Investigation of insert-hole interference fit influence on downhole drilling bit failure

Abstract

Down-the-hole drilling is a reliable method for drilling in medium-to-hard geological structures. The insert-hole fitting interference value emerges as one of the most important factors of down-the-hole drilling bit life. In this study, fatigue tests were conducted using a tungsten carbide pin press-fitted into a hole in the specimens of DIN 1.6580 steel (which is used for manufacturing the bit body) to quantify the effect of interference value on the bit body failure. Then, the process of bit-rock collision was simulated using three-dimensional finite element method taking into account the residual stresses in the insert and bit body to determine the optimal interference value. Simulations were performed considering the rotary motion of the bit in addition to impact for a single-insert as well as a commercial 3.5 inch bit. Results showed that the interference value influences fatigue life significantly. Also, the velocity and angle of impact have considerable influence on body stresses. Results of this study can greatly assist the design of insert-hole manufacturing tolerances in down-the-hole bits.

Stress distributions around the interference fit between a round pin and a perforated finite plate

“…The numerical analyses focused on 2D finite element models, since the aim of this work was to investigate the distribution of the contact stresses in the circumferential direction, disregarding the highly localized stress concentrations, which occur on the boundaries in the axial direction. As can be observed in several works from the literature [22,[28][29][30][31][32], the investigation of these stress peaks requires a very detailed, computationally expensive numerical model, and are strongly affected by the details of the geometry (fillets or chamfers on the hub). This test plan will examine the following variables, while keeping constant the width of the rectangular plate: the ratio between pin diameter and plate width, aspect ratio of the plate, pin offset along the longitudinal axis of the plate.…”

Connection force measurement of precision small interference components using ultrasound

“…However, the interfacial wear during the assembly process will affect the actual interference, resulting in significant errors in the theoretical analysis, thereby reducing the reliability of the assembly quality evaluation method based on the press-fit curve. 14…”

“…However, the interfacial wear during the assembly process will affect the actual interference, resulting in significant errors in the theoretical analysis, thereby reducing the reliability of the assembly quality evaluation method based on the press-fit curve. 14 At present, most studies focus on the design and assembly stage of interference components, but seldom pay attention to the detection stage. Due to wear and interference loss at the mating interface caused by the press-fit assembly, the press-fit curve cannot accurately reflect the contact state of the fitting surface.…”