AU651251B2 - A rock bolt nut - Google Patents

Description

651251

AUSTRALIA

Patents Act 1952 COMPLETE SPECIFICATION FOR A STANDARD PATENT

(ORIGINAL)

Regulation 3.2 o Name of Applicant: WEST FOOTSCRAY ENGINEERING PTY.LTD.

Actual Inventor(s): o Address for Service: Invention Title: DAVIES COLLISON CAVE, Patent Attorneys, 1 Little Collins Street, Melbourne, 3000.

"A Rock Bolt Nut" r666 Details of Associated Provisional Application No: PK 9033/91 filed 22 October, 1991 The following statement is a full description of this invention, including the best method of performing it known to me/us: -2- A ROCK BOLT NUT The present invention relates to rock bolt nuts.

A rock bolt conventionally comprises a threaded bar which may be of up to several metres in length. The bar is inserted into a hole drilled into the roof of a mine tunnel or the like, and is set at its upper end within the hole by a fast setting epoxy cement. The bar is then tensioned against the underside of the roof by a nut screwed onto the lower end of the bar and bearing against a washer and plate which in turn bear the underside of the roof. Conventionally, the upper end of the bar is set within the hole by screwing the bar upwardly into the hole to a position at which the upper end of the bar breaks a two part capsule of the cement and hardener within the upper end of the hole whereby the two components mix and rapidly set in order to anchor the upper end of 15 the bar, typically within about twenty seconds. It is conventional to use the same nut on the lower end of the bar initially to screw the bar into the hole prior to, and during, setting and, after the bar has been set, to thereafter tension the bar. For this purpose there has been proposed a nut of •conventional forrm, but with a resin plug at its outer end. The resin plug bears against the lower end of the bar and has a strength sufficient to withstand the forces generated during screwing of the bar into the hole and breakage of the epoxy capsule. As soon as the bar is set, increased torque applied to the nut dislodges the plug whereby the nut can then be screwed up along the bar into engagement with the underside of the washer to thereby tension the bar.

Although such a nut works satisfactorily in practice, it is relatively expensive to produce. The resin plug is cast within the end of the through bore and can take many hours before it is properly set.

According to the present invention there is provided a nut for use with a rock bolt, wherein the bore of the nut is closed at one end by a disk integrally formed with the body of the nut, the disk being connected to the nut body via a fracture zone which permits break out of the disk from the nut body 921019,rshspe.008,rockbolt-spe,2 -3to open the end of the bore for passage of the nut along the rock bolt at application of a predetermined torque.

In one embodiment of the invention, the fracture zone is defined by an annular groove machined into the disk in alignment with the internal bore of the nut. The depth of the groove facilitates accurate control of the torque characteristics at which the disk will be sheared from the nut body.

Advantageously the basic nut body including the disk is formed from a blank by forging and the internal bore of the nut body is then machined to facilitate subsequent tapping, machining of the groove being effected as part of the machining of the internal bore.

In other embodiments, the fracture zone is formed during forging and is 15 then machined to final thickness, preferably when machining the bore.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:- Figure 1 is a section through a nut in accordance with one embodiment of the invention; Figure 2 shows an enlarged detail of Figure 1; Figure 3 is a schematic section showing a blank after forging into a basic shape prior to machining and tapping; Figure 4 is a section through an alternative embodiment of nut; and Figure 5 shows an enlarged detail of Figure 4; and Figures 6 and 7 are views similar to Figure 5, but showing other alternative embodiments.

921019,rsspe.008,rockbotspe,3 In accordance with one preferred embodiment of the present invention, a nut for a rock bolt is formed at its outer end with an integral blanking disk 2 which closes the outer end of the threaded bore 4 of the nut. The disk 2 can be severed or otherwise fractured from the remainder of the nut upon application of a predetermined torque in order to permit the nut to perform setting and tensioning operations on a rock bolt as described previously. The disk 2 is formed integrally with the nut body as part of a forging operation to form the basic nut body and does not result in any significant increase in the manufacturing cost of the nut.

In one preferred manufacturing process, bar stock is cropped into individual blanks in a forging press. The blanks are then transferred to a forging die in which each blank is forged into a nut-like annular form (see S.Figure 3) with a hexagonal outer periphery and an inner bore which is closed 15 at one end by a disk. The peripheral surface of the bore is then machined to S-permit subsequent tapping. During the machining operation a peripheral Vshaped groove 6 is cut into the inner face of the disk, the outer wall 6a of the groove 6 forming a continuation of the machined bore. In this manner there is thereby formed an annular weakened zone or land between the base of the groove 6 and the outer surface of the disk 2 and the disk 2 will shear from the nut body in at annular zone upon application of increased torque after setting of the rock bolt. The depth of the groove 6 will determine the torque at which the disk 2 is sheared from the nut body and accurate control of the torque characteristics of the nut can be obtained by machining the groove to an appropriate depth. The machining operation to cut the groove 6 is part of a machining operation which is to be undertaken prior to tapping of the internal bore and results in relatively small increase in cost over that involved in manufacturing a standard nut with a through-bore.

In the embodiment of Figures 1 to 3, after forging the blank, the outer surface of the disk 2 is substantially flush with the outer end of the nut body, although other configurations are possible as will now be described.

921019,rshspe.008,rockbolt~sp,4 In the configuration as shown in Figures 4 and 5, the blank is formed into a configuration in which the disk 2 is located axially outwardly of the outer end of the nut body and is connected to the nut body by an annular land at the outer periphery of the disk. The internal periphery of the annular land is machined during the machining of the internal bore of the nut body to provide controlled torque characteristics for shearing of the land.

In the embodiment of Figure 6, the blank is formed into a configuration in which the disc 2 is located axially inwardly of the end of the nut body. The inner axial surface of the disc is machined to a predetermined thickness to provide controlled torque characteristics at which the outer periphery of the disc 2 will shear from the body of the nut.

In the embodiment of Figure 7, the blank is formed into a configuration 15 in which the disc 2 is flush with the end of the nut body and the outer surface S"of the body is formed with a chamfer 12 which defines with the inner surface of the disc 2 an annular land 14 at which the disc 2 will shear. The inner axial surface of the disc 2 is machined to provide a predetermined width for the land 14.

In the embodiments of Figures 4 to 7, when the disc shears upon application of the required torque the part of the land which remains on the nut after shearing tends to be located radially outwardly of the thread whereby the rock bolt does not need to tap that part, or at least that part does not extend significantly into the thread of the bolt. Thus, the nut is easily driven on the bolt after shearing of the disc.

Rock bolt nuts with integral fracture discs in accordance with the preferred embodiments of the invention can be manufactured for considerably less cost and more quickly than resin plug nuts as described previously.

The embodiments have been described by way of example only and modifications are possible within the scope of the invention.

Claims (5)

1. A nut for use with a rock bolt, wherein the bore of the nut is closed at one end by a disk integrally formed with the body of the nut, the disk being connected to the nut body via a fracture zone which permits break out of the disk from the nut body to open the end of the bore for passage of the nut along the rock bolt at application of a predetermined torque.

2. A nut according to claim 1, wherein the fracture zone comprises an annular thin-walled zone between the body and the disc.

3. A nut according to claim 2, wherein the nut body including the disc is formed from a blank by forging and the bore is machined to facilitate tapping, the thin-walled zone being sized by a machining operation.

4. A nut substantially as hereinbefore described with reference to the accompanying drawings.

5. A rock bolt for setting into the roof of a tunnel or the like, comprising a threaded bar having an upper end portion adapted to be set by a fast-setting cement into a hole drilled into the roof, and a nut according to any one of the preceding claims mounted at the lower end of the bar. DATED this 19th day of October, 1992. WEST FOOTSCRAY ENGINEERING PTY.LTD. By its Patent Attorneys DAVIES COLLISON CAVE 921019,rshspe.008,rockbl1Lspe,6 -7- ABSTRACT A rock bolt having an integral disc which closes one end of the bore. The nut body including the disk is formed from a blank by forging and the disk is connected to the nut body via a fracture zone which permits break out of the disk from the nut body at application of a predetermined torque to permit passage of the nut along the rock bolt. 921019,rshspe.0o8,rockboILspe,7