Core Guide to Rotary Drilling Rig Drill Rod Selection: Differences Between Friction Drill Rods and Mechanical Lock Drill Rods and Practical Applications

    The rotary drilling rig drill pipe is a core force-transmitting component connecting the power head and the drill bit, directly determining construction efficiency, hole quality, and equipment safety. Improper selection is prone to cause drill pipe damage, project delays, and economic losses. In actual construction, friction drill pipes and mechanical lock-type drill pipes are the two most commonly used types, which differ significantly in force transmission methods, structural characteristics, and applicable scenarios. Mastering their core differences is key to accurate selection.

    The friction drill pipe, also known as the friction pressure application type drill pipe, relies on the conical surface friction of multiple nested inner and outer tubes to transmit torque and drill pressure. Its outer keys are usually wear-resistant straight bars with three or six keys, without obvious pressure application locking points. Its prominent advantage is convenient operation, requiring no special unlocking steps, fast lifting and lowering of the drill string, low probability of carrying rods, and small failure rate. Additionally, it can be configured with five to six sections of drill pipes, and the hole depth is generally better than that of mechanical lock-type drill pipes, making it suitable for deep hole construction. Moreover, when the pressure reaches a certain value, it can automatically slide down, reducing the risk of drill tool damage. However, its shortcomings are also obvious: there is pressure loss during transmission, and it basically loses pressure application function under no load, with limited pressure application capability, unable to adapt to hard strata drilling, and only applicable to soft strata with lower uniaxial compressive strength, such as mud, clay, sand layers, loose gravel layers, and strongly weathered soft rock layers.

    The machine-lock drill rod forms a rigid connection by engaging the recessed pressure platform on the outer key with the inner key, achieving lossless force transmission. It also adopts a three-key or six-key structure with multiple fixed locking points. Its core advantage lies in strong pressure application, enabling efficient power transfer to the drill bit. It boasts broad formation adaptability, capable of handling both soft formations and hard formations such as well-cemented gravel layers, moderately weathered, and slightly weathered bedrock, making it the preferred choice for complex formations. However, it presents high operational difficulty, requiring an unlocking step before tripping out, with relatively slow hoisting and lowering speeds that demand skilled operators. Typically consisting of four sections, with special configurations reaching up to five, it has relatively limited hole-making depth and is prone to damage under forced pressure, necessitating regular maintenance.

    The core principles of selection must closely align with the three key factors: "drill rig tonnage - formation hardness - borehole depth." For soft formations, deep holes (typically over 30 meters), and a focus on construction efficiency, friction drill rods should be prioritized, suitable for small to medium-sized rigs of 15-36 tons. For medium-hard or harder formations and boreholes under 80 meters, mechanical-lock drill rods are required, with models selected based on the formation's compressive strength to match the rated torque and drilling pressure, avoiding overload fractures. Additionally, if the construction formation exhibits a composite geological profile of "hard upper and soft lower layers," composite full-mechanical-lock drill rods can be considered to meet both hard formation breaking and deep-hole drilling requirements.

    In summary, the friction drill pipe is characterized by its core features of "convenience, efficiency, and suitability for deep soft formations," while the mechanical lock drill pipe excels in "strong power transmission and adaptability to complex hard formations." Neither has an absolute advantage; instead, the optimal choice depends on precise matching with specific construction conditions to maximize equipment performance, reduce costs, and ensure safety and quality in construction.