Hollow Hexagonal Threaded Drill Rod

Updated: 2026-07-21

Overview

The hollow hexagonal inner thread drill rod is a critical component in percussive drilling systems, designed to withstand high-stress conditions in rock excavation. Its hexagonal external shape provides superior torque transmission compared to round rods, while the hollow core enables compressed air or water to flush drill cuttings. The internal threading allows secure connection to drill bits and other rods, forming a continuous drill string for deep-hole applications. First developed in the mid-20th century for hard rock mining, modern variants incorporate advanced heat treatments and anti-corrosion coatings. These rods are essential in quarrying, underground mining, and civil engineering projects where efficient drilling and durability are paramount. Leading manufacturers produce them in standardized lengths (typically 0.8–6 meters) with API or proprietary thread specifications.

Structure and Working Principle

The rod's hexagonal profile (usually 22mm, 25mm, or 32mm across flats) ensures positive engagement with drilling equipment's chuck mechanism, preventing slippage during rotation. The hollow center (8–20mm diameter) serves as a conduit for flushing media, which cools the bit and removes debris. Internal threads (commonly R32, T38, or T45 types) are precision-machined to maintain alignment under dynamic loads. During operation, the rod simultaneously transfers two energy forms: axial impacts from the hammer (5–20 kJ/blow) and rotational torque (200–1,500 Nm). The hexagonal design's flat surfaces distribute stress more evenly than round rods, reducing localized wear. High-quality rods feature upset forged ends—a forging process that thickens the threaded sections to withstand repeated impacts without deformation.

Key Features

1. **Material Strength**: Made from chromium-molybdenum alloy steels, these rods undergo quenching and tempering to achieve optimal hardness-toughness balance. Surface treatments like shot peening further enhance fatigue resistance. 2. **Geometric Advantages**: The hexagonal shape provides six driving surfaces, allowing up to 30% higher torque transmission than round rods of equivalent size. The inner thread's precision (typically ISO 7-1 standard) ensures vibration-resistant connections. 3. **Operational Efficiency**: Hollow design enables simultaneous drilling and cuttings removal, reducing downtime for hole cleaning. Some advanced models incorporate wear-resistant coatings (tungsten carbide or diamond-like carbon) on high-friction areas.

Application Areas

Primary applications include: - **Mining**: Drilling blast holes in hard rock (granite, basalt) for ore extraction - **Tunneling**: Creating pilot holes for tunnel boring machines or explosive placement - **Construction**: Foundation drilling for skyscrapers and bridges - **Water Wells**: Deep borehole drilling in geological surveys These rods are compatible with top-hammer and down-the-hole (DTH) drilling rigs from brands like Sandvik, Atlas Copco, and Boart Longyear. Selection depends on rock hardness (Mohs scale 5–8) and required hole depth (typically 10–50 meters per rod segment).

Maintenance and Precautions

Regular maintenance extends service life by 2–3 times. After each shift, clean threads with a wire brush and apply thread grease (lithium-based). Inspect for: 1. **Thread wear**: More than 1mm flank wear requires rethreading or replacement 2. **Body straightness**: Bend exceeding 1° per meter causes premature failure 3. **Internal cracks**: Use magnetic particle testing monthly Storage should be in dry, temperature-controlled environments to prevent rust. Stack horizontally with wooden spacers to avoid surface damage. Never weld damaged rods—this creates brittle zones prone to fracture.

B2B Procurement Guide

When sourcing these rods: 1. **Certifications**: Require ISO 9001/14001 and MSHA (mining safety) compliance 2. **Testing Reports**: Request Charpy impact test results (minimum 27J at -20°C) 3. **Customization**: Some suppliers offer rods with optimized thread profiles (e.g., wave thread for reduced stress concentration) 4. **MOQs**: Standard orders start at 50–100 units; bulk purchases (500+) often secure 15–20% discounts Leading Chinese manufacturers include Shandong XingTian, Hunan New Huide, and Zhejiang Zhengtong. For international procurement, verify Incoterms (FOB or CIF) and lead times (typically 30–60 days for customized orders).