Technical Field

[001] The invention broadly relates to a tensioning nut assembly for bolts, rock bolts and self-drilling rock bolts. More specifically, the invention relates to a system which enables a bolt or a rock bolt or a self-drilling rock bolt to have a high tensile load applied to it using a mechanical two stage nut system. A high tensile load applied to a bolt or rock bolt, often known as a high pre-tension load, can be used to support the roof and walls of mines and tunnels.

[002] Aspects of the invention also relate to a bolt and nut combination, and to a method of tensioning a bolt.

Background to the Invention

[003] Rock bolts and self-drilling rock bolts typically have threads on the trailing ends of the bolts and have nuts screwed onto the threads. The nut and thread are to enable the nut to be tensioned up against a bearing plate to support the rock face.

Commonly the thread on the bolt is cold rolled onto the bolt. There may be a threaded section on the trailing end of the bolt, or the entire length of the bolt. Where the thread is just on the trailing end of the bolt, the threaded section typically is between 100 and 300 mm in length.

[004] The thread profiles on rock bolts vary depending on the application. Typically rock bolts used in the underground coal industry have cold rolled metric threads, such as M24 or M30 threads where the thread pitch is 3mm and 3.5mm respectively. The disadvantages of these metric cold rolled threads are that:

• a hot rolled bar with deformations on it is used to form a rock bolt, and the deformations on the bar have to be removed over a length of approximately 100-300mm so that a metric type thread can be cold rolled on it, and this bar peeling operation adds to manufacturing cost and removes steel which reduces the bar tensile strength;

• a thread can then be cold rolled onto this peeled section of the bar, and this involves forcing dies into the bar to displace metal to form a thread. In the case of rolling threads with a fine thread pitch onto the bar, forcing dies with narrow sharp edges into a bar results in wear on the dies, and creates stress concentrations in the bar such that the root diameter of the cold rolled thread is the weakest part of the rock bolt; • fine pitched, cold rolled threads have relatively narrow, sharp edges on the crest of the thread that can easily be damaged in the underground mining environment;

• fine pitched, cold rolled threads can become damaged and or difficult to unscrew if they are used with rotary percussive drilling as used in hard rock mining and tunnelling;

• if a shear pin is used in a nut which is screwed onto a cold rolled fine pitched thread, the shear pin may be positioned anywhere across the thread profile, i.e. on the crest, the sides, or at the base of the thread profile, and this can affect the torque at which the shear pin fails.

[005] In contrast, rock bolts used in the underground metal mining, tunnelling, and civil engineering industries can include coarse pitched threads, either cold rolled rope ' type threads, or hot rolled buttress type threads.

[006] Cold rolled rope type threads typically have a much coarser thread pitch, typically 12.7mm, or greater, and have wide and rounded thread profiles that can withstand the rough handling associated with the underground metal mining industry. These coarse rope type threads can also withstand the hammer action of rotary percussive drilling without damage or without threads locking up, and are therefore preferred for most hard rock applications. Finally, cold rolled rope type threads create less wear on cold rolling threading machinery, and these rope type threads also have less stress concentrations in the bar itself, compared to metric threads with finer thread pitches.

[007] Hot rolled threads also have a coarse thread pitch where the thread pitch is typically

10mm or greater. These hot rolled threads typically have a buttress type thread profile and the thread ribs are typically only on the top and bottom of the bar, not on the sides of the bar. The ribs are typically more than 4mm wide, and they have a flat top. These ribs are hot rolled onto the bar in the hot rolling mill and the ribs on the top of the bar are aligned with the ribs on the bottom of the bar to form a thread spiral or thread helix along the bar. The advantage of hot rolled threads is that they can be continually formed on the whole length of the bar at very high speed in the hot rolling mill, and there is no additional processing cost for thread forming.

Moreover, because the threads are formed when the bar is hot, the stress concentrations in the bar are low. In addition, these hot rolled threads are formed as aligned ribs on the outside of a bar so the hot rolled thread does not reduce the bar diameter and hence does not weaken the bar. Finally, a hot rolled thread is very robust and can easily withstand the rough handling associated with the mining and tunnelling industries.

[008] The disadvantage with both cold rolled rope type threads and hot rolled buttress type threads, is that they have a relatively coarse thread pitch of typically 10mm or greater, and it is difficult to generate a high pre-tension load on a rock bolt with a coarse thread pitch.

[009] However, there is a growing demand from geotechnical engineers and mining

engineers to install rock bolts with a high pre-tension load. This pre-tension load provides an active force pushing the bearing plate against the rock face, and it is achieved by tensioning up a nut on the trailing end of a rock bolt. This active pretension force helps to reduce rock movement and improves rock stability.

[010] This active pre-tension force can be achieved by anchoring a rock bolt in the top of a borehole, and then tensioning up a nut on the trailing end of the rock bolt. The leading end of the rock bolt, typically the top of the bolt, can be anchored in the borehole by either using a mechanical anchor device, or by using a fast set chemical resin. If the top of the bolt is then anchored in the borehole, then tensioning up the nut will create a tensile force in the rock bolt between the nut and the anchor point. This tensile load in the bolt can then be locked into the bolt by subsequently fully encapsulating the bolt with resin or cement grout.

[011] This pre-tensioning of rock bolts can be achieved by using a two speed chemical resin cartridge whereby the leading end of the cartridge contains a fast set resin, and the trailing end of the resin cartridge contains a slow set resin. The resin cartridge is pushed into the borehole by the rock bolt, and then the rock bolt ruptures the frangible plastic film of the resin cartridge. The resin is then mixed by the bolt being rotated and the fast set resin cures and hardens at the top of the borehole, thus anchoring the top of the bolt in the borehole. The nut on the trailing end of the bolt can then be tensioned up before the slow set resin has cured and hardened. The two speed resin cartridges enable a pre-tensioned load to be applied to rock bolts easily.

[012] The pre-tensioning load that can be generated in rock bolt depends on: the friction between the nut and the bolt thread; and, the friction between the contact face of the nut and the contact face of the domed ball washer the nut is typically pushing against; and, the applied nut tensioning torque; and, the thread pitch of the bolt that the nut is advancing along. [013] Various methods have been used to reduce the friction described above. The bolt thread and the nut thread can have a light coating of oil to reduce friction. Molybond or other low friction coatings can also be applied to these threads. The end face of the nut typically presses against a low friction plastic washer which is inserted between the nut face and the domed ball face, and this reduces the friction between these two steel components.

[014] In addition, hydraulic drilling machines can apply a reasonably high rotational torque of between 300 and 400 Nm. Even higher nut tightening torques of up to 600Nm can be applied to generate high tensile loads in rock bolts. Nevertheless, the nut tightening torque is typically limited by the drilling machine being used, although torque multipliers can be used in some cases.

[0 5] Therefore, once friction has been minimised and torque has been maximised as described above, the controlling factor determining the tension that can be generated in a rock bolt depends upon the pitch of the thread on the rock bolt, known as the thread pitch. The thread pitch on the bolt is the same as the thread pitch inside the nut so that the nut can be rotated and screwed along the bolt thread.

[016] Typically the thread pitch on rock bolts is approximately 3.0mm for cold rolled

threads used in the underground coal industry, 12.7mm for cold rolled rope threads used in the underground metal mining industry, and 10mm or greater for hot rolled threads used in the civil, tunnelling, and metal mining industries. The advantage of using fine pitched threads is that the tension that can be generated in a rock bolt increases as the thread pitch gets smaller or finer. For example, for any given coefficient of friction between the nut and the bolt, and any given applied torque to the nut, a 3.0mm thread pitch will generate a higher tensile load in the rock bolt than a 12.7mm thread pitch.

[017] For rock bolts used in the underground coal industry with commonly 3.0mm thread pitches, the tensile load generated in the rock bolts is typically between 2 and 7 tonnes.

[018] However, a major factor that can limit the amount of tension that can be generated in a rock bolt is damage to the threads on the rock bolt. Damage can occur to the threads on rock bolts due to the harsh mining and tunnelling environment and, as discussed above, fine pitched threads are more susceptible to thread damage than coarse pitched rope threads or coarse pitched hot rolled threads. If a fine pitched thread is damaged, then the amount of tension that can be generated, in a rock bolt with this thread, may be less than the tension that can be generated in a rock bolt with an undamaged coarse pitch rope thread.

[019] Furthermore, fine pitched threads provide a lower bond capacity with resin or grout compared to coarse pitched rope threads.

[020] Finally, fine pitched threads are unsuitable for use with rotary percussive drilling

because fine pitched threads can become difficult to unscrew after they have been subjected to the hammer action of rotary percussive drilling.

[021] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this specification.

Summary of the invention

[022] It is an objective of the current invention to overcome or at least ameliorate one or more of the above described limitations of threads used on bolts and rock bolts, and the problems of tensioning nuts on those threads.

[023] The present technology in one example form provides a method of tensioning a nut for a bolt that is inserted in a borehole, including the steps of: threading a sleeve onto the bolt, the sleeve having an internal thread and an external thread that is finer than the internal thread; threading a nut onto the sleeve; drilling at least one hole through the nut, sleeve, and into the bolt; inserting a pin into the at least one hole; applying a torque to the nut to rotate the bolt until the bolt is anchored; continuing to apply a torque to the nut to advance the sleeve on the bolt, until an axial movement of the sleeve on the bolt is stopped by the ground, rock face, or a bearing plate against the rock face; and increasing the torque applied to the nut to advance the nut on the sleeve.

[024] By its rotation the bolt can be adapted to mix a resin to anchor the bolt in the

borehole, and/or to activate a mechanical anchoring device.

[025] The example technology also provides a two stage tensioning system for a bolt, the system including an inner component and an outer component, the inner component having an internal surface and an external surface, the internal surface of the internal component being adapted to engage an external surface of the bolt and allow the internal component to travel axially on the bolt during a first stage of tensioning, and the external surface of the internal component being adapted to engage an internal surface of the outer component and allow the outer component to travel axially on the internal component during a second stage of tensioning, whereby an axial travel during the first stage requires a lower torque than an axial travel during the second stage.

[026] The example technology comprises a mechanical two stage nut system including an inner sleeve which screws on to the thread on the end of a rock bolt and a larger outer nut which screws onto a fine pitched thread on the outside of the inner sleeve. The inner sleeve and the outer nut are prevented from rotating with respect to each other by a shear pin or other similar device. Typically the shear pin will go all the way through the nut and the sleeve and into the bolt where it locks the nut and sleeve assembly on to the bolt. The outer nut can then be used to rotate the bolt, either to mix a resin cartridge, or to activate a mechanical anchor on the leading end of the bolt, or to rotate a self-drilling rock bolt. Once the bolt is anchored in the borehole, either with a resin cartridge or with a mechanical anchor device, rotational torque can then be applied to the nut to break the shear pin between the bolt and the inner sleeve such that the inner sleeve and the outer nut both start to rotate about the bolt thread and advance along the bolt thread. Once the leading end of the nut starts to push a bearing plate against the rock face to be supported, the rotational torque required to continue to rotate the nut increases rapidly. Eventually the steel sleeve cannot advance along the bolt thread any further because friction exceeds the mechanical advantage of the bolt thread pitch. However, the thread between the steel sleeve and the nut has a fine thread pitch and hence has a higher mechanical advantage than the thread on the bolt which has a coarser thread pitch. In addition, the thread between the steel sleeve and the nut has a greater radius from the central axis of rotation, than the thread between the steel sleeve and the bolt. Consequently, continued rotation of the nut causes the shear pin between the nut and the steel sleeve to fail enabling the nut to advance along the fine thread on the steel sleeve.

[027] As the nut advances along the steel sleeve thread, it compresses the bearing plate against the rock face and generates a tensile load in the rock bolt between the nut and the bolt anchor. Since the thread between the nut and the steel sleeve has a very fine thread pitch, and is much finer than the typical thread pitch found on rock bolts, then the tensile load or the pre-tensile load that can be applied to rock bolts, is much greater using the current invention than is possible with conventional rock bolt threads. [028] In one form, the present technology provides a nut assembly for tensioning a bolt, including:

an inner sleeve having an inner surface and an outer surface, the inner surface having a first thread portion having a first thread pitch adapted to form a first threaded connection between the inner sleeve and the bolt, and the outer surface having a second thread portion having a second thread pitch; and an outer nut member having a third thread portion adapted to cooperate with the second thread portion of the inner sleeve, to form a second threaded connection between the nut member and the inner sleeve;

wherein the first thread pitch is longer than the second thread pitch such that the first threaded connection provides greater axial movement per thread turn than the second threaded connection.

[029] A further aspect of the invention provides a nut and bolt assembly comprising a bolt having an external thread portion; and a nut assembly according to the present technology in threaded connection with the external thread portion.

[030] A yet further aspect of the invention provides a method of tensioning a bolt within a borehole, comprising:

providing a nut and bolt assembly according to the present technology;

inserting the bolt of the nut and bolt assembly within the borehole; applying torque to the nut member to cause the nut and bolt assembly to rotate together up to a first torsional load at which a first failure in the shear pin between the bolt and the inner sleeve occurs;

continuing to apply torque to the nut member to advance the nut assembly along the external thread portion of the bolt to apply tension to the bolt, up to a second torsional load at which a second failure in the shear pin between the inner sleeve and the nut member occurs; and

continuing to apply torque to the nut member to advance the nut member along the second thread portion of the inner sleeve to apply additional tension to the bolt.

[031] Further optional aspects of the invention are described below, and are as set out in the claims.

[032] The nut may have a leading end, and a radial extension such as a circumferential flange extending inwardly at the leading end. [033] The sleeve can have at least one through hole extending between the internal surface and the external surface, the at least one through hole being adapted to align with a corresponding through hole provided through the nut, the aligned through holes being adapted to receive a pin which resists a relative rotation between the nut and the sleeve, until a torque which causes said rotation between the nut and the sleeve causes the pin to fail.

[034] The pin can be adapted to be inserted into a corresponding hole in said bolt, to resist a relative rotation between the sleeve and the bolt, wherein the pin fails when a torque which causes said relative rotation between the sleeve and the bolt is continued to be applied after the bolt is fully anchored.

[035] The bolt can be a rock bolt, for example a self drilling rock bolt.

[036] The external thread of the sleeve can have a thread pitch of about 1.5mm, and the internal thread of the sleeve can have a thread pitch of between 3mm to 20mm.

[037] The sleeve can have an outward radial extension on a trailing end of the sleeve, for example a circumferential flange.

[038] The present technology can be used with rock bolts or with self-drilling rock bolts but is not so limited.

[039] Persons skilled in the art would appreciate that different embodiments of the

invention could be used with bolts, rock bolts, self-drilling rock bolts, or any other bar or bolt that needs to be tensioned or be pre-tensioned.

[040] 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 element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[041] Where the specification refers to a "bolt" or to a "rock bolt" or to a "self-drilling rock bolt" or to a "hollow rock bolt" or to a "hollow self-drilling rock bolt" it is to be understood that the invention includes all such variations and modifications of the above including any long elongate used for bolting.

[042] Where the specification refers to a "bar" or to a "deformed bar" or to a "steel bar" or to a "main bar" or to a "support bar" or to a "main tensile support bar" it is to be understood that the invention includes all such variations and modifications of a bar including both solid and hollow bars, and bars made from any suitable material, and bars which are used as the main tensile support bar in a rock bolt.

[043] Where the specification refers to a "nut" it is to be understood that the invention

includes all such variations and modifications of the above, including any device that can be screwed along a thread.

[044] Where the specification refers to a "sleeve" or to a "threaded sleeve" it is to be

understood that the invention includes all such variations and modifications of a sleeve including any cylindrically shaped member with an external male thread and with an internal female thread.

[045] Where the specification refers to a "nut and sleeve assembly" or to a "mechanical nut and sleeve assembly" it is to be understood that the invention includes all such variations and modifications of a nut with an internal threaded sleeve that can be screwed on to a bolt or onto a rock bolt.

[046] Where the specification refers to a "nut tensioning system" or to a "mechanical two stage nut system" it is to be understood that the invention refers to any mechanical assembly of a nut with an internal sleeve that are assembled onto a bolt or onto a rock bolt .

[047] Where the specification refers to a "mechanical anchor" it is to be understood that the invention refers to any mechanical anchor that can be used to anchor a bolt in a hole.

[048] Where the specification refers to a "shear pin" it is to be understood that the

invention refers to any device or component that can be fitted to a nut, or to a sleeve, or to a nut and sleeve assembly, to initially prevent the nut, or the sleeve, or the nut and sleeve assembly, from rotating about a thread, but at a pre-determined torque value the shear pin will fail enabling the nut, or the sleeve, or the nut and sleeve assembly, to advance along the thread.

Brief Description of the Drawings

[049] In order that the present invention may be more clearly understood, preferred

embodiments will be described with reference to the following drawings and examples.

[050] Figure 1 is a schematic isometric view of the nut tensioning system of a first

embodiment, assembled on a rock bolt. [051] Figure 2 is a schematic isometric exploded view of the nut tensioning system assembled on a rock bolt.

[052] Figure 3 is a schematic sectional view of the nut assembly.

[053] Figure 4 is a schematic side view of the nut shown in Figure 3.

[054] Figure 5 is a schematic isometric view of the trailing end of the nut shown in Figure 4.

[055] Figure 6 is a schematic side view of the sleeve shown in Figure 3.

[056] Figure 7 is a schematic isometric view of the trailing end of the sleeve shown in Figure 6.

[057] Figures 8 and 9 are, respectively, schematic side views and isometric views of the nut and sleeve when assembled.

[058] Figure 10 shows a sectional view of a bolt with the nut assembly screwed on to the bolt, and where a shear pin prevents rotation of the nut and sleeve assembly along the bolt.

[059] Figure 11 shows a sectional view of the bolt and nut assembly of Figure 10, where the shear pin between the sleeve and bolt has failed and the nut assembly has advanced along the bolt thread.

[060] Figure 12 shows a sectional view of the bolt and nut assembly of Figure 10 where the shear pin between the sleeve and bolt has failed and the shear pin between the sleeve and the nut has also failed, and the nut has advanced along the sleeve thread.

[061] Figure 13 shows a section view of a nut and bolt assembly according to a second embodiment, assembled on a rock bolt.

Detailed Description of the Preferred Embodiments

[062] Herein described is a nut tensioning system for bolts, rock bolts and self-drilling rock bolts that utilises a mechanical assembly on the trailing end of the bolt, whereby the mechanical assembly comprises an inner threaded sleeve which has an internal female thread that is matched to the male thread on a rock bolt such that the sleeve can screw onto the rock bolt. The inner threaded sleeve also has an external male thread around the outside of the sleeve. The thread pitch of this external male thread is finer than the thread pitch of the internal female thread inside the sleeve. Typically the thread pitch of this external male thread on the outside of the sleeve is 1 5mm but is not restricted to this thread pitch. [063] The mechanical assembly also comprises a nut which has an internal female thread that matches the external male thread on the sleeve, such that the sleeve can be screwed into the nut. The internal female thread in the nut has the same very fine thread pitch as the male thread on the outside of the sleeve. The internal female thread in the nut does not need to extend all the way along the internal hole in the nut, and the nut has an end bearing face at the leading end of the nut.

[064] The mechanical assembly also comprises one or more shear pins which are inserted into holes drilled through the nut, and through the steel sleeve and into the bolt.

[065] In operation, the inner sleeve is screwed into the nut from the trailing end of the nut until the sleeve meets the end bearing face at the front of the nut. The nut and sleeve assembly can then be screwed onto the thread on a rock bolt with the female thread in the sleeve matching the male thread on the bolt. Once the nut and sleeve assembly has been screwed on to the bolt, then one or more holes are drilled through the nut and sleeve assembly and into the bolt, such that one or more shear pins can be inserted into those drilled holes. The shear pin or pins then secure the nut and sleeve assembly on to the rock bolt to lock the nut, sleeve and bolt against relative rotation.

[066] The nut can then be used to rotate a rock bolt up to a torque value equivalent to the shear strength of the shear pin or pins.

[067] Typically, for resin anchored rock bolts, the nut and sleeve assembly can then be used to rotate a rock bolt to break and mix a resin cartridge to anchor the bolt inside the borehole. Once the bolt is anchored inside the borehole, further rotation of the nut and sleeve assembly will cause the shear pin or pins to fail at a location between the sleeve and the bolt, and allow the nut and sleeve assembly to advance along the bolt thread until the leading face of the nut encounters resistance to further axial movement along the bolt from the domed ball washer pressing against a bearing plate which is in turn pressing against the rock face to be supported. This resistance to further axial movement of the nut along the bolt will cause the torque required to rotate the nut to increase. Eventually the torque applied to the nut will exceed the shear strength of the shear pin between the nut and the sleeve. The full torque of the drilling machine can then be used to rotate the nut about the sleeve. Since the thread between the nut and the sleeve has a much finer thread pitch than the thread between the sleeve and the bolt, then the nut will advance further along the thread on the sleeve, until the full rotational torque of the drilling machine is applied to the nut and the nut cannot be rotated any further. The maximum drilling machine torque is therefore applied to the nut about a thread with a fine thread pitch, so that the tension generated in the bolt is maximised.

[068] Alternatively, for mechanically anchored rock bolts, and for mechanically anchored self-drilling rock bolts, the nut and sleeve assembly can be used to activate a mechanical anchor on the leading end of the rock bolt to anchor the bolt in the borehole. Once the bolt is inside the borehole, the nut and sleeve assembly are used to rotate the bolt, which causes the mechanical anchor on the leading end of the bolt to expand and anchor the bolt in the borehole. Further rotation of the nut and sleeve assembly then causes the shear pin or pins to fail and allow the nut and sleeve assembly to advance along the bolt thread until the leading face of the nut encounters resistance to further axial movement from the domed ball washer pressing against a bearing plate which is in turn pressing against the rock face to be supported. This resistance to further axial movement of the nut along the bolt will cause the torque required to rotate the nut to increase. Eventually the torque applied to the nut will exceed the shear strength of the shear pin between the nut and the sleeve. As this shear pin fails, the full torque of the drilling machine can then be used to rotate the nut about the sleeve. Since the thread between the nut and the sleeve has a much finer thread pitch than the thread between the sleeve and the bolt, then the nut will advance further along the thread on the sleeve, until the full rotational torque of the drilling machine is applied to the nut and the nut cannot be rotated any further. The maximum drilling machine torque is therefore applied to the nut about a thread with a very fine thread pitch so that the tension generated in the bolt is maximised.

[069] Furthermore, the new nut tensioning system can be used with a wide range of bolts, rock bolts and self-drilling rock bolts, and can also be used with a wide range of threads on those bolts. The female thread in the inner sleeve is matched to the male thread on the bolt, and therefore the nut tensioning system can be used with any threaded rock bolt.

[070] Typically the thread pitch of rock bolt threads is between 3mm and 20mm. However, the thread pitch of the thread between the nut and the sleeve is finer, and is typically 1.5mm. This finer pitch thread could easily be damaged if used on a rock bolt.

However, by having this very fine pitched thread inside the nut and sleeve assembly the thread is protected from damage.

[071] For consistency, in the Figures, item numbers refer to the same feature or design component. [072] One embodiment of the nut assembly for tensioning of rock bolts, including self- drilling rock bolts, is shown in Figures 1 to 12.

[073] Figures 1 and 2 show a mechanical tensioning system 1 for a rock bolt that includes a nut 2 with an inner threaded sleeve 3, which are assembled on the rock bolt 4. A shear pin 5 is fitted into an aligned hole in the nut 1 and in the sleeve 3 and in the bolt 4, such that the shear pin 5 goes all the way through the side of the nut 2 and through the sleeve 3 and into the bolt 4. The external drive surfaces of the nut 2 are shown as being hexagonal but it can be any suitable shape.

[074] As best seen in Figures 6 and 7, the sleeve 3 has an external male thread 7

provided on its external surface 17 and an internal female thread 8 provided on its internal surface 18. The trailing end of the sleeve 3 has drive slots 9 which enable the sleeve 3 to be screwed into the nut 2.

[075] Figure 3 shows a sectional view of the nut 2 with the sleeve 3 screwed into it.

[076] The internal female thread in the nut typically does not extend all the way along the central hole in the nut so that a bearing face approximately 5mm to 10mm thick, as measured in the longitudinal direction along the nut, can be formed at the leading end of the nut. This leading end of the nut 2 can have an extension which extends radially inwardly.

[077] In the embodiment shown in Figure 3 the radial extension is a circumferential flange

10. The circumferential flange 10 serves as a bearing face against a domed washer or bearing plate (not shown) on the rock bolt which bears against the rock face, and further limits the advance of the sleeve 3 in relation to the nut 2. However the extension 10 need not be circumferential. A washer (not shown) can be further provided adjacent the leading end of the nut 2 to provide a bearing surface against the rock face, instead of or in addition to the extension 10.

[078] The internal female thread 6 in the nut 2 does not extend all the way through the nut 2. The internal female thread 6 in the nut 2 has a fine thread pitch, for example a thread pitch of about 1.5mm. The sleeve 3 has an external male thread 7 which has a matching fine thread pitch, such that the sleeve 3 can be screwed into the nut 2 along thread 6, allowing for a relative axial movement between the nut 2 and the sleeve 3. The sleeve 3 is shown as being fully contained within the nut 2, which is desirable but not essential. The sleeve 3 also has an internal female thread 8 which can be cooperate with the external threads on the rock bolt, to allow a relative axial movement between the sleeve 3 and the bolt 4. [079] Figure 4 shows a side view of the nut 2 shown in Figure 3. The nut 2 has an internal female thread 6 which has a very fine thread pitch. A hole 11 is drilled through the side of the nut to accommodate a shear pin 5.

[080] Figure 5 shows an isometric view of the trailing end of the nut 2 with an internal female thread 6.

[081] Figure 6 shows a side view of the sleeve 3 which has an external male thread 7 which also has a fine thread pitch such that the sleeve 3 can screw into the female thread 6 in the nut 2. The internal thread 8 in the sleeve 3 has a relatively coarse pitch. Figure 6 also shows a hole 12 through the sleeve to accommodate a shear pin 5, and a drive slot 9 to enable the sleeve 3 to be screwed into .the nut 2. There can be multiple sets of the aligned holes on the nut, sleeve, and bolt, to each

accommodate a shear pin.

[082] Figure 7 shows an isometric view of the trailing end of the sleeve 3 which has drive slots 9 that enable the sleeve 3 to be screwed into the nut 2. Figure 7 also shows the internal thread 8 and the external thread 7 as well as the hole 12 for the shear pin 5.

[083] Figures 8 and 9 show the nut and sleeve when assembled.

[084] Figures 10 to 12 show successive stages of tightening of the nut assembly on a rock bolt.

[085] The arrangement enables the rock bolt to be rotated to mix a chemical resin

cartridge or to activate a mechanical anchor device on the leading end of the rock . bolt, such that the leading end of a rock bolt can be anchored in a borehole. Once the leading end of the rock bolt is anchored in the borehole, the invention enables the nut assembly to advance along the bolt thread and tighten up against a bearing plate supporting a rock face until a pre-set torque value is exceeded. Once the preset torque value is exceeded, a shear pin between the nut and the internal sleeve fails and enables the nut to advance along the fine pitched thread on the outside of the sleeve. The nut advances along the fine pitched thread on the outside of the sleeve until the maximum torque of the drilling machine is reached, at which point the nut cannot be advanced any further along the sleeve thread. By rotating the nut along a very fine pitched thread using the maximum torque of the drilling machine, ensures that maximum tension is generated in the rock bolt.

[086] Figure 10 shows a sectional view of the bolt 4 with the nut 2 and the sleeve 3

assembled onto the bolt 4 and with a shear pin 5 securing the nut 2 and sleeve 3 to the bolt 4. Figure 8 also shows that the shear pin 5 is inserted into a through hole 11 in the nut 2 and in a corresponding and aligned through hole 12 in the sleeve 3, and further into a corresponding aligned hole 13 in the bolt 4. Figure 10 also shows that the internal thread 8 in the sleeve 3 is threadably engaged with the thread on the bolt 4, and that the internal thread 6 in the nut 2 is threadably engaged with the external thread on the sleeve 7. In this arrangement a torque can be applied to the nut 2, to rotate the nut 2 and hence the bolt 4 for drilling, or to rotate and mix a resin cartridge (not shown), or to expand and fix a mechanical anchor (not shown) at the leading end of the rock bolt 4.

[087] A torque which can be applied to the nut 2 tends to cause a relative rotation between the sleeve 3 and the bolt 4, but this relative rotation is resisted by the shear pin 5. When bolt 4 is fully anchored, further application of the torque eventually causes the pin 5 to fail by shearing the pin at a location between the sleeve 3 and the bolt 4. The sleeve 3 and the nut 2 are then permitted to rotate relative to the bolt 4.

[088] Figure 11 shows a sectional view of the bolt 4 where the nut 2 and sleeve 3 have advanced together along the thread on the bolt 4, by shearing and failing the shear pin at position 15. In this arrangement the bolt 4 has been anchored in the borehole either by a mechanical anchor or by resin hardening (not shown), and cannot be rotated by the nut 2. Consequently further rotation of the nut 2 by the drilling machine (not shown), causes the shear pin 5 to fail at position 15 and the nut 2 and sleeve 3 can advance along the bolt 4 thread and begin to tighten up against a domed washer and against a bearing plate (not shown).

[089] Figure 12 shows a sectional view of the bolt 4 where the nut 2 has advanced with respect to the sleeve 3. In this arrangement the nut 2 has rotated the sleeve 3 along the bolt 4 thread as far as possible before the torque applied to the nut 2 exceeds the shearing strength of the pin 5, causing the shear pin 5 to shear and fail at position 14 between the sleeve 3 and the nut 2. However, the torque can continued to be applied, so that the nut 2 can be further advanced along the fine pitched threads 6 and 7 and further tighten up against a domed washer and against a bearing plate (not shown), to generate a high tensile load to the bolt 4.

[090] Figure 13 illustrates a nut assembly incorporating a different embodiment of the sleeve. As shown, the sleeve 3 can include an outward radial extension 20 located adjacent the trailing end 19 of the sleeve 3. The outward radial extension 20 of the sleeve 3 is adapted to abut the trailing end of the nut 2. The outward radial extension 20 can be a ring provided around the sleeve 3, but this is not required. [091] This arrangement counters a tendency for the sleeve 3 to fracture in the region of the drive slots 9 (shown in Figures 1 and 2) under extreme loads. For example, this tendency can occur when the nut 2 has been screwed completely or almost completely along the length of the sleeve 3, and then subjected to a large load, e.g. about 27 to 30 tonnes. The load on the nut 2, and in turn the sleeve 3, can cause the sleeve 3 to expand and fracture at a weakened location in the sleeve 3, e.g. the drive slots 9. The outward extension 20 of the sleeve 3 therefore helps reinforce the sleeve 3 against such expanding and fracturing.

[092] In preferred embodiments, the nut tensioning system is made from steel and the nut and the sleeve are designed to fit onto rock bolts. The internal thread in the sleeve is designed to suit the existing threads on rock bolts, as well as giving engineers the option of using coarse pitched rope type threads on all rock bolts.

[093] In preferred embodiments, the failure or. break out torque of the shear pins can be adjusted to suit the specific application. A single shear pin could be used, or multiple shear pins could be used depending on the torque required to fail the shear pins. One shear pin could be used between the nut and the sleeve, and another shear pin could be used between the sleeve and the bolt.

[094] In preferred embodiments, a low friction washer (not shown) is used in front of the nut to minimise friction between the nut and the domed ball washer (also not shown) to maximise the tension developed in the bolt.

[095] In preferred embodiments, the nut and the sleeve will remain together until the nut and sleeve assembly have advanced along all of the bolt thread free length and some compression, approximately 1-2 tonnes, of the domed ball washer and bearing plate (not shown) has occurred. Failure of the shear pin between the nut and the sleeve will only occur after this point, such that further axial movement of the nut will be limited to typically less than 20mm.

[096] In preferred embodiments, the present invention is used with rock bolts and self- drilling rock bolts but is not so limited and could be used with any bolts.

[097] In preferred embodiments, the present invention could be used with either right hand threads or left hand threads.

[098] It should be noted that the present invention enables a high tensile load to be

applied to rock bolts between the nut and the anchor point. The rock bolt may be left like this, or the rock bolt may then be fully encapsulated with cement grout or resin. In either case, the rock bolt is considered to be pre-tensioned because an active tensile force has been created which is supporting the rock face. The tensile load generated by the invention will vary depending on specific conditions but is normally between 5 tonnes and 10 tonnes.

[099] The presently described technology thus enables a nut tensioning system to be used on a bolt or rock bolt or self-drilling rock bolt using fine pitched threads, but where these threads are fully protected from possible damage.

[0100] More particularly, the invention not only enables the very fine threads on the internal sleeve and in the nut to be protected against damage from the rough handling common in the mining and tunnelling industries, but also enables any lubrication or low friction coating on these very fine pitched threads to fully protected, such that bolt tensioning is maximised.

[0101] In practice, the nut tensioning system enables a high pre-tensile load to be applied to rock bolts and self-drilling rock bolts by using the existing torque capacity of drilling machines. In particular, the present technology enables a greater pre-tensile load to be applied to a rock bolt than is possible using existing threads on rock bolts.

[0102] Moreover, the present technology enables cold-rolled rope type threads or hot-rolled buttress type threads, both which have a coarse thread pitch of typically 10mm or greater, and which are often easier to roll and form and generate less wear and tear on thread forming machinery, to be used on rock bolts and still have the tensioning benefits of using a very fine pitched thread of approximately 1.5mm.

[0103] Furthermore, the present technology enables coarse pitched threads which can resist damage due to aggressive handling, to be used on rock bolts and still have the advantages of a fine pitched thread.

[0104] The present technology provides the tensioning benefits of using a fine thread pitch nut of approximately 15mm without the risk of damage to that fine pitched thread because the fine pitched thread is inside the nut and sleeve assembly.

[0105] The present technology provides an internal fine pitched thread that can be coated with a suitable lubricant to maximise the pre-tensile force that can be generated, and that lubricant is also protected from being washed off or removed.

[0106] The present technology can be fitted to existing threaded rock bolts without a

change to installation machinery.

[0107] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described and as claimed. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.