The threaded extension rods are the most variety, the most complete specifications, and the best economic benefit of all the rock drilling tools with a large quantity. The article” The types of threaded extension rods” also classified different types of threaded extension rods, and also mentioned that the development of rock drilling machinery to high power and high frequency, will stimulate the marketing demand for threaded extension rods. Anyway, the basic requirements for extension drill rods will not change.

Basic requirements for threaded extension rods

1. The combination of high fatigue strength and fracture toughness.

The fracture of threaded extension rods is mainly fatigue fracture. The main reason is that the drill rods have to bear the high-frequency impact force and axial thrust of the rock drill. Moreover, the threaded extension rod is a long and thin rod, the drill rods often sustain bending stress in oblique rock drilling and horizontal rock drilling, because of the influence of the self-weight of the drill tool group. Therefore, the material of the threaded extension rods must have high fatigue strength and fracture toughness.

2. Good elasticity and wear resistance.

The drill rod often works under different bending states, the process of rock drilling, whether it is horizontal, vertical, or oblique operations, due to the change of the rock drill thrust, the inhomogeneity of the rock, or the existence of cracks, holes, and incorrect operation. If the bending condition is severe or the drill rods’ elasticity is not good, it will cause the drill rods to bend and break. Cause early failure. Therefore, the drill rods must have good elasticity.

The threaded structure of the two ends of the threaded extension rods is an important part of connecting the whole set of rock drill tools and transmitting the rock drilling stress. The threaded part is the most important part of the failure of the threaded extension rods, and its failure mechanism is the result of fretting wear and impact wear successively. Therefore, the wear resistance of the threaded part is often the determining factor for the life of the threaded extension rod.

3. Good corrosion resistance.

The main factor of fatigue fracture in drill rods is inner hole corrosion of threaded extension rods due to the combined action of high frequency and high stress and water (often acidic water). Therefore, the threaded extension rods must have good waterproof corrosion resistance.

4. High straightness and rigidity.

The straightness and rigidity of the threaded extension rod are important guarantees for high rock drilling efficiency. In rock drilling, the drill rod should work under the rotating state of 250r/min. If the straightness is low or the rigidity is insufficient, the drill rods will work under the eccentric blow, and part of the rock drilling energy is lost. While bending stress is produced, which increases the probability of the drill rods’ fatigue failure. The higher the straightness and the better the rigidity of the drill rod, the higher the efficiency of transferring the rock drilling stress, the faster speed of the rock drilling .

5. Good structural design.

The threaded extension rods are high-frequency and high-stress transfer rods, the section change of the component can cause stress change and stress concentration, the greater the section change, the lower the efficiency of stress transmission and the greater the failure stress. Therefore, the principle of designing is avoiding or reducing excessive section changes as much as possible to guarantee good structure.

The threaded extension rod service life is not long restricted by the complex stress state and harsh working conditions. The basic requirements for threaded extension rods are also to improve their service life.