WO2024044795A1

WO2024044795A1 - Threaded rock bolt and nut assembly with asymmetric thread

Info

Publication number: WO2024044795A1
Application number: PCT/ZA2023/050044
Authority: WO
Inventors: Greig KNOX
Original assignee: Innovative Mining Products (Pty) Ltd
Priority date: 2022-08-20
Filing date: 2023-08-01
Publication date: 2024-02-29

Abstract

The invention provides a cable anchor support system which includes a cable anchor of a first steel material, a section of which is formed with a male thread, a nut of a second steel material which has a hole which is formed with a female thread, wherein the first steel material is harder than the second steel material, and wherein a base of the female thread is at least 1.5 times that of a base of the male thread.

Description

THREADED ROCK BOLT AND NUT ASSEMBLY WITH ASYMMETRIC THREAD

FIELD OF THE INVENTION

[0001] The invention relates to generally to a cable anchor and more specifically to a cable anchor and a nut assembly.

BACKGROUND OF INVENTION

[0002] A typical cable anchor support system includes a cable anchor which has a mechanical anchor at one end and a tensioning means at the other end. The anchor body carries a faceplate. [0003] When the anchor is installed in a rock hole and the mechanical anchor is expanded to anchor the anchor body in the hole, the tensioning means is actuated to tension the body between the anchor and the faceplate, pulling the faceplate into load supportive contact with the rock face. The tensioning means typically includes a nut which is torque-tightened against the faceplate. [0004] A problem emerges along a threaded interface between the nut and the bolt. Due to movement in the rock face, the force imposed on the faceplate increases over time. As the faceplate bears down on the nut, this increasing force translates into an increase in shear stress across the female and male threads of the nut and the bolt respectively.

[0005] As the cable anchor is made from a steel of a higher tensile strength than the nut, the female threads of the nut tend to shear before the male threads of the cable anchor. With the female threads failing to hold the nut in a longitudinally fixed position relatively to the cable, the support system losses its load supportive capacity.

[0006] The invention at least partially solves the aforementioned problem.

SUMMARY OF INVENTION [0007] Hereinafter the following terms bear the following meanings in respect of a male or a female thread in cross section:

“width” refers to a width of a crest of the thread;

“base” refers to a width of the thread between points of intersection of a leading flank and a trailing flank of the thread with a root of the thread; and “area” refers to an area of the respective thread defined between the base, the crest, and the leading and trailing flanks. [0008] A cable anchor support system which includes; a cable anchor of a first steel material, a section of which is formed with a male thread; a nut of a second steel material which has a hole which is formed with a female thread; wherein the first steel material is harder than the second steel material; and wherein a base of the female thread is at least 1.5 times that of a base of the male thread (i.e. , a ratio of the base of the female thread to the male thread of 3:2).

[0009] Preferably the ratio of the base of the female thread to the male thread is 2:1.

[0010] Preferably, the first steel material is a steel material with an ultimate tensile strength of at least 1500Mpa [0011] Preferably, the second steel material is EN8 with an ultimate tensile strength of 550Mpa.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The invention is described with reference to the following drawings in which: Figure 1 is a view in longitudinal section of a cable anchor support system in accordance with the invention;

Figure 1A is a magnified view of a part of the system highlighted in a dotted circle on Figure 1 ; Figure 2 is a view in elevation of a proximal end section of the system of Figure 1 ;

Figure 3 is an isometric view of a cable anchor from a proximal end, with a nut engaged to the proximal end section;

Figure 4 diagrammatically illustrates a view in cross section through a male and female thread of the proximal end of the cable anchor and the nut respectively, showing a plurality of sections of the male and female thread; and

Figure 5 diagrammatically illustrates a view in cross section through a male and female thread of the proximal end of the cable anchor and the nut respectively, showing a pair of sections of the male thread and a single section of the female thread.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0013] Figure 1 illustrates a cable anchor support system 10 in accordance with the invention. [0014] The cable anchor support system 10 includes an elongate multi-strand steel cable anchor 12, of a first steel material, which extends between a distal end 14 and a proximal end 16.

[0015] The cable anchor has a proximal end portion 18, ending at the proximal end 16, which is formed with a male thread 20.

[0016] In this particular non-limiting example, the cable anchor support system 10 includes a tubular steel sleeve 22 which is swaged to the cable anchor 12.

[0017] The system 10 also includes a grout inlet assembly 26 comprising a spherical seat 26.1, with a domed leading end 28, and a barrel 26.2. A cylindrical passage 32 runs through the assembly 26, connecting its ends. On a side wall of the barrel, there are one or more grout inlets 34 that allow communication between the exterior of the barrel and the passage.

[0018] The cable anchor 12 passes through the passage 32, connecting it to the grout inlet assembly 26. [0019]To hold the grout inlet assembly 26 on the cable anchor 12, a nut 36 is utilized. The nut features a hole 38 with a female thread 40, which engages the male thread 20 along the proximal end portion. Attaching the nut between the grout inlet assembly and the proximal end 16 of the cable anchor 12 holds the grout inlet assembly in place. [0020] A mechanical anchor 41 , engaged to a leading end section of the cable body 12, a grout tube 42, which encloses a substantial length of the cable anchor 12, and a faceplate 44 completes the cable anchor support system 10.

[0021] Figure 1 illustrates the grout tube 42 in its full extent. However, for ease of illustration, the grout tube is shown truncated in Figure 2.

[0022] In use, the cable anchor support system 10 is inserted in a rock hole (not shown) with the distal end 14 of the cable body 12 leading.

[0023]To secure the distal end within the rock hole, the mechanical anchor 38 is actuated to radially expand into resistive contact with walls of the rock hole.

[0024] Once the distal end 14 of the anchor body 12 is securely anchored within the rock hole, the nut 36 is rotated along the male threads 20 of the proximal end portion 18, causing it to move upwards. This upward movement brings the spherical seat 26.1 into contact with a flared section 46 of the grout tube 42, which, in turn, contacts the faceplate 44. Finally, the faceplate comes into contact with the rock face adjacent to the rock hole, completing the anchoring process.

[0025] Subsequently, by applying torque and tightening the nut, the cable body 12 is tensioned between the mechanical anchor 38 and the nut 36. This tensioning action exerts force on the faceplate 44, bringing it into load- supportive contact with the rock face. [0026] The interaction of the tubular sleeve 22 with the grout tube 42 prevents the cable body 12 from twisting during this action. However, as the configuration of the rock anchor support system 10 is merely exemplary, with the invention characterised by the configuration of the male and female threads of the cable body 12 and the nut 36 respectively, an explanation as to the precise mechanics on how this is achieved is unnecessary.

[0027] The cable anchor support system 10 is now prepared for the grouting process. Grout is introduced into the system through the grout inlets 34 located on the barrel 26.2. Once inside, the grout flows out at an upper end 48 of the grout tube, creating a cascading effect within the annular space between the grout tube and the walls of the rock hole.

[0028] Once the grout hardens within the annular space, the system 10 is fully installed and ready to play its rock supportive function.

[0029] Due to movement in the rock face, the force exerted on the faceplate 44 increases over time. Consequently, the faceplate applies greater pressure on the nut 36, leading to an escalation in shear stress within the female and male threads of the nut and the bolt, respectively (40, 20).

[0030]As the cable anchor is made from a steel of a higher tensile strength, for example a tensile strength of at least 1500Mpa, than the nut (which has a tensile strength of for example 550Mpa), the male and female threads are asymmetrically formed. Each section of the female thread (respectively designated 40.1 , 40.2, 40.3) of the nut 36 is formed to have a larger area in cross section (and a larger base) when compared to a counterpart section of the male thread (respectively designated 20.1, 20.2, 20.3). This is best illustrated in Figures 4 and 5.

[0031] Referring to Figures 4 and 5, each female thread section (40.1 , 40.2, 40.3) has a cross-sectional area, designated A, which is defined between a base-line B, a crest-line C, and leading and trailing flanks (respectively designated 50.1 and 50.2), and each male thread section has a cross-sectional area, designated a, which is defined between a base-line b, a crest-line c, and leading and trailing flanks (respectively designated 52.1 and 52.2).

[0032] By adapting the female thread 40 to have a higher mass (as defined by the proportional dimensions of either cross-sectional area or base length) than the counterpart male thread 20, it compensates for the lower inherent tensile strength of the steel material used in the nut 36, ensuring that the female and male threads will fail at the same load.

Claims

1. A cable anchor support system which includes a cable anchor of a first steel material, a section of which is formed with a male thread, a nut of a second steel material which has a hole which is formed with a female thread, wherein the first steel material is harder than the second steel material, and wherein a base of the female thread is at least 1.5 times that of a base of the male thread

2. A cable anchor support system according to claim 1 wherein a ratio of the base of the female thread to the male thread is 3:2.

3. A cable anchor support system according to claim 1 or 2 wherein a ratio of the base of the female thread to the male thread is 2:1

4. A cable anchor support system according to anyone of claims 1 to 3 wherein the first steel material has an ultimate tensile strength of at least 1500Mpa.

5. A cable anchor support system according to anyone of claims 1 to 4 wherein the second steel material has an ultimate tensile strength of 550Mpa.