Due to the harsh working environment, the hollow hexagonal tapered drill rod often has various failure modes during drilling. When these failure modes appear, understanding the common positions of the drill rod failure in advance can help us to analyze the specific problems. Let’s learn together today.

The common positions of the drill rod failure mainly refer to the damage or fracture at the shank end, the root of the shank collar, the middle of the shank collar, the front of the shank collar, the rod, the taper section, etc. The different failure forms and reasons are as follows.

1. Fracture at the shank end

There are two failure forms of shank end. One is that due to the low hardness of the shank end, under the impact of the rock drill piston, it is easy to cause the end upsetting phenomenon. The other is that under the high-frequency impact of the rock drill, local caving occurs at the end of the shank end.

2. Fracture at the root of the shank collar

The positions where the root of the shank collar starts to break are on its transition arc. The fracture plane is very uneven, and there is a relatively smooth crack development area at the corner or around the corner. However, the ratio of the total area of this area to the whole fracture area is very small, and the brittle fracture area is very large.

3. Fracture in the middle of the shank collar

The fracture in the middle of the shank collar generally occurs in the place where the cross-section of the flushing hole changes suddenly, commonly known as the “bell mouth”. This “bell mouth” has two shapes: one is due to the expansion of the diameter of the flushing hole in the middle of the shank collar during the process of forging the shank collar, and the unreasonable design of the forging die or insufficient machining allowance. When the swollen inner hole continues to be subjected to the axial extrusion force at the shank collar, a “bell mouth”-shaped flushing hole is formed. The other is to insert a forging needle into the flushing hole when forging the shank collar to prevent the flushing hole in the middle of the shank collar from being blocked due to metal forging deformation. The diameter of the flushing hole will shrink when the metal expands, forming another shape of a “flare” under the push of the needle.

4. Fracture at the front part of the shank collar

Fracture often occurs mostly on the front edge of the transition arc between the shank collar and the rod. Most cracks occur on the outer surface and most of them are on hexagonal edges and corners.

5. Fracture of the rod

During the drilling process of the drill rod, the rod may break, but most fractures occur at 20-400mm in front of the shank collar. Some cracks on the fracture surface arise from the outer surface, and the fatigue arc zone expands from the outside to the inside, forming an external fatigue fracture. Some cracks occur on the inner surface of the rod, and the fatigue arc zone expands from the inside to the outside, resulting in internal fatigue fracture.

6. Broken taper section

The taper section often breaks at its smaller end due to its production process and technology. You can roughly analyze the force characteristics of the drill rod and the cause of failure based on your understanding of the common positions of the drill rod failure, and different fracture shapes. It is not only conducive to the improvement of process technology and other aspects but also to prolonging the service life of the drill rod.