Mine roof bolt with resin control surface

A resin bonded mine roof bolt having an elongated rod with a first end, a second end, and a resin control surface positioned on the rod between the rod first and second ends. The resin control surface has a channel extending along the length of the bolt for distributing resin and interrupted spiral threads for mixing resin. When installed in a mine roof bore hole with curable resin, the resin control surface mixes the resin and partially fills the bore hole to minimize the amount of resin needed to anchor the bolt.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mine roof bolt anchored in a bore hole by resin bonding and, more particularly, to a mine roof bolt bearing a resin control layer that exerts a compressive force on resin within a bore hole.

2. Description of Related Art

The roof of a mine conventionally is supported by tensioning the roof with 4 to 6 feet long steel bolts inserted into bore holes drilled in the mine roof that reinforce the unsupported rock formation above the mine roof. The end of the mine roof bolt may be anchored mechanically to the rock formation by engagement of an expansion assembly on the end of the mine roof bolt with the rock formation. Alternatively, the mine roof bolt may be adhesively bonded to the rock formation with a resin bonding material inserted into the bore hole. A combination of mechanical anchoring and resin bonding may be employed by using both an expansion assembly and resin bonding material.

When resin bonding material is used, it penetrates the surrounding rock formation to adhesively unite the rock strata and to firmly hold the roof bolt within the bore hole. Resin is typically inserted into the mine roof bore hole in the form of a two-component plastic cartridge having one component containing a curable resin composition and another component containing a curing agent or catalyst. The two-component resin cartridge is inserted into the blind end of the bore hole, and the mine roof bolt is inserted into the bore hole such that the end of the mine roof bolt ruptures the two-component resin cartridge. Upon rotation of the mine roof bolt about its longitudinal axis, the compartments within the resin cartridge are shredded and the components are mixed. The resin mixture fills the annular area between the bore hole wall and the shaft of the mine roof bolt. The mixed resin cures and binds the mine roof bolt to the surrounding rock.

The typical diameter of a mine roof bore hole is one inch. Mine roof bolts anchored with resin bonding are often ¾ inch in diameter, and more recently, ⅝ inch in diameter. The mine roof bolt is generally centered within the bore hole creating a circular annulus that becomes filled with bonding resin. The larger diameter bolts (¾ inch) offer performance advantages over ⅝ inch bolts in that the annulus provided between the bore hole wall and a ¾ inch bolt is smaller than that of smaller diameter bolts. A smaller annulus provided between the bolt and the bore hole wall improves mixing of the resin and catalyst in the annulus. In addition, when the resin cartridge is shredded upon insertion of the mine roof bolt and rotation thereof in an annulus larger than ⅛ inch (as for mine roof bolts having less than ¾ inch diameter installed in one inch bore holes), the shredded cartridge can interfere with the resin and catalyst mixing. Poor mixing results in an inferior cured resin and results in poor bond strength between the bolt and bore hole wall. A phenomenon of “glove fingering” occurs when the plastic film that forms the cartridge lodges in the bore hole proximate the surrounding rock, thereby interrupting the mechanical interlock desired between the resin and bore hole wall. This phenomenon can also manifest itself in the bolt inserting longitudinally through only one compartment of the two-compartment package. When this occurs, sections of the uncured resin and unmixed catalyst are evident in the bore hole, thus affecting the overall bond strength of the bolt resin system to the surrounding rock. In addition, the larger annulus created by using a ⅝ inch bolt in a one-inch bore hole requires more resin to bond the bolt to the rock than does a larger diameter bolt, thereby adding to the cost of installing a smaller diameter bolt. While one solution might be to proportionally reduce the size of the bore hole to less than one inch, this is not practicable. The mine roof drilling equipment already in use is conventionally produced for drilling one-inch bore holes. Moreover, there are significant technical difficulties in drilling small diameter bore holes in mine roofs.

Despite these drawbacks of using mine roof bolts having a diameter of less than ¾ inch, the popularity of smaller diameter mine roof bolts is increasing. A ⅝ inch bolt is more lightweight and easier to use than a ¾ inch bolt and can be produced at lower cost.

A mine roof bolt that overcomes the need for extra resin and avoiding the glove fingering problem of smaller diameter bolts installed in one-inch bore holes is disclosed in U.S. Patent Application Publication No. 2005/0134104. The bolt includes an elongated rod that forms the main structure of the mine roof bolt. A portion of the rod between a drive head and the end of the bolt is coated with a layer of material having a lower specific gravity than the rod, such as a polymer. The polymeric coating layer includes interrupted raised threads that help with mixing of resin in the mine roof bore hole. The coating on the mine roof bolt also helps to fill a portion of the annulus at a minimal increase in weight to the bolt, thereby minimizing the amount of resin that is required for bonding the bolt to rock strata. This coated mine roof bolt can be produced from a ⅝ inch metal rod with a polymeric coating layer about 1/16 inch thick.

One problem with the coated mine roof bolt is in the distribution of resin along the length of the bolt prior to curing. Conventional resin systems typically cure in 5 seconds to 2 minutes and are manufactured with design parameters specifically for larger annulus bolts. Once mixing is complete and curing begins, the resin should be distributed along the length of the bolt to maximize interlock between the bolt and the surrounding rock. In some instances, the resin may not be distributed as needed along the bolt before curing begins, due to deficiencies in the flow parameters of the resin, the mechanical limitations of the installation equipment, and the insertion strength of the bolt.

In addition, the portion of the coating in between the spiral threads is generally smooth. These smooth surfaces provide only minimal structure to engage with the cured resin.

Accordingly, a need remains for a resin-bonded mine roof bolt where the resin mixing and distribution is controlled by the bolt and that is particularly suited as a small diameter mine roof bolt.

SUMMARY OF THE INVENTION

This need is met by the mine roof bolt of the present invention that includes an elongated rod having a first end, a second end and a resin control surface located on the rod at a position between the first and second ends. The resin control surface defines a channel extending between ends of the resin control surface in a direction generally parallel to the longitudinal axis of the rod. In one embodiment, the resin control surface is a layer positioned on the rod with the channel defined in the layer. The rod may be metallic, and the layer may be polymeric. Alternatively, the rod and resin control surface may be a unitary structure.

In another embodiment, the resin control surface includes a plurality of raised, interrupted threads, with the threads bearing ridges. The portions of the resin control surface in between the threads may also include ridges. The ends of the threads may be aligned longitudinally along the rod with the channel spaced apart from the aligned thread ends.

When the mine roof bolt of the present invention is installed in the mine roof bore hole, a frangible curable resin cartridge is inserted into the bore hole. The mine roof bolt is inserted into the bore hole and ruptures the resin cartridge. The mine roof bolt is rotated about its longitudinal axis such that the resin control layer contributes to mixing of the contents of the resin cartridge and to distributing resin along the bolt through the channel. The resin control layer also compresses the resin between the mine roof bolt and the bore hole wall. The ridges on the interrupted threads and/or portion of the resin control surface therebetween provide enhanced surface area for interlocking of resin with the bolt and surrounding rock. In addition, the spiral threads urge the resin toward the first end upon rotation of the mine roof bolt.

The mine roof bolt of the present invention may be produced by providing an elongated rod and applying a resin control surface to the rod intermediate a first and second end of the rod. A drive head or drive nut is attached to the second end of the rod. The resin control surface may be a polymeric layer and may be applied to the rod by injection molding, or the resin control surface may be unitary with the rod and is formed thereon via casting or machining of the rod.

DETAILED DESCRIPTION OF THE INVENTION

A complete understanding of the present invention will be obtained from the following description taken in connection with the accompanying drawing figures, wherein like reference characters identify like parts throughout.

Referring toFIGS. 1-4, there is illustrated a mine roof bolt10for securing in a bore hole drilled in a rock formation (not shown) to support the rock formation that overlies an underground excavation such as a mine passageway or the like. The bore hole is drilled to a pre-selected depth into the rock formation as determined by the load bearing properties to be provided by the mine roof bolt10.

The bolt10includes an elongated rod12, typically being metallic, having a first end14for positioning in the blind end of a bore hole and a second end16. A drive head18is attached to second end16for extending into the mine passageway from the open end of the bore hole. In one embodiment, as shown inFIG. 3, the drive head18includes a shoulder20and a plurality of drive faces22. The rod12and drive head18typically are integrally produced from steel or other metals. Drive head18shown in the drawings is only one example of a drive head. Other suitable drive heads include internally threaded nuts that are threaded onto the rod second end16, which may further incorporate a stop mechanism for post-curing tensioning of the bolt10.

A portion of the elongated rod12between the first and second ends14,16includes a resin control surface24. In one embodiment, the resin control surface24is a separate layer of material provided on the rod12. This is not meant to be limiting as the resin control surface24may be integral with the rod12, such as by having been produced thereon via casting or machining or the like. The elongated rod12may be a smooth rod or a textured rod such as rebar, with a smooth rod being shown in the drawings herein. In one embodiment of the invention, the resin control surface24extends between a first surface end26at a position adjacent the rod first end14, such as about one inch from the rod first end14, and a second surface end28positioned adjacent the rod second end16, such as within one to two inches of the rod second end16. Other lengths of the resin control surface24may be selected relative to the length of the bolt10, depending on the roof anchoring needs.

The resin control surface24defines a channel30extending between first and second surface ends26,28. As shown inFIG. 3, channel30is open at surface ends26,28, to allow curable resin to flow into and out from the channel30. In one embodiment, the width of the channel30is about 2 to 20% of the circumference of the resin control surface24. The resin control surface24may further include a plurality of raised spiral threads32, each spiral thread32being discontinuous with an adjacent thread32. The spiral threads32may be textured with ribs or ridges34as shown (FIG. 4), or the threads32may be smooth. Four ridges34are shown on each thread32, but this is not meant to be limiting as fewer or more ridges34may be included thereon. In addition, portions of the resin control surface24between spiral threads32may also be textured, such as with ribs or ridges36. Ridges36are shown as being generally disposed circumferentially around resin control surface24, but this is not meant to be limiting as other patterns or features of ridges36are encompassed by the present invention. The spiral threads32of the resin control surface24urge resin upwardly into the bore hole upon rotation of the bolt10during mixing of resin. The ridges34and36further assist in mixing and distributing the resin around the mine roof bolt10and provide surfaces for mechanical interaction with cured resin, thereby enhancing the interlock between the bolt10and surrounding rock. The channel30shown inFIG. 3has smooth surfaces. However, this is not meant to be limiting. In another embodiment, as shown inFIG. 5, bolt110includes resin control surface124defining channel130. Channel130includes ridges132or other features providing texture thereto. Such features also assist in mixing resin and provide additional surfaces for mechanical interaction with cured resin. All references to bolt10hereinafter are applicable to bolt110.

The mine roof bolt10of the present invention may be produced by coating the elongated rod12with a flowable polymer so that the coating has a thickness such as about at least 1 mm. The polymer is allowed to solidify on the elongated rod12and texturing is applied to the exterior of the polymer to form the channel30, spiral threads32and ridges34,36. The coating step may be performed by dip coating, injection molding and/or hot forging of the polymer resulting in an outer layer of a low density hard coating of the resin control surface24on an inner portion of higher density material (e.g., steel) of the elongated rod12. When the resin control surface24is formed from a polymer, the low density hard coating that is applied increases the overall diameter of a portion of the bolt10with a minimal increase in weight. Hence, while realizing the weight advantages of polymers as compared to metals used in an elongated rod12, such a composite bolt10can be advantageously sized to provide improved mixing of resin by creating a smaller annulus between the bolt in the location of the resin control surface24and the rock surrounding a bore hole. Likewise, with reduced annulus dimensions, less resin is required for bonding the bolt10within a bore hole with concomitant reduction in the size and quantity of shredded resin packaging film that remains after mixing.

In one embodiment of the invention, the elongated rod12is a smooth rod and the polymer coating is produced by molding to create the channel30, spiral threads32and ridges34,36. Typically, the thickness of the coating is sufficient to minimize the annulus between the resin compression layer and the bore hole wall at less than ⅛ inch or less than 1/16 inch. This reduces the overall weight of a similar sized mine roof bolt, produced from a higher density material (e.g., steel), particularly if the coating is a polymer of low density, such as about 2.0 g/ml or less.

In accordance with the present invention, the mine roof bolt10may be installed in a mine roof to provide support to a rock formation. In one embodiment of the method of supporting a mine roof, the mine roof bolt10is installed by inserting a frangible resin cartridge into a bore hole and inserting the mine roof bolt10into the bore hole. The drive head18of mine roof bolt10extends out of the bore hole. A post resin cure tensioning drive nut may be threaded onto the second end16of rod12until the tensioning drive nut cannot be advanced further along second end16when the tensioning drive nut abuts a stop or the mine roof itself, thereby inducing tension in the bolt. Continued rotation of the tensioning drive nut imparts rotation to the bolt and mixing of the resin. When the rod first end14contacts a resin cartridge in a bore hole, the cartridge ruptures releasing a curable resin. The mine roof bolt10is rotated about its longitudinal axis so that the resin control layer24and any exposed portion of elongated rod12mixes the contents of the resin cartridge. Resin released from the cartridge flows down along the bolt, particularly via channel30, which enhances the rate at which resin is distributed along the length of the bolt10. Channel30allows resin to flow directly down the length of the bolt. In this manner, resin is rapidly distributed along the length of the bolt before the resin cures. In addition, the resin control surface24compresses the resin between the exterior of the mine roof bolt10and the bore hole wall.

The resin control surface24serves several functions during installation of the mine roof bolt10and after it is installed in a mine roof. As the bolt10is rotated about its longitudinal axis, the spiral threads32on the resin control surface24urge resin upwardly toward the blind end of a bore hole, while the channel30allows for resin to be distributed along the length of the bolt. In this manner, these two features of the resin control surface24(threads32and channel30) together ensure distribution of resin along the bolt10. Distribution of resin along the length of the bolt ensures good bonding between the mine roof bolt10and the surrounding rock. Sufficient resin is required in the annulus between the mine roof bolt10and the bore hole wall to completely fill the annulus and allow for some of the resin to fill cracks and crevices in the rock to enhance the interlock between the rock and the mine roof bolt10.

The resin control surface24also serves to mix resin. The spiral threads32and the ridges34,36provide mixing surfaces to enhance mixing of the curable resin.FIG. 6depicts another embodiment of the present invention. Mine roof bolt210is similar to bolts10,110, except that second surface end28of resin control surface224is positioned intermediate first end14and second end216of rod212. Second end216of rod212is threaded to accept a tensioning nut218. Bolt210is installed in a mine roof similar to bolts10,110and may be tensioned after resin curing by threading nut218toward the first end14.

The length of resin control surface224may be selected based on the anchoring needs. Bolt210is suitable for use with less resin than bolts10,110and may function as a point anchor. In particular, bolt210may be installed in a bore hole with curable resin where the resin is provided primarily in the annulus between resin control surface224and the bore hole wall. A resin retaining ring (not shown), such as described in U.S. Pat. No. 4,865,489 or 5,181,800 both incorporated herein by reference, may be provided at a position intermediate the rod first and second ends14,16(such as adjacent the second surface end28) to retain resin in the annulus surrounding the resin control surface224. The resin retaining ring may be incorporated with the resin control surface224, such as by molding a resin retaining ring with surface224to produce an integrally formed resin retaining ring.

While the present invention has been described with reference to particular embodiments of a mine roof bolt and methods associated therewith, those skilled in the art may make modifications and alterations to the present invention without departing from the spirit and scope of the invention. Accordingly, the foregoing detailed description is intended to be illustrative rather than restrictive. The invention is defined by the appended claims, and all changes to the invention that fall within the meaning and the range of equivalency of the claims are embraced within their scope.