AU2006100511B4 - Plastic nut, for use with rock bolts - Google Patents

Description

AUSTRALIA

Patents Act 1990 PHILLIP HANFORD BOOT COMPLETE SPECIFICATION INNOVATION PATENT Invention Title.

Plastic nut, for use with rock bolts The following statement is a full description of this invention including the best method of performing it known to us:- Cross-Reference to Related Applications The present application claims priority from Australian Provisional Patent Application No 2005903020 filed on 9 June 2005, the content of which is incorporated herein by reference.

Field of the Invention The present invention relates to a plastic nut particularly, but not exclusively, for use with rock bolts or dowel bolts in the reinforcement of earth strata such as in underground mining, tunnelling in coal mines and also in general strata reinforcement in earth embankments. This invention is particularly concerned with fibre reinforced plastic nuts for use with fibre reinforced plastic bolts or dowels and a system to protect the plastic bolts or dowels from damage during installation.

Background of the Invention The present invention is directed to the anchoring of fibre reinforced composite (FRP) rods or rock bolts used to reinforce earth and rock strata in mining, tunnelling or general earth embankment stabilisation operations. Such a device can be used in tensioning fibre reinforced composite rods in any suitable application.

Fibre reinforced composite materials are becoming more popular in replacing steel in specialised uses, particularly, although not limited to, the reinforcement of rock or earth strata where the reinforcement or support of that strata is of a temporary nature.

One use of fibre reinforced rods is in the coal mining industry where underground roadways or tunnels are excavated to facilitate the main mining operation.

The tunnels have to be reinforced for safety reasons, and traditionally this is done with steel rods called rock-bolts or rock-dowels.

A typical rock-bolt or dowel used in coal mines is usually a rod of 20mm 22mm diameter and varying length from 900mm 1800mm, which is inserted in a predrilled hole of approximately 28mm diameter and encapsulated in a binding cementicious material, usually a resin material.

In many cases the rod has a threaded end that projects out of the hole where a washer and nut are attached to the rod, and after encapsulation, the nut is tightened down to exert a pressure on the strata surface.

Due to the much lower shear strength of composite materials it is common for these FRP reinforcing rods to be used in strata where a later excavation is required to mine or excavate the material that the bolts are supporting.

When a steel rock bolt is used to reinforce this part of the strata, expensive damage can occur to the excavating equipment used in the later excavation and to equipment, such as conveyor belts, used in conveying the excavated material. For example, if a steel rock bolt damages a conveyor significant repair costs may be incurred in replacing the conveyor belt which may be several km long and in "down time" when the mine is unable to be operated, or operates at reduced capacity.

Fibre reinforced composites (FRP) rock bolts have become popular because of the lower shear strength characteristics of those materials and because they do not damage the excavating or conveying equipment.

FRP rock bolts are also used in other applications such a "soil nailing" where the bolts when used for temporary purposes can be easily broken or cut up and removed at a later date when or if required.

In coal mining the use of fibre reinforced dowels or bolts is limited to the mining or "later to be excavated" side of the access tunnel or what is commonly called the "cuttable" side, steel dowels are usually used in the other non "cuttable" side and the roof of the access tunnel.

The sequence of dowel installation is as follows. First a hole is drilled in the strata to the required depth using a drill bit. Next, the drill bit is removed and a socket spanner placed in the drill chuck. A two part resin binding agent contained in a flexible capsule of varying length is then inserted into the hole. The capsule keeps the two components separate. Then the dowel, including a plate and a nut partially screwed onto the threaded end, is partly inserted into the hole.

The nut is then engaged by the drill chuck and spun vigorously whilst being pushed further into the hole, thus mixing the two resins together. The nut has a cap which prevents it from being screwed further down the dowel thread during the spinning operation, the dowel is then held motionless for a number of seconds whilst the resin solidifies.

When the resin is hardened the nut is turned further down on the now rigidly held dowel which breaks out the cap at a pre-determined torque value and allows the nut to be tightened, creating force on the washer plate and strata surface until the desired torque value is obtained. This value is determined by the skill of the drill rig operator.

The torque value is typically accomplished by guesswork, which can be quite difficult as working conditions are usually tricky. Mines tend to be poorly lit and the equipment is robust and very strong. Typically the operator cannot see if he has damaged the dowel by over tightening of the nut, nor can he tell if the encapsulation is adequate or successful.

In many cases, the machines that excavate the access tunnel also install the reinforcement dowels at the same time. These machines have drilling rigs positioned on the back of the machine and the sides and the roof of the tunnel are reinforced with dowels as the tunnel forming machine advances.

The drilling rigs are operated hydraulically and basically designed to install steel dowels. However, when installing the fibre reinforced dowels on the "cuttable" side, a problem arises due to the high torque performance of the drill rig required for the steel dowels and the low torque values of the FRP dowels.

The strength of the installation drill rig and the significant difference in shear and torque values between steel and FRP dowels, results in the FRP dowel being easily damaged unless the operator is experienced, skilled and very careful. In extreme cases the head of the dowel is twisted off.

Some mines have "automatic bolters" mounted on the tunnelling machine, which cannot be used effectively because of the difference in torque values between steel and FRP dowels.

To overcome this problem some FRP dowel manufacturers have developed what is known as a "thrust" dowel. This type of dowel has an enlarged nut shaped head but has no thread to exert force onto the strata surface. The installation drill rig simply pushes the head of the dowel hard into the hole until the encapsulating binder solidifies.

To overcome this problem some FRP dowel manufacturers have developed what is known as a "thrust" dowel. This dowel has an enlarged nut shaped head but has no thread to exert force onto the strata surface. The installation drill rig simply pushes the head of the dowel hard into the hole until the encapsulating binder solidifies.

However, when using a thrust dowel the operator cannot tell if there is sufficient load onto the strata or more importantly if the encapsulation has worked satisfactorily, which is critical for the safety of mining personnel. Hence there are safety issues with the use of thrust dowels.

Although the thrust dowel overcomes the damage to the FRP bolt problem, the performance of this dowel is significantly inferior to the threaded dowel in that the force applied to the strata surface is less than one third to that of the threaded dowel.

One aim of the present invention is to provide a fibre reinforced rock-dowel product in the form of a nut, that when installed to the required torque value, can allow the rock-dowel to be installed by the same machines designed to install steel rockdowels preferably without damage to the FRP dowel and without special skills or significant additional operator care.

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 before the priority date of each claim of this application.

Summary of the Invention In accordance with a first broad aspect of the present invention there is provided a nut made from a plastics material for use with a rock-dowel or the like, the nut having a central bore and defining a first, typically relatively wider, barrel portion having an internally threaded bore and a second, typically relatively narrower, nut portion also defining a central bore the second portion defining a nut section for engagement with a socket, wherein an area of weakness is provided between the first and second portions such that the second portion breaks from the first portion when a particular torque value is applied to the nut and wherein a frangible cap separates the internally threaded bore of the barrel portion from the bore of the nut section.

In use, the torque value is set to be less than the breaking torque of the rockdowel so that the nut portion shears off during installation in preference to the dowel, thus preventing damage to the dowel.

The nut is typically made from a fibre reinforced plastics material and moulded in one operation.

The invention also provides a combination of a nut embodying the first aspect of the present invention and a rock-dowel wherein the torque value is set to be less than the breaking torque of the rock-dowel.

The first portion of the nut which is the main structural barrel that exerts pressure is preferably and predominately a conical in shape with the rear nut section being shaped to fit a hexagonal drill nut socket attached to the installation rig.

It is preferred that at least some of the flats of the nut portion are weakened by defining recesses or through apertures or other areas of reduced thickness such that in use, when the required torque value is reached and the nut portion held in the drill chuck socket shears off, the nut also preferably disintegrates leaving the structural barrel portion only of the nut in place.

It is preferred that the barrel portion of the nut also has flats formed on it that can facilitate variations to the torque value by nut socket means at a later date, if required.

The flats formed on the front barrel shaped section are either larger in diameter or so configured that the socket used to torque up the hexagon shaped nut section cannot engage it during the installation operation.

A circlip ring could also be used to prevent the installation rig chuck socket from engaging any part other than the hexagon end section which is designed to shear off.

The nut is preferably manufactured in one operation by injection moulding means from a glass reinforced thermoplastic material. The nut will have an internal thread formed inside it to match the thread section of the rock-dowel.

The nut may also have a "break out" cap at the hexagon shaped end, this cap is designed to break out during the installation phase after the encapsulation binder solidifies. During the injection moulding process weaknesses in the form of recesses are formed into the nut at the junction of the barrel and the hexagon sections in what is called a "core pulling slide" operation.

The core pulling operation operates at least two slides transverse to the length of the nut and must be retracted as these parts of the nut moulding die must retract before further de-moulding of the nut is undertaken. The dimensions of these weakened region and apertures is variable and controls the shear value of the parting of the two nut sections.

The desired torque value of the shear of the two nut sections can vary with the ultimate torque value of the dowel, for example, for a 20mm diameter fibreglass dowel the shear value is approximately 55 ftlbs 100 ftlbs. A 22mm diameter dowel has a higher ultimate torque value and would range between 75 ftlbs 120 ftlbs.

The torque value for the nut can also vary with the type of resin used in the dowel and also the design and quantity and quality of reinforcement used to manufacture the dowel. This means that the nut shear torque value can range from ftlbs 150 ftlbs.

At the lower torque shear values it was found that a simple groove formed between the two nut sections that determines the shear value was reduced to a 0.6 1 mm wall thickness.

When using a glass fibre reinforced thermoplastic to injection mould the nut, the glass fibre length is around 400 microns or .4mm at a fibre content level of between 50%. The speed and high injection pressure used in the process to mould the nut causes the fibre contained in the now liquid resin to shear and break up when forced through these small narrow section thicknesses. This causes the fibre content and reinforcing performance to become unevenly distributed and result in an unreliable shear value for the separation of the two nut sections.

To overcome this problem and make the shear value consistent, apertures may be moulded into the weakened portion between the nut and barrel sections so that a series of columns of more even sections were formed, these column type sections snap or shear off separating the nut and barrel sections reliably at the desired shear value as the glass fibres contained in the plastic is evenly distributed The structural action of concentrating stress in designated areas of the separation region causes the separation process to be quite different.

When the rear hexagon shaped section is sheared off, the thread inside that section must break up and not act as a lock nut, preferably this part disintegrates into small pieces and falls from the drill chuck socket. In some cases to facilitate the disintegration process the hexagon section around this part of the thread is reduced in strength during the moulding operation.

It is not essential that the rear hexagon shaped part of the nut has an internal thread as long as the internal diameter is larger than the diameter of the threaded section, however it may be important that the rear hexagon section of the nut does in fact shatter and disintegrate. The reason for this is so that it does not get caught in the drill chuck socket necessitating the installation operator to clear the socket out.

The disintegration of the hexagon section is caused by different recesses formed into the flat sections of the hexagon sections, these recesses must not affect the shear value of the nut separation but may in some cases be formed integrally with the separation recesses during the moulding operation Throughout this specification 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.

Description of the Drawings Fig 1 is an elevation of a threaded plastic rock dowel with mixing vanes, a washer plate and a nut, showing sections of the dowel enlarged; Fig 2 is a section showing a typical rock-dowel, nut and plate assembled in a pre-drilled hole;

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Fig 3 is a partial section through the nut of Figure 2 with weaknesses (apertures) moulded into the junction of the barrel and hexagon sections and also recess into the flat hexagon area; Fig 4 is a section through the nut of Figure 2 showing weakened region recesses and apertures; and Figure 5 is a plan view of the hexagon end of the nut showing the weakening recessed region at the juncture of the barrel and hexagon ends and the moulding slides to form the recesses; Detailed Description of the Invention In Figures 1 and 2 is shown a GRP rock-dowel 2. The rock-dowel 2 has a dowel shank 2a having a diameter of about 20-22mm, defining a longitudinal axis and a threaded end 2b at the head or proximal end of the dowel. Three groups 2d, 2e and 2f of projecting mixing vanes are spaced along the shank 2a. Also shown in Figure 1 is a washer plate 3 and a nut 13.

Figure 3 shows the cooperating nut 13 in more detail. It includes a circular generally annular barrel section 120 having a truncated hemi-spherical end portion 121 connected to a co-axial hollow hexagonal section 122 by a recessed annular weakened portion 123. The hollow hexagonal section 122 may or may not be internally threaded.

The barrel section 120 has an internal thread 124. A breakout cap 126 that allows the dowel 2 to be spun during the spinning and mixing stage separates the interior of the barrel section 120 from the hexagonal section 122. This cap 126 will break out at a set torque level significantly less than the torque level required to shear the weakened portion 123 and separate the hexagon 122 and barrel 120. Also shown are apertures 128 in the weakened portion 123 and column sections 130 (refer to Figure 4) which combine to allow the hexagon section 122 to separate from the barrel section 120 at the required torque value.

The position of the breakout cap 126 as shown in Figs 3 and 4 is close to the junction 123 of the hexagon section 122 and the barrel section 120. This is preferable in that when these two sections part and the nut 13 is in the final position, the amount of projection of the threaded end 2b of the dowel will be as small as possible.

The lesser projection of the threaded portion 2b is particularly significant when the installation operator has used what is termed a "thrust" installation. In such an operation, immediately on completion of the dowel spinning stage, the dowel 2 is thrust hard against the washer plate 3 and strata surface 5 and held whilst the resin in the gap 4a is hardening. The nut 13 when finally tightened will only advance several threads and not project any further than approximately 25 mm out from the end of the barrel section 120 or project no more than the original position of the hexagon section 122 before it separates from the barrel 120.

The position of the cap 126 in the nut 13 may also be critical for other reasons.

The size and configuration of the hexagonal section 122 is dictated in coal tunnels by the size of the steel nut used in the other non cuttable areas as the operators refuse to constantly keep changing the drill chuck socket to suit plastic nuts. Traditionally with both steel and plastic nuts the position of the cap has been at the very end of the hexagonal section 112 or at the top of the nut 13. The hexagonal size restriction has created a problem for plastic nuts in that to obtain a high cap breakout value the whole or part of the top of the hexagon breaks away with the cap yielding enormous variations and unreliable cap breakout values and additionally jamming in the socket. By placing the cap in between the hexagonal nut 122 and barrel section 120 as shown in Figs 3 and 4 the cap can be thickened for higher breakout values without affecting the hexagonal section 122.

Figure 4 shows another view of the nut 13 showing recesses 132 (Jeremy 130 is the column sections) moulded into some of the flats of the hexagon section 122 to facilitate the disintegration or collapsing of the hexagon section 122 after it separates from the barrel section 120. Disintegration of the hexagon section 122 is not essential and in most cases the recesses 132 may be omitted as long as the sheared off nut section 122 does not become jammed in the socket Figure 5 shows a plan view of the barrel section 120, the apertures 128 and the columns 130. Also shown is the method of forming the apertures 128 and the columns 130 by horizontal slides 8 with projections 7 that form the apertures or holes. The apertures 128 do not have to actually penetrate the full section but preferably do penetrate enough to significantly weaken the section so it cannot add significantly to the torque value. The columns 130 may take any convenient configuration but should be approximately equal in shear strength for reliability.

More specifically, the nut is manufactured in one operation by injection moulding means from a glass reinforced thermoplastic material. The internal thread 124 formed inside the nut match the thread section 2b of the rock-dowel.

During the injection moulding process the apertures 128 and weakened region 123 are formed into the nut at the junction of the barrel and the hexagon sections in what is called a "core pulling slide" operation. The Transverse moving slides 8 are shown to demonstrate the moulding action of the nut mould die (not shown) and how the apertures 128 and weakened section 123 are formed.

The core pulling operation operates at least two slides 8 transverse to the length of the nut and must be retracted as these parts of the nut moulding die must retract before further de-moulding of the nut is undertaken. The dimensions of these recesses 123 and 128 are variable and controls the shear value of the parting of the two nut sections.

Manufacturing the nut in a single operation has significant cost advantages over a two or more step operation.

The size of the apertures 128 and weakened region 123 will vary with the required torque value required which will vary with the diameter of the rock-dowel 2 and the ultimate strength of the dowel shank 2a.

A significant feature and advantage of this reinforcing system is that if the encapsulation is faulty or insufficient the hexagon and barrel section will not separate.

Instead the whole head of the dowel will shear off as the torque value of the dowel will be exceeded before the torque value of the separating barrel 120 and hexagon 122 sections is reached. The operators are thus instantly alerted to the dowel head and shank failure and must install another replacement dowel with a longer resin capsule to complete the reinforcement.

In this way the reinforcing system is fail safe in that operators no longer have to be too careful and tentative when tightening the nut and are confident that the dowel 2 is undamaged and a minimum force of approximately 2 tonnes 4 tonnes load is applied to the strata 5 surface. This system speeds up the installation operation and removes the remedial repair work required when the reinforcement system is not installed correctly and the strata 5 begins to collapses In use, firstly a hole 4 is drilled in the strata 5 to the required depth, the drill bit is then removed from the drill chuck and replaced by a socket spanner. A two part resin binding agent contained in a flexible capsule of varying length is inserted into the hole 4. The capsule keeps the two components separate. Then the dowel 2, including a plate 3 and a nut 13 partially screwed onto the threaded end is partly inserted into the hole.

The nut is engaged by the drill chuck socket and spun vigorously whilst being pushed further into the hole, thus breaking the capsule and mixing the two resins together. The breakout cap 126 prevents the nut 13 from being screwed further down the dowel thread during the spinning operation, the dowel is then held motionless for a number of seconds whilst the, now mixed, resin solidifies.

When the resin is hardened the nut 13 is turned further down on the now rigidly held encapsulated dowel which breaks out the cap at a pre-determined torque value and allows the nut to be tightened, creating force on the washer plate and strata surface. In Figure 2,the nut 1 is shown in the second stage of installation after the nut cap (not shown) has been broken out and the dowel 2 is locked in by the encapsulation and hardened resin 4a.

The next stage of installation is the tightening of the nut 1 down onto the plate 3 and strata surface 5. The apertures 128 and weakened section 123 allow the hexagon 122 to shear off at a prescribed torque value, which may vary depending on the conditions. Sometimes there can be a steel or plastic mesh between the strata surface and the washer, this is to hold back coal from falling out from areas between the spacings of the dowel installation.

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. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims (4)

1. A nut made from a plastics material for use with a rock bolt, rock-dowel or the like, the nut having a central bore and defining a first, typically relatively wider, barrel portion having an internally threaded bore and a second, typically relatively narrower, nut portion also defining a central bore, the second portion defining a nut section for engagement with a socket, wherein an area of weakness is provided between the first and second portions such that the second portion breaks from the first portion when a particular torque value is applied to the nut, and wherein a frangible cap separates the internally threaded bore of the barrel portion from the bore of the nut section.

2. A nut as claimed in claim 1 wherein the nut consists solely of a fibre reinforced plastics material.

3. A nut as claimed in any preceding claim wherein the thickness of the frangible cap ranges from 2.5mm to 6mm.

4. A nut as claimed in any preceding claim wherein the torque value required to shear the first and second portions ranges between 55 ftlbs 150 ftlbs A nut as claimed in any preceding claim wherein the nut is manufactured in one operation by injection moulding means from a glass reinforced thermoplastic material and wherein, during moulding, weaknesses, in the form of recesses, are formed into the nut at the junction of the barrel and nut section in a "core pulling slide" operation. Dated this ninth day of June 2006 Phillip Hanford Boot Patent Attorneys for the Applicant: F B RICE CO