Drill Rod Thread Types

Table of Contents

In the construction industry and geological exploration, the drill rod is an essential drilling tool, and its performance and use effect directly affect work efficiency and operation quality. The thread type of the drill rod, as a critical link in the design and manufacturing of the drill rod, has a non-negligible impact on the performance of the drill rod.

What is thread?

A thread is a kind of helical structure. It is an important mechanical element and industrial component, playing an important role in the development of modern science and technology. It can connect, fix, and transmit force and is widely used in machining, construction, transportation, electronic equipment, and other fields.

Application of threads on drill rods

The application of threads on drill rods is mainly on threaded extension rods, which are hollow circular or hollow hexagonal rods with connecting threads at both ends or one end of the rod and are also the main part of the threaded connection drilling tool. As thread types play a critical role in various industrial applications, different thread types are suitable for different application scenarios and provide solid and reliable connections and fastening effects. This article will introduce several common drill rod thread types and discuss their characteristics and scope of application.

drill rods

Evolution of thread forms

The process of applying threaded connection forms to rock drilling tools has gone through many evolutions. In the form of threaded connections, conical pipe threads, old-style trapezoidal threads, and serrated threads have been adopted successively. Extension rod rock drilling, often using the shoulder seat energy transmission serrated thread. Before 1940, shoulder-seated energy-transferring drill bits were used in tunnel boring to connect to drill rods with 25.4mm 4-button lamp-seated threads. These thread forms were all eliminated because of small pitch, small pressure-bearing contact area, the high tendency of stress concentration, fast wear, low service life, fast energy loss, processing, and loading and unloading difficulties.

After 1950, the wave thread of the 25.4mm 2 buckles appeared. As the cross-section of the drill rod becomes wider, the helix angle becomes smaller, the pressure on the thread surface increases, and the loading and unloading are laborious. Therefore, it is generally only used when the thread diameter is less than or equal to 32mm. However, in a tunnel or drifting, because the drilling rock hole is not deep, generally do not need to extend the drill rod and is also used for threads with a diameter of 38mm. With the emergence of high-frequency, high-power rock drills, the thread deepened, and the pitch increased. After 1960, there was a large-lead antialiasing thread- HL thread. This thread form is made of a combination of serrations and arcs and is also known as a composite thread. Later, due to the emergence of hydraulic rock drills, in 1970, the Swedish Secoroc and Sandvik companies simultaneously proposed a new FI-type thread, commonly known as trapezoidal or T-shaped thread, mainly used for threaded connections with a diameter of more than 38mm.

In recent years, the S thread has emerged. It is also known as the C thread, a kind of double-head trapezoidal thread. The S-thread tooth angle and T thread are the same, but their pitch is larger than the T thread. Compared with the T thread, this thread has smaller torque for tightening and dismounting and a larger pressure-bearing and wear-resistant area. It is a more suitable thread connection structure for large-section thread extension drilling tools.

Thread type for threaded extension rods

Corrugated thread (R thread)

Corrugated threads are the longest-used, more mature, and most widely adopted thread structure in threaded extension drilling tools. Its structure and geometric dimensions have been revised many times and formally incorporated into ISO international standards and national standards of various countries. At the same time, it is the main form of thread structure in the range of medium and small cross-section rock drilling tools, and it is the thread type with more varieties, specifications, and quantities in the rock drilling tools, adapted to a broader range of rock drilling machine models and drilling applications.

Characteristics are as follows:

A. After surface strengthening treatment such as carburizing, it has sufficient wear resistance. 

B. The loss of energy transfer is small. 

C. The loading and unloading are more convenient. 

D. It can be processed by a special lathe, and the manufacturing cost is low.

Compound thread (HL thread)

Since its appearance in 1960, the composite thread has been widely used for quite a long time and has been adopted in many national and regional rock drilling tools and ISO standards. It synthesizes the advantages of corrugated thread and serrated thread. The disadvantage is that the friction at the corner of the thread is too high, and the processing is quite difficult. The thread type is applied to larger-diameter thread connections and is becoming replaced by T threads.

Characteristics are as follows:

A. Large pressure area, high energy transfer efficiency. 

B. Slow wear, large allowable wear amount, long service life.

C. Convenient loading and unloading.

Trapezoidal thread (T thread)

Due to its stable helix angle, low tightening and unclamping torque, large pressure-bearing contact surface, and volume available for wear, it has excellent application performance and long drilling life. Therefore, it has gradually become the main threaded connection structure on the matching drilling tools for medium and large cross-section threaded connections.

The scope of application of different thread types of drill rods

Classification of threadNominal diameter applicableScope of application for rock drilling
Corrugated thread22、25、28、32、38Medium and small diameter rock holes
Trapezoidal thread38、45、51Medium and large diameter rock holes
Compound thread32、38、45Medium and large diameter rock holes

In engineering applications, selecting the proper drill rod thread type is critical. Different thread types can meet different connection needs, thus ensuring the performance and safety of the equipment. With the continuous development of technology, threaded connection technology is also constantly innovating, providing more reliable and efficient connection solutions for various industries.

How to choose the appropriate drill rod thread type?

Choosing the right drill rod thread type is critical to ensuring a smooth drilling operation. When choosing, you need to consider the following factors:

  • Drilling depth: drilling operations at various depths have distinct requirements for drill rods, necessitating choosing the appropriate thread type based on the specific situation.
  • Ground conditions: the hardness, stability, and other factors of the ground will affect the use effect of the drill rod. Choosing the suitable thread type according to the ground conditions is essential.
  • Operation requirements: different drilling operations have different performance requirements for drill rods, such as torque transmission capacity and anti-slip ability. Selecting the appropriate thread type according to the operational requirements is crucial.

In addition, when selecting a drill rod, it is also necessary to pay attention to its material, size, and accuracy requirements. High-quality drill rod material can improve the strength and wear resistance of the drill rod. Appropriate size and accuracy can ensure a good match between the drill rod and the drill bit and improve drilling efficiency.


Selecting the appropriate thread type is crucial for ensuring the performance and stability of the drill rod. In practical applications, it is necessary to comprehensively consider factors such as the operating environment, working conditions, and drill rod size to select the most suitable thread type.

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