Product Description
In underground mining and long-hole drilling operations, the straightness of a borehole and the durability of your drill string directly determine productivity and cost per meter. RockHound’s ST68 Extension Rods and Guide Tubes are purpose-built to meet the extreme demands of modern hydraulic rock drills operating in hard and abrasive rock formations.
The ST68 thread profile features a significantly larger cross-sectional contact area compared to conventional T-series threads, delivering higher bending stiffness and superior energy transmission with minimal loss. Whether you are running a long-hole stoping operation or deep-hole bench drilling, RockHound ST68 tools are engineered to keep your drill string on trajectory — reducing deviation, minimizing rod breakage, and lowering your total drilling cost.
Material & Expertise
Premium Alloy Steel: ZK22CrNi3Mo (23CrNiMo)
Every ST68 Extension Rod and Guide Tube produced by RockHound starts with ZK22CrNi3Mo (23CrNiMo), a high-performance alloy steel specifically selected for its outstanding combination of fatigue strength, impact toughness, and case-hardening response.
| Property | Benefit in Service – Top Hammer Drill Rods |
|---|---|
| High fatigue strength | Withstands millions of high-frequency percussion cycles; significantly reduces risk of premature failure under cyclic loading in continuous drilling operations |
| Superior impact toughness | Resists cracking and brittle fracture in variable ground conditions, including fractured rock and sudden impacts |
| Excellent hardenability | Allows deep, uniform carburization for long-lasting wear resistance on surface while maintaining tough core |
| Corrosion resistance | Performs reliably in wet underground environments; minimizes internal bore pitting and external rust in high-humidity or corrosive flushing water conditions |
Want to understand why 23CrNiMo outperforms other common rod steels on the market? Read our in-depth comparison: Rock Drill Rod Material: 23CrNiMo vs Sanbar64
Advanced 20-Hour Carburizing Heat Treatment
Material quality alone is not enough. RockHound subjects every ST68 rod and tube to a rigorous 20-hour carburizing heat treatment cycle. This extended process builds a super-hard, wear-resistant outer case while preserving a tough, ductile core — the optimal structure for tools that must simultaneously resist surface abrasion and absorb massive shock loads with every drill stroke.
This is not a shortcut process. The extended cycle time ensures consistent case depth, uniform hardness distribution, and maximum metallurgical integrity across the entire length of the tool.
Learn more about why heat treatment duration matters: 20-Hour Heat Treatment in Rock Drilling Tools
Key Features
- ST68 High-Stiffness Thread Profile — Larger contact area and higher bending rigidity than T51, designed for powerful hydraulic drifters and deep-hole applications where thread fatigue and connection loosening are common failure modes. See how we achieve this precision: Thread Processing of Rock Drilling Rods
- Superior Hole Straightness Control — The ST68 Guide Tube’s enlarged outer diameter (D87 / 3½”) fills the annular space between tool and borehole wall, acting as a mechanical guide to resist lateral deflection and dramatically reduce hole deviation.
- Full-Length Carburization — Unlike tools with localized surface treatments, our ST68 rods are carburized along their entire length, ensuring uniform hardness, wear resistance, and corrosion protection from coupling to coupling.
- Optimized Flushing Geometry — The diameter-to-hole ratio of the D87 tube is engineered to maintain efficient cuttings evacuation, preventing packing and reducing the risk of stuck tools.
- Precision-Machined Couplings — Tight manufacturing tolerances on all mating surfaces minimize energy loss at each connection point in the drill string, transferring maximum impact energy to the drill bit.
Technical Specifications
ST68-D87-ST68 Guide Tube — Standard Size Reference
| Product Type | L (mm) | L (ft) | D (mm) | D (inch) | Product Code | Weight (kg) |
|---|---|---|---|---|---|---|
| ST68 Guide Tube | 1525 | 5′ 0″ | 87 | 3 7/16″ | 2A0-8715-2B2B | 35.5 |
| ST68 Guide Tube | 1830 | 6′ 0″ | 87 | 3 7/16″ | 2A0-8718-2B2B | 41.6 |
- Thread System: ST68 (2 3/4″)
- Custom Lengths Available: Yes — contact RockHound for non-standard configurations
- Recommended Drilling Diameter: 102 – 115 mm
- Compatible Drill Rigs: Heavy-duty hydraulic long-hole drifters (e.g., Atlas Copco Simba series, Sandvik DL series)
Applications
The ST68 system is the recognized standard for demanding long-hole drilling environments. Typical applications include:
- Underground Long-Hole Stoping — Delivers the hole accuracy required for large blast patterns at increasing level heights, reducing dilution and improving ore recovery.
- Raise Drilling & Boxhole Boring — Maintains directional control in upward and inclined drilling geometries where gravity-assisted deviation is a constant challenge.
- Tunneling & Pipe Umbrella / Forepoling — Ensures straightness in weak or fractured geological formations where conventional drill strings wander unpredictably.
- Deep-Bench Blast Hole Drilling — Ideal for open-pit and underground bench operations requiring holes exceeding 20 meters, where cumulative deviation becomes critical.
- Geothermal & Exploration Drilling — Where precision trajectory and tool longevity over long footage are both essential.
Maintenance Guide
Proper maintenance of your ST68 drill string extends service life significantly and protects your investment in capital equipment. Follow these professional best practices:
- Thread Lubrication Before Every Use — Apply a high-quality, molybdenum-based thread grease to all male and female connections before coupling. This prevents galling, reduces friction-induced heat, and eases disassembly after deep-hole drilling.
- Eliminate Blank Firing — Never operate the rock drill under no-load conditions (with no button bit engaged against rock). Blank firing sends uncushioned shock waves through the rod string, concentrating stress at thread roots and leading to rapid fatigue cracking.
- Inspect After Every Shift — Check all threads for signs of deformation, chipping, or corrosion pitting. Inspect rod bodies for visible cracks, especially at the shank end and mid-body. Remove any suspect rod immediately to avoid in-hole breakage.
- Rotate Rods in Service — Rotate drill rods within your working inventory to distribute fatigue loading evenly. Avoid running the same rod in the same position in the string every cycle.
- Correct Storage Conditions — Store rods horizontally on wooden racks in a dry environment. Apply a light coat of anti-rust oil to exposed metal surfaces for any storage period exceeding two weeks.
- Monitor Drill Rig Parameters — Consistently excessive percussion pressure or rotation speed beyond the drill rig manufacturer’s specification accelerates thread wear and rod fatigue. Always operate within rated parameters.
How to Choose the Right ST68 Tool
Choosing between an ST68 Extension Rod and a Guide Tube — and selecting the correct length — depends on several operational factors:
| Selection Factor | Guidance – ST68 D87 Guide Tube & Rod System |
|---|---|
| Rock Hardness | For UCS > 150 MPa (very hard abrasive rock), prioritize full-length carburized 23CrNiMo rods for maximum wear resistance and extended service life |
| Hole Depth | Holes > 15 m benefit significantly from Guide Tubes (D87) in the string to control deviation. The deeper the hole, the more tubes you should incorporate in the drill string |
| Blast Pattern Accuracy | High-precision stoping with tight collar-to-toe tolerances demands Guide Tubes throughout the entire drill string to minimize deviation and ensure consistent burden/toe spacing |
| Drill Rig Torque | Verify your drifter’s rated torque and impact energy are compatible with ST68 thread specifications; underpowered rigs cannot fully utilize the ST68 system’s torque transmission and may cause premature wear or failure |
| Flushing Medium | Both water and air flushing are compatible with the ST68 D87 tube geometry; confirm adequate flow rate for the selected hole diameter to maintain effective cuttings removal and cooling |
For a comprehensive breakdown of extension rod types, thread systems, and material selection criteria, consult our expert guide: What Is an Extension Drill Rod? Types, Materials & Selection Guide
About RockHound
RockHound is a specialist global supplier of rock drilling tools for the mining and construction industries. We combine advanced materials science, precision manufacturing, and deep application knowledge to deliver drill rods, guide tubes, bits, and accessories engineered for the world’s most demanding ground conditions.
FAQ
ST68 features a substantially larger cross-sectional area, greater root radius, and higher bending stiffness than T51. It is engineered for next-generation, high-power hydraulic drifters and deep-hole applications where T51 connections would be exposed to unacceptable levels of bending stress and fatigue loading.
An extension rod transmits percussive energy and rotation through the drill string. A Guide Tube (also called a Drill Tube) performs the same function but features a significantly larger outer diameter — in this case D87 (3½") — that fills the annular space in the borehole and physically guides the drill string, suppressing lateral deflection and maintaining hole straightness in long-hole applications.
Only if the thread profiles and dimensional tolerances are fully compatible. Mismatched threads — even within the same nominal designation — can lead to poor energy transfer, accelerated thread wear, and in-hole breakage. RockHound recommends using a complete matched drill string for critical long-hole applications.
Yes. While these are our standard production lengths, we manufacture custom lengths based on your specific drill rig configuration, bench height, and operational requirements. Contact our technical team with your application details for a custom quotation.
Service life varies with rock type, drill rig parameters, and maintenance practices. In typical underground hard-rock applications with proper lubrication and inspection protocols, RockHound ST68 tools consistently outperform standard market alternatives due to our 23CrNiMo material specification and extended 20-hour carburizing process.
Yes. Our engineering team is available to assist with complete drill string design, tool selection, and troubleshooting for specific drilling applications. Reach out through our contact page for project-specific guidance.