AU2003257529A1 - Plaster washer plate for rockbolting, soil nailing and the like - Google Patents

Description

AUSTRALIA

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

Plaster washer plate for rockbolting, soil nailing and the like The following statement is a full description of this invention including the best method of performing it known to us:m:\speci\110000\116-117\116439cmpjmd.doc Field of the Invention The present invention relates to a plastic washer plate for use with rockbolts or dowelbolts in the reinforcement of earth strata such as in underground mining, tunnelling operations, earth or rock excavations and in general stabilisation of earth embankments. This invention is particularly concerned with plastic washer plates for use with fibre reinforced plastic bolts.

Background of the Invention In earth stabilisation, underground mining and tunnelling operations rockdowels and rockbolts are used to reinforce the roof and walls of the tunnels or the sides of the embankment. Washer plates are used in conjunction with the rock dowel or rockbolt to assist in the containment and stabilisation of the surface of the relevant strata.

Generally rockbolts and rockdowels are round in section ranging from 20mm diameter and in length from Im 12m, and are usually made of steel or fibreglass reinforced plastic (FRP). Some bolts have an expansion shell on one end and threaded end on the other so that when the bolt is inserted into the predrilled hole, activation of a nut on the thread causes the expansion shell to open and lock into the walls of the hole. The nut reacts against a large diameter washer which in turn imparts the pressure applied by the nut onto the surface around the entry point of the hole. In most cases the bolt is then encapsulated in the hole with a Portland cement mixture.

Some bolts are pre-tensioned to relatively high values prior to the encapsulation.

A rockdowel does not have an expansion shell and works in a different way in that encapsulation of the rockdowel in the predrilled hole by a suitable fast setting resin or cement is carried out before the activation of the nut. In this way there is no pretensioning but the nut is still activated and applies pressure through the washer to the surface around the entry point of the pre-drilled hole.

As it is not pre-tensioned, the dowel acts as a reinforcement member after the encapsulation process, therefore both rockbolts and rockdowels can use a similar washer plate made from steel or plastic.

The washer plate is usually positioned under the threaded head of the rockbolt and when nut is placed and tightened on the bolt the force is transmitted via the washer plate onto the surface of the material to be stabilised.

Generally washer plates can be rectangular or circular and vary from 100mm 500mm in size. The bolt is threaded through a centrally located parallel walled hole in the washer prior to insertion of the bolt into the pre-drilled hole.

When the washer plates are made of plastic, the hole that the bolt passes through is relatively small and has parallel sides, i.e. the hole is approximately the same diameter on both sides of the plate. The nut has a predominantly square face to bear directly onto the solid portion of the washer plate provided around the central hole so that the nut cannot split the washer plate. In the past, various conical shaped nuts have been unsuccessfully tried without success, the sharper angle of the conical head tends to split the washer plate due to the semi horizontal angled force that is applied.

In practice, more than half of the bolt holes drilled in tunnels and mines are not drilled at the optimal 90 degree angle to the surface of the tunnel wall/roof. This is mainly due to the type of equipment used. The result is that when the bolt carrying the washer is inserted into the predrilled hole, the washer plate which bears on the surface of the tunnel surrounding the hole, is not square or at 90 degrees to the bolt.

This angle is further adversely affected by any irregularities of the strata surface.

Angles deviating by up to 20 degrees from the normal 90 degree may be encountered.

These angles can apply severe bending stresses to the shank of the bolt between the encapsulation and the nut. This in turn limits the effective strength of the threaded exposed end of the bolt and the engaging nut.

Although these factors affect all rockbolt materials, fibre reinforced plastic bolts are most severely affected as they are relatively weak in shear and torsion when compared to steel.

Fibre reinforced plastic (FRP) bolts are more expensive than steel bolts and are typically only used in situations where they have distinct advantages over steel bolts such as in the development tunnels of long wall coal mining where the walls of these tunnels are subsequently re-excavated by large machines. Because of their low shear value the FRP bolts do not damage the mining equipment during the re-excavation of the coal. The FRP bolts used in this way are a sacrificial item.

Another important application is in tunnelling, mining and earth stabilisation situations where steel is too corrosive and heavy and FRP bolts can be safely used and will last indefinitely.

Even in some embankment stabilisation projects because of future extensions, contractors require cuttable bolts so that excavating equipment is not damaged at a later date.

In both applications, a FRP nut is also used, these are usually produced by an injection moulded process so the fibre length is quite small. Also the fibre content is limited by the manufacturing process and cannot easily be increased beyond 40% by weight.

Because of the very low shear values of FRP, the nut thread strength is low, usually less than half the tensile value of the bolt shank. For example, a 20mm diameter FRP bolt can have a shank tensile value of 15 tonnes and a nut thread tensile strength of only 6 to 8 tonnes. By increasing the fibre content in the bolt shank the shank strength can go as high as 30 tonnes but the nut thread strength will not increase beyond 8 tonnes.

Hence, there are two basic problems associated with FRP bolts which detrimentally affect the strength of the bolts and limit FRP bolts' performance in many applications. One problem is the angle misalignment and irregularities of the substrate material, and the other is the poor nut thread tensile strength.

As a steel bolt and steel nut can have a nut thread tensile strength as high as the shank tensile strength and a very high shear strength, FRP bolts experience great difficulty when competing with steel in many applications where cost is not the major issue.

Plastic is a weak material in shear and this makes the threads vulnerable to be sheared off at relatively low forces. This applies to both the thread in the nut and the thread on the rockbolt.

The angle and pitch of the threads significantly affect the shearing off force required, while the pitch affects the amount of thread that can be sheared off the angle of the thread affects the direction of the shear forces.

If the thread angle is shallow, then these shear forces are deflected into the nut at approximately 90 degrees to the thread angle, this is resisted by the hoop strength of the nut until it causes the nut to split.

If the nut say is made of steel then it has sufficient hoop strength to contain the forces and the failure mode will be markedly different and may be the shearing off of the threads on the bolt shank, or whichever becomes the weakest element, however if the nut is made from plastic then either the nut will split or the threads on either the nut or the bolt will shear off, at a relatively low force.

The threads in plastic products are for cost reasons moulded with the nut and with the bolt. Being moulded in plastic material, the threads will not mate as well as steel, this is due to differential material shrinkage and other tolerance issues.

The aim of the present invention is to address the problems with the use of FRP bolts addressed above and provide a means of improving their performance.

Summary of the Invention According to the present invention there is provided a washer plate made of a plastics material particularly for use with rock bolts and dowel bolts for the reinforcement of earth strata in underground mining, tunnelling operations, earth or rock excavation and stabilisation of embankments defining an aperture for receiving a rock bolt and an annular seat for receiving a nut or head of the bolt, characterised in that one face of the washer plate defines a high strength annular seat insert which defines a hole for receiving the shank of a bolt In one preferred embodiment a portion of the washer plate on the opposite face of the washer of the seat is cut away or chamfered. By using a nut with a typically part-spherical boss shaped to swivel in the annular seat the cut away underneath the ring avoids the problem of misalignment of the pre-drilled holes. However, the cut away portion severely reduces the performance of the washer plate and the nut could easily be forced through the now relatively larger and weaker hole were it not for the high strength annular seat insert which overcomes this problem. By altering the sectional shape of the seat, for example by making it act as a tapered seat at a shallow or positioned at a low engaging angle to the conical shaped nut, an improvement in nut and bolt thread strength of higher than 50% 100% can be achieved.

Typically, the portion of the washer plate underneath the seat is cut away to allow misalignment angles of up to 20 degrees or higher. In some instances where much higher misalignment angles are required the boss must be shaped to continue to engage the annular seat, this is made possible by altering the shape of the nut in the section or part between the part-spherical shaped end part of the nut and the typically hexagonal end which engages a spanner or the like. Instead of the shape of being parallel to the length of the nut a reverse curved shape or profile is deployed to allow the annular seat to continue to engage the nut continuously allowing much greater misalignment angles to be used and the pressure upon the nut to also be continuously maintained A less preferred alternative to the part spherical seat and chamfer/cut away is to have a larger hole similar in diameter to the internal dimension of the top part of the hole, however, this would weaken the performance of the washer plate more than the cut away or chamfer.

The high strength annular seat insert may be made of metal, most preferably steel or stainless steel, or it may be made of carbon fibre or another material with high hoop strength. It can be shaped in various ways to match the nut's boss or simply be similar to an ordinary flat steel washer for lower cost. It is the internal diameter in relation to the external diameter of the head of the nut and the strength of the set insert that is important.

It is also preferred that the annular seat insert is slightly recessed below the surface of the plastic material, protecting it from corrosion and abrasion against the nut when the nut is spun vigorously to mix the encapsulating resin within the pre-drilled hole.

The head of the bolt which contacts and sits in the annular seat may be curved, part spherical, or conical.

The annular seat is preferably curved shaped or tapered to accommodate the head of the bolt.

The annular seat insert is preferably of a material with a high tensile hoop strength and should be at the correct angle or of the correct size to create that angle (as large a diameter as possible) and to impart an increasing force onto the side of the conical shaped nut as the force between the washer plate and the substrate surface also increases. It achieves this by the wedge action of the angle of the conical nut against the seat material so that these forces generated by the direct force onto the face of the nut are internalised back into the engagement of the nut and bolt threads.

The internal diameter of the seat is important to the performance of the rock bolt. Ideally, it should be only marginally smaller than the outside diameter of the shaped nut. The wedge action is higher if the angle is lower and the force that the ring seat can impart onto the conical nut is directly related to the angle that the force is being applied and is directed back at a higher angle into the threads and thus decreases the shear forces on those threads.

The wedge action that the washer seat and insert impart onto the nut compresses the nut onto the shank of the bolt, allowing more thread surface contact and giving the nut an enormous increase in hoop strength. This increases the compressive force onto the bolt shank and decreases the shear forces acting on the threads.

This effect can be accentuated by decreasing the hoop strength of the plastic nut by cutting longitudinal slits into the external surface of the nut boss, this then allows the tapered seat insert and seat to compress the diameter of the nut boss and applies a greater compressive force onto the threads and the bolt shank.

Therefore the shallower the angle of the tapered washer plate seat the greater the compressive force imparted onto the threads. The ideal angle is between 1 degree to 18 degrees to the longitudinal direction of the nut.

By way of example a flat 3 mm thick steel seat ring with an external diameter of mm and an internal diameter of 46.5 mm will improve the performance of the nut dowel thread strength when combined with a conical nut having an external diameter of mm by more than Another preferred feature of the washer plate of the present invention is a series of thin plastic projections at the bottom of the spherical seat that grips the shank of the bolt or dowel and prevent separation of the plate from the bolt head.

The purpose of this feature is to prevent the bolt head from contamination of the mined material such as coal and damage to further processing or conveying equipment by ensuring that the bolt head, which is small enough to pass through a typical coal mines screening equipment by itself, remains attached to the much larger bolt or dowel.

Typically the washer plate will be reinforced by a series of radially extending ribs. In a preferred feature notches are defined in the radiating ribs to allow the outer part of the washer plate to deform/hinge to accommodate large deformations in the strata surface.

Brief Description of the Drawings A specific embodiment of the invention will now be described by way of example only, and with reference to the accompanying drawing in which: Fig 1 is a front view of a washer plate; Fig 2 is an end view of the washer plate of Figure 1; Fig 3 is a cross section through the washer plate of Figure 1 on A-A; Fig 4 is a schematic drawing showing the washer plate in use with a nut and bolt shank; Fig 5 is an elevation drawing of the shaped nut and a circular tapered seat ring defined on the nut; Fig 6 is a drawing showing the hexagonal end of the nut; and Figure 7 shows an alternative nut embodiment.

Detailed Description of a Preferred Embodiment Referring to the drawings, Figures 1 to 3 show a washer 1. Figures 5 and 6 show a nut 2 and Figure 4 shows the nut 2 threaded onto a rock bolt 3 shown in dashed outline, passing through the washer 1. The rock bolt is made of fibre reinforced plastic, as is the nut. The washer is made of polypropylene or polyethylene which may be fibre reinforced.

The washer 1 is made of a plastics material and as can be seen particularly in Figures 2 and 3, includes a generally conical skin 10 supported by a series of radially extending ribs 12 and inner and outer reinforcing annular ribs 14, 16 in the form of rings. Notches 13 (best seen in Figure 4) are defined in the radiating ribs 12 to allow the outer part of the washer plate between the inner and outer ribs to deform/hinge to accommodate large deformations in the strata surface.

With reference to Figures 3 and 4 in particular, it can be seen that the washer defines a central through hole or aperture 18 which extends from the front face la of the washer to the opposite face. The diameter of the hole is a maximum at the front face then gradually reduces to define a generally part spherical curved annular seat before tapering outwardly to define a generally conical cut away or chamfered portion 19 on the opposite side of the washer to the seat 15. A annular washer or annular seat insert in the form of a ring 4 made from steel is embedded in the washer 1 adjacent the front side of the through hole.

It is important that the seat insert 4 size be marginally smaller than the diameter of the nut 2 so as to minimise the angle of the wedge action. The bigger the diameter of the conical shape, the more bearing surface area is engaged.

As best seen in Figures 1 and 3, a series of thin plastic projections 24 are defined at the bottom of the spherical seat and project into the aperture 18. In use when a bolt head is inserted into the seat the projections grip the shank of the bolt or dowel and prevent separation of the washer plate 1 from the bolt head. This prevents the bolt head from contaminating the mined material such as coal and damage to further processing or conveying equipment by ensuring that the bolt head, which is small enough to pass through a typical coal mines screening equipment by itself, remains attached to the much larger bolt or dowel and to the washer.

As shown in Figure 5 the nut 2 has a generally curved part-spherical front surface or boss 20 for engagement with the part spherical curved seat 15 in use, and a hexagonal shaped back 22 to facilitate winding on of the nut onto the bolt shank 3.

With reference to Figure 6 longitudinal cuts 5 are defined in the boss of the nut to facilitate the reduction in circumference of the nut 2 by the wedge action of the seat ring 4 which in turn applies a compressive force to the bolt shank and nut threads (not shown) forcing the threads into more uniformity and containing the hoop stress in the boss of the nut.

The nut 2 has a part spherical shape so that it can articulate up to 20 degrees in any direction (refer to figure 6) and still fit into the circular seat ring 4.

The ring can be made of steel, stainless steel, GRP, carbon reinforced plastic or any suitable material that is structurally sufficient to contain the hoop stress in the nut boss.

In use as shown in Figure 4 the washer 1 can be placed over rock bolts and accommodate those which are up to 200 from the normal. The high strength ring prevents the nut 2 from being pulled through the washer which would other wise occur due to the weakening of the washer by due to the cut away conical portion 19. The wedge action that the seat and seat insert imparts onto the nut compresses the nut onto the shank of the bolt, allowing more thread surface contact and giving the nut an enormous increase in hoop strength. This increases the compressive force onto the bolt shank and decreases the shear forces acting on the threads and increases the effective thread strength of the bolt shank.

This effect is accentuated by the longitudinal slits 5 cut into or formed in the external surface of the nut boss, which allow the tapered seat to compress the diameter of the nut boss and applies a greater compressive force onto the threads and the bolt shank. The notches 13 in the radiating ribs to allow the outer part of the washer plate to deform/hinge to accommodate large deformations in the strata surface.

With reference to Figure 7, in some instances where much higher misalignment angles are required the boss is shaped to continue to engage the annular seat and the seat insert at angles greater than 200. As shown in the drawing this is made possible by altering the shape of the boss of the nut in the section or part between the part-spherical shaped end part of the nut 2a and the hexagonal end 2b. Instead of the shape of being parallel to the length of the nut a reverse curved shape or concave profile 40 is defined to allow the annular seat to continue to engage the nut at much greater misalignment angles whilst maintaining continuous pressure upon the nut.

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 (16)

1. A washer plate made of a plastics material particularly for use with rock bolts and dowel bolts for the reinforcement of earth strata in underground mining, tunnelling operations, earth or rock excavation and stabilisation of embankments defining an aperture for receiving a rock bolt and a seat for receiving a nut or head of the bolt, characterised in that one face of the washer plate defines a high strength annular seat insert which defines a hole for receiving the shank of a bolt.

2. A washer plate made of a plastics material as claimed in claim 1 wherein the high strength annular seat insert is made of metal, most preferably steel or stainless steel, or carbon fibre.

3 A washer plate made of a plastics material as claimed in claim 1 or claim 2 wherein the portion of the washer plate underneath the annular seat insert is cut away or chamfered to allow misalignment angles of up to 20 degrees or higher.

4. A washer plate made of a plastics material as claimed in any one of claims 1 to 3 wherein the annular seat insert is slightly recessed below the surface of the plastic material.

A washer plate made of a plastics material as claimed in any one of claims 1 to 4 wherein the annular seat is curved or otherwise shaped or tapered to accommodate the head of a bolt.

6. A washer plate made of a plastics material as claimed in any one of claims 1 to further defining a ring of flexible projections extending into the aperture.

7. A washer plate made of a plastics material as claimed in any one of claims 1 to 6 wherein the washer plate comprises a conical skin supported by a plurality of radiating ribs and wherein the radiating ribs are notched to allow the washer to hinge or flex about the notches.

8. A washer plate made of a plastics material as claimed in any one of claims 1 to 7 wherein the seat is tapered at an angle of between 1 degree to 18 degrees to the longitudinal direction of the nut, perpendicular to the plane of the washer.

9. A combination of a washer plate made of a plastics material as claimed in any preceding claim and a bolt head including a boss for inter-engaging with the annular seat wherein the boss which contacts and sits in the annular seat is curved or part spherical.

A combination of a washer plate made of a plastics material and a bolt head as claimed in claim 9 .wherein an internal diameter of the seat is marginally smaller than the outside diameter of the boss.

11. A combination as claimed in claim 10 wherein longitudinal slits are formed in the external surface of the nut boss.

12. A combination of a washer plate made of a plastics material particularly for use with rock bolts and dowel bolts for the reinforcement of earth strata in underground mining, tunnelling operations, earth or rock excavation and stabilisation of embankments defining an aperture for receiving a rock bolt wherein one face of the washer plate defines an annular seat including a high strength annular seat reinforcing insert, the seat defining a hole for receiving the shank of a bolt, and a nut or the like threaded to receive the rock bolt, the nut including a boss configured for engagement with the annular seat wherein the boss which contacts and sits in the annular seat may, in use, be compressed by the annular seat insert onto a rock bolt onto which the nut is threaded.

13. A combination as claimed in claim 12 further including a rock bolt onto which the nut is threaded.

14. A combination as claimed in claim 13 wherein longitudinal slits are defined in the external surface of the boss which allow the annular seat to compress the diameter of the nut boss and applies a greater compressive force onto the threads and the bolt shank in use.

A combination as claimed in any one of claims 9 to 14 wherein the boss of the nut defines a first part-spherical convex portion distal from a tool engaging end of the nut and a second outwardly extending concave portion between the tool engaging end and the first part. 13

16. A washer plate made of a plastics material substantially as hereinbefore described with reference to the accompanying drawings. Dated this twenty-third day of October 2003 Phillip Hanford Boot Patent Attorneys for the Applicant: F B RICE CO