Threaded bar & rock bolts formed therefrom

THREADED BAR & ROCK BOLTS FORMED THEREFROM FIELD OF THE INVENTION

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The scope of this invention encompasses threaded bars and particularly a special thread pattern that works exceedingly well with hot rolled threaded bars. Additionally, it covers rock bolts derived from such hot rolled threaded bars.

BACKGROUND ART

Threaded bars find extensive utility in various engineering fields, including as reinforcement for concrete and as raw material for rock bolts. These bars are designed to endure substantial tensile forces, applied either through a nut threaded onto the bar or through concrete, resin, or grout that engages with the bar’s threads. This keying interaction ensures load transfer.

However, failures can occur due to several reasons: the threads may not be robust enough, causing the nut to strip the threads from the bar; the minimum diameter of the bar, influenced by thread depth, may be too small to bear the load; inadequate thread height relative to the bar diameter may fail to provide sufficient keying; or insufficient thread spacing may prevent the formation of a strong enough grout body. Any of these failures can have disastrous results.

Hot rolling entails forming the thread on the bar while it is still hot at a rod and bar mill. The thread is created perpendicular to the roll’s exit angle, which is constrained by the forming roll sizes and the gear meshing action. Longitudinal formation prevents sides of the bar from forming a thread, leading to either longitudinal ribs due to excess material or gaps in the thread due to insufficient material.

Conversely, cold rolling involves threading a cold round bar by rotating it between threading rolls aligned parallel to the bar. This process can make high-tensile steel brittle.

Different types of threaded engineering bars have distinct properties and associated defects. For example, "ALL THREAD" bar is cold rolled and typically brittle. "GEWI BAR" features a continuous single-start, coarse-pitched 10-12mm thread for a 20mm nominal diameter bar but tends to loosen nuts due to vibrations. Consequently, it is not ideal for mine roof support.

"REID BAR" also employs a continuous single-start, coarse-pitched thread and shares similar disadvantages with the "GEWI BAR" due to nut vibration issues.

"FREYSSI BAR" displays a double-start, coarse-pitch thread with a nominal diameter of 20mm and a pitch of 12mm. It’s essentially a continuous thread.

REID and FREYSSI BARS are pre-rolled with side grooves, making the bars oval before threading. This shape transforms back to round during threading, preserving high thread height and preventing material interference from the minor diameter of the oval bar into the thread.

A normal metric thread with a 3mm pitch tends to clog in the harsh environments where rock bolts are used. Hence, a 5mm rope thread, often cold rolled onto bars after peeling deformations, is preferred.

This traditional method of peeling and cold forming threads is just one way to create suitable threads on bars for nuts.

Other methods include cold rolling unpeeled bars or material from bar deformations, or hammering deformations to make the bar round before cold rolling the thread.

Novelty searches post-priority date revealed an International Patent Application PCT/AU01/00286 disclosed in WO 01/68287 (Gray). It describes a hot rolled threaded bar with semi-circular ribs tapering at the ends, ensuring that opposite sides of the bar remain protrusion-free and each rib extends circumferentially about 25% of the bar, aiding load transfer to resin, grout, or nut.

The manufacturing challenge with hot rolled threaded bars lies in maintaining roll alignment to achieve true helices. Typically, up to 10% rejection rates occur due to roll mis-alignment or mis-synchronisation. Short ribs exacerbate this issue compared to longer circumferential thread portions.

This led to the creation of separate organisations, each with distinct roles.

Rod and bar mills start with cold steel billets, producing long hot rolled bar lengths, typically around 12 meters, the maximum transportable via road. Customers include steel merchants or rock bolt manufacturers who cut, thread, and assemble bolts with necessary accessories.

Steel merchants may cut but not otherwise process the hot rolled threaded bar before selling it to wholesale distributors or rock bolt manufacturers. The latter distribute to end users, like mines and tunneling sites.

Rock bolts are generally fabricated in sizes increasing by 300mm from a minimum of 1.2m up to 12m.

Typically, rock bolts are delivered assembled with an anchor at the tip and a nut threaded at the end by the manufacturer before dispatch.

OBJECT OF THE INVENTION

The primary aim is to design an improved threaded bar with a thread pattern suited for effective keying, hot rolling, and harsh environments.

Another goal is cost reduction in rock bolt manufacturing by using hot rolled threaded bars directly without further processing and altering distribution methods. This strategy can potentially lower current rock bolt costs by approximately A$1.00 - A$2.00.

SUMMARY OF THE INVENTION

The invention proposes a single start helical thread pattern for rock bolts, reinforcing bars, and similar engineering bars. This pattern features every second turn of the thread absent, rendering each half-turn nearly 50% of the bar's circumference, enhancing load transfer capabilities.

This thread pattern yields a strong threaded bar as described.

The invention also covers rock bolts made from unprocessed hot rolled threaded bars with accessories threadably engaged.

The process of assembling rock bolts includes receiving a hot rolled threaded bar from a supplier, cutting it to length if needed, obtaining accessories from another supplier, and assembling the bolt on site.

(i) Receive a hot rolled threaded bar with partial threads from a supplier.

(ii) Cut the bar to the required length, if necessary.

(iii) Obtain accessories from another supplier that fit the threaded bar.

(iv) Assemble the rock bolt by engaging the accessories with the threaded bar.

Embodiments of the invention are shown in the accompanying figures:

Figure 1: Perspective view showing one side of the preferred hot rolled threaded bar.

Figure 2: Perspective view of the other side of the same bar.

Figure 3: Plan and inverted plan views of the bar.

Figure 4: Side elevation showing the side illustrated in Figure 2.

Figure 5: Side elevation showing the side illustrated in Figure 1.

Figure 6: Transverse cross-sectional view of the bar.

Figure 7: Cross-sectional view of a single thread with a specific profile.

Figure 8: Similar view to Figure 7 showing an alternative thread profile.

Figure 9: Longitudinal view, partly cross-sectioned, showing a rock bolt made from the threaded bar, grouted in a borehole.

Figures 1-6 illustrate a bar 1 with thread 2 and side gaps 3, where every second turn of the thread is absent, creating a discontinuous but coarsely spaced thread.

Although the thread appears coarse, the pitch is preferably 5mm. The minimum bar diameter remains largely unreduced, and the thread height provides efficient load transfer and keying.

The rock bolt 10, as shown in Figure 9, is made from a bar 1 that is simply cut to length. It includes a load plate 11, washer 12, and nut 13, with optional expansion device 19 for enhanced load transfer in a grouted borehole 14.

The nut 13 has a standard 5mm pitch but alternately engages only with certain threads on the rock bolt 10, enhancing load transfer through a longer nut, if necessary.

Possible thread profiles include trapezoidal (Figure 7) or semi-circular (Figure 8), and can be right or left-handed.

The described bar offers key advantages: high deformation height and circumferential extent, excellent keying and load transfer. Additionally, hot rolling enables the use of high tensile steels, ideal for mining with high yield strength requirements, and simplifies fabrication without additional processing, leading to significant cost savings.

Overall, this streamlined manufacturing and distribution process reduces bar transportation and handling costs. The advanced bar structure eliminates the need for further processing, making it straightforward to supply only the necessary accessories.

This innovation promises to reduce rock bolt costs by approximately A$1.00 - A$2.00 per unit.

The embodiments described are illustrative, and modifications can be made within the scope of the invention. For example, the washer 12 might be omitted, with the nut 13 featuring a domed leading end instead.

The term "comprising" is inclusive, meaning "having" or "including" not "consisting only of".

SN stands for "Store Norfors", named after its initial application in Store Norfors, Sweden. Alternatively, "SN" can stand for "Soil & Nail".

SN rock bolts, known as SN-Bolt, SN Rock Bolt, or SN Anchor, are used in mining, tunneling, and specialized civil engineering. A complete SN bolt set typically includes a threaded bar, a plate, a spherical washer, and a nut.

There are three principal anchoring methods for rock bolts: mechanical, grouted, and friction. The most prevalent is the fully grouted rebar, a steel threaded bar commonly grouted with cement or resin. A cable bolt, made from steel wires formed into a strand or rope, is typically installed in boreholes with cement grout.

For complex ground conditions like inconsistent fills, boulders, and weathered rock, high duty, ultra-high-strength heat-treated rock bolts offer superior anchoring effects. These rock bolts use premium hot-rolled steel rebars or deformed bars, treated with inductive heat technology (our patent). They are ideal for tunneling, mining, slope stabilization, and foundation support.

Our high-strength rock anchor bolts, available in grades MG500,600,700,785,830 Mpa, are stronger than standard hot-rolled rebars. This strength minimizes rock deformation, limits breakage areas, and withstands sudden impact pressure, ensuring mining operation safety.

Mechanical Properties

Steel Grade

Yield Strength

Tensile Strength

Elongation After Fracture

Elongation under Max Force

Impact Absorb Energy

Rel/MPa

Rm/MPa

A/%

Agt/%

kv2/J

NOT LESS THAN

Industrial Standard

MG335/MG400

335

500

15

40

MG500

500

700

22

100

Our company Standard

600

600

750

19

9

90

700

700

850

17

7.5

80

Want more information on 32mm hot rolled thread bar? Feel free to contact us.

General Dimension

Surface Type

Nominal Diameter

Nominal Cross Section

Weight per meter

Top strength level

Levorotation,

no longitudinal rib

16

201.06

1578

800

Levorotation,

no longitudinal rib

18

254.47

1998

800/1080

Levorotation,

no longitudinal rib

20

314.16

2466

800

Levorotation,

no longitudinal rib

22

380.13

2984

800

Levorotation,

no longitudinal rib

25

490.88

3853

800/1080

Levorotation,

no longitudinal rib

28

615.75

4834

800

Levorotation,

no longitudinal rib

32

804.25

6313

800/1080

Production Process

1. 600MPa ~ 700MPa

Raw material — Heat Treatment — Mechanical Property Test — Sawing/Cutting — Reduce diameter — Screw thread — Test — Packaging — Stock in

2. 335MPa ~ 500MPa

Raw Material — Reduce diameter — Screw thread — Test — Packaging — Stock in

Application

• Coal mining
• Hard rock mining
• Civil construction
• General construction
• Geotechnical and Shoreline
• Bridges
• Dams
• Infrastructure
• Tunnels

Safety Certificate of Approval for Mining Products

MODEL/TYPE: MSGLW-335/16,18,20,22,25

MSGLW-400/16,18,20,22,25

MSGLW-500/16,18,20,22,25

MSGLW-600/16,18,20,22,25

MSGLW-700/16,18,20,22,25

More information for our certificates, please contact [email protected]. Thank you!

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