Stress State and Working Conditions of Threaded Extension Rod

Table of Contents

As one of the indispensable tools in modern engineering, the stress state and working conditions of threaded extension rods directly affect the stability and efficiency of the project. In rock drilling, mining, and other operations, threaded extension rods must withstand not only the high-frequency impact from the rock drill piston but also the reaction force from the rock. These complex stress conditions necessitate threaded extension rods to possess excellent strength, toughness, and durability. At the same time, different working conditions also pose severe challenges to the performance of threaded extension rods. Therefore, having a deep understanding of the stress state and working conditions of threaded extension rods is crucial for selecting appropriate drill rod materials, optimizing design solutions, and extending service life.

High-frequency tensile and compressive stress

threaded extension rod

When drilling, threaded extension rods are mainly subjected to the combined effects of the high-frequency impact of the rock drill piston and the propeller’s thrust. At the same time, at the moment of impact drilling, the rock also generates a reaction force that is transferred to the drill rod through the drill bit. Therefore, the drill rod is subjected to axial high-frequency compression pressure and high-frequency tensile stress during the rock drilling process. These two high-frequency stresses in the form of stress waves are the main factors causing drill rod fracture or fatigue damage.

Torsional stress

When drilling rock, the rock drill drives the entire drilling tools to rotate, and the weight of the drill rod, the obstruction of rock chips and rock walls, and the resistance of the rock at the bottom of the hole all cause the drill rod to be subjected to torsional stress when drilling rock.

Bending stress

A threaded extension rod is a slender rod. They are primarily used in high-power rock drilling machinery and for deep-hole rock drilling. There are two major types of threaded extension rods. One type is the extension rod, primarily used for open-pit rock drilling (with a downward working mode) and underground ore mining (with an upward working mode). It belongs to medium-depth rock drilling, with blast hole depths ranging from 6 to 50 meters. During operation, multiple drill rods are connected using a coupling sleeve. Therefore, when the extension rod is working downward (generally in a vertical state), its force is mainly the impact force, the thrust of the rock drill, and the self-weight of the drill rod group. When working vertically upward, it is affected by the impact force, the thrust of the rock drill, and the self-weight of the drill rod group. When working obliquely upward, such as drilling fan-shaped blast holes, in addition to the above forces, it is also affected by the bending stress generated by the oblique component and self-weight. The other type is the drifter rod, primarily used for drilling horizontal tunnel rock, classified under shallow hole rock drilling, with blast hole depths less than 6 meters. Since the drifter rod is a single horizontal rod, it mainly experiences bending stress due to the impact force and thrust from the rock drill and the weight of the drill rod. Under similar specifications (diameters), the slenderness ratio of the drifter rod is greater than that of the extension rod, and the drifter rod is a single horizontal rod. Therefore, the bending stress of the drifter rod when working is much greater than that of the extension rod, and the service life of the drifter rod is relatively lower than that of the extension rod.

The corrosive effect of wet rock drilling on rods

r32 threaded extension rod

Open-pit drilling typically uses dry drilling with dust collection or dust protection at the hole mouth. In contrast, wet rock drilling is used for underground rock drilling, shaft work, and some downward rock drilling. Wet rock drilling water, acidity, and alkalinity are generally between pH = 5 ~ 8, whether acidic, alkaline, or neutral water on the rod corrosion, and the corrosion fatigue strength of steel is lower than the dry fatigue strength. Therefore, the corrosive effect of water reduces the fatigue strength of the drill rod, which is another significant factor contributing to the reduction of drill rod service life.

Abrasion of the rock wall and rock dust in the borehole

In dry and wet rock drilling, the outer surface of the drill rod is exposed to abrasion from the rock wall, rock chips, and rock dust within the borehole. This abrasion causes the diameter of the drill rod to decrease over time. The hexagonal shape of the drill rod can even be ground down to a round shape, reducing the contact area for force distribution. However, this abrasion also acts to mitigate fatigue by creating tiny cracks on the outer surface of the drill rod, which reduces the initiation and propagation of fatigue cracks. Therefore, this kind of wear has little effect on the service life of the drill rod. When rock chips or rock powder get into the threaded portion of the drill rod, because the hardness of the rock is higher than the hardness of the steel, so abrasive wear is formed. It significantly speeds up the wear of the threaded part, consequently reducing the lifespan of the drill rod.


After discussing the stress state and working conditions of the threaded extension rod, it becomes evident that it must exhibit exceptional durability and stability. Whether subjected to high-frequency tensile and compressive stress, torsional stress, or bending stress, the threaded extension rod needs to maintain robust performance. At the same time, facing different working conditions, such as high temperatures and corrosive environments, the performance and service life of the threaded extension rod is significantly tested. Therefore, for technicians, comprehending and mastering the stress conditions and operational requirements of the threaded extension rod is crucial. This knowledge not only aids in selecting more appropriate drill rod materials and designs but also significantly extends the service life of the drill rod. Moreover, it enhances project safety and efficiency effectively. In the future, as technology and engineering requirements continue to improve, the performance and reliability of threaded extension rods will be further improved.

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