Composition of Threaded Extension Rod

Table of Contents

Threaded extension rods are a common component in building construction and other fields. It is a hollow circular or hollow hexagonal rod with connecting threads at both ends or one end of the rod. It is also the main component of the threaded extension supporting tools.

Composition of threaded extension rod

extension rod

The threaded extension rods consist of the rod, thread, and its transition zone. The thread structure and geometric dimensions are the main structures and parameters.


The rod body of the threaded extension rod is a circular or hexagonal hollow steel material without any change in geometric size and is available in all sizes except B19mm. Since the threaded extension rods have to be threaded on the hollow steel rods and are mostly used for drilling medium and deep rock holes, the geometric dimensions of the steel section and its precision must meet and exceed the standard technical requirements to ensure that the center hole is free of water and air and to maintain proper rock drilling. In addition, the straightness and precision of the rod body, to a certain extent, determines the performance of threaded extension rods and drilling life but also affects the quality of drilling holes. In particular, threaded extension rods used for drilling medium and deep holes require stricter straightness and precision.

Connecting thread

There are three main thread types for threaded extension rods, R thread, HL-shaped thread, and T thread. The threaded extension rod is formed by processing both ends of the corresponding hollow steel rod into threads of certain specifications (including its transition zone) and the formation of the rod. Therefore, it has different threads combined with the size and structure of the hollow steel rod body to produce threaded extension rods with different drill rod structures for a variety of purposes, such as extension rods, light extension rods, tunnel drill rods, MF rods, and other structural threaded extension rods.

Rod transition area

The connection part between the thread of the threaded extension rod and the rod usually has a transition area where the cross-sectional shape and geometric dimensions change according to the geometric structure and the technical requirements of machining. The geometric structures of the thread transition zone have eccentric groove type, flat transition groove type, progressive thread type, upsetting thread opening type, flat cone combined transition groove type, and thread direct retraction type. The first five transition structures are all conducive to slowing down and dispersing the general tendency of stress concentration in this area, thereby reducing the probability of breakage and failure and improving the rock drilling life of the threaded extension rod. Choose the structure according to the thread diameter size, the geometry of the rod, the material condition and use, processing technology, and equipment to determine the status quo.

Other size parameters

Drill rod length

Under normal circumstances, the length of the threaded extension rod is its total length. However, for threaded shank rods, the length should be the length from the root of the drill shoulder to the threaded end; for MF rods, the length is the length from the root of the female thread at the female threaded end (including the rebate groove) to the male threaded end of the rod. The selection of the length of the threaded extension rod has a great relationship with the cross-sectional size, geometric structure, use method, and rock conditions of the drill rod. According to various rock drilling conditions, the shortest length of the rod is 250mm, the longest is 6100mm, or even longer.

Toggle groove

During the drilling process of the threaded extension rod, to facilitate the removal of the circular threaded extension rod under specific circumstances (when the drill is stuck or used with a non-independent rotary rock drill), the circular rod body near the threaded end of the circular drill rod is the toggle groove should be machined.

End faces

The end structure of the threaded extension rod plays an essential role in ensuring that the drill rod connection has a good contact surface, the coaxiality of the overall connection after the rod is connected, and the sealing performance of the joint against water and air.

As an indispensable tool in engineering construction, the threaded extension rod has excellent mechanical properties and simple operation. Reasonably determining the parameters of the drill rod is the key to ensuring project quality. To better take advantage of the threaded extension rod, construction workers should be proficient in its use skills, reasonably select models, and pay attention to regular inspection and maintenance.

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