Abstract:
A fastener system for fixing hardened plates includes a fastener element having a frusto-conical head and a threaded shaft portion, wherein a smallest outer diameter of the frusto-conical head is adjacent the threaded shaft, and a largest outer width of the shaft is smaller than a smallest diameter of the head.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a fastener system for fixing a hardened plate and for minimising wear on structural elements.  
       BACKGROUND OF THE INVENTION  
       [0002]     Plates of hardened material, such as steel and steel alloys, are often used to minimise the effect of wear on structural elements of a piece of equipment. Often, these hardened plates are called wear plates because the material of the plate is selected for its resistance to wear. Wear plates are also known to be made of other materials, such as rubber and ceramics. Examples of equipment which use wear plates include hoppers, bins and chutes used in rock handling equipment and in an ore processing plant. This sort of equipment can be exposed to wear in the form of sliding and/or gouging abrasion. The wear plates act as a sacrificial element so that the plates are worn rather than the structural element of the equipment. The plates can be readily exchanged once worn, thus extending the life of the working equipment.  
         [0003]     The wear plates are typically square in shape such that they can be tiled across the surface of the structural element to be protected from wear. Each wear plate is provided with four bolt holes, one near each corner.  
         [0004]     Recently, harder materials have been used to form a wear plate. Such a harder wear plate is a sheet of very hard material (such as a steel alloy having a Brinell hardness of approximately 500 HB) which is attached to the surface of the equipment which would otherwise be subject to wear. It is possible to perform cutting, welding, drilling and machining operations on such steels, however the difficulty of such operations increases with the hardness of the material. For example, when drilling steel of this hardness a tungsten carbide drill bit is usually required, and the feed and speed rates of the drill bit need to be carefully selected, thus requiring complex expensive drilling machinery.  
         [0005]      FIG. 1  shows a cross sectional view of a wear plate  10  with a hole  12  for fixing the wear plate  10  to a structural element (not shown) according to a known system. The surface  14  on the wear plate  10  is exposed to wear. Therefore, the opposing surface  16  will abut the structural element. In this system, a standard counter sunk bolt  18 —as shown in  FIGS. 2 and 3 —is provided. The counter sunk bolt  18  has an externally threaded shaft  20  which extends from a frusto-conical head  22 . The head  22  is provided with a hexagonal hole  24  for receiving an Allen key to hold or turn the bolt  18  during fastening etc.  
         [0006]     Obviously, the structural element would be provided with a hole for receiving the shaft  20  of the bolt  18 . The hole  12  in the structural element could be either an internally threaded hole to threadingly engage the thread on the shaft  20 , or could merely be a throughway such that a nut (not shown) can be threaded onto the shaft  20 .  
         [0007]     The hole  12  in the wear plate  10  is shaped to allow the bolt  18  to be recessed with respect to the wear surface  14 . Accordingly, the hole  12  comprises a first cylindrical portion  26 , a frusto-conical portion  28  and a second cylindrical portion  30 . The first and second cylindrical portions  26 ,  30  are dimensioned to receive the head  22  and the shaft  20  of the bolt  18 , respectively. It should be noted that the frusto-conical portion  28  is dimensioned to compliment the frusto-conical shape of the head  22 . Furthermore, as shown in  FIG. 2 , the half-opening angle θ of the frusto-conical head  22  is 45°.  
         [0008]     The bolt  18  is recessed to minimise the amount of wear which the bolt head  22  will experience. This is at a price to the wear plate  10  because the recessing of the head  22  leaves a void in the cylindrical portion  30 . This in turn allows material to catch on the wall of the cylindrical portion  30  which increases wear around the hole  12 . It is noted that prior to this invention the bolt  18  was a standard “off-the-shelf” bolt, typically having a hardness which is much less than the hardness of more recent harder ware plates.  
         [0009]     Due to the properties of the material used in the wear plates  10 , forming the hole  12  is an expensive and time consuming process. Generally, the hole  12  can be created in at least two drilling operations using a small drill bit to create the second cylindrical portion  30 , and a larger drill bit with a conical tip to create both the first cylindrical portion  26  and the frusto-conical portion  28 .  
         [0010]     The procedure for lining a piece of equipment with wear plates is, for each wear plate in turn, as follows: 
        1. the wear plate is located on the surface of the equipment;     2. the four bolts are inserted through their respective holes in the wear plate and through corresponding holes in the equipment; and     3. threading nuts onto each of the bolts.        
 
         [0014]     Given that there can be in excess of 100 wear plates lining, for example, the walls of an ore chute, this is clearly a very time consuming task. Plant equipment downtime ultimately costs the plant due to lost productivity. For this reason it is highly desirable that the time taken to line a piece of equipment with wear plates be reduced.  
       SUMMARY OF THE INVENTION  
       [0015]     According to one aspect of the present invention, there is provided a fastener element comprising a frusto-conical head and a threaded shaft, wherein the smallest outer diameter of the frusto-conical head is adjacent the threaded shaft, and the largest outer width of the shaft is smaller than the smallest outer diameter of the head.  
         [0016]     Typically the half opening angle of the frusto-conical head is within the range of 5° to 20°, preferably the half opening angle of the frusto-conical head is about 15°.  
         [0017]     In one embodiment the frusto-conical head is hardened, preferably to a Brinell hardness of at least 300 HB.  
         [0018]     In one embodiment the fastener element further comprises a shoulder between the head and the shaft, the shoulder having a diameter larger than the outer width of the shaft and smaller than the smallest outer diameter of the head.  
         [0019]     According to a variation of this aspect of the present invention there is provided a fastener element comprising a frusto-conical body and an internally threaded hole extending through the body and substantially parallel to the centre line of the cone shape, wherein the smallest outer diameter of the body is adjacent an opening to the threaded hole, and the diameter of the threaded hole is smaller than the smallest outer diameter of the body.  
         [0020]     Typically the half opening angle of the frusto-conical body is within the range of 5° to 20°, preferably about 15°.  
         [0021]     In one embodiment the body is hardened a Brinell hardness of at least 300 HB.  
         [0022]     In one embodiment the fastener element further comprises a shoulder between the body and the internally threaded hole, the shoulder having a diameter larger than the diameter of the hole and smaller than the smallest outer diameter of the body.  
         [0023]     According to another aspect of the present invention there is provided a fastener element comprising a frusto-conical head and a threaded shaft, wherein at least the frusto-conical head is hardened to a Brinell hardness of at least 300 HB.  
         [0024]     Preferably the head has a wearing surface remote from the threaded shaft, the wearing surface being continuous.  
         [0025]     In a variation to this aspect of the invention there is provided a fastener element comprising a frusto-conical body and an internally threaded hole extending through the body and substantially parallel to the centre line of the cone shape, wherein the body is hardened to a Brinell hardness of at least 300 HB.  
         [0026]     It is typical for the half opening angle of the frusto-conical head/body is within the range of 5° to 20°.  
         [0027]     Typically the body has a continuous wearing surface at the largest outer diameter of the body.  
         [0028]     According to a further aspect to the present invention there is provided a fastener system comprising:  
         [0029]     an elongate shaft; and, a head portion having a generally annular shape, a hole, and means for engaging the inside of the hole with the shaft, the head portion being engaged with the shaft,  
         [0030]     wherein the head portion is relatively harder than the shaft.  
         [0031]     Preferably the means for engaging the inside of the hole with the shaft is an internal thread of the hole and an external thread of the shaft.  
         [0032]     Preferably the head portion is hardened prior to the head portion being engaged with the shaft, typically by heat treatment. In one embodiment the head portion is hardened to a hardness of at least 300 HB.  
         [0033]     Typically the head portion is at least partly frusto-conical in shape.  
         [0034]     In one embodiment the smallest diameter of the head portion is larger than the largest diameter of the shaft.  
         [0035]     In one embodiment an adhesive is applied to the shaft in the portion of the external thread which engages the internal thread of the head portion. Preferably the adhesive is an anaerobic adhesive.  
         [0036]     In one embodiment one end of the shaft is provided with a hole for receiving a tool such that the bolt system can be prevented from rotating during installation of the bolt system.  
         [0037]     Typically the thread extends the entire length of the shaft.  
         [0038]     According to a further aspect of the present invention there is provided a hardened plate comprising a hole therein for receiving a complementary fastener element for fastening the hardened plate to a work piece, the hole being frusto-conical in shape, wherein the plate has a Brinell hardness of at least 300 HB.  
         [0039]     In a variation of this aspect there is provided a hardened plate for cooperating with one or more other plates to form a wear plate layer, the hardened plate comprising:  
         [0040]     a hole part on an edge or corner of the plate, the hole part in combination with one or more other hole parts of one or more other hardened plates forming a complete hole, the complete hole being continuously tapered in shape to form a frusto-conical hole, the complete hole for receiving a complementary fastener element for fastening the hardened plate to a work piece,  
         [0041]     wherein the hardened plate has a Brinell hardness of at least 300 HB.  
         [0042]     Typically the frusto-conical hole has a half opening angle is within the range of 5° to 20°.  
         [0043]     Preferably the hole is formed by rotating a high temperature cutting device about a central axis at an acute angle to a surface of the plate.  
         [0044]     In one embodiment the hole part is either a half hole or a quarter hole.  
         [0045]     In one embodiment the hardened plate comprises at least one beveled edge wherein in use the hardened plate can be arranged such that each of the at least one beveled edge abuts a complementary beveled edge of a like hardened plate. Preferably the angle of inclination of the beveled edge is within the range of +15° to +75°, and wherein the angle of inclination of the complementary beveled edge can be within the range of −75° to −15°.  
         [0046]     Typically the angle of inclination of the beveled edge is +45°.  
         [0047]     In another aspect of the present invention there is provided a hardened slate system for minimising wear on a structural element, the system comprising at least two hardened plates, each having at least one beveled edge and one or more frusto-conical holes for receiving fastening elements,  
         [0048]     wherein the hardened plates can be arranged such that the at least one beveled edge of a first hardened plate abuts a complementary beveled edge of a second hardened plate, the fastening elements being provided for securing the hardened plates to the structural element;  
         [0049]     wherein the hardened plates have a hardness of at least 300 HB.  
         [0050]     Preferably the first hardened plate has a single beveled edge inclined in a first direction.  
         [0051]     Typically the second hardened plate has two opposing beveled edges, one beveled edge being inclined in a second direction and the other beveled edge being inclined in a third direction, wherein the angle of inclination of the first direction is complementary of the angle of inclination of the second direction.  
         [0052]     Usually the second hardened plate is arranged adjacent to the first hardened plate such that the beveled edge of the first hardened plate abuts and overlaps a beveled edge of the second hardened plate so as to at least partly secure the second hardened plate to structural element.  
         [0053]     In one embodiment a third hardened plate may be provided having a beveled edge. Preferably a row of hardened plates can be formed, the row having one first hardened plate and one third hardened plate.  
         [0054]     Typically the second hardened plate is disposed between the first and third hardened plates, such that the row of hardened plates comprises three hardened plates.  
         [0055]     In one embodiment the row of hardened plates is one of a number of like hardened plates.  
         [0056]     Preferably the plates have a Brinell hardness of at least 300 HB.  
         [0057]     According to another aspect of the present invention there is provided a method for forming a wear plate, the method comprising:  
         [0058]     providing a hardened plate having a Brinell hardness of at least 300 HB; and  
         [0059]     creating at least one frusto-conical hole having a half opening angle,  
         [0060]     wherein the at least one frusto-conical hole is suitable for receiving a fastener element having a frusto-conical portion complementary to the frusto-conical hole.  
         [0061]     Preferably the at least one frusto-conical hole is created by high temperature cutting. Typically the high temperature cutting is achieved using either a plasma cutter or an oxyacetylene cutter.  
         [0062]     In one embodiment creating at least one frusto-conical hole comprises:  
         [0063]     providing a plasma cutter;  
         [0064]     inclining the cutting head of the plasma cutter to the half opening angle, relative to the normal of the proximal surface of the hardened plate, of the at least one frusto-conical hole; and  
         [0065]     rotating the cutting head about the hole centre.  
         [0066]     In one embodiment creating at least one frusto-conical hole can comprise either creating a complete hole within the hardened plate or creating a part hole on an edge or corner of the hardened plate.  
         [0067]     In one embodiment the method further comprises the step of cutting the hardened plate along a line intersecting at least one of the frusto-conical holes to form two or more wear plates having a part hole on an edge or corner of the wear plate.  
         [0068]     Preferably the half opening angle of the frusto-conical hole is within the range of 5° to 20°.  
         [0069]     In one embodiment the method further comprises:  
         [0070]     beveling at least one beveled edge, and  
         [0071]     wherein the hardened plate can be arranged such that each of the at least one beveled edge abuts a complementary beveled edge of a like hardened plate.  
         [0072]     According to a further aspect of the present invention there is provided a method for manufacturing a fastener system, the method comprising the steps of: 
        (i) forming a head portion having a generally annular shape and a hole;     (ii) heat treating the head portion to increase its hardness;     (iii) providing an elongate shaft; and,     (iv) engaging the head portion with the shaft to form the fastener system.        
 
         [0077]     Preferably the head portion has an internal thread inside the hole. Preferably the elongate shaft is provided with an external thread. Preferably the head portion is engaged with the shaft by threading the head portion onto the shaft.  
         [0078]     Typically step (i) involves machining the outer profile of the head portion, drilling a hole and tapping an internal thread in the hole.  
         [0079]     In one embodiment step (iv) involves applying an adhesive to the engaging threads between the head portion and the shaft.  
         [0080]     According to another aspect of the present invention there is provided a fixing system for fixing a first element to a second element, the fixing system comprising:  
         [0081]     a frusto-conical hole extending through the first element;  
         [0082]     a first fastener element having a frusto-conical portion which complements the frusto-conical hole, the frusto-conical portion having its largest diameter remote from the second element, the frusto-conical portion having a hardness of at least 300 HB; and,  
         [0083]     a second fastener element having an engagement means for engaging the first fastener element,  
         [0084]     wherein to fix the first and second elements together the first fastener element is positioned such that the frusto-conical portion is located within the frusto-conical hole, and the second fastener element is engaged with the first threaded portion such that the first and second elements are fixed together.  
         [0085]     According to yet another aspect of the present invention there is provided a fixing system for fixing a first element to a second element, the fixing system comprising:  
         [0086]     a frusto-conical hole extending through the first element;  
         [0087]     a first fastener element having a frusto-conical portion which complements the frusto-conical hole, the frusto-conical portion having its largest diameter remote from the second element; and,  
         [0088]     a second fastener element having an engagement means for engaging the first fastener element,  
         [0089]     wherein one of the first fastener element and the second fastener element has a shaft extending into the second element,  
         [0090]     wherein the largest diameter of the shaft is less than the smallest diameter of the frusto-conical portion,  
         [0091]     wherein to fix the first and second elements together the first fastener element is positioned such that the frusto-conical portion is located within the frusto-conical hole, and the second fastener element is engaged with the first threaded portion such that the first and second elements are fixed together.  
         [0092]     According to a further aspect of the present invention there is provided a fixing system for fixing a first element to a second element, the fixing system comprising:  
         [0093]     a frusto-conical hole extending through the first element;  
         [0094]     a first fastener element having a frusto-conical portion which complements the frusto-conical hole, the frusto-conical portion having its largest diameter remote from the second element, wherein the first element is a plate with a hardness of at least 300 HB; and,  
         [0095]     a second fastener element having an engagement means for engaging the first fastener element,  
         [0096]     wherein to fix the first and second elements together the first fastener element is positioned such that the frusto-conical portion is located within the frusto-conical hole, and the second fastener element is engaged with the first threaded portion such that the first and second elements are fixed together.  
         [0097]     Preferably the engagement means is a thread for threadingly engaging a corresponding thread of the first fastener element.  
         [0098]     Typically the first fastener element is a bolt and the second fastener element is a nut. Alternatively the first fastener element is a retainer and the second fastener element is a bolt.  
         [0099]     Usually the narrowest diameter of the frusto-conical portion is wider than the width of a shaft of the bolt.  
         [0100]     Typically the frusto-conical portion of the bolt is hardened, preferably to a hardness of at least 300 HB.  
         [0101]     Preferably the depth of the frusto-conical head is approximately equal to the thickness of the first element about the frusto-conical hole.  
         [0102]     In some embodiments additional fastening elements and/or washers may be used in the fixing system.  
         [0103]     Typically when the first fastener element is located within the frusto-conical hole, the frusto-conical portion of the first fastener element is flush with a wearing surface of the first element.  
         [0104]     In a further aspect of the present invention there is a fastener element comprises a frusto-conical portion and a threaded portion, wherein the smallest outer diameter of the frusto-conical portion is adjacent the threaded portion, and the largest outer width of the threaded portion is smaller than the smallest outer diameter of the frusto-conical portion. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0105]     In order for the invention to be more easily understood, embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0106]      FIG. 1  is a cross section of a wear plate according to a prior art fixing system;  
         [0107]      FIG. 2  is an elevation view of counter-sunk bolt which is used in conjunction with the wear plate of  FIG. 1 ;  
         [0108]      FIG. 3  is a plan view of the counter-sunk bolt of  FIG. 2 ;  
         [0109]      FIG. 4  is a schematic cross section view of a first embodiment of a fixing system according to the present invention;  
         [0110]      FIG. 5  is a cross section view of the wear plate of  FIG. 4 ;  
         [0111]      FIG. 6  is of an elevation view of an embodiment of a bolt according to the present invention used in the fixing system of  FIG. 4 ;  
         [0112]      FIG. 7  is a plan view of the bolt of  FIG. 6 ;  
         [0113]      FIG. 8  is a bottom view of the bolt of  FIG. 6 ;  
         [0114]      FIG. 9  is a schematic cross section view of a second embodiment of a fixing system according to the present invention;  
         [0115]      FIG. 10  is a cross section of an elevation of a retainer of the fixing system as shown in  FIG. 9 ;  
         [0116]      FIG. 11  is a bottom view of the retainer of  FIG. 10 ;  
         [0117]      FIG. 12  is a schematic plan view of four wear plates according to an embodiment of the present invention;  
         [0118]      FIG. 13  is a schematic plan view of a wear plate of  FIG. 12 ;  
         [0119]      FIG. 14  is a side elevation of an alternative embodiment of a bolt according to the present invention;  
         [0120]      FIG. 15  is a cross sectional side elevation of a retainer according to the present invention, with a coating applicable to fastener elements of the present invention;  
         [0121]      FIG. 16  is a schematic representation of the retainer of  FIG. 15  in use;  
         [0122]      FIG. 17  is a top view of an alternative embodiment of a bolt according to the present invention;  
         [0123]      FIG. 18  is a side view of the alternative embodiment of a bolt of  FIG. 17 ;  
         [0124]      FIG. 19  is a bottom view of an alternative embodiment of a bolt of  FIG. 17 ;  
         [0125]      FIG. 20  is an exploded view of a fastener system according to another embodiment of the present invention;  
         [0126]      FIG. 21  is a cross section view of the fastener system of  FIG. 20 ;  
         [0127]      FIG. 22  is an end view of the fastener system of  FIG. 20 ;  
         [0128]      FIG. 23  is a cross section of the fastener system of  FIG. 20  together with a nut fastening a wear plate to a structural element;  
         [0129]      FIG. 24  is a side view of a fastener system according to a fastener embodiment of the present invention;  
         [0130]      FIG. 25  is an end view of the bolt system of  FIG. 23 ;  
         [0131]      FIG. 26  is an exploded view of the bolt system of  FIG. 23 ; and,  
         [0132]      FIG. 27  is a cross section of the bolt system of  FIG. 23  together with a nut fastening a wear plate to a structural element.  
         [0133]      FIG. 28  is a plan view of a wear plate system according to yet another embodiment of the present invention;  
         [0134]      FIG. 29  is a side view of the wear plate system of  FIG. 28 ; and  
         [0135]      FIG. 30  is a schematic side view of the wear plate system as viewed along the line A-A in  FIG. 28 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0136]      FIG. 4  shows a schematic cross sectional view of a fixing system  50  according to one embodiment of the present invention. The fixing system  50  fixes a first element, such as a wear plate  52  to a second element, which may be a structural element  54  such as the wall of a hopper or the bin of an earth moving truck or other equipment. The wear plate  52  has a first surface  56  which is exposed to wear, and a second surface  58  which, in use, abuts the structural element  54 .  
         [0137]     The wear plate  52  is made of a wear resistant material having a Brinell hardness of at least 300 HB. Typically BISALLOY 500™ or similar is used, which has a typically Brinell hardness of 500 HB.  
         [0138]     A fastener, such as bolt  62 , having a head  64  and a threaded shaft  66 , is used to secure the wear plate  52  to the structural element  54 . The threaded shaft  66  extends through a hole  68  in the structural element  54 . A nut  70  is threaded onto the shaft  66  to fix the wear plate  52  to the structural element  54 . It will be appreciated that additional nuts and/or washers may be used in combination with the nut  70 . Alternatively, the hole  68  in the structural element  54  may be internally threaded to engage the threaded shaft  66 . In this alternative, the head  64  of the bolt  62  may be provided with, for example, a hexagonal hole to allow the bolt  62  to be rotated. Further alternative means of securing the fastener with out use of a nut will be evident to the skilled addressee.  
         [0139]     As shown in  FIG. 5 , the wear plate  52  is provided with a hole  60  which is frusto-conical in shape such that the diameter of the opening on the first surface  56  is larger than that on the second surface  58 . The hole  60  can easily be formed in the material of the wear plate  52  using, for example, a plasma cutter or an oxyacetylene cutter. Where a plasma cutter is used, the hole  60  may be formed by inclining the head of the plasma cutter at an angle corresponding with the half opening angle of the desired frusto-conical shape for the hole  60  and then rotating the head of plasma cutter about the centre line for the hole  60 .  
         [0140]     The hole  60  may be formed such that the cut is made from the second surface  58  toward the first surface  56 . Cutting in this manner minimises the exposure of the first surface  56  to heat during cutting which may result in deterioration of the wear plate  52  material.  
         [0141]     Forming the known wear plate  10  as shown in  FIG. 1  involves separate equipment to perform the cutting operation and the drilling operation. This would obviously involve significant handling time in transferring the plate from a cutting station to a drilling station. In addition, the drilling operation would take at least approximately 10 minutes per hole and could take up to 30 minutes including set up time. As discussed above, drilling the hole  12  requires the use of special drill bits. In addition, a coolant fluid must be used to remove heat generated during the drilling operation.  
         [0142]     A plasma cutter can be computer numerically controlled, allowing the cutting and hole-forming operations to be performed at the same station. Using a plasma cutter, the hole  60  can be created in approximately 10 seconds. These time savings will ultimately result in significant cost savings.  
         [0143]     In one alternative, the circumferential surface  65  of the head  64  may be provided with at least partly radially protruding ramps (not shown) which engage the wear plate  52  about the hole  60  as the head  64  is inserted into the hole  60 . During the working life of the fixing system  50 , the ramps resist any unintended rotational movement of the bolt  62  within the hole  60 .  
         [0144]     Alternatively or additionally, where a plasma cutter is used to create holes  60 , the cutting operation will often result in grooves being formed on the surface of the cut. These grooves can engage, for example, the head  64  of the bolt  62  and provide resistance to any unintended rotational movement of the bolt  62  within the hole  60 . Corresponding grooves could be formed on the head  64  of the bolt  62 .  
         [0145]     In yet another alternative the head may be oval shaped in perpendicular cross section as shown in FIGS.  17  to  19 . In this embodiment the semi-major axis x of the top of the head is greater than the semi-major axis y. For example x may be 47 mm and y may be 45 mm. The semi-major axis x of the bottom of the head is also greater than the semi-minor axis y. For example x may be 37 mm and y may be 35 mm. The hole in the wear plate would ideally be correspondingly oval shaped. This embodiment has the advantage of preventing rotation of the bolt when it is seated in the hole. For the avoidance of doubt, the term frusto-conical is intended to cover a frustum part of a cone which is not necessarily a right circular cone, such as where the base of the cone is oval shaped.  
         [0146]     FIGS.  6  to  8  show the bolt  62 . The head  64  of the bolt  62  is also frusto-conical in shape to compliment the shape of the hole  60 . The half opening angle α of the conical frustum in each-of the hole  60  and the head  64  is, in this embodiment, 15°. However, it will be appreciated that the angle a would be greater than or equal to about 5° and usually less than or equal to about 20°. Angles larger than 20° will work, but are less desirable. The head  64  is machined and hardened such that it resists wear. In addition, the top surface  72  of the bolt  62  is substantially flat.  
         [0147]     The process of forming the bolt  62  may involve through hardening (that is, quenching and tempering) of at least the head  64 .  
         [0148]     Typically, the head  12  will be of a hardness comparable to the hardness of the object being fastened by the bolt system  10  in use. Where the bolt system  10  is used to fasten the hardened wear plate  30  to the structural element  32 , the head  12  would be hardened to a Brinell hardness of 300 HB or more, preferably 350 HB or more.  
         [0149]     In a preferred embodiment, the head  12  would be of a hardness more than the hardness of the wear plate. For example, when used to secure wear plate of a hardness of 350 HB, then the hardness of the head  12  would be about 400 HB. Where the wear plate has a hardness of 500 HB the head  12  is hardened to at least 500 HB, preferably harder.  
         [0150]     It is noted that cutting the hole  60  may leave imperfections in the surface forming the hole  60 . To alleviate problems that may arise from the bolt  62  from not seating correctly in the hole  60  due to these imperfections, the half opening angle of the conical frustum of the hole  60  may be slightly greater than the half opening angle α of the head  64  of the bolt  62 . The difference may be about 1-2° or less. This will allow the base of the head  64  to engage with the base of the hole  60  first. Slight deflection of the hole surface/head will allow for an increase in the area of contact as the nut  70  threaded on the bolt is tightened. Furthermore any gap left will be relatively small and is likely to be filled with fines from the inside of the bin/hopper which will act as a cement further increasing holding ability of bolt  62 .  
         [0151]     To assemble the fixing system  50 , the holes  60 ,  68  are aligned and the shaft  66  is inserted through both holes  60 ,  68 . The nut  70  is then threaded onto the shaft  66  and tightened. The head  64  becomes wedged in the hole  68  as the nut is tightened. Because the half opening angle α of the conical frustum of each of the hole  60  and the head  64  is shallow, the friction between the head  64  and the wear plate  52  rises rapidly. This friction is usually sufficient to prevent the bolt  62  from rotating as the nut  70  is rotated to tighten/loosen the clamping force.  
         [0152]     As shown in  FIG. 4 , the depth of the frusto-conical head  64  is preferably approximately equal to the thickness of the wear plate  52  about the hole  60 . Furthermore, as shown in  FIG. 4 , when the fixing system  50  is assembled the top surface  72  of the bolt  62  is substantially flush with the first surface  56  of the wear plate  52 .  
         [0153]     The fixing system  50  of the present invention is advantageous over the existing fixing systems for hardened plates because the method of forming the hole in the plate is considerably easier, quicker and requires less expensive equipment. The new bolt of the present invention is just as effective in fastening the hardened plate and can be used with the new hole in the hardened plate of the present invention.  
         [0154]     In  FIG. 9 , a fixing system  150  according to a second embodiment is shown. The fixing system  150  fixes a first element, such as a wear plate  152 , to a second supporting or structural element  154 . The wear plate  152  has a first surface  156  which is exposed to wear, and a second surface  158  which, in use, abuts the structural element  154 .  
         [0155]     A hole  160 , of frusto-conical shape, is provided in the wear plate  152 . The diameter of the opening on the first surface  156  is larger than that on the second surface  158 . The fixing system  152  includes an retainer  162  having a frusto-conical shape which complements that of the hole  160 . The retainer  162  may be selected to have a depth equal to the thickness of the wear plate  152 . An internally threaded hole  164  extends through the retainer  162 . The same slight difference in angles between the head of the bolt and the hole may be employed with the nut and the hole to alleviate problems caused by imperfections in the surface of the hole.  
         [0156]     To fix the wear plate  152  to the structural element  154 , a bolt  170 , having an externally threaded shaft  172 , is inserted through a hole  168  in the structural element  154 . The thread on the shaft  172  engages the internal thread of the retainer  162 . The retainer  162  is drawn into the hole  160  in the wear plate  152  as the bolt  170  is threaded into the retainer  162 .  
         [0157]     The friction force between the retainer  162  and the wear plate  152  increases with the tensile force along the shaft  172 . Thus, as the bolt  170  is tightened the friction force will prevent the retainer  162  from rotating.  
         [0158]     As shown in  FIG. 10 , the half opening angle β of the conical frustum in each of the hole  160  and the head  162  is, in this embodiment, 15°. However, it will be appreciated that the angle β will usually be in the range of 5° to 20°.  
         [0159]      FIGS. 10 and 11  show a cross section view and bottom plan view of a retainer  262  according to another embodiment. In this embodiment, the internally threaded hole  264  is offset with respect to the centre of the retainer  262 . In some cases, the hole  168  in the structural element  154  may not line up with the hole  160  in the wear plate  152 . The retainer  162  can be rotated prior to engagement with the bolt  170 , thus aligning the centre of the holes  160 ,  162 .  
         [0160]     It will be appreciated that the threaded shaft  172  may be longer than the combined thickness of the wear plate  152  and the structural element  154 . Accordingly, once the fixing system  150  is assembled, the threaded shaft  172  may protrude from the top surface  166  of the retainer  162 . The protruding portion will wear away until the threaded shaft  172  and the retainer  162  are flush at the top surface  166 .  
         [0161]      FIG. 12  is a plan view showing schematically four wear plate  52  “tiled” together over a structural element (not shown).  FIG. 13  is a plan view of one such wear plate  52 . As shown in  FIG. 13 , there are three types of holes in the wear plates  52 . The first, a complete hole  360  provided within the wear plate  52 ; a second, half hole  362  provided on an edge of the wear plate  52 ; and, a third, quarter hole  364  provided on a corner of the wear plate  52 . As shown in  FIG. 13 , two half holes  362  can be aligned to receive either the head  64  of a bolt  62  or a retainer  162 . Similarly, four quarter holes  364  can be aligned to receive either the head  64  of a bolt  62  or a retainer  162 . While not shown in the figures, a half hole  362  and two quarter holes  364  may be aligned.  
         [0162]     It will be appreciated that variations of wear plates  52  can be formed. For example, wear plates  52  may be formed having any combination of complete holes  360 , half holes  362  and quarter holes  364  as required. Furthermore, it will be appreciated that, where wear plates  52  having half holes  362  and/or quarter holes  364  are used, it will be necessary to provide lateral support to prevent the wear plates  52  separating. Such lateral support may be provided by complete holes  360  and/or external supports. The location and number of the complete holes  360  would be determined by the lateral support required, and the shear stress applied to the shafts  72 ,  172  of the bolts  70 ,  170  within those complete holes  360 .  
         [0163]     An example of one method of forming individual wear plates  52  can be described in connection with the wear plates  52  shown in  FIG. 13 , as follows: 
        1. providing a large plate of hardened material;     2. forming nine holes in the large plate using, for example, a plasma cutter;     3. cutting the large plate along two orthogonal lines which intersect the holes to form four wear plates  52 , each having a complete hole  360 , two half holes  362  and a quarter hole  364 .        
 
         [0167]     Obviously, other combinations of holes and cuts, and the number of wear plates  52  formed can be created as desired. It will be appreciated that some allowance for loss of material may be required when cutting the plate through the holes formed in step 2 of the method described above. Thus, for example, the size and/or shape of the holes formed in step 2 may need to be adjusted.  
         [0168]     It will be understood to persons skilled in the art that the fixing system may be used in applications where access to the bolt head can be impeded during the service life of the bolt. Furthermore, the hardened plate may be other material.  
         [0169]     In the embodiment shown in  FIGS. 9 and 10 , the internally threaded hole  164  in the retainer  162  extends through the retainer. Alternatively, tile hole  164  may be a blind hole such that the bolt  170  only extends part way into the retainer  162 . In this alternative embodiment, a hexagonal hole, for example, may be provided which extends into the retainer  162  from the top surface  166 . Accordingly, an Allen key may be inserted into the retainer  162  to assist preventing the retainer  162  from rotating during assembly of the fixing system  152 . This embodiment of the retainer is useful where an externally threaded stud is fixed to the structural element and inserted through the hole  160  to perform the role of the bolt  170 .  
         [0170]     Referring to  FIG. 14 a  variant of the bolt shown in  FIG. 6  is shown. In this variant bolt  462  has a shoulder or step  402  between the head  464  and the threaded pull shaft  466 . The step  402  is typically sized to fit within the hole  68 . This allows for a larger sized bolt head and step which is more resistant to shearing forces.  
         [0171]     Referring to  FIGS. 15 and 16 , which show a variant retainer  162 . The variant retainer  162  is the same as retainer  162  except it has a hard plastic coating  502 . As seen in  FIG. 16  as the clamping force is applied to the retainer it causes the plastic  502  to flow into the hole  68  as indicated by  504 . This provides a seal through the hole  68 . The same coating can also be applied to bolt  162  or  462  so that the respective coating can also flow into the hole  68  in the supporting element  154 . This is useful where materials having a high sulphide content exposed to water. This can produce an acidic liquid that if it flows through the hole  68  it can damage the supporting structure and/or components behind the supporting structure.  
         [0172]     FIGS.  20  to  23  show a fastener system  610  according to another embodiment of the present invention. The fastener system  610  can be in the form of the threaded bolt  62  having an external thread for engaging an internally threaded nut. However the fastener system  610  can be in the form of a fastener without thread remote from a head  612 . In this embodiment the fastener system  610  is a bolt system and comprises a heat treated head  612  of generally annular shape and having an internally threaded hole  614 , and an externally threaded shaft  616 . The bolt system  610  is used, in combination with a nut  638  to fastening, for example, a hardened wear plate  630  to a structural element  632 . It will be appreciated that is fastener system need not be exclusively used for wear plates.  
         [0173]     The head  612  is through hardened to reduce the rate of wear due to abrasion. It will be appreciated that the head  612  can be initially formed to shape and subsequently hardened to the desired hardness. The shaft  616  can be made of a mild steel.  
         [0174]     Typically, the head  612  will be of a hardness comparable to the hardness of the object being fastened by the bolt system  10  in use. As before, where the bolt system  10  is used to fasten the hardened wear plate  630  to the structural element  32 , the head  12  would hardened to a Brinell hardness of 300 HB or more, preferably 350 HB or more.  
         [0175]     Preferably, the head  612  would be of a hardness more than the hardness of the wear plate.  
         [0176]     The bolt system  610  is assembled by threading the head  612  onto the shaft  616 . Typically an adhesive is applied to the engaging threads of head  612  and shaft  616 . The adhesive is preferably an anaerobic adhesive, such as a thread locker. As shown in  FIG. 21 , the head  612  is threaded onto the shaft  616  such that, immediately after assembly, an end portion  618  of the shaft  616  extends beyond surface  620  of the head  612 .  
         [0177]     The head  612  in this embodiment has a frusto-conical shape such that the head  612  can be received in a complementary hole  634  in the hardened wear plate  630 . When the head  612  is located within the hole  634  in the hardened wear plate  630 , the surface  620  of the head  612  is approximately flush with the wearing surface  636  of the wear plate  630 . Accordingly, the end portion  618  of the shaft  616  will protrude from the wearing surface  636 . As the shaft  616  is made of a mild steel, in use the end portion  618  will quickly wear to approximately the level of the wearing surface  636 .  
         [0178]     The end portion  618  of the shaft  616  can optionally be provided with a hexagonal hole  622  which can receive an Allen key. Thus, during installation, the bolt system  610  can be prevented from rotating as a fastener is threaded onto the shaft  616 .  
         [0179]     FIGS.  24  to  27  show a bolt system  710  according to a second embodiment of the present invention. The bolt system  710  comprises a head  712  and an externally threaded shaft  716 . As shown in  FIG. 26 , the head  712  is generally annular in shape and the head  712  has an internally threaded hole  714  for receiving the shaft  716 .  
         [0180]     The bolt system  710  is assembled in the same manner as the bolt system  610  of the previous embodiment. That is, the head  712  is threaded onto the shaft  716  and an adhesive is applied to the engaging threads of head  712  and shaft  716 .  
         [0181]     The head  712  is shaped such that there is a cylindrical portion  724  and a frusto-conical portion  726 . In use, the head  712  is received in a hole  734  having complementary shape in a wear plate  730 .  
         [0182]     As shown in  FIG. 27 , the bolt system  710 , in combination with a nut  738 , applies a clamping force to the wear plate  730  and a structural element  732 .  
         [0183]     The head  712  can be threaded onto the shaft  716  such that the end portion  718  is substantially flush with the wearing surface  720  of the head  712 . It will also be appreciated that in an alternative embodiment the head may be threaded onto the shaft such that the end portion is recessed with respect to the wearing surface of the head. While this alternative embodiment is feasible, it is less desirable since there will be fewer engaging threads between the head and the shaft.  
         [0184]     The bolt systems  610 ,  710  can both be manufactured by:  
         [0185]     (i) forming a head portion  612 ,  712  having a generally annular shape and an internally threaded hole  614 ,  714 , by machining the outer profile of the head portion, drilling a hole and tapping an internal thread in the hole;  
         [0186]     (ii) heat treating the head portion  612 ,  712  to increase its hardness;  
         [0187]     (iii) providing an externally threaded elongate shaft  616 ,  716 ; and,  
         [0188]     (iv) threading the head portion  612 ,  712  onto the shaft  616 ,  716  to form the bolt system  610 ,  710  and applying an adhesive to the engaging threads between the head portion  612 ,  712  and the shaft  616 ,  716 .  
         [0189]     Thus, the head portion  612 ,  712  is formed separately of the shaft  616 ,  716 . This can be of advantage where it is desirable for the bolt head to be hardened, but for the shaft of the bolt to retain ductile.  
         [0190]     It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the scope of the invention. Instead of hexagonal hole  622 , the end portion shaft can be provided with an alternative means for holding the bolt system  610 ,  710  from rotating during installation. Parallel flats sections can be on either side of the shaft  616 ,  716  for bearing against an open ended spanner or similar tool. Alternatively, a hole may be provided to receive a tool, such as a Torx® drive. In a further alternative, a threaded element is temporarily threaded onto the end portion  618 ,  718  to abut the surface  620 ,  720  of the head  612 ,  712 . The threaded element has a handle means for applying a torque to oppose rotation of the bolt system  10 ,  110  during installation. It will further be appreciated that embodiments of the bolt system may be provided in which friction between the head and the wear plate are used to prevent rotation of the bolt system during installation.  
         [0191]     FIGS.  28  to  30  show a wear plate system  810  which is used to prevent wear on a surface of a structural element (not shown). The wear plate system  810  has a number of rows  812 , each of which has a number of hardened plates such as wear plates  814 ,  816 ,  818  that provide a continuous cover. The wear plates  814 ,  816 ,  818  are exposed to wear rather than the structural element and are as described above. As such, the wear plates  814 ,  816 ,  818  are sacrificial elements. Each row  812  consists of a top wear plate  814 , a number of intermediate wear plates  816  and a bottom wear plate  818 .  
         [0192]     As shown in  FIG. 92 , each wear plate  814 ,  816 ,  818  has at least one beveled edge  820 ,  822 . The direction of inclination of the beveled edges of adjacent wear plates is such that there is overlap of one wear plate an adjacent wear plate within the same row  812 . For example, as shown in  FIG. 30 , one intermediate wear plate  816 ′ has a beveled edge  820  which is positively inclined while the adjacent intermediate wear plate  816 ″ has a beveled edge  822  which is negatively inclined. The beveled edge  822  is complementary of the beveled edge  820 , such that a substantially flat wearing surface S of the wear plate system is formed by the wear plates  814 ,  816 ,  818 .  
         [0193]     In the embodiment shown in FIGS.  28  to  30 , the angle of inclination of the beveled edges  820 ,  822  is −45° and +45°, respectively, to the normal to the wear surface S. Accordingly, each intermediate wear plate  816  is provided with a positively inclined beveled edge  820  and a negatively inclined beveled edge  822 .  
         [0194]     The top wear plate  814  is provided with a single positively inclined beveled edge  820 , while the bottom wear plate  818  is provided with a single negatively inclined beveled edge  822 . The opposite edge  824  of each of the top and bottom wear plates  820 ,  822  have substantially no inclination.  
         [0195]     To secure the wear plates to the structural element, the top wear plate  814  and each intermediate wear plate  816  is provided with two holes  826 , while the bottom wear plate  818  is provided with four holes  826 . Each hole  826  is provided with a fastening element  828 .  
         [0196]     The combination of the fastening elements  828  and the overlapping of the wear plates  814 ,  816 ,  818  secure the top wear plates  814 ,  816 ,  818  to the structural element.  
         [0197]     As the bottom wear plate  818  overlaps on top of the adjacent intermediate wear plate  816 , the bottom wear plate  818  is provided with four holes  826  for receiving four fastening elements  828 .  
         [0198]     The fastening elements  828  depicted in the figures are bolts as above. Accordingly, the holes  826  are frusto-conical in shape. It will be appreciated that other alternative fastening elements may be used and that the holes  826  will be shaped to receive the fastening element.  
         [0199]     The side edges  830  of each wear plate  814 ,  816 ,  818  which are not adjacent to another wear plate  814 ,  816 ,  818  within the same row  812  have no inclination.  
         [0200]     The wear plate system  810  of the present invention, when compared with the standard wear plate attachment arrangement, uses fewer bolts. Accordingly, for the present invention there is reduced time for fixing the wear plate system  810  to a structural element.  
         [0201]     It will be understood to persons skilled in the art of the invention that many modifications may be made without departing from the spirit and scope of the invention. For example, the angle of inclination of the beveled edges can alter, providing that the modulus of the two angles add up to 90°. In another embodiment, the beveled edge  820  may be positively inclined to +20° to the normal to the wear surface S, while the (complementary) beveled edge may be negatively inclined to −70°. This arrangement is satisfactory since: |20°|+|−70°|=90°. The beveled edge may be in the range of 15° to 75°. Accordingly, a complementary beveled edge may be in the range of 75° to 15°.  
         [0202]     Furthermore, the person skilled in the art will also appreciate that the wear plate system may be arranged such that adjacent rows have overlapping edges.  
         [0203]     The wear plates  814 ,  816 ,  818  according to the embodiment shown in the figures are generally square in shape. It will be appreciated that wear plates may be provided in other shapes. For example, the wear plates may be other quadrilateral shapes (such as rectangular, or a non-regular quadrilateral), or triangular.  
         [0204]     While the term frusto-conical has been used in the specification in connection with conical portions having a circular cross section, it will be appreciated that other cross sections may be employed such as elliptical or a polygon.  
         [0205]     Throughout this specification, except where the context requires otherwise due to express language or necessary implication, the words “comprise” or variations such as “comprises” or “comprising” are used in an inclusive sense; that is, to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.