Product Description
RockHound T35 Extension Rod (Extension rod, T35 – Round 39 – T35, MF type) is a core component of the top-hammer percussive drill string, purpose-built for underground long-hole production drilling and bench drilling in medium-hard to hard rock formations.
Manufactured from ZK22CrNi3Mo chromium-nickel-molybdenum alloy steel and subjected to a full-length gas carburizing 20-hours heat treatment, each rod achieves a high-hardness wear-resistant case over a tough, resilient core — the optimal metallurgical balance for withstanding millions of high-frequency percussive impact cycles without fatigue fracture.
The Round 39 mm (1½”) cross-section provides exceptional column rigidity, minimizing borehole deviation in long, deep holes and ensuring consistent flushing annulus geometry for efficient drill cuttings removal. The precision-machined T35 trapezoidal buttress thread (T-thread, 35 mm nominal) delivers superior torque transmission, minimal thread wear, and rapid uncoupling — directly reducing non-productive rod change time and lowering Cost per Meter (CPM).
As a male-male (MF) extension rod, it is designed for use with T35 coupling sleeves, and integrates seamlessly with T35 shank adapters, T35 Speed Rods (MF rods), retrac button bits, flat face button bits, and guide bits — forming a fully optimized T35 drill string for maximum rig uptime and penetration rate.
Key Features
| # | Key Features | Content – Benefits & Technical Advantages |
|---|---|---|
| 1 | ZK22CrNi3Mo Alloy Steel | High-grade Cr-Ni-Mo carburizing steel — delivers exceptional fatigue strength, superior toughness, and outstanding wear resistance, perfectly suited for the high cyclic loading of percussive top hammer drilling. |
| 2 | Full-Length Gas Carburizing | Complete through-carburization of the entire rod creates a deep, high-hardness wear-resistant case over a tough, ductile core. This prevents brittle fracture under repeated percussive impacts and extends service life by 40–60% compared to standard heat-treated rods. |
| 3 | T35 Trapezoidal Buttress Thread (T-Thread) | Precision-machined T35 trapezoidal buttress thread ensures secure joint make-up, minimal thread wear, highly efficient percussive energy transfer, and rapid uncoupling — significantly reducing rod change time and downtime. |
| 4 | Round 39 mm Section – High Rigidity | The Rnd 39 round cross-section maintains a consistent flushing annulus, maximizes bending stiffness, and minimizes borehole deviation — ideal for deep long-hole drilling, benching, and production applications. |
| 5 | Optimized Flushing Channel (Ø ≈14.5 mm) | Precision-engineered internal flushing bore (≈14.5 mm) provides ample water flow for effective cuttings removal and bit cooling, reducing the risk of borehole blockage, bit overheating, and premature failure. |
| 6 | Mandrel-Rolled Inner Bore | Mirror-smooth inner surface achieved through mandrel rolling eliminates scratches, hot-piercing marks, and stress risers — dramatically reducing the initiation and propagation of internal fatigue cracks for maximum rod durability. |
| 7 | Full T35 Drill String Compatibility | Plug-and-play compatibility with T35 shank adapters, T35 coupling sleeves, T35 MF Speed Rods, retrac button bits, flat face button bits, and guide bits — ensures seamless integration into existing T35 top hammer drilling systems. |
Technical Specifications
| Product Code | Thread Type | Rod Section | Length (mm) | Length (foot) | Diameter (mm) | Weight (kg) |
|---|---|---|---|---|---|---|
| 290-3912-7575 | T35 | Round 39 | 1220 | 4′ | 39 | 10.4 |
| 290-3915-7575 | T35 | Round 39 | 1525 | 5′ | 39 | 12.8 |
| 290-3918-7575 | T35 | Round 39 | 1830 | 6′ | 39 | 15.3 |
Applications
Underground Long-Hole Production Drilling (Primary Application)
Optimized for sublevel stoping, long-hole open stoping (LHOS), and fan drilling in underground metal mines. The Round 39 mm section and full-length carburized T35 thread deliver the rigidity, fatigue life, and thread integrity required for multi-pass, deep-hole production drilling at high rig utilization rates.
Bench Drilling (Primary Application)
Reliable performance in surface quarry benching and open-pit production drilling for small- to medium-diameter holes. Stable column behavior and consistent thread engagement ensure repeatable penetration rates in varying rock hardness.
Development Drifting & Tunneling (Secondary Application)
Suitable as an auxiliary or supplementary system in underground development headings where T35 tooling is already in use across a fleet. Note: For primary drifting and tunneling operations, R-series or SR-series drill strings are typically the recommended configuration.
Infrastructure & Precision Anchoring
Applicable for rock bolting pre-drilling, grout hole drilling, and cable anchor hole preparation in civil and infrastructure projects where a T35 system is specified.
Maintenance & Best Practices
1. Thread Lubrication — Apply Before Every Coupling Make-Up
Apply a certified anti-seize thread compound (e.g., copper-based or molybdenum disulfide (MoS₂) grease) to all T35 thread faces before every coupling make-up. This minimizes galling, reduces frictional heat at the thread contact zone, and prevents cold welding under high-torque conditions.
2. Thread Wear Inspection — Check at Every Shift
Inspect T35 threads for signs of “mushrooming” (thread face plastic deformation), flank wear, pitch erosion, or shoulder damage after each shift. Replace rods where thread profile deviates from the T35 tolerance band — worn threads dissipate percussive impact energy and accelerate bit and coupling wear.
3. Eliminate Dry Running & Rattling (Blank Firing)
Never operate the rock drill without adequate feed pressure or with a loosely coupled drill string. Dry running or blank firing (operating without drill bit contact) generates destructive bending loads and vibration-induced fatigue cracking — the primary cause of mid-rod and thread-root fractures.
4. Flushing Water Pressure & Flow Rate
Maintain the correct flushing water pressure and flow rate as specified for your drill rig. Insufficient flushing fails to clear drill cuttings, causing bit regrinding wear, borehole collapse, and accelerated rod thread wear from abrasive particle re-circulation.
5. Storage — Horizontal Racking in Dry, Covered Areas
Store extension rods horizontally on steel rod racks, never in a standing/leaning position (causes permanent bending set). Keep in a dry, covered environment to prevent surface corrosion and thread oxidation; apply wax or rust-inhibiting oil to thread ends before long-term storage.
Related Reading:Rock Drill Rods: Types,Work,Choose & Maintenance
How to Select the Right T35 Extension Rod
| # | Key Features | Content – Benefits & Technical Advantages |
|---|---|---|
| 1 | ZK22CrNi3Mo Alloy Steel | High-grade Cr-Ni-Mo carburizing steel — delivers exceptional fatigue strength, superior toughness, and outstanding wear resistance, perfectly suited for the high cyclic loading of percussive top hammer drilling. |
| 2 | Full-Length Gas Carburizing | Complete through-carburization of the entire rod creates a deep, high-hardness wear-resistant case over a tough, ductile core. This prevents brittle fracture under repeated percussive impacts and extends service life by 40–60% compared to standard heat-treated rods. |
| 3 | T35 Trapezoidal Buttress Thread (T-Thread) | Precision-machined T35 trapezoidal buttress thread ensures secure joint make-up, minimal thread wear, highly efficient percussive energy transfer, and rapid uncoupling — significantly reducing rod change time and downtime. |
| 4 | Round 39 mm Section – High Rigidity | The Rnd 39 round cross-section maintains a consistent flushing annulus, maximizes bending stiffness, and minimizes borehole deviation — ideal for deep long-hole drilling, benching, and production applications. |
| 5 | Optimized Flushing Channel (Ø ≈14.5 mm) | Precision-engineered internal flushing bore (≈14.5 mm) provides ample water flow for effective cuttings removal and bit cooling, reducing the risk of borehole blockage, bit overheating, and premature failure. |
| 6 | Mandrel-Rolled Inner Bore | Mirror-smooth inner surface achieved through mandrel rolling eliminates scratches, hot-piercing marks, and stress risers — dramatically reducing the initiation and propagation of internal fatigue cracks for maximum rod durability. |
| 7 | Full T35 Drill String Compatibility | Plug-and-play compatibility with T35 shank adapters, T35 coupling sleeves, T35 MF Speed Rods, retrac button bits, flat face button bits, and guide bits — ensures seamless integration into existing T35 top hammer drilling systems. |
Related Production
FAQ
The T35 extension rod is a high-performance drilling tool primarily used for long-hole drilling, tunneling, and underground mining. At RockHound, our T35 rods are engineered to provide high rigidity and precision, making them ideal for sublevel stoping and bench drilling in medium to hard rock formations.
An Extension Rod (MM) has T35 male threads on both ends and requires a T35 coupling sleeve to connect to adjacent rods or the shank adapter. A Speed Rod (MF Rod) has a male thread on one end and an integrated female thread (socket) on the other, eliminating the need for a separate coupling sleeve. Extension rods (MM) are more common in long-hole drilling setups; MF rods are favored in bench drilling for faster rod changes.
Use a T35 coupling sleeve (full bridge or standard design) with an outer diameter of approximately 55–56 mm. Full-bridge couplings offer enhanced energy transmission efficiency and are recommended for high-demand long-hole applications.
Yes. The T35 thread is a standardized industry thread profile compatible with T35 shank adapters from all major OEM and aftermarket suppliers (Epiroc, Sandvik, Furukawa, Montabert, etc.), provided thread class and pitch are consistent.
ZK22CrNi3Mo is a premium chromium-nickel-molybdenum alloy steel known for its exceptional toughness and fatigue resistance. This material allows RockHound’s T35 rods to withstand high-impact energy and bending stresses, significantly reducing the risk of rod breakage compared to standard carbon steel alternatives.
We provide three standard lengths to fit various drilling rig configurations and space constraints:
1220 mm (4 ft) – Ideal for confined underground spaces.
1525 mm (5 ft) – A versatile mid-range option.
1830 mm (6 ft) – Optimized for maximum efficiency in larger galleries. All our rods feature a Round 39 (39mm) cross-section for superior straightness.
RockHound also support OEM/ODM customization to meet the different customer needs for varying drill rod lengths.
Standard T35 extension rods have male threads on both ends and require a coupling sleeve to connect. In contrast, MF (Male/Female) rods feature an integrated female socket on one end. While MF rods simplify the drill string and reduce energy loss, standard T35 rods from RockHound offer more flexibility in component replacement and are often preferred for specific long-hole drilling setups.
To maximize longevity, RockHound recommends:
Consistent Lubrication: Use high-quality thread grease on all T35 connections.
Proper Feed Pressure: Maintain correct pressure to avoid "rattling" the threads.
Regular Inspection: Check for thread wear or heat cracks daily.
Corrosion Protection: Store rods in a dry area to prevent rust on the precision-machined ZK22CrNi3Mo surface.
Yes. Our T35 extension rods are manufactured to international industry standards. They are fully compatible with standard T35 shank adapters, coupling sleeves, and T35 button bits (both standard and Retrac designs) from major global brands.