Surface control bearing plate

Abstract

A surface plate for supporting generally planar loads, specifically mine roofs. The present invention generally relates to a surface plate that is capable of supporting a mine roof. Surface plates in accord with the preferred embodiments of the present invention comprise a plurality of embossments. The embossments provide the surface plates with increased strength when compared against the prior art. In the preferred embodiments, one of the plurality of embossments is a continuous perimeter embossment that generally parallels the outer perimeter of the surface plate. Another of the plurality of embossments is a circular embossment located at the center of the surface plate wherein a central opening through which a mine roof bolt is received is located. The preferred embodiments may further comprise a plurality of radial embossments that extend from the central embossment to the perimeter embossment. These radial embossments may be arranged in a regular pattern around the central embossment. In one preferred embodiment that is generally rectangular in shape, four radial embossments intersect and extend from the central embossment towards the perimeter embossment and intersect with the perimeter embossment in proximity to the corners of the surface plate.

Description

CLAIM OF PRIORITY

[0001] This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 60/278,549 filed Mar. 23, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to load support or bearing equipment and more particularly to load-bearing surface plates adapted to provide support for a generally planar surface. This invention is particularly applicable to mine roof surface plates.

[0004] 2. Description of the Background

[0005] Each year there are numerous reported injuries and multiple fatalities from ground falls in underground coal mines. The vast majority of these injuries and deaths result from relatively minor falls of rock from the roof and ribs in coal mines.

[0006] The activity that is most at risk from these minor falls is roof bolting. A large fraction of injuries from roof collapses occurred during the roof bolting cycle. More importantly, over 75% of both the reported wall and roof fall accidents occurred at or near the working face of the mine shaft.

[0007] In underground mining operations, it is necessary to support the roof of the mine shaft so that it will not cave in. To provide the necessary support, large mine bolts are typically set into the roof of the mine by mechanical means, resin, or a combination of the two. The mine bolt will often support a bearing plate that is capable of supporting the planar surface of the mine roof that is very proximal to the mine bolt.

[0008] A mine roof bolt is an anchoring device that is typically set into the roof of a mine to aid in supporting the roof of the mine. A bearing plate is a primarily planar plate that is used to support portions of a mine roof that are proximal to the mine roof bolt. Generally, mine roof bolts extend through an opening in the center of a bearing plate and are secured against the bearing plate to provide support for the mine roof or walls or other generally planar surface. A surface plate is a large, primarily planar plate that is used to support large portions of a mine roof in conjunction with a mine roof bolt and a bearing plate. An embossment is a three-dimensional surface or protrusion surface that is raised away from the plane of the bearing plate or surface plate.

[0009] Surface plates are generally much larger than bearing plates and may be used, generally in conjunction with a bearing plate, to stabilize a larger area of a mine roof that a normal bearing plate would. A primary function of surface plates is preferably to maintain and control the surface around the roof bolt and bearing plate. Essentially, the surface plate is designed to maintain a much larger area around the mine bolt than a bearing plate. Surface plates are preferably used in conjunction with the standard bearing plates such that these two components work in concert to maintain the roof of a mine. One difficulty with fabricating larger surface plates is the additional weight of such a structure. A need exists to construct large surface plates with adequate stiffness and support characteristics that are not too heavy to employ easily. Surface plates, with adequate stiffness characteristics, can help minimize or even eliminate progressive roof and rib failures.

[0010] These and other limitations to current implementations of surface plates are preferably addressed through the use of the present invention. These and other objectives and advantages of the present invention will become readily apparent to persons skilled in the art from the following description of the preferred embodiments.

SUMMARY OF THE INVENTION

[0011] In accordance with the present invention, there is provided an apparatus for the support of planar surfaces, such as the roof or walls of a mine shaft. Surface plates in accord with the present invention have improved strength and stiffness characteristics compared with the prior art due to at least one but generally a plurality of embossments protruding from the surface of the plate. Generally, the plurality of embossments comprise a perimeter embossment around the perimeter of the surface plate, a central embossment centered about a central opening, and a plurality of embossments extending between the central embossment and the perimeter embossment.

[0012] Surface plates in accord with the present invention allow for the stabilization and support of mine ceilings. The geometry and orientation of embossments on the surface plates in accord with the presently preferred embodiments generally provide such surface plates with higher deflective strength and stability when challenged with a planar load than prior art surface plates. Such improvements provide greater stability and safety for the mining industry.

[0013] In the presently-preferred embodiments of the present invention, a surface plate may be placed on the bolt in conjunction with a bearing plate so the surface plate contacts, bears against, or rests parallel to the mine roof. The surface plate of the present invention may be constructed of any substance commonly known in the art. One such presently preferred substance is sink steel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The following figures provide further detail of certain of the presently preferred embodiments of the present invention:

[0015] FIG. 1 displays a preferred embodiment of the present invention depicting a surface plate comprising a generally square or rectangular shape and four radial embossments;

[0016] FIG. 2A displays a cross-sectional view of the surface plate of FIG. 1 along Section 2A;

[0017] FIG. 2B displays a cross-sectional view of the surface plate of FIG. 1 along Section 2B;

[0018] FIG. 2C displays a cross-sectional view of the surface plate of FIG. 1 along Section 2C;

[0019] FIG. 3A displays a preferred embodiment of the present invention depicting a surface plate comprising a circular shape with a perimeter embossment and four radial embossments;

[0020] FIG. 3B displays a preferred embodiment of the present invention depicting a surface plate comprising a generally square or generally rectangular shape with a perimeter embossment and eight radial embossments;

[0021] FIG. 3C displays a preferred embodiment of the present invention depicting a surface plate comprising a generally square or generally rectangular shape and six radial embossments;

[0022] FIG. 3D displays a preferred embodiment of the present invention depicting a surface plate comprising a circular shape and eight radial embossments; and

[0023] FIG. 4 depicts the test setup utilized in the stiffness experimental testing of certain preferred embodiments.

[0024] FIG. 5 displays a preferred embodiment of the present invention depicting a surface plate comprising a generally square or rectangular shape, four radial embossments and rounded corners.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant to the preferred embodiments of the present invention, while eliminating, for purposes of clarity, other elements that may be well known. Those of ordinary skill in the art will recognize that other elements are desirable and/or required in order to implement the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. The detailed description will be provided hereinbelow with reference to the attached drawings.

[0026] A plurality of surface plate geometries are contemplated within the scope of the present invention. While data primarily for generally square plates will be discussed, one of skill in the art will recognize that additional embodiments of the present invention would be generally rectangular, circular, or of any other geometry that was appropriate for the specific circumstance confronting the practitioner, as shown by way of example in FIGS. 1, 2A, 2B, 2C, and 2D. In addition, while the corners of the surface plate of the depicted presently-preferred embodiments of the present invention are portrayed as clipped along a diagonal line, it is also contemplated that the corners of the surface plate may be pointed so that the sides of the plates intersect at right angles as in a square or rectangular shape.

[0027] In a presently-preferred embodiment of the present invention, the surface plates may vary in size preferably from about 14 inches by 14 inches for a square plate to about 24 inches by 24 inches. One skilled in the art would appreciate that these dimensions could be increased or decreased to address particular interpretations. Another presently-preferred embodiment of the surface plate comprises a square plate having the overall dimensions of 15½ inches by 15½ inches. Other presently-preferred embodiments may have circular shapes with similar dimensions.

[0028] Preferred embodiments of the present invention provide for the inclusion of embossments in the structure of a surface plate. The embossments enhance the ability of a surface plate to support a planar load. Certain preferred embodiments of the present invention possess a plurality of embossments of varied width and length in the structure of a surface plate, including an embossment around the perimeter of the surface plate.

[0029] Generally, preferred embodiments of the present invention allow for a central opening in a surface plate through which a roof bolt may be secured into the mine roof. By way of example, one presently preferred embodiment, illustrated by a generally square surface plate 100, is displayed in FIG. 1. In this preferred embodiment, the surface plate 100 has a first surface 104 that is generally planar, except for a plurality of embossments 108, 112, 116 extending therefrom. The surface plate 100 also has a second surface, comprising the opposite side of the surface plate. Note that the second surface, although generally planar, will have the inverse of the plurality of embossments from surface 104 which form recessed arrays as the second surface. This plurality of embossments 108, 112, 116 are preferably incorporated into the structure of the first surface 104 of the surface plate 100, and are disposed around a central opening 120. As shown in FIGS. 2A, 2B, and 2C, such embossments 108, 112, 116 comprise portions of the surface plate that have been molded, stamped, pressed, or otherwise formed such that they extend a certain height 117, 119, 121 from the first surface of the surface plate and have a certain width 108, 113. The geometry of the embossments may be varied within the scope of the present invention, such that the face of a particular embossment may be rounded, flat, pointed, or otherwise shaped. A circular embossment 116 is preferably centrally situated about a central opening 120 in the plate that allows for the inserting of a bolt through the plate. The central embossment 116 is preferably raised approximately {fraction (5/16)} of an inch in height from the first surface 104 of the surface plate 100. The face of the central embossment 116 is generally flat or planar such that the face of the central embossment 116 is parallel to the first surface 104 of the surface plate 100. The central embossment 116 allows the present invention to be placed between or over a bearing plate and the mine roof or other planar load. One of ordinary skill in the art will recognize that the face of the central embossment 116 may be of a different geometry than float and may have embossments for on its surface. In one presently-preferred embodiment, the diameter of the central embossment is ten times that of the central opening, such that a 6 inch by 6 inch bearing plate only bears against the central embossment and an 8 inch by 8 inch bearing plate bears almost entirely against the central embossment. As shown in FIGS. 2A, 2B, and 2C, the embossments 112, 116 preferably protrude away from the planar load that is being supported by the present invention.

[0030] In one presently-preferred embodiment, the diameter of the central embossment 116 is 10 inches although this diameter may be changed to account for the size of the bearing plate. The bearing plate employed with the present invention may be oriented such that all sides of the bearing plate are parallel to the surface plate. Alternatively, it is further contemplated that the bearing plate may be offset rotationally by any number of degrees.

[0031] In a presently-preferred embodiment as depicted in FIG. 1, an additional embossment 112 is also located proximal to the perimeter of the entire second surface of the bearing plate. As such, as shown in FIGS. 1, 3A, 3B, 3C, and 3D, in certain preferred embodiments, the perimeter of the perimeter embossment 112 is preferably the same shape as the perimeter of the surface plate 100, e.g. square if the surface plate 100 is square, as in FIG. 1. However, the geometry need not match the geometry, e.g. the perimeter embossment 112 may be circular while the surface plate 100 is square. The perimeter embossment 112 is, in certain preferred embodiments, as depicted in FIG. 1, set back from the edges of the first surface 104 of the surface plate 100 by approximately ½ inch. For certain preferred embodiments, such as depicted in FIGS. 1, 2A, 2B, and 2C, the perimeter embossment 112 is preferably raised ½ inch in height 119 from the face of the surface plate 100. The width of the perimeter embossment 112 as depicted in FIG. 1, is preferably 1 inch. One of ordinary skill in the art can appreciate that additional widths, heights, and sizes of the perimeter embossment are possible and desirable depending on the size and loading on the surface plate 100. Notably, the perimeter embossment 112 is preferably proximal to the edge of the surface plate.

[0032] A presently-preferred embodiment, as depicted in FIG. 1, further provides for embossments 108 extending radially from the central embossment 116 to the perimeter embossment 112, such that the radial 108, central 116, and perimeter 112 embossments may be viewed as forming a continuous embossment. A plurality of radial embossments 108 may emanate from the central embossment 116. In a presently-preferred embodiment shown in FIG. 1, the radial embossments 112 emanate from the central embossment 116 at regular angular spacing and extend outward to the perimeter embossment 112 perpendicular to the outer perimeter of the central embossment 116 at the location that the radial embossment 108 intersects the central embossment 116. One skilled in the art will appreciate, however, that the radial embossments 108 may emanate from the central embossment 116 in a variety of orientations as shown, for example, in FIG. 3.

[0033] In one preferred embodiment as depicted in FIGS. 1, 2A, 2B, and 2C, the radial embossments 108 are 1¼ inches in width 113. The radial embossments 108 are preferably raised by {fraction (5/16)} of an inch in height 117 from the first surface 104 of the surface plate 100 at the location 109 of the radial embossment 108 that intersects 109 with the central embossment 116. The radial embossments 108 are preferably raised by a tapered amount away from the first surface 104 of the surface plate 100 such that a radial embossment 108 is raised by ½ inch in height 117 at the location 111 of the intersection between the radial embossment 108 and the perimeter embossment 112. The preferred embodiment for a square surface plate as depicted in FIG. 1 comprises four radial embossments 108 that emanate from the central embossment 116 at approximately ninety degrees from one another. Each of the radial embossments 108 in this preferred embodiment, as depicted in FIG. 1, radiate from the central embossment 116 towards a corner 101 of the surface plate 100.

[0034] Various orientations and numbers of radial embossments are contemplated within the scope of the present invention. As shown in FIGS. 1 and 3, surface plates may have 4, 6, or 8 radial embossments emanating at regular spacing from the central embossment to the perimeter embossment. FIG. 3A depicts a circular surface plate embodiment of the present invention 204 with a circular perimeter embossment 208, six radial embossments 212, a central embossment 216, and a central opening 220. FIG. 3B depicts a rectangular surface plate embodiment of the present invention 226 with a rectangular perimeter embossment 228, eight radial embossments 232, a central embossment 236, and a central opening 224. FIG. 3C depicts a square surface plate embodiment of the present invention 242 with a rectangular perimeter embossment 240, six radial embossments 232, a central embossment 248, and a central opening 252. FIG. 3D depicts a circular surface plate embodiment of the present invention 258 with a circular perimeter embossment 260, eight radial embossments 256, a central embossment 264, and a central opening 268. These examples are meant to be illustrative of the present invention and not limiting. Those of skill in the art will recognize additional variations on the spacing, shape, and number of radial, central and perimeter embossments.

[0035] In one presently-preferred embodiment, shown in FIG. 5, the surface plate 500 has a first surface 504 that is generally planar but for a plurality of embossments 508, 512, 516. A perimeter embossment 508 extends around the surface plate proximal to the outer perimeter of the surface plate 500 approximately {fraction (1/2 )} inch from the perimeter. This perimeter embossment 508 is raised ½ inch from the first surface 504. A central embossment 516 is located centrally on the plate 500 and on the first surface 504 and is generally symmetric about an opening 520 defined by the plate 500 on the central embossment 516. The central opening 520 is designed to accept a bolt and is at least the diameter of the mine roof bolts being used in the application for which the surface plate 500 is being used. The central embossment 516 has a diameter approximately ten times the diameter of the central opening 520. The central embossment 516 protrudes {fraction (5/16)} of an inch from the first surface 504 in the same direction that the perimeter embossment 508 extends from the first surface 504. A 6 inch×6 inch bearing plate will thus contact only the central embossment 516 when used with the surface plate 500 in conjunction with a roof bolt. A 8 inch×8 inch bearing plate will also, generally, only contact the central embossment 516 but will overhang from the diameter of the central embossment 516 and thus may contact a radial embossment 512 near where the radial embossments 512 intersect the central embossment 516. In the preferred embodiment depicted in FIG. 5, the central embossment 516 is at minimum 1¼ inches from the perimeter embossment 508. The perimeter embossment 508 is approximately 1 inch in width at its widest point and is approximately ½ of an inch from the perimeter edge of the surface plate 500 and the first surface 504.

[0036] In the preferred embodiment depicted in FIG. 5, four radial embossments 512 extend from the central embossment 516 to the perimeter embossment 508. In this regard, the radial embossments 512, the perimeter embossment 508 and the central embossment 516 are connected to form one nearly continuous embossment on the first surface 504. A second surface 524 opposite the first surface 504 on the surface plate 500 comprises a generally planar surface with recesses corresponding to the perimeter embossment 508, the central embossment 516 and the radial embossments 512. The second surface 524 thus has the inverse topography as the first surface 504.

[0037] The four radial embossments 512 protrude from the first surface approximately {fraction (5/16)} of an inch from where they intersect with the central embossment and generally decrease in height as they extend from the central embossment 516 to the perimeter embossment 508 such that the radial embossments are ½ of an inch in height where they intersect with the perimeter embossment 508. The radial embossments are approximately 1¼ inch in width where they intersect the first surface 504. A cross-section of the perimeter embossment 508 is generally triangular with a generally small flat surface 509 at the apex of the triangle at the greatest height of the embossment 508 above the first surface 504, except in the corner intersections of the perimeter embossment 508 and the radial embossments 512 where a larger flat surface is formed 510. The radial embossments 512 also have a generally triangular cross-section with a small flat surface 513 at their apex at the greatest height they attain above the first surface 504. The central embossment 516 is generally rectangular in cross-section and forms a generally planar surface upon which a bearing plate may bear.

[0038] The surface plate 500 depicted in FIG. 5 is generally 15½ inches×15½ inches in size. One skilled in the art will recognize, however, that the size of the plate 500 may be varied in accord with the needs of the application and desires of the user. Note that the corners 528 of the surface plate 500 are rounded as are the corners 532 of the perimeter embossment 508.

[0039] Note that the embossments 508, 512, 516 will form four areas 536 of the first surface 504 that are bounded by the central embossment 516, two radial embossments 512 and the perimeter embossment 509. The intersections between the embossments 508, 512, 516 also are rounded.

[0040] Several comparative tests were performed using presently-preferred embodiments of the present invention with radial embossments and a circular surface plate with a perimeter embossment. The plates are preferably fabricated from 20 gauge (0.0359 inch thickness) sink steel. Two preferred embodiments of the present invention having square shapes were tested each having different arrangements of radial embossments. Each embodiment was a 17 inch square steel surface plate. A third surface plate used in the tests was a 19 inch diameter circular surface plate having a perimeter embossment. One preferred embodiment of the present invention had four radial embossments (similar to FIG. 1) and one preferred embodiment had eight radial embossments (similar to FIG. 3B). Two variations of the square radial embossment surface plate were used. One preferred embodiment comprises radial embossments that emanate from the central embossment to the corner of the perimeter embossment diagonally across the face of the surface plate. A second preferred embodiment comprises radial embossments that emanate from the central embossment to the center of a face of the perimeter embossment parallel to the sides of the surface plate. All surface plates have a 1 inch diameter center hole or opening.

[0041] A laboratory test was performed to evaluate performance of the surface plates. FIG. 4 shows the basic test set up 300. The test involved setting a surface plate 316 on two supports 304, 308 (two-point test) 14 inches apart then applying a load 312 to the center of the surface plate 316. The center load 312 was applied through a 6 by 6 inch bearing plate 320. Both loads 312 and deformations of surface plates were measured during these tests. The square surface plates 316 were tested with the supports 304, 308 parallel to the edges of the surface plate 316 as well as at a 45 degree angle to the surface plate 316 to evaluate the diagonal surface plate 316 properties.

[0042] A load-deformation relationship was measured for each surface plate and several variable characterizing the strength of the surface plate were measured. Table 1 gives the stiffness and yield and peak strengths for the surface plate tests. The stiffness was calculated as the secant from the “no load” condition to the yield strength. The energy is defined as the area under the load-deformation curve up to yield. 1 Strength, Stiffness, Energy, lbs Deformation, Yield, Yield, Area, Plate Type Peak Yield Yield, in lbs/in in-lbs in2 Square (17 in) 886 800 0.6 1400 220 289 4 radial embossment, parallel Square (17 in) 1375 1290 1.7 760 1100 289 4 radial embossment, diagonal Square (17 in) 550 450 0.4 1133 94 289 8 radial embossment, parallel Square (17 in) 825 725 1.4 520 500 289 8 radial embossment, diagonal Circular 520 310 0.2 1531 34 284 (19 in)

[0043] Much of the deformation, surface plate yield, and failure result from bending of the surface plates. Some deformation may result from punching of the bearing plate into the center of the surface plates.

[0044] The four radial embossment square surface plate has the highest yield and peak load while the circular surface plate has the lowest strength. The four radial embossment square surface plate has particularly higher strength as compared to the other tested surface plates. The surface area of the circular surface plate is 283 square inches, while the surface area of the square surface plates was 289 square inches.

[0045] Among the preferred embodiments of the present invention, the four radial embossment square surface plate is significantly stronger than the eight radial embossment square surface plate. The four extra radial embossments in the eight radial embossment square surface plate parallel to the surface plate edges may act to weaken the surface plate essentially by providing a weak point or crease where the surface plate may bend. This result highlights the fundamental importance of the internal structure of the surface plates. The strength of the presently-preferred surface plates of the present invention is much higher when tested across the diagonals. This is believed to be the result of more material and structure in the corners that adds strength to the surface plate across the direction of bending.

[0046] The circular surface plate and the 17-inch four radial embossment square surface plate have approximately the same stiffness with the circular surface plate having slightly higher stiffness. Much of this stiffness appears to come from the external or edge structure, namely the perimeter embossment, of the surface plates. The stiffness of a prior art square flat plate without any internal embossments was measured at 35 pounds/inch. The stiffness of the square and circular surface plates was thus significantly higher than the stiffness of a prior art plate.

[0047] Although the invention has been described in terms of particular embodiments in an application, one of ordinary skill in the art, in light of the teachings herein, can generate additional embodiments and modifications without departing from the spirit of, or exceeding the scope of, the claimed invention. Accordingly, it is understood that the drawings and the descriptions herein are proffered only to facilitate comprehension of the invention and should not be construed to limit the scope thereof.

Claims

1. A surface plate for use in supporting a mine roof, said surface plate comprising:

a first surface adapted to face away from said mine roof, wherein said first surface defines a central opening capable of receiving a mine roof bolt, and said first surface further comprising:

a first embossment centrally located on said first surface, wherein said first embossment comprises a circular raised plane surrounding said central opening, further wherein said raised circular plane is generally parallel to said first surface;

a second embossment located on said first surface, wherein said second embossment comprises a continuous raised surface that is proximal to the outer perimeter of said first surface;

a plurality of radial embossments, wherein each of said plurality of radial embossments extend from said first embossment to said second embossment and intersect with said first embossment and said second embossment; and

a second surface adapted to face said mine roof, wherein said second surface is generally planar, further wherein said second surface comprises a complementary face to said first surface comprising a plurality of recesses corresponding to said first and second embossments and said plurality of radial embossments.

2. The surface plate of claim 1, wherein said surface plate is rectangular, circular, or elliptical.

3. The surface plate of claim 2, wherein said plurality of radial embossments is selected from the group consisting of four, six, and eight.

4. The surface plate of claim 3, wherein said plurality of radial embossments are spaced at regular intervals around said first embossment.

5. The surface plate of claim 1, wherein said first embossment is raised {fraction (5/16)} of an inch from said first surface.

6. The surface plate of claim 1, wherein said first embossment has a diameter that is ten times larger than the diameter of said central opening.

7. The surface plate of claim 1, wherein said second embossment continuously parallels the outer perimeter of said first surface.

8. The surface plate of claim 1, wherein said second embossment is ½ inch from an outer edge of said surface plate.

9. The surface plate of claim 1, wherein said second embossment is one inch in width.

10. The surface plate of claim 1, wherein said second embossment is raised ½ of an inch from said first surface.

11. The surface plate of claim 1, wherein said plurality of radial embossments are raised from said first surface by ½ of an inch at the locations where said plurality of radial embossments intersect said second embossment.

12. The surface plate of claim 12, wherein said plurality of radial embossments are raised from said first surface by {fraction (5/16)} of an inch at the locations where said plurality of radial embossments intersect said first embossment.

13. The surface plate of claim 12, wherein said plurality of radial embossments are raised a tapered amount, further wherein said tapered amount is {fraction (5/16)} of an inch at the locations where said plurality of radial embossments intersect said first embossment and ½ of an inch at the locations where said plurality of radial embossments intersect said second embossment.

14. The surface plate of claim 13, wherein said tapered amount varies smoothly between {fraction (5/16)} of an inch and ½ of an inch.

15. A surface plate for use in supporting a mine roof, said surface plate comprising:

a first surface adapted to face away from said mine roof, wherein said first surface defines a central opening capable of receiving a mine roof bolt, further wherein said first surface is generally square, and wherein said first surface further comprises:

a first embossment centrally located on said first surface, wherein said first embossment comprises a circular raised plane surrounding said central opening, further wherein said raised circular plane is generally parallel to said first surface;

a second embossment located on said first surface, wherein said second embossment comprises a continuous raised surface that generally parallels the outer perimeter of said first surface;

four radial embossments, wherein each radial embossments extends from said first embossment to said second embossment and intersects with said first embossment and said second embossment, further wherein said four radial embossments are spaced regularly around said first embossment, further wherein each radial embossment extends from said first embossment to said second embossment and intersects said second embossment at a corner of said second embossment wherein said corner of said second embossment corresponds in location to a corner on said outer perimeter of said first surface; and

a second surface adapted to face said mine roof, wherein said second surface is generally planar, further wherein said second surface comprises a complementary face to said first surface comprising a plurality of recesses corresponding to said first and second embossments and said four radial embossments.