Causes of Early Failure of Light Shank Rods

Table of Contents

What is a Light shank rod?

The light shank rod is a hexagonal hollow steel rod with a shank shoulder at one end and a thread at the other end, commonly used specification H25x4.7m, shank adapter length 159mm, thread specification R32mm, whose function is to transfer the impact energy of the rock drill to the drill bit, so that the drill bit can impact the broken rock and realize the drilling of shell holes. It is widely used in underground resources mining such as iron ore mines and coal mines to drill vertical shafts.

shank rod

During the rock drilling process, the light shank rod not only bears the tensile and compressive stress caused by the impact but also bears the rotating torsional stress and bending stress. The stress situation is complicated, and the quality requirements are relatively high due to the small space, high dust concentration, and poor visibility of the rock drilling site.

If the drill rod fails early and abnormally, it will not only increase the construction cost but also increase the operation time, construction difficulty, labor intensity, and safety risk. What are the reasons for the failure of the light shank rod?

Causes of early failure of light shank rod

The failure forms of light shank rods are shank rounding, shank fracture, drill shoulder fracture, rod fracture, thread fracture, early wear of the thread, etc.

Early wear and rounding of the edges and corners of the shank

1. The size of the hexagonal opposite sides of the drill rod shank is too small, resulting in an excessive gap between the shank and the hexagonal rotating sleeve of the rock drilling machine, resulting in early wear loss of the shank edges and corners.

2. The size of the hexagon in the hexagonal rotating sleeve of the rock drill is too large, so the gap between the rotating sleeve and the shank of the drill rod is too large, resulting in early wear loss of the shank edges and corners.

Shank fracture

1. There is hollow steel treatment rolling defects such as inclusions, segregation, indentation, folding, and transverse cracks in the shank.

2. The shank reaming opposite side size is small, uneven deformation, reaming in damaged drill rod inner wall.

3. The rear R part is the place where the drill rod section area changes sharply, and if the R is small or too large due to the unreasonable design of the die R or the processing is not in place, the forging filling is not sufficient, etc., which leads to the formation of stress concentration at this location or poor coordination with the hexagonal turning sleeve of the rock drill.

4. When the drill shoulder is forged and heated, overheating occurs.

5. The coaxial on both sides of the drill shoulder is too large.

6. The R size of the rotating sleeve of the rock drill is too small or too large, resulting in a poor fit with the rear R of the drill rod shoulder.

7. The braze holder of the umbrella drilling machine is damaged, or the clearance is too large, and the drill rod swings a lot in use, resulting in bending.

Shoulder fracture

1. Hollow steel has rolling defects such as shrinkage cavities, inclusion, and segregation.

2. The structural design of the shoulder mold is unreasonable, or the forging shoulder heating temperature is too low, resulting in the formation of a bell mouth defect in the middle of the shoulder.

3. The hollow steel core hole is eccentric, the ellipse is serious, and the forging deformation of the drill shoulder is uneven.

4. The heating temperature of the shoulder forging is too high, or the heating time is too long, resulting in overheating or overburning.

5. If the quenching and tempering length of the shank exceeds the shoulder, the quenching temperature is too high, or the tempering is insufficient.

Rod fracture

1. There are inclusions, segregation, high or low rolling temperature, heavy eccentricity, and ellipse in the raw material of drill rod hollow steel.

2. The maximum superimposed stress is borne at a distance of 180 mm to 300 mm from the end face of the shank. At the same time, it is the heat-affected zone formed when the shoulder is forged and heated, which causes the strength of the drill rod in this area to be reduced significantly due to the existence of the heat-affected zone.

3. The surface hardness of the hollow steel is too high or too low, resulting in the mechanical properties of the drill rod not reaching the best state.

4. The operator does not use it properly. Too much-pushing force and incorrect use of the brazier lead to the bending of the drill rod; the drill rod body is knocked during use so that the surface traces form a crack source of fracture, etc.; the drill rod is bent and deformed after use, and has not been straightened and continue to use, etc.

Thread breakage and early wear

1. The main factors affecting the early fracture of the thread are the existence of non-metallic inclusions in the hollow steel raw material, composition segregation, high or low rolling temperature, and other internal quality defects; brazing when the threaded part of the forging heating temperature is too high or high temperature for too long, resulting in the coarse organization or low-temperature forging; thread quenching temperature is too high or tempering insulation is insufficient, high hardness, etc.; drill bit and drill rod thread are poor; users use high air pressure and large propulsion force when drilling holes, non-standard operations such as long-time empty punching and non-straight punching.

2. The factors that affect the early wear of threads include the small outer diameter of drill rod threads, shallow teeth, and other dimensional deviations, resulting in poor fit with drill bit threads; low thread heat treatment hardness, reducing the wear resistance; larger inner diameter of drill bit threads, shallow teeth, and other dimensional deviations, resulting in a poor fit between the drill bit and drill rod, resulting in early wear; long idle time for users, the thread temperature rises rapidly and leads to a decrease in thread hardness or ablation of threads, which makes the early wear of the thread invalid.

To sum up, as a rock drilling tool, the light shank rod is also a wearing part, the service life of which depends not only on the intrinsic quality of the rod but also has a close relationship with the standard operation of the user, reasonable adjustment of the parameters of the rock drilling machine and factors such as air pressure and water pressure.

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