Impact drill rod Knowledge（extension drill rod）

When selecting a drill rod for an impact rock drill, mining engineers are faced with many factors that can be considered comprehensively, including the drill rod diameter, cross-sectional shape, drill steel type and heat treatment process. The selection is not only affected by technical conditions, but also by regional preferences and market supply. There are two basic types of drill rods currently produced: the first is with a forged shank adapter, and the second is with threads on each end. The first is suitable for integral, tapered or threaded drill heads. The second is used in the case of a shank adapter, such as only one drill rod for a rail-type rock drill, or a rod assembly drill rod connected to a threaded rod sleeve.

Types of Drill Rods:

Hollow drill steel is hot rolled from a billet with a round center full metal core into round or hexagonal cross-sections and different lengths. The chemical composition of the steel must be well selected and accurately controlled to suit the type of drill rod required and the heat treatment method used. After rolling to the required size, it is stretched to reduce the diameter and then the metal core is removed. High carbon steel containing 1% carbon and 1% chromium, with small amounts of manganese and molybdenum, has strong fatigue resistance and can be locally heat treated and welded. They are used to manufacture drill rods with shank adapters, including integral drill rods. The high-frequency quenching process involves rapidly heating the workpiece to 900°C and then rapidly cooling it in water. This changes the metal structure and also causes compressive stresses in the surface layer. The high-frequency method can be used to locally heat treat the cone, shank adapter and thread, making the drill rod flexible and able to withstand large bending and rough handling. Sandblasting is a cold work hardening process that removes surface defects and prolongs life. Low and medium carbon steels containing 0.2-0.27% carbon, 2-3% chromium or nickel, and manganese or molybdenum are used to make adapter rods, adapter tails, adapter sleeves and drill bit bodies. They are generally fully carburized. During carburization, a bundle of 200-300 drill rods is suspended in a cage and treated in a carbon-rich atmosphere at 925°C in a pit furnace for about 6 hours. Due to carburization, the basic chemical composition and properties of the outer layer change, causing volume increase and compressive stress. This process occurs over the entire surface including the internal flushing holes, improving the durability, fatigue strength, hardness and stiffness of the drill rod, as well as wear and corrosion resistance. When wet drilling, it is naturally important to improve wear and corrosion resistance in order to protect the flushing holes. It is reported that about 95% of the adapter parts used for underground wet drilling are carburized as a whole, including the adapter sleeve and drill bit threads. When this type of steel is partially carburized, it is difficult to perform local annealing because "annealing" zones will be formed. It is a technically difficult process to weld carbide sheets in carburized steel to make an integral drill rod, but some manufacturers have succeeded. The heavy-duty integral drill rods they produce are used in high-power rock drills for mechanized rock drilling. It is reported that their service life is three times that of high-carbon steel, such as 900 meters versus 300 meters. Another low-carbon alloy steel has particularly good machinability. It contains nickel and chromium. Generally, this steel is also carburized as a whole. Medium carbon (0.42%) carbon chromium-nickel steel is used by some factories to make long, heavy-duty integral drill rods. The outer surface and flushing holes can be phosphated to prevent rust. Protective wax can also be applied during storage. Corrosion and rust can cause premature fatigue cracks. When drilling abrasive hard rocks with wet rock drills, fully carburized drill rods are generally preferred. However, when drilling in open-pit steps with joints or broken rock formations, the drill rod is in danger of severe bending, and the carburized drill rod may be broken. High-frequency quenching drill rods can better adapt to this situation. Because the high-frequency quenched drill rod has greater toughness, it is not easily damaged by accidental or intentional impact. At the same time, it is not easy to loosen the thread due to insufficient propulsion force, causing local heating and damage. Compressed air blowing may also cause local heating and surface erosion. High-frequency quenched drill rods are not prone to this phenomenon, but thread wear is prone to occur. Drill rods with shank adapters When the blast-hole is very shallow, for example, below 6 meters, or as the blast-hole deepens, a drill rod group of different lengths is used, or a single drill equal to the depth of the blast-hole is widely used. This type of drill rod is forged with a shank adapter. The drill head is either integral with the drill rod or a movable drill head connected by a cone or thread. The drill rod with a shank adapter is made of hexagonal drill steel with opposite side dimensions of 19mm, 22mm or 25mm, and is used for hand-held rock drills, air leg rock drills and mechanized drilling vehicles for drilling holes in working faces or rock bolts. As mentioned above, this type of drill rod is generally made of high-carbon chromium-containing alloy steel. This alloy steel can withstand bending without breaking or permanent deformation. The end of the shank adapter (if the drill rod is not fully carburized) is quenched separately to withstand the impact stresses caused by the piston and the rotating torque. If the drill head is connected by a cone, it can also be heat treated separately. The integral drill rod structure is to make the total wear life of the tungsten carbide equivalent to the fatigue life of the drill rod. However, in very abrasive rocks, this approach is not practical and it is better to use a movable drill head. Mechanized rock drilling with high-pressure water flushing cuttings should be equipped with a shank adapter seal to prevent the flushing water from entering the rock drill, because for pneumatic rock drills, this situation may affect lubrication and cause ice. At least one manufacturer offers an integral drill rod with a ball tooth drill head instead of a slotted drill head. In broken or fractured rocks, it is suitable to use an integral drill rod with a cross drill head because there is a risk of jamming when drilling in these rocks. However, the slotted drill head has the outstanding advantage of being easy to sharpen.

Connecting the shank adapter:

 When drilling deep holes with a rod, the rod tail is inserted into the shank adapter sleeve of the rock drill, and the rod sleeve is connected to the first drill rod in the drill rod group. Since the rock drill manufacturers manufacture rock drills with various structures of drill sleeves, there are many types of rod tails. The simplest is the hexagonal shouldered tail, and the others vary in complexity, with bosses or splines. The rod tail must transmit the impact energy, rotary torque and propulsion force to the drill rod, and its rear end face, thread and spline or shoulder must have high wear resistance. The shock wave generated by the piston propagates at the speed of sound in steel (about 5000 m/s) and a frequency of 60 times/s. A small displacement will occur at the threaded connection, and the wear caused must be minimized by wear-resistant steel. The wear-resistant steel used must have the characteristics of becoming brittle but not losing fatigue strength. The most suitable steel is low-carbon chromium steel or nickel-chromium steel, and the commonly used heat treatment is full carburization. When drilling shallow holes, it is better to connect a rail-type rock drill and a live drill bit to the adapter rod tail than to use a single drill rod with a shank adapter, especially when drilling large diameter holes in hard rock and using high-powered rock drills. This method allows the adapter rod tail, adapter sleeve, drill rod and drill bit to be replaced when necessary. However, since the adapter sleeve passes through an open drill holder, there is a disadvantage that the push length is lost equal to the length of the adapter rod tail. When drilling downward step holes, the first drill rod must be disconnected from the shank adapter before adding the adapter. When using an adapter sleeve, it may fall off, so it is sometimes preferred to use a shank adapter with internal threads. Unless all female threads are used, the adapter sleeve should be connected to the lower end of the drill rod. Compared with the use of an adapter sleeve, a more rigid connection is formed between the shank adapter and the first drill rod. If large bending stresses are caused in the shank adapter due to blast-hole deflection, the rigid connection is more likely to be damaged. There is a "slender" transition section between the tail of the drill bit and the threaded part, which is said to make the drill bit elastic and able to withstand bending stress. The flushing medium (water or compressed air) enters the blast-hole through the water needle of the rock drill or an independent rotating device. When high-pressure water (more than 8 bar) is used for flushing, an independent rotating device must be used. Modern hydraulic rock drills have an integrated flushing system at the end of the drill bit sleeve.

Connecting drill rods:

Deep hole drilling drill rods are made of chromium-molybdenum steel containing 2~3% chromium or nickel, and can be hexagonal or round. The diameter of the rod body and the threaded part of the large-section drill rod is the same, while the diameter of the threaded end of the light drill rod is larger. Although the lighter drill rod has the risk of deflection of the blast-hole due to the reduced rigidity, it is more convenient to use light drill rods when drilling upward. Light drill rods require complex forging processes, and carburizing is the basic method to obtain higher fatigue strength. Round drill rods with a diameter of more than 32mm are used for drilling deep holes on heavy drilling vehicles. When connecting and unloading heavy drill rods for drilling deep holes, use effective auxiliary devices to mechanize the connection and unloading of drill rods.

Threads:

The "R" thread has a constant thread angle of 20° and a constant pitch of 0.5 inches measured from the axis of the drill rod. It is used for small-size drill rods with diameters of 22mm to 28mm. For high-powered rock drills, it is possible to overtighten the "R" thread, and the degree of tightening depends on the impact energy, rotational torque, and the resistance created by the rock and the thrust. Compared with the "R" thread, the "T" thread has a larger thread angle and its pitch increases with increasing diameter. It has the characteristics of balanced tightness and is used for drill rods with diameters of 38mm and 45mm. The "C" thread is suitable for larger equipment, such as drill rods with diameters of 51mm or 57mm. It has a double-start thread and its thread profile angle is similar to that of the "T" thread. The thread profile of "Hi-Leed" is sawtooth-shaped, and its connection and disassembly performance is between "R" and "T" threads. It is used for drill rods with diameters between 25mm and 57mm, and its thread angle is between "R" and "T" threads. When rock drilling is easier, the threaded part on the connecting rod can be made twice as long, so that when the first section of the thread is worn, it can be cut off, but special attention should be paid when cutting off to avoid local heating and tempering of the drill rod. In some cases, the drill rod has reached its fatigue life after thread wear. The selection of single-end or double-end thread is determined by the inspection and trial of the rock drill and rock conditions. to determine, but thrust is an important parameter. Ingersoll-Rand makes a special thread that is threaded along the entire length of the rod so that when one end is worn, it can be cut off and then chamfered and reused. It is reported to have a lifespan five times greater than that of a rod threaded only at the end. The threads are rolled, which gives them a high shear strength, and the surface is hardened for toughness and wear resistance. A fairly steep helix angle is used so that they can be loosened with minimal torque. Available in 32mm, 38mm and 44mm.

Improvements in drill rods:

The new generation of percussive rock drills, especially those powered by hydraulic power, are able to compete successfully with rotary and down-the-hole drills in many aspects of open-pit drilling. These advances have occurred in parallel with improvements in drill rods and drill bits. As drilling speeds increase, drill rods must be more resilient and usually heavier. Obviously, handling devices and automatic control of heavy drill rods are important and necessary auxiliary equipment. The life of drill rods is mainly affected by the amplitude of the stress wave, so long stress waves with small amplitude and uniform distribution are most favorable. Hydraulic percussive rock drills produce such waves. Their pistons are small and long in diameter compared to the short and thick pistons of pneumatic rock drills. It is estimated that a 15% saving in drill rod consumption costs is possible by using hydraulic rock drills. Other favorable factors are that the power of the rock drill can be adjusted relatively easily according to the matching of drilling speed and drill rod cost. In addition, the use of automatic eye opening and anti-jamming devices can also prevent or at least minimize the occurrence of jamming. It is estimated that the basic investment accounts for 25-30% of the rock drilling cost, spare parts and maintenance account for 22-33%, wages account for 12-25%, energy consumption accounts for 2-6%, and drill rods and drill bits account for 20-22%. In the above-mentioned expenses, there are great differences between mines. However, if the total rock drilling cost of ore is $2/ton, the cost of drill rods and drill bits is $0.4/ton. If drill breaks often occur during rock drilling, it will increase costs and delays. In large mines, even a small saving in drill bit and drill rod consumption can bring considerable benefits. In order to minimize costs, it is very important to discuss with the rock drilling rod manufacturer and carefully select the drill rod according to production and use conditions.

Maintenance of drill rod:

Drilling machine: Ensure that the drill tail sleeve is intact and the drill bit is sharp. Turn the rod and use the rods in the rod group in turn to make the threads in the rod group wear evenly. Use the rod sleeve according to the corresponding life of the rod thread - a new rod sleeve is matched with a new rod. When dry drilling, clean and lubricate the threads with special grease, and only use straight drill rods. Heating the drill rod before use in cold weather conditions will extend its life. Drill carefully and use 1/4~1/2 of the rock drill power when drilling. If the drill rod is not aligned, it must be re-drilled. Use the optimal thrust. Excessive thrust will cause the drill rod to bend and shorten its life, and it can also block the drill head purge hole and cause wear or damage to the drill head carbide plate; insufficient thrust will cause heating at the connection, damage the connection, excessive wear or loosening of the drill head carbide plate. The rod sleeve "snaps" relative to the drill holder, which will damage the rod sleeve. Ensure flushing means that there is always enough flushing water to quickly remove rock chips. Be careful when undocking -- use 1/4 of the push speed when undocking to prevent sticking. Use a good wrench to undo the rod. Hammering or using a pipe wrench will damage the hardened surface. Pitfalls can cause fatigue fractures. Use a special gauge to measure the wear of the threads on the extension sleeve and the rod. When the wear exceeds the specified limit, it must be scrapped. Storage If it is to be stored after use, it must be treated with a rust inhibitor and not stored near water or dust. Corrosion is a big problem in underground mines.