Failure analysis of the drifter rods

(1)abrasionof R32 threads of drifter rods

In general, the normal failure mode of the drifter rods should be severe abrasionof R32 threads, that is, the thread crestsare ground to be thin or flat, or fatigue breakage occurs due to severe abrasion after the drilling meters is high. The abrasion causing the failure of R32 threadsinclude fretting abrasionand impact abrasion. In drilling operations, with the continuous drilling progress,the abrasion of R32 thread continues to increase due to the replacement of old drill bits, and the fitting clearance between R32 threads and old or new bits increases. The fretting abrasiongenerated from good fitting clearance at the beginning will gradually transitionto the impact abrasion when the fitting clearance is large. The Impact abrasion will lead to rapid abrasion of the threads, resulting in rapid failure of drill rods. In addition, under the impact and rotation of high-power hydraulic rock drills, line contact or side contact will be formed between R32 threads and the old or new drill bits, resulting in local high temperature, hardness reduction, wear resistance reduction, and even formation of erosion pits on thread surfaces, which turns into crack sources.

(2) breakage of R32 threads of drifter rods

Samples of abrasion R32 threads was boiled in 70% hydrochloric acid for one hour, then washed with hot water and dried, as shown by (a) and (b)in Figure 11. It can be seen from the figure that the fatigue is the most serious at the root and the second thread of the R32 threads of the drifter rods, and the erosion pits appear. It shows that in the process of rock drilling, the stress on these two parts of the drill rods is the largest, resulting in local high temperature to form erosion pits, so the drifter rodsare very easy to break in these two parts.

macroscopic appearance of the sample of failed R32 threads in 70% hydrochloric acid

During drilling processes, the operator generally uses a drill rig equipped with a drill pipe and a φ76R-32 button bit or a reaming bit to drill the central hole on the face of the tunnel. Because  tensile and compressive stress, torsional stress, bending stress, propulsion resistance and frictional force are generated by the φ76-R32 button bit and the reaming bit on the R32 thread of the drill rod, it is easy to break the R32 thread, mainly at the part where the R32 thread connects the drill bit.

Figure 12  φ76-R32 drill bit with 12 buttons and a reaming bit

The connection of R32 threads of drifter rodsto drill bits

When drilling tools from different manufacturers are used in the same mine,R32 threads of drifter rodsare excessively exposed when connecting with drill bits, which is hard to guaranteesuitable fit tolerance, as shown in Figure 13, resulting in failure by premature abrasion or fracture of R32 threads of drifter rods.

Fracture and abnormal abrasion at connection parts of T38 or R38 threadsof drill rods and shank adapters

When drifter rodsare used, the rootsand the second threads of R32 threads of drifter rodsbear the most concentrated stress, and are most easily broken. T38 or R38 threads connecting drill rods and shank adapters rarely break, and the wear is relatively light. If there is fracture and abnormal abrasion, the analysis should focus on that whether the parameters of drill rigs and the hydraulic rock drills are suitable to the changes in the surrounding rock conditions,such aspropulsion pressure of drill rigs, impact and rotating pressure of hydraulic rock drills, etc.

The fracture of rod bodies

The fracture of rod bodies rarely occurs in drilling operations. Generally, it is necessary to analyze whether rod bodies have defects caused by external forces, or drill rods are operated in abnormal ways in drilling operations.

The bending of drill rods

When severely bent during drilling operations, the drill rods cannot be used without straightening. Bending generally occurs nearR32 threads due to the complex surrounding rock conditions, resulting in the failure of the drill rods because of bending caused by hole deviation or stucking in cracks, etc.

Figure 13  excessive expose of threads of drill rods due to bad connection to drill bits