291 significantly enhanced. In the crater formed by dual spherical inserts, the depression between the inserts connects with the fractured zones around the penetration points. For dual conical inserts, the crater shows a characteristic where the depth of the inter-tooth region approaches that of the penetration point areas. This further illustrates the reinforcing effect of depth on "synergistic rock-breaking by dual inserts." Moreover, the crater morphology from dual inserts is distinct from the "near-circular" shape typical of single inserts. It consistently presents an elliptical form, with the penetration points precisely located at the foci of the ellipse. This is the result of rock fractures preferentially propagating along the "line connecting the two inserts" under the combined load of the dual inserts, confirming the "spatial synergy" in rock-breaking by dual-insert configurations. (a) Crater area (b) Cuttings mass Figure 10 - Changes in crushing pit area and rock mass under double-inserted teeth penetration velocity and depth As shown in Figure 10, the statistical data indicate that both the crater area and the cuttings mass gradually decrease as the penetration velocity increases. Furthermore, the crater area and cuttings mass at d=2.5 mm are significantly smaller than those at d=4.0 mm. It is noteworthy that during single-insert penetration, the crater area and cuttings mass for conical inserts are considerably smaller than those for spherical inserts. However, during dual-insert penetration, the crater area and cuttings mass formed by conical-conical dual inserts are generally larger than those for spherical inserts under the same experimental conditions. This discrepancy primarily stems from the different crack propagation modes induced by different insert geometries during penetration. Conical inserts are more conducive to the propagation of radial cracks, which consequently makes them more effective at causing spalling on the rock surface. 4. CONCLUSIONS This study systematically analyzed the rock-breaking performance of spherical and conical inserts in disc-like hybrid bits through triaxial tests under in-situ stress. The main conclusions are as follows: (1) Conical inserts show higher efficiency in initial penetration and reducing rockbreaking resistance. Under identical conditions, their brittleness index is significantly lower 0.6 0.9 1.2 1.5 1.8 10 20 30 40 50 60 70 80 90 Crater area/mm2 Penetration velocity/mm·min-1 Spherical teeth (d=2.5mm) Spherical teeth (d=4.0mm) Conical teeth (d=2.5mm) Conical teeth (d=4.0mm) (a) 0.6 0.9 1.2 1.5 1.8 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 Mass of rock fragments/g Penetration velocity/mm·min-1 Spherical teeth (d=2.5mm) Spherical teeth (d=4.0mm) Conical teeth (d=2.5mm) Conical teeth (d=4.0mm) (b)
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