What Is a T38 Extension Rod?
In high-intensity hard rock drilling projects such as open-pit quarrying, underground mining and tunnel excavation, Top Hammer Drilling is the most widely used drilling method. Within the entire drill string, the T38 Extension Rod plays a key role in transferring energy. This article covers its definition, structural classification, applicable conditions and practical function.
1. Definition and Core Function
The T38 Extension Rod is a consumable force-transferring rod used in medium-to-deep hole drilling. In the industry it is also commonly called a hydraulic drill rod or high-duty drill rod. “T38” refers to the trapezoidal thread (Trapezoidal Thread) with an outer diameter of 38 mm (1.5 in) at the rod ends. Through the threads at both ends of the rod body, one end connects to the shank adapter and the other to the drill bit, transferring the impact energy and torque of the hydraulic rock drill to the bit to break rock.
As the first rod in a standard drill string, the T38 thread system itself helps improve drilling precision. MF rods with a speed-rod design produce straighter holes and, under favorable rock conditions, a penetration rate about 10% higher than a rod-and-coupling-sleeve combination system.
Because it must withstand thousands of high-pressure impacts per minute and severe torque friction, a high-quality T38 drill rod is generally made from high-strength alloy steel (such as premium chrome-nickel-molybdenum alloy steel) and undergoes full carburization (Carburization) heat treatment. Material selection and heat treatment are equally critical—only with proper heat treatment can the material’s performance be fully realized. After treatment, the rod surface is hard and wear-resistant while the core retains high toughness, avoiding early fracture under high fatigue stress. For buyers, this means more stable fatigue resistance in hard rock conditions and fewer rod-change shutdowns.
2. Core Structural Classification
Based on working conditions and carrier configuration, T38 drill rods are mainly divided along two structural dimensions.
2.1 By End Connection Type: MM Rods and MF Rods
- MM type (Male-Male): Both ends have external threads. When extending the drill string, an additional T38 coupling sleeve is required. This configuration is relatively economical and flexible in combination.
- MF type (Male-Female speed rod): One end has a male thread, the other is directly flared with an integrated female thread, allowing direct make-up without a coupling sleeve. With one less threaded contact surface, energy transfer efficiency is higher, the hole is straighter, and rod breaking is faster—making it the mainstream choice for automated rock drilling rigs (such as Epiroc Boomer and Sandvik DD series). If the rig targets single-shift advance and rod-breaking efficiency, MF rods are usually the more cost-effective choice.
2.2 By Cross-Section Shape: Round Rods and Hexagonal Rods
- Round rods: The most common shape, with a standard outer diameter mostly of 39 mm. Lighter overall weight and lower resistance to flushing water or air during flushing make them well suited to fast advancement in medium-to-deep open-pit bench holes.
- Hexagonal rods: Hexagonal cross-section, with across-flats typically 32 mm or 35 mm. The hexagonal shape facilitates gripping for easier installation and removal; the section is stiffer than round rods with better bending resistance, and provides better flushing disturbance during rotation, so it is more widely used in underground tunnel excavation.
2.3 Tapered Design and Cooling Channels
Many T38 drill rods use a tapered design, with one end larger in diameter than the other, to facilitate coupling and decoupling during drilling. Some products also feature internal cooling channels that circulate air or water to help cool the bit during extended drilling sessions.
3. Applicable Conditions and Rock Types
T38 drill rods typically cover lengths from 600 mm to 6400 mm to suit different hole depth requirements. They are suitable for surface mining, quarrying and construction, and adapt well to different geological conditions. Common applicable rock types include:
High-Abrasive Hard Rock (Granite, Basalt, Quartzite): Ideal for formations where wear resistance is critical. T38 rods with full carburization deliver maximum thread life under high-frequency impact.
4. Why the T38 Specification Is So Widespread
In the drilling tool industry, the T38 thread is a “golden specification”: it connects upward to large-diameter deep-hole tools such as T45/T51 and downward to small-diameter drifting tools such as R32/R28.
- Matched hole range: T38 drill rods are mainly used for holes of 64 mm to 89 mm (2.5 to 3.5 in). This range is exactly the core size for medium-to-deep hole blasting in mines, tunnel rock bolt support and micro-pile construction.
- Matched mainstream rigs: Whether Epiroc, Sandvik or Furukawa, most medium-sized rock drilling rigs on the market use T38 as standard or primary specification. T38 products made to process technology consistent with mainstream European levels can be matched with Sandvik, Epiroc/Atlas Copco, Furukawa and other rigs, so customers do not need to adjust rig configuration for T38 tools—they can be put into service right out of the box.
Specifications
| Thread Type | Body Section | Common Lengths (mm) | Bit Diameter | Compatible Drifter Models |
|---|---|---|---|---|
| T38 MM / MF | R39 Round | 3050 / 3660 / 4310 | 64mm – 89mm | Epiroc Boomer 282, Sandvik DD321 |
| T38 MM / MF | H35 Hex | 1220 / 1525 / 1830 | 64mm – 76mm | Furukawa HD712, Montabert |
5. Compiled by ShandiKe
The T38 Extension Rod is the core drill rod in medium-to-deep hole top hammer drilling, balancing high rigidity, energy transfer efficiency and versatility. Choosing high-quality T38 rods and following proper thread grease lubrication helps reduce cost per meter and improve construction efficiency. For long-hole drilling, a complete top hammer tool set usually includes MF rods, extension rods, coupling sleeves, shank adapters and drill bits; only when all components are properly matched can high penetration rates, straight high-quality holes and low overall production costs be achieved. If you are looking for stable supply and matching solutions for T38 tools, selection can be tailored to hole diameter, rig model and specific working conditions.
