AU5396601A - Nut break out system - Google Patents

Description

EVELYN FRANCES GRAY

AUSTRALIA

Patents Act 1990 SPECIFICATION FOR THE INVENTION ENTITLED "NUT BREAK OUT SYSTEM" a a a* a a a I ft f This invention is described in the following statement: NUT BREAK OUT SYSTEM Field of the Invention The present invention relates to a nut break-out system and relates particularly, though not exclusively, to a nut break-out system for use with rock bolts.

Background to the Invention Nut break-out systems are an integral part of rock bolting installation systems in the mining and tunnelling industries. Nut breakout systems are particularly used on rock bolts which are installed using resin cartridges or resin injection systems.

The resin is used to anchor the rock bolt in a borehole in the rock, and the resin is normally supplied in a resin cartridge which contains two components; namely a mastic compound and a catalyst compound. These two compounds are normally separated by a thin plastic film within the resin cartridge.

The rock bolting operation consists of drilling a borehole to the required depth, and then inserting the resin cartridge into the borehole. The rock bolt is then pushed into the borehole and this forces the resin cartridge to the back of the borehole. Further pushing of the rock bolt into the borehole causes the plastic membrane containing the resin cartridge to rupture, and the bolt is then rotated and pushed right to the back of the borehole. This process also ruptures the plastic membrane separating the catalyst from the mastic, and the rotation of the bolt causes the two compounds to be mixed thoroughly together.

This process also causes the mixed resin to flow back down the borehole and completely encapsulate the bolt. The mixed resin then cures and hardens, typically within 10 seconds to 60 seconds, and this hardened resin then securely anchors the rock bolt in the borehole.

-3- Once the bolt is securely anchored in the borehole, the final process of the rock bolt installation procedure is to tighten the nut up firmly against the bearing plate against the rock surface. However, the nut is usually locked onto the bolt such that the nut and bolt can be rotated together to mix the resin during the resin mixing stage of the installation process. The existing mechanisms that are used to lock a nut onto a rock bolt are designed to "break-out" above a certain applied torque. In this manner, once the bolt is securely anchored into the borehole by the cured resin, further rotation of the nut by the drilling machine by applying a high torque, causes the locking mechanism on the nut to "break-out" and the nut can then be tightened up against a bearing plate. These nut locking systems are known as "nut break-out systems" or "break-out nuts".

Many different systems have been developed to enable rotation of rock bolts to mix resin, and then allow subsequent nut tightening to occur. Some of the nut break-out systems that have been used in Australia are outlined below: 1. Square Drive. This simple system uses a separate square drive on the end of a rock bolt or cable, such that a separate drive dolly is used over the square drive to rotate the bolt and mix the resin; and then another hexagonal drive dolly is used to tighten up the nut. This system has the advantage that the drive is positive and the square drive can withstand very high applied torque loads without premature break-out of the drive, and is one of only two nut break-out systems to provide an absolute guaranteed nut break-out system. The major disadvantage is the requirement to change drive dollies and this can waste precious time in underground mining operations when many thousands of bolts are being installed.

2. Crimped Nut. This system uses a nut where the threads on the back of the nut have been damaged by "crimping". The crimped or damaged threads initially prevent the nut from advancing onto the bolt, but once a high torque is applied, the nut is forced onto -4the bolt. This has the advantage that it is cheap and simple, but has severe disadvantages; namely that the break-out torque achieved is highly variable, the threads in the nut can be permanently damaged, and in some cases the residual torque required to advance the nut onto the bolt can be very high. This in turn, limits the amount of pretensioning that can be generated in the bolt.

3. Resin Plug. This system uses a resin plug in the end of the nut. The advantage of this system is that it is simple, and the nut can simply be screwed onto the bolt, and the resin plug is pushed out of the nut once a high torque is applied. The disadvantage of this system is that resin debris often contaminants the threads after the resin plug is pushed out of the nut, and the nut is therefore not completely free running. It is also difficult to generate very high break-out torques using this system. Another disadvantage of this system is that the resin plug itself can then fall into the drive dolly and create debris which can ultimately clog the drive dolly after many bolts have been installed.

4. Shear Pin into the Bolt. This system uses a shear pin which is inserted into a hole drilled through the nut and into the bolt.

The advantage of this system is that the shear pin then locks the nut onto the bar, and different pins can be used to generate different breakout torques. However, a disadvantage of this system is that a hole has to be drilled both in the nut and into the bolt itself and this is a costly exercise in itself. Also if the shear pin does not shear through cleanly it can damage the thread in the nut and the thread on the bar, and the nut will not then run freely onto the bolt. This again restricts the tensile load that can be generated in the bolt. Finally, the break-out torque is unreliable and can cover a wide range of break-out torques. In addition, it is difficult to generate very high break-out torques with this system, because if the shear pin is very strong, it can severely damage the bolt threads when it finally does break-out.

Shear Pin across the Nut. This is similar to the system above, except that the shear pin is only located in the nut. A hole is drilled all the way through a nut and a shear pin is placed in this hole such that the nut can only be partially screwed onto the bolt. Once the bolt has been rotated and the resin mixed and cured, further rotation of the nut causes the shear pin to bend and then break, thus allowing the nut to be screwed onto the bolt. The advantage of this system is that the nuts can be drilled separately from the bolts, and then be screwed onto the bolts later. The disadvantages are that it still requires expensive drilling operations to be undertaken, and the nuts are not securely locked onto the bolts. Another disadvantage is that, as the shear pin initially bends and deforms, it can damage the initial threads at the end of the bolt, thus making it difficult or impossible to screw anything else onto the end of the rock bolt. Finally, the break-out torque is unreliable as with the system above, and can cover a wide range of break-out torques. In addition, it is difficult to generate very high breakout torques with this system, because if the shear pin is very strong, it can severely damage the bolt threads when it finally does break-out.

6. Double Locking Nuts. This system has a main nut and a smaller threaded nut which are screwed together tightly in the factory. The advantages are that there is no expensive drilling required and no debris is left to impede the thread or clog the drive dolly. The disadvantages are that the break-out torque is unreliable and it requires two threaded nuts rather than one.

7. Retained Washer in the Nut. This system has a thin steel washer retained in the end of a recessed nut by a series of crimps over the end of the recess. At high torque, the steel washer pushes past the crimps and allows the nut to run freely along the bolt. The advantages are that it the nut is free running after the washer is pushed out of the nut, and that the nut can be fabricated separately from the bolt and be screwed onto the bolt later. The disadvantages are that this system can still damage the ends of the threads on the end of the bolt thus preventing other support brackets and nuts being screwed onto the bolt; and, the washer creates debris in the drive dollies and can eventually clog them up completely.

8. Stop Washer and Reverse Direction. This system has a stop washer welded or otherwise fixed onto the rock bolt behind the nut, such that in operation the nut is rotated in a direction such that it will contact the stop washer. The welded stop washer is immoveable and therefore the bolt is forced to rotate with the nut. When the resin is mixed and cured, the drilling machine is rotated in the opposite direction such that the nut screws away from the stop washer and tightens up against the bearing plate. The advantages of this system are that it is absolutely guaranteed to work correctly without premature break-out of the nut; and, the nut is free to run up the thread on the bolt once the direction is reversed. It can withstand as much torque as the drilling machine can apply including shock loading, without failure. The disadvantages of this system are that it does require welding of the bolt once the nut has been screwed onto the bolt. It also can necessitate that at least one nut length of thread be left on the end of the bolts below the stop washer to allow other services to be screwed onto the end of the bolt. Finally, the system requires use of a steel stop washer and a slightly larger nut than normal such that the nut is bigger than the stop washer.

All of the above systems have advantages and disadvantages and different rock bolt manufacturers prefer to use their own in-house developed systems. Nevertheless all of the above systems, except for system above, have some mechanism to prevent the nut from screwing onto or off, a rock bolt. Most of these break-out systems have one or all of the following disadvantages: they can damage the thread on the nut or the bolt, and the nut is therefore not free running on the bolt after nut breakout (eg shear pins, retained washer systems); the shear pins, or resin plugs or steel washers have to withstand both a torsional force as well as a pushing or bending force of both the nut trying to rotate, and the end of the bolt trying to push through or past the pin, plug or -7washer. Therefore the pins, plugs or washers have to be strong enough to resist both forces, but may be too strong to provide a clean break or shear when the nut does finally break-out.

they create debris that can clog drive dollies (eg resin plug, retained washer systems), or can create debris that can clog the thread on the bolt itself (resin plug); they have unreliable break-out torques and can have premature break-out, or in some cases when using hand held bolters, may not break-out at all; the break-out torque cannot be adjusted on-site (except for systems above which can simply apply whatever torque the machine can generate).

Objects of the Invention It is an object of this present invention to provide a nut breakout system which overcomes all of the disadvantages with existing nut break-out systems listed above, as well as providing some additional benefits for the installation and performance of rock bolts.

The present invention provides a break-out mechanism which does not interfere, restrict or damage the threads in the nut or the bolt in any way. In addition, it does not generate any debris to clog the threads or the drive dollies used to install the bolts.

Moreover, the present invention provides a nut break-out system whereby the break-out means is only subjected to shear and tensile forces and is not required to provide resistance to the end of the bolt trying to push past a conventional break-out system. Additionally, if shear pins are used as the break-out mechanism they can be spaced at a maximum practical distance from the axis of rotation of the bolt and the nut, thereby reducing the torsional shear strength required in the shear pin thus allowing the shear pin to be made of cheaper materials than shear pins used in current break-out nuts.

The force required to push the rock bolt into the borehole and through the resin cartridge are all transferred through the nut and the threads on the bolt only.

The present invention provides a system which does not require drilling of the nut or the bolt as with some conventional nut break-out systems and which can be expensive in terms of both materials and labour, or the use of expensive shear pins. The present invention also provides a system where the break-out torque can be adjusted on-site. The nut can also be assembled very rapidly onto the bolt either on-site, or in the factory thus saving labour assembly costs.

However, once the system is assembled onto the bolt, it cannot come off the bolt accidentally or through transport or through vibration.

Finally, the present invention provides a system whereby the load bearing area of the nut is maximised within practical limits, unlike the rotational contact surfaces between conventional nuts, domed balls, and bearing plates, which are small. Moreover, rotational contact is guaranteed to occur between the load bearing area of the nut and its mating, load bearing, contact face with the washer. This ensures that maximum tensile force is generated in the bolt from an applied torque to the nut.

Summary of the Invention According to the present invention there may be provided a nut break-out system for use with a non-circular threaded section, said system including a nut adapted to threadably engage with said noncircular threaded section, a washer member adapted to be slidably, but non-rotatably, located, in use, on said threaded section to abut said nut and a frangible locking means linking said nut and washer member to form a break-out mechanism, which frangible locking means, in use, will prevent rotation of said linked nut and washer member along said threaded section to a predetermined torque and said frangible locking means breaking at said predetermined torque to allow rotation of said nut and slidable movement of said nut along said threaded section.

Preferably said frangible locking means includes at least one shear pin axially mounted along said nut and said washer member. A preferred feature of the invention is to have a domed head on the surface of the washer member remote from said nut.

In one embodiment of the invention, at least one slot or hole is provided in said nut and washer member and, in use, said at least one slot or hole is aligned with one other by rotation of the nut with respect to the washer member for insertion of one or more shear pins into the aligned said at least one slot or hole.

The shear pins initially secure both the nut and the washer member to the threaded section, and subsequently transmit sufficient torque to the threaded section to enable the threaded section to be rotated during installation with resin cartridges when the threaded section is used as a rock bolt. Finally, the shear pins are designed to break once the resin has cured and the bolt is firmly anchored in the borehole, to allow the nut to be fully tightened up.

The slots or holes in the nut and the washer member are typically formed during manufacture of the nut and the washer member.

The shear pins can be installed in the factory or on-site, and additional shear pins can be simply added or removed on-site as required.

In a practical embodiment, a plurality of slots or holes in the nut and the washer member are provided which are designed to retain the shear pins such that the shear pins can simply be pushed sideways into the slot, typically by hammering.

In a preferred embodiment, the predetermined break-out torque of the shear pin or pins is between 50Nm and 400Nm.

In a further preferred embodiment, the shear pins are made from fibreglass or plastic such that the effects of potential corrosion on steel shear pins is eliminated, and the manufacturing cost of the shear pins is minimised.

The shear pins may be elongate members of any suitable cross sectional shape. The cross sectional shape may not be a uniform shape or size along its axial length and may have bulbed ends or be tapered to ensure that the pins do not pull through the slots or holes.

The shear pins are positioned substantially parallel with the axis of rotation of the threaded section and are also positioned a maximum practical radial distance away from the axis of rotation of the threaded section.

The nut and the washer member are preferably formed from steel or ductile iron.

The nut may have a substantially hexagonal cross sectional profile and a section with a larger diameter, substantially circular cross sectional profile.

In a preferred embodiment, the washer member may have a substantially circular cross sectional profile such that where the washer member contacts the nut, both the nut and the washer member are substantially of the same diameter. The contact faces between the nut and the washer member could be planar, curved, partially hemispherical, or designed with mating teeth. The contact face of the washer member and a bearing plate can be partially hemispherical.

In a preferred embodiment, said washer member has a noncircular central hole that is designed to fit neatly over said non-circular threaded section such that it will easily slide along the bar but it will not rotate around the bar. However, any suitable non-rotational device could be used, for example, a washer with a "lug" and a bolt with a suitably sized keyway.

In a further preferred embodiment, the diameter of the nut and the washer member is greater than the diameters of equivalently sized conventional nuts and washer members.

In a further practical embodiment, particularly for high torque or shock loading applications, the contact interface between the nut and the washer member is provided with one or more mating teeth, which -11are designed to lock together in one rotational direction and to unlock in the opposite rotational direction.

It is preferred that said threaded section is formed as a hot rolled threaded bar.

The invention can be used with bars or bolts that do not have a perfectly circular cross sectional profile, but is not so limited and could for example be used with a round bolt with a suitable keyway.

Description of the Drawings In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings, in which:- Figure 1 is a side view of a nut break-out system made in accordance with a first embodiment of the present invention located on a non-circular threaded bar or bolt; Figure 2 is cross-sectional view along and in the direction of arrows 2-2 of Figure 1; Figure 3 is a similar view to that of Figure 1 without the noncircular threaded bar or bolt; Figure 4 is an end view in the direction of arrow 4 shown in Figure 3; Figure 5 is an end view of the washer of the nut break-out system shown in Figure 3; Figure 6 shows a longitudinal view, a cross sectional view and a perspective view of a shear pin used with the nut break-out system shown in Figure 1; Figure 7 is a similar view to that of Figure 1 showing a shear pin securing the washer to the nut; Figure 8 is a similar view to that of Figure 7 with the shear pin sheared through after the nut has been rotated with respect to the washer; -12- Figure 9 is a side view of a second embodiment of a nut break-out system showing an assembled nut and washer with a series of asymmetric mating teeth; Figure 10 shows the washer contact face with the nut of the nut break-out system shown in Figure 9; Figure 11 shows the nut contact face with the washer nut of the nut break-out system shown in Figure 9; and Figure 12 is an expanded view of the asymmetrical mating teeth profile around the circumference of Figures 10 and 11 with positions of the mating teeth indicated by the letters A, B, C, and D.

Detailed Description of the Preferred Embodiments The invention will be described with respect to the manufacture of rock bolts, but the invention is not limited to this application and could be applied to any threaded bar. The invention is particularly applicable to hot rolled threaded bars but again is not so limited.

Many hot rolled threaded bars have external cross-sectional shapes which are not perfectly circular. This is often the result of having a circular core, and then having ribs which extend from the core on both the top and the bottom of the bar. Other hot rolled bars have deliberate longitudinal ridges or "flash" on opposite sides of the bar. This flash makes it easier to hot roll a circumferential rib all the way around the circumference of a hot rolled bar.

In the case of hot rolled threaded bars, the ribs rolled onto the top and the bottom of the bar are aligned such that they form a thread form. This is achieved by having synchronised rolls in the hot rolling mill such that the ribs are formed in their correct position with respect to each other.

Most rock bolts have a nut and a domed ball on one end of the bolt. The nut is used to tighten the bolt up against the bearing plate and hence the supported rock face. The domed ball is used to provide -13angular movement between the nut and the bearing plate such that irregular rock surfaces can be supported without causing excessive shear forces on the nut and the end of the bolt. A conventional nut is hexagonal and has a flat, load bearing, end face that contacts the domed ball. A conventional domed ball has a flat, load bearing contact face with the nut, and a curved, partially hemispherical face that contacts the bearing plate. The hemispherical contact face against the bearing plate can provide angular movement between the domed ball and the bearing plate and still maintain full and uniform contact with it.

In some cases, a flat, low friction washer, is installed between the nut and the domed ball.

In the preferred embodiments of the present invention the basic configuration of a nut and domed ball system with a non-circular cross section of hot rolled threaded bars can be achieved in a simple manner.

In the drawings, the same numerals have been used to designate similar integers in each Figure to avoid duplication of description.

In the first embodiment shown in Figures 1 to 8 there is shown a nut break-out system 10 for use on a generally non-circular threaded bar 12 having a series of ribs 14 extending away from the core 16 of the bar 12. The ribs 14 are formed from the same material as the core 16. A washer 18 has a non-circular central hole 20 and slides onto non-circular threaded bar 12. Washer 18 preferably includes a domed head 22 with a partially hemispherical contact face to contact a bearing plate (not shown) and has another contact bearing face 24 for abutting against a nut 26. Washer 18 can have a curved contact face instead of the flat bearing face 24 shown to maximise the surface area of this contact face within practical limits. The contact faces between nut 26 and washer 18 are mating faces such that they are substantially the same size, shape and curvature. Conventional domed balls on current rock bolts with a nominal diameter of 24mm are typically either 46mm in -14diameter or 50mm in diameter. The present invention uses a washer that is at least 50mm in diameter and is preferably 60mm in diameter but is not so limited. The surface area of this contact face can also be increased by making the contact face curved rather than planar. The contact face of domed head 22 allows angular movement between washer 18 and the bearing plate (not shown).

Nut 26 has a hexagonal drive section 28 (or any other suitable profile) and a flange section 30 with a central threaded bore 32.

The nut 26 is screwed onto the bar 12. Nut 26 has a bearing face section 34 which is designed to contact the bearing face 24 on washer 18. A plurality of slots 36 are located around the circumference of the flange section 30 of nut 26. A corresponding number of slots 38 are also located around the circumference of washer 18. The slots 38 in washer 18 are substantially the same size and shape as the slots 36 in nut 26. Slots 36,38 may also be substituted by holes, grooves or keyways. These slots 36,38 are typically 3 to 10mm in width and of any suitable length. They can be any suitable cross sectional shape and are located as close as practicable to the outside circumference of washer 18 and flange section 30. Slots 36,38 are substantially parallel with the axis of rotation of the nut 26 and bar 12.

A plurality of shear pins 40 are then located in aligned slots 36 and 38 such that a portion of one pin is in slot 36 and the remaining portion of the same pin is in slot 38. A shear pin 40 in its correct position is shown in Figure 7. Shear pins 40 can be made from any suitable material and would typically be between 3 and 10mm in diameter and be less than 50mm long. The shear pin or pins could be round, square, rectangular or any suitable shape to fit in both nut 26 and washer 18. Shear pins 40 securely attach washer 18 to nut 26.

Washer 18 cannot rotate around bar 12 because both the bar 12 and hole 20 in washer 18 are non-circular. Washer 18 can only slide along the bar 12.

In use, washer 18 is firstly slid onto threaded bar 12 to a desired location. Nut 26 is then screwed onto bar 12 such that the full length of the nut is screwed onto bar 12. Nut 26 is then rotated further around bar 12 such the slots 36 in nut 26 and slots 38 of washer 18 are aligned with each other. A shear pin 40 is then inserted into one or more of the slots 36,38. In practice, each shear pin 40 would be hammered into the slots 36,38 with the shear pin 40 being retained in slots 36,38 by friction. The shear pins 40 therefore firmly attach nut 26 to washer 18. Nut 26 cannot slide along bar 12 because it is threaded onto bar 12. Washer 18 cannot rotate around bar 12 because its noncircular central hole fits closely around non-circular bar 12. Nut 26 and washer 18 are therefore firmly attached to bar 12 for transport and installation as a rock bolt.

Once a borehole (not shown) for a rock bolt has been drilled and a resin cartridge has been installed into it, the bar 12 is inserted into the borehole and a drive dolly (not shown) is located over the hexagonal section 28 of nut 26.

The drilling machine (not shown) attached to the other end of the drive dolly, then rotates and pushes the nut 26 and bar 12 into the borehole and through the resin cartridge and thoroughly mixes the resin. The shear pin or pins 40 prevent nut 26 from rotating on bar 12 during this resin mixing stage of the operation. The size, number and shear strength of the shear pin or pins 40 are designed to provide sufficient torsional capacity to enable the bolt to be rotated and mix the resin at this stage. It should be noted that the shear pins 40 do not have to withstand any axial thrust since they are positioned parallel with the axis of rotation of the bolt. In contrast, other conventional break-out nuts have shear pins positioned at right angles to the axis of rotation of the bolt and they often have to withstand axial thrust, as well as a shear force from the torsional load of the drilling machine. In the present invention, the axial thrust from the drill rig (not shown) is transferred from the nut 26 to the bar 12 via the threads 14 in nut 26 and on bar 12 only.

-16- Once the resin has been thoroughly mixed by rotation of the bolt and the resin has been allowed to cure and harden, the drilling machine then applies sufficient torque to nut 26 to shear and break the shear pins 40 as shown in Figure 8. Once the shear pins 40 are broken, nut 26 can then be rotated until the torsional capacity of the drilling machine is reached, and nut 26 is hence tightened up against washer 18, and hence the bearing plate, and in turn the rock surface, to be supported.

It should also be noted that the contact areas 24,34 between nut 26 and washer 18 is relatively large, such that the contact pressure is kept to a practical minimum. For example, a conventional M24 nut used on a nominal 24mm rock bolt is a uniform hexagonal nut with dimensions of only 35mm across the flats and 40mm across the hexagonal. This nut has a contact face area with the domed ball of approximately 460 square mm. When an applied torque is used on such a nut, to generate a tensile force in the bolt of between 2 and 10 tonnes, this can create a contact pressure with the domed ball of between and 210 MPa. This higher contact pressure of 210 MPa is close to the yield strength of many steels and this can cause the nut to deform and grind itself into the domed ball thus increasing the total frictional resistance between the nut and the domed ball.

In contrast, if the present invention has a nut 26 and a washer 18 approximately 60mm in diameter, this provides a contact face area of approximately 2300 square mm. If the tensile force generated in the bolt is the same 2 to 10 tonnes as in the above example, the contact pressure between the nut and washer is only 8 to 42 MPa. Even this higher contact pressure is well below the yield strength of steels. Consequently, this reduction in contact pressure minimises deformation of nut 26 and prevents an increase in the total frictional resistance between nut 26 and washer 18.

It should be noted that rotational movement can only occur between the nut and the washer contact faces 24,34 with the present 17invention. These faces can then be coated with a low friction coating if required to further reduce the total frictional resistance of the between the nut 26 and washer 18, and hence enable higher tensile forces to be generated in the bolt than would otherwise be possible.

The present invention also enables the torsional resistance of the break-out system to be adjusted on-site by simply adding or removing shear pins The present invention enables the nut 26 and washer 18 to be assembled on-site or in the factory, but once they are assembled onto the bar 12, they are securely attached and will not come off bar 12.

The present invention is believed to be the only nut and domed ball system where the domed ball is securely retained in position on the end of the bolt. This is an advantage since conventional domed balls can slide off rock bolts when they are being up-turned during handling onsite.

The present invention can also be used where extreme break-out torque is required and or where the break-out mechanism may be subjected to shock loading. In this case a simple shear pin or multiple shear pins still may not provide sufficient resistance to shock loading, such as for example, where the bolt is used as a self drilling rock bolt and the break-out mechanism has to withstand all the torsional forces required for drilling a rock bolt hole. A high performance version of the invention is described the second embodiment shown in Figures 9 to 12 which is designed for applications where extreme break-out torque is required.

In this embodiment a plurality of asymmetric mating teeth 44 are provided on the contact interface 46 between nut 26 and washer 18.

The mating teeth 44 are designed to lock together the nut 26 and washer 18 in one rotational direction, and are designed to unlock in the opposite rotational direction. The advantage of this embodiment is that it provides an absolute positive lock between nut 26 and washer 18 during the installation cycle of bar 12 when the resin needs to be -18thoroughly mixed by rotation of bar 12. It will resist shock loading or the full rotational torque of the drilling machine without premature nut breakout. Once the resin has been thoroughly mixed and cured, the machine rotates nut 26 in the opposite direction and causes nut 26 to rotate relative to washer 18 and hence can screw nut 26 along bar 12.

The "break-out", or the relative rotational movement is designed to occur at the interface between washer 18 and nut 26. The break-out force required to cause relative movement at this interface is increased by the use of mating teeth 44. For example, if one or more mating teeth 44 are used on washer 18 and nut 26, and the teeth 44 are asymmetrical such that one face on the teeth has an angle of greater than 45 degrees and typically is greater than 65 degrees, and the other face on the teeth is less than 30 degrees and is typically less than degrees, then rotational movement in a direction to cause the steep faces on the mating teeth to contact would cause them to lock together and further rotational movement could not occur. However when rotational movement occurs in the opposite direction to cause the flat inclined faces on the mating teeth to contact, then rotational movement can still occur by the teeth moving along the inclined faces.

Therefore the invention can have a break-out system that provides a positive lock between the nut 26 and washer 18 in one rotational direction, and also provides an effective break-out means in the opposite rotational direction.

For example, in the case of self drilling bolts it is imperative that nut break-out does not occur as the rock bolt hole is being drilled. However if the self drilling rock bolt hits a hard band in the roof during the drilling operation, it can generate a severe shock load on the nut break-out system. In this embodiment it is possible to drill a rock bolt hole by rotational movement that causes the mating teeth 44 on the nut 26 and washer 18 to lock together against their steep mating faces. Thus nut break-out cannot occur. Once the rock bolt hole is drilled it is possible to 19subsequently tighten up nut 26 by rotating nut 26 in the opposite rotational direction thus enabling relative movement between nut 26 and washer 18 along the flat inclined teeth faces.

There are two further advantages of using this embodiment.

The first advantage is that nut 26 can only be rotated in one direction once the mating teeth 44 between nut 26 and washer 18 come into contact with each other. Therefore once nut 26 has been tightened up to its full torsional force, it will be impossible to unscrew. Therefore nut 26 has a self-locking mechanism, such that nut 26 will not come loose due to vibration or rock movement around the nut.

Secondly, rotation of nut 26 with respect to washer 18 in a direction where the slightly inclined faces of the asymmetrical mating teeth 44 slide across each other, will initially cause nut 26 and washer 18 to move apart from each other in an axial direction by a distance equivalent to the height of the asymmetrical mating teeth 44. Once nut 26 and washer 18 have separated by the maximum height of the asymmetrical mating teeth 44, further rotation of nut 26 with respect to washer 18 will cause nut 26 and washer 18 to move towards each other in an axial direction. This axial movement backwards and forwards of washer 18 with respect to nut 26, will occur at high speed when using a drilling machine to tighten up nut 26, and this will cause an axial "hammer" action or force to be generated at the contact face between nut 26 and washer 18. This hammer action will assist to apply a high torque to nut 26 and hence generate a high tensile force in bar 12.

The mating teeth 44 can be any suitable design that will lock in one rotational direction and will unlock in the other rotational direction. For example, a suitable screw thread would achieve the same objective. In this embodiment it is possible to hold the washer 18 and nut 26 together in an axial direction by use of the design of the mating teeth 44, or by the use of a plastic pin or by any suitable means.

The present invention therefore provides a nut break-out system that does not require any drilling or post processing machining 20 operations either on the nut 26 or on the bar 12. The nut 26 and washer 18 can simply be assembled onto the bar 12 in the field if necessary, and can be held together by shear pins 44 which are simply pressed or hammered into the appropriate slots 36,38 in the nut 26 and washer 18.

Additional shear pins 44 can be installed to increase the break-out torque if required.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions, and compounds referred to or indicated in this specification (unless specifically excluded) individually or collectively, and any and all combinations of any two or more of said steps or features.

Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising" will be understood to imply the inclusion of a stated integer or group of integers, but not to the exclusion of any other integer or group of integers.

Where the specification refers to a "nut" it is to be understood that the invention includes all such variations and modifications of the above, and any other member that could be used to screw onto a thread form on a bolt or a bar.

Where the specification refers to a "washer member" or to a "domed ball" it is to be understood that the invention includes all such variations and modifications of the above, and any other member that could be used to provide angular movement between a bearing plate and a rock bolt.

Where the specification refers to a "non-rotational device" or to a "domed ball" it is to be understood that the invention includes all such variations and modifications of the above, and any other member -21that could not be rotated around the longitudinal axis of a rock bolt or bar.

Where the specification refers to a "rock bolt" or to a "threaded bar" or to a "hot rolled threaded bar" it is to be understood that the invention includes all such variations and modifications of the above, and any other elongate member that has a full or partial thread form along all or part of its length.

Where the specification refers to a "hole" or to "holes" it is to be understood that the invention includes all such variations and modifications of a "hole" but is not limited to these alone and includes one or many "slots", "grooves", or "keyways".

Where the specification refers to a "shear pin" or to "shear pins" it is to be understood that the invention includes all such variations and modifications of a "shear pin" but is not limited to these alone and includes one or many "pins", "rods", "bars", "roll pins", "wires", "clips" or any elongate member that is designed to break or shear above a predetermined applied shear force.

Where the specification refers to "teeth" or to "mating teeth" it is to be understood that the invention includes all such variations and modifications of "teeth" but is not limited to these alone and includes one or many "gears", "mating gear teeth", "teeth", "lugs", "projections", "pins", "clips" or "screw threads" or any device that will provide a positive lock between the nut and the washer member or domed ball in one rotational direction and will unlock in the opposite rotational direction.

1. A nut break-out system for use with a non-circular threaded section, said system including a nut adapted to threadably engage with said non-circular threaded section, a washer member adapted to be slidably, but non-rotatably, located, in use, on said threaded section to abut said nut and a frangible locking means linking said nut and washer

Claims (8)

1. A nut break-out system for use with a non-circular threaded section, said system including a nut adapted to threadably engage with said non-circular threaded section, a washer member adapted to be slidably, but non-rotatably, located, in use, on said threaded section to abut said nut and a frangible locking means linking said nut and washer 22 member to form a break-out mechanism, which frangible locking means, in use, will prevent rotation of said linked nut and washer member along said threaded section to a predetermined torque and said frangible locking means breaking at said predetermined torque to allow rotation of said nut and slidable movement of said nut along said threaded section.

2. A nut break-out system according to claim 1, wherein said frangible locking means includes at least one shear pin axially mounted along said nut and said washer member.

3. A nut break-out system according to claims 1 or claim 2, wherein the surface of the washer member remote from said nut comprises a domed head.

4. A nut break-out system according to claim 1, wherein at least one slot or hole is provided in said nut and wasier member and, in use, said at least one slot or hole is aligned with one other by rotation of the nut with respect to the washer member for insertion of one or more shear pins into the aligned said at least one slot or hole. A nut break-out system according to any one of claims 1 to 4, wherein a plurality of slots or holes in the nut and the washer member are provided to retain said shear pins to enable said shear pins to be pushed sideways into the slot.

6. A nut break-out system according to anyone of claims 2, 4 or wherein the predetermined break-out torque of the shear pin or pins is between 50Nm and 400Nm.

7. A nut break-out system according to claim 5, wherein said shear pins are elongate members positioned substantially parallel with the axis of rotation of the threaded section and a maximum practical radial distance away from the axis of rotation of the threaded section.

8. A nut break-out system according to claim 7, wherein the cross- sectional shape of said shear pins is of variable shape and size along their axial length and wherein the at least one end is bulbed or tapered.

23- 9. A nut break-out system according to claim 8, wherein the shear pins are made from fibreglass or plastic. A nut break-out system according to any one of the previous claims, wherein the nut and the washer member are formed from steel or ductile iron. 11. A nut break-out system according to claim 10, wherein the nut is of substantially hexagonal cross-sectional profile and a section with a larger diameter, substantially circular cross sectional profile. 12. A nut break-out system according to any one of the previous claims, wherein the washer member has a substantially circular cross sectional profile such that where the washer member contacts the nut, both the nut and the washer member are substantially of the same diameter. 13. A nut break-out system according to claim 12, wherein the at least one contact face between the nut and the washer member is selected from planar, curved, partially hemispherical, or wherein said contact face is designed with mating teeth. 14. A nut break-out system according to any one of the previous claims, wherein the washer member has a non-circular central hole designed to fit neatly over said non-circular threaded section such that it will easily slide along the bar but will not rotate around the bar. A nut break-out system according to claim 14, wherein the contact interface between the nut and the washer member is provided with one or more mating teeth, whereby said tooth is designed to lock together in one rotational direction and to unlock in the opposite rotational direction. 16. A nut break-out system according to any one to the previous claims, wherein the threaded section is formed as a hot rolled threaded bar. 17. A nut break-out system according to any one of claims 1 to 16, substantially as hereinbefore described with reference to any one of the accompanying drawings.