 with an integrally- formed circular flange 24 at its lower end. The upper extent of the body 23 is threaded. The body 23 extends through the opening in the centre of the disc 22, a friction washer 25 being interposed between the flange 24 and the disc 22. In the upper part of the housing, a friction washer 26, pressure disc 27 and a pair of spring washers 28 are mounted about the body 23. The assembly is completed by an adjustment nut 29 engaged with the threaded part of the body 23. A hexagonal bore is broached along the longitudinal axis of the body 23, a hexagonal spindle 30 being fixedly received within that bore. The spindle 30 extends through the openings 15, 16 in the lower part 12 and lower socket 14, and projects a short distance beyond the lower socket 14. The restraining assembly is thus rotatably mounted upon the friction disc 22, the frictional resistance to rotation being determined by the compressive force exerted on the spring washers 28 by the adjustment nut 29. In use, a bolt 1 is threadedly engaged with a threaded bore in an electrical connector in conventional fashion. The drive unit 10 is then fitted to the bolt 1 , the head 3 of the bolt 1 being received within the lower socket 14 and the spindle 30 extending into the blind bore 5, and specifically into the hexagonal cross- section lower region of that bore 5. A conventional drive tool is then inserted into the upper socket 13 and the drive unit 10 is rotated. By virtue of the engagement of the lower socket 14 with the head 3 of the bolt 1 , the bolt 1 rotates with the drive unit 10. Continued rotation brings the tip of the bolt 1 into engagement with the electrical conductor it is intended to fasten. As the bolt 1 is rotated further, the resistance to rotation increases and a point is reached at which the neck 4 of the bolt 1 shears. When this happens, despite the fact that rotational force continues to be applied directly to the head 3 of the bolt 1 , because the restraining assembly maintains a mechanical connection between the drive unit 10 and the stationary shank 2 of the bolt 1 , the frictional engagement between the restraining assembly and the friction disc 22 damps or retards the further movement of the drive tool and prevents sudden and uncontrolled movement as can take place using conventional shearable fasteners and drive tools. When the torque applied to the head 2 of the bolt 1 exceeds the predetermined shearing torque, the neck 4 yields quite suddenly, but this frictional engagement retards the consequential acceleration of the head 2 and constrains its movement. This leads to a considerably reduced risk of injury to the user. After shearing, the drive unit 10 is disengaged from the shank 2 of the bolt 1 by withdrawing the spindle 30 from the blind bore 5. The sheared-off head 3 and neck 4 of the bolt 1 then either fall from, or can easily be withdrawn from, the lower socket 14.

Claims: Claims 1. A drive tool for use in association with a shearable fastener comprising an operative part and a head part shearable therefrom upon application of a predetermined torque, said drive tool comprising a first assembly adapted to be engaged with the head part of the fastener so as to apply rotational drive thereto, and a second assembly which engages the operative part of the fastener when said first assembly is engaged with the head part of the fastener, wherein said first assembly and said second assembly are coupled such when the operative part and the head part of the fastener are joined the first assembly and the second assembly rotate together, and when the head part shears from the operative part the first assembly is capable of restricted rotational movement relative to the second assembly, whereby sudden movement of the drive tool upon shearing of the fastener is inhibited or prevented. 2. A drive tool as claimed in Claim 1 , wherein the coupling of said first assembly and said second assembly is frictional, the degree of frictional force between the first and second assemblies being sufficient to inhibit relative movement of the two assemblies. 3. A drive tool as claimed in Claim 1 , wherein the coupling of said first assembly and said second assembly is by means of resiliently or elastically deformable components. 4. A drive tool as claimed in Claim 1 , wherein said first assembly and said second assembly are coupled by a damped motion, using a hydraulic shock absorber or the like. 5. A drive tool as claimed in any preceding claim, wherein engagement of the second assembly with the operative part of the fastener is by means of a rod or the like depending from the second assembly and engaging in a bore or recess in the operative part. 6. A drive tool as claimed in Claim 5, wherein the rod or the like is of non- circular cross-section, and the lowermost extent of the bore into which the rod or the like extends is of similar dimension and cross-section so that the rod or the like is non-rotatably received within the operative part, and the upper extent of the bore is of larger dimension and/or circular cross-section so that the head part of the fastener, once sheared from the operative part, can rotate freely about the rod or the like. 7. A drive tool as claimed in Claim 5 or Claim 6, wherein the rod or the like is aligned coaxially with the axis of rotation of the fastener so that the axis of rotation of the drive tool is preserved, even after the fastener has sheared. 8. A drive tool as claimed in any preceding claim, which is formed integrally with a handle or the like enabling manual operation of the tool. 9. A drive tool as claimed in any one of Claims 1 to 7, which is adapted for connection to a separate tool such as a socket wrench or the like. 10. A drive tool substantially as hereinbefore described and as illustrated in Figures 3 to 6 hereof.

Inventors: BARNETT GARY, HOLLICK DAVID JOHN, BARNETT, GARY, HOLLICK, DAVID, JOHN
Applicants: BARNETT GARY, HOLLICK DAVID JOHN, TYCO ELECTRONICS LTD UK, BARNETT, GARY, HOLLICK, DAVID, JOHN, TYCO ELECTRONICS UK LIMITED
Application year: 2001
Main classification: B25B23
IPC codes: B25B23, F16B31