Background Art In the field of mechanical engineering bolt fastenings are frequently used because they can be tightened to a predetermined torque setting and then locked in that torque setting by the application of a locking nut. The locking nut is threaded on to a free end of the screw-threaded shank of the bolt, and tightened against the retaining nut.

The common practice is to hold the nut against rotation with a first spanner, and to tighten the locking nut with a second spanner, thereby ensuring that the high compression achieved between the locking nut and the retaining nut does not significantly alter the torque setting to which the original bolt has been tightened.

Frictional resistance between the retaining nut and locking nut prevents slackening of the two nuts one relative to the other, and the high compression between those two nuts forces their screw-threads into high frictional locking contact with the screw thread of the shank of the bolt, imposing an axial strain on the cooperating screw- threads to achieve that locking.

One shortcoming of the use of a conventional locking nut to lock a bolt in position as above described is that it requires a free portion of the threaded shank of the bolt to extend from the fastening to receive the lock nut. The use of a lock nut is not therefore available as a locking solution in situations where the entire threaded shank of a bolt is received in a threaded bore.

It is also a disadvantage with the use of conventional lock nuts that the locking forces are created by imposing axial stresses on the screw threads of the shank and retaining and locking nuts. If the locking nut is over tightened, there is a risk of stripping the

screw-thread of the nuts, the shank, or both. It is an object of the invention to overcome the above disadvantages.

There are known a wide range of bolts which have segmented screw-threaded shaft portions that can be distorted outwardly into radial contact with a cooperating screw- thread. No such proposals have, however, been used in the context of locking bolts except for US 3202194 which uses a resiliently compressible wedge to force apart the two halves of a two-segment screw-threaded shaft to a diameter greater than that of the nut into which the bolt is to be threaded. The nut draws together the two halves to their original effective diameter and in so doing compresses the wedge, thereby achieving a locking effect. In US 3202194 the user is not able to unlock the bolt for easy removal or adjustment. Any turning of the bolt relative to the nut, even during initial threading of the nut onto the bolt, is against the frictional locking force established by the wedge.

The invention The invention provides a locking bolt having the features of claim 1. Superficially the bolt has an appearance similar to that of a conventional bolt with a screw-threaded shaft and bolt head. One portion of the threaded shaft, preferably the end portion remote from the bolt head, is however segmented. It may be divided into two segments by a single diametrical slot formed in the axial end portion of the shaft, or it may be divided into four equal segments by a pair of mutually perpendicular diametrical slots; or it may be divided into six or more segments by three or more diametrical slots. The bolt is used to create a fastening in conventional manner, either by passing it through apertures in components to be secured together and applying a retaining nut, or by screwing it into a threaded bore and then tightening it to the desired torque setting. That torque setting may be precisely determined for example by use of torque wrench, or it may be determined entirely subjectively by the user.

After the necessary tightening of the bolt, it can be locked in position by advancing the expansion bolt axially along the axial hole formed in the bolt head and shaft, to cause the radial expansion of the segmented portion of the screw-threaded shaft. That

expansion may be a measurable radially outward movement of the segmented portion of the shaft, or it may simply comprise the establishment of a large compressive force in the radially outward direction from the segmented portion onto the surrounding screw thread of the nut or bore in which the bolt is received.

If the bolt head is held in position with a spanner as the locking bolt is tightened, then the tightening can be achieved without affecting in the slightest the torque setting to which the original fastening has been tightened. Turning of the expansion bolt can be achieved by means of a screwdriver engaging in a slotted recess at one end of the expansion bolt, or by inserting a hex key into a hexagonal key recess at the head end of the expansion bolt. The recess does not have to be hexagonal. Other security and non-security-shapes of recess are equally possible. What is important is that the expansion bolt can be physically advanced axially along the axial hole by the key engagement between the screwdriver or other tool and the recess at the end of the bolt, to create an extremely strong compressive force in the radially outward direction directly between the individual segments of the threaded shaft and the surrounding nut or threaded bore. Once tightened in this way, the locking nut cannot be moved relative to the nut or threaded bore until the expansion bolt is rotated in the reverse direction and slackened.

Preferably driving the expansion bolt into the axial hole in a direction away from the bolt head causes expansion of the segmented portion of the screw-threaded shaft. In such a construction the axial hole is preferably a blind bore extending from the bolt head towards the distal end of the threaded shaft, terminating in a tapered internal wall portion within the segmented portion of the screw threaded shaft, and driving the expansion bolt down the axial hole and into contact with the tapered internal hole portion causes expansion of the segmented portion of the screw-threaded shaft.

The use of a screw thread to advance the expansion bolt axially along the axial hole in the locking bolt makes it possible to generate a very high radial force between the shaft segments and the nut or threaded bore, so that very secure locking of the locking bolt can be achieved. On the other hand if all that is required after locking the bolt is a mild resistance to rotation, then that can be established using the bolt of the

invention simply by advancing the expansion bolt along the axial hole with a much reduced torque being applied to produce that movement.

Drawings Figure 1 is a plan view of the two portions of a locking bolt according to the invention with the expansion bolt removed from the locking bolt; Figure 2 is an axial section through the bolt of Figure 1, with the expansion bolt in position and tightened to lock the bolt with a threaded bore; Figure 3 is a plan view from above of the head of the locking bolt of Figure 2; and Referring first to Figures 1 to 3 of the Drawings, there is illustrated a locking bolt according to the invention comprising a tubular bolt member 1 and an expansion bolt 2. The bolt member 1 comprises a shaft 3 formed with an external screw-thread 4 and a bolt head 5. The bolt head 5 is hexagonal (see Figure 3) but could be any alternative shape, such as square. Down the centre of tubular bolt member 1 is a blind bore 6 which terminates at the end remote from the head 5 in a frustroconical wall portion 7 tapering inwardly towards the axis of the bolt member 1.

A single diametrical slot 8 at the end of the shaft 3 remote from the head 5 divides the end portion of the shaft 3 into two segments 9. Those segments 9 can be forced apart by the insertion of the expansion bolt 2 into the blind bore 6. The end of the blind bore 6 passing through the head 5 is internally screw-threaded at 10, and the expansion bolt 2 has an external cooperating screw-thread 11. The bolt 2 can be driven down into the blind bore 6 by the insertion of a hex key into a hexagonal recess 12 formed in one end of the expansion bolt 2. The opposite end of the expansion bolt 2 is tapered at 13, and the cooperating tapers 13 and 7 generate a strong expansion force on the segmented end portions 9 of the tubular bolt member 1 when the expansion bolt 2 is driven down into the blind bore 6.

Figure 2 shows how the bolt of the invention can be used to secure together two components 14 and 15. First of all the tubular bolt member 1, with the expansion bolt 2 slackly received in the blind bore 6, is used like a conventional bolt to fasten

together the two components. Those two components are illustrated in Figure 2 as being two plates, but it will be readily understood that the bottom plate 15 could be replaced by a conventional nut. When the necessary tightening of the fastening has been achieved, by tightening the head 5 to a desired torque setting, the tubular bolt member 1 is held against rotation by a spanner around the head 5, and a hex key is used to tighten the expansion bolt 2 and force apart the two segmented portions 9 to press their screw threads radially outwardly against the internal screw threads of the bottom plate 15 in which the bolt is received, to lock the bolt in the bore of the plate 15.