GB2563002B

GB2563002B - A rock bolt coupling system, and method of manufacture

Info

Publication number: GB2563002B
Application number: GB1708201.7A
Authority: GB
Inventors: Robert Corbett Michael
Original assignee: Rand York Castings Pty Ltd
Current assignee: Rand York Castings Pty Ltd
Priority date: 2017-05-22
Filing date: 2017-05-22
Publication date: 2019-06-26
Anticipated expiration: 2037-05-22
Also published as: GB2563002A; ZA201803353B; GB201708201D0

Description

A rock bolt coupling system, and method of manufacture

Field ofthe invention

The invention relates to rock bolts for use in mining and tunneling applications.

Background to the invention

Resin bonded rock bolts or roof bolts are widely used in the mining and tunnelling industries for underground strata control. According to ASTM standards, rock bolts are hot-rolled bars with a thread and nut at one end, to be used as a means to support the surrounding rock in the underground excavations or alternatively to hold down equipment or foundations.

In order to meet performance requirements, deformed bar is used to manufacture the rock bolts to facilitate adequate mixing of the resin and thereafter provide sound bonding of the resin to the steel. The deformations are in the form of ribs protruding from the core of the bar which results in a larger outer rib diameter. According to ASTM standards, a threaded coupling system must be capable of achieving the actual tensile and ultimately the yield values of the rock bolt, with which they are to be used. The coupling system needs to be stronger than the bar itself so that there is no breakage of the coupling before the bar.

In many instances, the mining height is lower or smaller than the length of the rock bolts required to provide adequate support of the surrounding rock. Cable anchors, which are relatively flexible, as well as parallel threaded coupled bolts, have been used to alleviate this problem.

Cable anchors require large diameter drill holes to accommodate the fittings and fixtures whilst installation is difficult and time consuming. These two factors are costly. In addition to this, cable anchor installations are generally filled with a cement based grout which requires time to cure before becoming fully effective and be able to provide the ultimate tensile load.

The use of rock bolts which utilise parallel threaded couplings, partly overcomes the costs of cable anchors but they in themselves have certain cost implications such as larger holes needing to be drilled to facilitate the diameter of the separate coupling element, which is substantially larger in diameter than the bar and results in greater resin usage, higher drilling costs and installation times.

It is imperative that a coupling system utilised in a rock bolt will, at least, equal the ultimate tensile load of the deformed bar. A parallel threaded male and female coupling system is intrinsically reliant on tolerances between the male and female thread and as a result the diameter and length ofthe coupling is designed, in consideration ofthe tolerances, to meet the ultimate tensile load requirements. The steel used in the coupling element is of higher grade steel which increases the costs.

The outer diameter of parallel threaded couplings would therefore, of necessity, be considerably larger than the outer rib diameter of the bar. This means that a drill hole diameter required to accommodate the diameter of the coupling would be considerably greater than the diameter of a drill hole diameter which would be required to accommodate only the outer rib diameter of the bar.

Drilling of larger diameter drill holes is more costly and requires an increased volume of resin resulting in further costs. Furthermore, there is a possibility that the required annulus of resin between the deformed bar and the sides of the drill hole will be exceeded, and cross-threading can occur.

Coupled bolt systems have generally been made up of two lengths of rods each having a male thread machined on one end and joined together by means of a separated coupler having a female thread machined in each end so that the male and female threads are complementally joined to each other albeit that the threads may be parallel or tapered. In some instances coupled bolts may be made of more than one coupling system.

Machining of the parts and the threads of male and female threads of a coupling system is the traditional method of manufacturing these rock bolts. This is a time consuming and costly process. Furthermore, the tensile load capability is reduced as a result of the removal of material during the process.

Tapered threading eliminates the disadvantages of parallel threading in that the tapered male thread interlocks with the tapered female thread. Parallel male and female threading does not perform in this way and therefore the diameter and length must be, of necessity, larger to achieve the ultimate load of the rock bolt.

Taper threads have been used in the construction industry, but do not have tensile needs. A coupling system of a rock bolt is required to at least equal the ultimate load of the rock bolt. A taper which improves male to female thread interlocking with improved ultimate load qualities and in addition give rise to added benefits in the dimensions of forged parts, is not readily available. Currently available rock bolts require a greater diameter and length to achieve the ultimate load of the rock bolt. Parallel threading may only be machine cut, must be square such that the male and female abut with each other, otherwise it will not perform optimally.

Some of the existing art in the field will be described below.

Patent US5131204 is directed at the civil and construction industry and employs a taper threaded coupling in a reinforcing steel rod, in conjunction with an adhesive or soldering to prevent loosening of the screw connection. It counteracts dynamic loads on the connection or “swinging”. The patent specifies a taper thread of between 3° and 10°. The applications and objectives of the above patent are different to that of the invention disclosed herein, in that the tensile load or performance of the coupling system is not a factor.

Patent US5169183 relates to exploration drilling in the petroleum industry and may employ the tapered coupling system in order to increase the angle at which drilling can be conducted. The coupling system includes a taper thread of any profile but preferably a trapezoidal thread. This patent is not related to the mining and tunnelling industries, and the tensile load or performance of the coupling system is not a factor.

Patent W02000/19056 relates to percussion drilling and employs a tapered coupling system. The ‘thickening’ of the material of the socket or female end abutting the drill bit skirt reduces the rate at which breakages occur at the drill bit end of the drill rod. Rock bolts are not analogous to drilling or drill rod elements.

All three the above patents include coupling systems which do not employ a hot forged male and female portion and utilise a taper thread for different applications and objectives.

Patent WO200325345 relates to a rock bolt having a coupling system which has a female end which is upset to a required diameter but does not employ a tapered threaded system.

Patent ZA2014/08984 relates to a roof bolt for use in the mining industry, which is formed by a hot forging process. The patent belongs to the applicant in respect of the invention disclosed herein. The roof bolt or rock bolt described in the patent is not configured to have a tensile strength at least the same or more than the solid deformed bar, and does not have an ultimate load of at least the same as or more than the ultimate load strength of the solid deformed bar to which it is coupled. The applicant is of the opinion that the invention is directed towards novel and inventive improvements ofthe above rock bolt.

Hot forging of steel is a well-known process but when applied to the invention herein described, may be considered new and inventive. It eliminates the need for any machining requirements and substantially increases the load capacity ofthe coupling system.

The applicant is of the opinion that the invention provides an improved method of manufacturing a rock bolt coupling system which overcomes and/or partially alleviates the abovementioned problems, and a rock bolt coupling system which has improved load performance and eliminates tolerances.

Summary of the invention

According to an aspect of the invention, there is provided a method of manufacturing a rock bolt coupling system which has an ultimate load of at least the same as or more than the ultimate load strength of a solid deformed bar to which it is coupled, the method including: multistage hot forging, a first length of deformed bar to form a conical male portion at an end region thereof which includes a fully formed male thread having a taper angle between 3° and 8°; and multistage hot forging a second length of deformed bar into a female portion at an end region thereof including a socket having a fully formed female thread having a taper angle between 3° and 8°, wherein the female thread is complemental to the male thread and the socket has an outer diameter substantially equal to or larger than the outer rib diameter of the deformed bar.

According to a further aspect ofthe invention, there is provided a rock bolt coupling system manufactured according to the above method, wherein the coupling system has a tensile strength at least the same as but preferably more than the solid deformed bar, wherein the coupled rock bolt includes at least two lengths of deformed bar, an end ofthe first length of the deformed bar including a male portion conically formed with a fully formed male thread having a taper angle of between 3° and 8°, and an end of the second length of the deformed bar having a coupling portion formed with a socket and a fully formed female thread having a taper angle of between 3° and 8° which is complemental to the male thread, the socket having an outer diameter substantially the same as the outer rib diameter of the deformed bar.

The socket may be forged such that the outer diameter of the socket may be substantially the same as the outer rib diameter of the deformed bar. The upset socket or larger diameter may assist in preventing the breaking of the rock bolt coupling system where the root diameter is smaller, which would be at the coupling joint. In the preferred embodiment of the invention, the socket is concentric, and in turn the coupling system is concentric, thereby preventing and/or limiting breakage of the rock bolt at the thinnest or weakest point.

The taper threaded male and female portions may fully interlock with each other, thereby assisting in eliminating tolerances and providing increased load performance. The tapered threads of between 3° and 8°, may improve the interlocking of the male and female threads and thereby increase the ultimate load performance of the rock bolt. The male and female portions may be configured such that they do not abut with each other when coupled. The male portion and socket may be configured such that there is a space between the end of the male portion and the distal end of the socket when the male and female portions are coupled together which assists in interlocking the threads fully. In this preferred embodiment of the invention, one or more of the threads on the male portion may be exposed. The exposable threads may be exposed externally once the male and female portions are engaged and coupled together. The minor or root diameter of the exposed threads of the male portion, which is located between the end of the female portion and the deformed bar, may be larger than the core diameter of the deformed bar, further strengthening the coupling system, and increasing elongation capability.

The fully formed tapered male and female threads, which by way of the resultant Thermo-Mechanical strengthening, may further increase the load capability of the male and female threads. A rock bolt coupling system having a load capacity which is greater than the deformed bar affords the ability to achieve the maximum yield of the rock bolt.

The hot forged male and female tapered threads with increased strength may permit the coupling portion diameter to be substantially the same as the outer rib diameter of the deformed bar.

Hot forging to form the male and female portions complete with formed threads may eliminate the need for costly and performance inhibiting machining to be used to manufacture the rock bolt coupling system. Hot forging the threading makes it stronger than machining the threading, as no material is lost or removed.

During the hot forging process, the conical shape of the male portion may be suitably formed so that the root diameter of the exposable threads formed will be greater than the core diameter of the deformed bar. On the other hand, using machined threads in the male portion may result in a root diameter of the exposed threads which is less than the core diameter of the deformed bar and therefore will be the weakest segment of the coupling assembly.

The tapered male and female threads, during assembly, may locate more readily and in environments where installation of the rock bolt is more difficult and/or onerous, they may reduce the possibility of cross threading which allows for easier installation.

Installation of coupled rock bolts takes considerably less time to install than it takes to install a cable anchor.

Brief description ofthe drawings

The invention will now be explained by way of reference to the following drawings, in which:

Figure 1 is a side view of a rock bolt including a rock bolt coupling system, showing a cross-section through the rock bolt coupling system;

Figure 2 is a side view of the male portion of the rock bolt coupling system, showing a cross-section through the male thread of a male portion; and

Figure 3 is a side view of the female portion of the rock bolt coupling system, showing a cross-section through the female thread and socket of a female portion.

Detailed description ofthe drawings

It should be appreciated to those skilled in the art that, without derogating from the scope of the invention as described, there are various alternative embodiments or configurations or adaptions ofthe invention and its features.

Referring to Figure 1, the rock bolt coupling system 10 has a tensile strength at least the same as but preferably more than the solid deformed bar 12, wherein the coupled rock bolt assembly 14 includes at least two lengths of deformed bar 12, an end of the first length 12a ofthe deformed bar 12 including a male portion 16 conically formed with a fully formed male thread 18 having a taper angle of between 3° and 8°, and an end of the second length 12b ofthe deformed bar 12 including a female portion 26 having a coupling portion formed with a socket 20 and a fully formed female thread 22 having a taper angle of between 3° and 8° which is complemental to the male thread 18, the socket 20 having an outer diameter substantially the same as the outer rib 24 diameter of the deformed bar 12.

Referring to Figure 3, the socket 20 is forged such that the outer diameter of the socket 20 is substantially the same as the outer rib 24 diameter of the deformed bar 12. The upset socket 20 or larger diameter assists in preventing the breaking of the rock bolt coupling system 10 where the root diameter is smaller, which would be at the coupling joint. The socket 20 is concentric, and in turn the coupling system is concentric, thereby preventing and/or limiting premature failure ofthe rock bolt assembly 14.

Referring again to Figure 1, the taper threaded male and female portions 16, 26 fully interlock with each other, thereby assisting in eliminating tolerances and providing increased load performance. The tapered threads improve the interlocking ofthe male and female threads 18, 22 and thereby increase the ultimate load performance ofthe rock bolt assembly 14. The male and female portions 16, 26 are configured such that they do not abut with each other when coupled. The male portion 16 and socket 20 are configured such that there is a space 28 between the end of the male portion 16 and the distal end of the socket 20 as well as a further space 28a between the end of the female portion 26 and the deformed bar 12 when the male and female portions 16, 26 are coupled together which assists in interlocking the threads fully, and which is the safeguard against abutment. One or more ofthe threads on the male portion 16 are exposed, the exposable threads 30 being exposed externally once the male and female portions are engaged and coupled together. The minor or root diameter of the exposable threads 30 of the male portion 16, which is located between the end of the female portion 26 and the deformed bar 12, is larger than the core diameter of the deformed bar 12, further strengthening the coupling system 10, and increasing elongation capability.

The fully formed tapered male and female threads 18, 22, which by way of the resultant Thermo-Mechanical strengthening, further increase the load capability of the male and female threads 18, 22. A rock bolt coupling system 10 having a load capacity which is greater than the deformed bar 12 affords the ability to achieve the maximum yield of the rock bolt 14.

The hot forged tapered male and female threads 18, 22 with increased strength permit the coupling portion diameter to be substantially the same as the outer rib 24 diameter of the deformed bar 12.

Hot forging to form the male and female portions 16, 26 complete with formed threads eliminate the need for costly and performance inhibiting machining to be used to manufacture the rock bolt coupling system 10. Hot forging the threading makes it stronger than machining the threading, as no material is lost or removed.

During the hot forging process, the conical shape of the male portion 16 is suitably formed so that the root diameter of the exposable threads 30 formed will be greater than the core diameter of the deformed bar.

The tapered male and female threads 18, 22, during assembly, locate more readily and in environments where installation of the rock bolt assemblies 14 are more difficult, onerous and/or arduous, they reduce the possibility of cross threading which allows for easier installation. A rock bolt 14 which includes the rock bolt coupling system 10.

Installation of the coupled rock bolts 14 with the rock bolt coupling system 10 takes considerably less time to install than it takes to install a cable anchor.

Claims

1. A method of manufacturing a rock bolt coupling system which has an ultimate load of at least the same as or more than the ultimate load strength of a solid deformed bar to which it is coupled, the method including: multistage hot forging, a first length of deformed bar to form a conical male portion at an end region thereof which includes a fully formed male thread having a taper angle between 3° and 8°; and multistage hot forging a second length of deformed bar into a female portion at an end region thereof including a socket having a fully formed female thread having a taper angle between 3° and 8°, wherein the female thread is complemental to the male thread and the socket has an outer diameter substantially equal to or larger than the outer rib diameter of the deformed bar.

2. The method as claimed in claim 1, wherein the socket is forged such that it is concentric, and in turn the coupling system is concentric.

3. The method as claimed in any one of the preceding claims, wherein during the hot forging process, the conical shape of the male portion is suitably formed such that the root diameter of exposable threads formed will be greater than the core diameter of the deformed bar, wherein the exposable threads are exposed externally once the male and female portions are engaged and coupled together.

4. A rock bolt coupling system, wherein the coupling system has a tensile strength at least the same as or more than the solid deformed bar, wherein the coupled rock bolt includes at least two lengths of deformed bar, an end of the first length of the deformed bar including a male portion conically formed with a fully formed male thread having a taper angle of between 3° and 8°, and an end of the second length of the deformed bar having a coupling portion formed with a socket and a fully formed female thread having a taper angle of between 3° and 8° which is complemental to the male thread, the socket having an outer diameter substantially the same as the outer rib diameter of the deformed bar.

5. The system as claimed in claim 4, wherein the socket is concentric, and in turn the system is concentric.

6. The system as claimed in either claim 4 or 5, wherein the male portion is conical in shape.

7. The system claimed in any one of claims 4 to 6, wherein one or more of the threads on the male portion include exposable threads, the exposable threads being exposed externally once the male and female portions are engaged and coupled together.

8. The system claimed in claim 7, wherein the root diameter of the exposable threads is greater than the core diameter of the deformed bar.

9. The system claimed in any one of claims 4 to 6, wherein the male and female portions are configured such that they do not abut with each other when coupled, wherein the male portion and socket are configured such that there is a space between the end of the male portion and the distal end of the socket when the male and female portions are coupled together.