From the principles of rock crushing to the design of drilling tools

Basic theory of rock crushing

1. Basic force of tools on rocks

When using tools to crush rocks, no matter how the tools act (impact, pressure, cutting) to crush the bottom rock, the rock in front of the teeth will be subjected to a pressure (vertical or horizontal). Therefore, pressure is the basic force on the rock.

1) Under the combined action of normal force and tangential force, the stress under the pressure head will be the superposition of the stress under the two separate loads.

2) Under the combined action of normal force and tangential force, an uneven stress state is generated. The compression zone I in all directions decreases with the increase of tangential load, and tensile zone II and transition zone III appear. There are both compressive stress and tensile stress in the transition zone.

3) There is an optimal ratio between normal force and tangential force. In other words, when the cutting teeth crush rocks obliquely, there is an optimal force angle for each rock, and the drilling effect is best at this time.

2. Basic phenomena of rock crushing by tools

1) Regardless of the tool, load, or material, when the tool penetrates the rock, a dense core is first generated in front of the tool, which is formed by local crushing or plastic deformation of the material under huge pressure.

2) The penetration depth does not increase evenly with the increase of load. At the beginning of the load increase, the penetration depth increases at a certain ratio. When a certain critical value is reached, a sudden leap occurs, and the rock around the dense core collapses to form a crushing pit.

3) The crushing pit is funnel-shaped. Regardless of the form of the pressure head, the method of penetration, and the type of rock, the funnel vertex angle β does not change much, generally between 60 and 75 degrees. The harder the rock, the larger β.

Ⅰ) Surface crushing zone: The contact pressure between the tool and the rock is much smaller than the hardness of the rock, and the tool cannot be pressed into the rock. At this time, the rock crushing is caused by contact friction work, and the rock crushing efficiency is low.

Ⅱ) Fatigue crushing zone: The contact pressure increases. Although it is smaller than the hardness of the rock, it can destroy the intercrystalline connection of the rock. After multiple loadings, fatigue cracks develop and intersect, resulting in coarse-grained separation.

Ⅲ) Volume crushing zone: However, the contact pressure is greater than the hardness of the rock, and the tool effectively invades the rock, forming a crushing pit, resulting in volume crushing, and separating large pieces of rock debris.

Critical condition for volume crushing: axial pressure ≥ 1.2 to 1.5 times the uniaxial compressive strength of rock

3. Factors affecting the efficiency of rock breaking by impact penetration

1) Influence of free surface

- The existence of a free surface near the pressure head is conducive to penetration; the load required for the pressure head to penetrate the rock increases as the distance between the pressure head and the free surface increases.

- The reasonable distance between two adjacent pressure heads depends on the size of the maximum crushing pit formed. When the external load center distance is greater than or equal to (D1+D2)/2, the O1CO2 part may not be crushed or cannot be pushed out, thus forming a rock ridge between the two cutting edges. When the external load center distance is too small, the two compaction zones are too close, which is equivalent to expanding the compression zones, making it more difficult for the rock to undergo large shear between the adjacent pressure heads. The appropriate spacing is between (D1+D2)/3~(D1+D2)/2.

The diameter of the crushing pit: for brittle rock, D:d=5~8; for plastic rock, D:d=3~4

Based on upon principle, Drillmaster designs drilling tools with scientific and reasonable drilling teeth angle and position arrangement. That's why the construction efficiency and quality can be improved by using our drilling tools.

2) Influence of loading speed

When the load reaches the rock's crushing strength, within a certain impact frequency range, the rock breaking speed increases in direct proportion to the impact frequency. When the impact frequency increases to a certain value, the rock breaking speed decreases. For a certain rock, there is an optimal impact frequency.

When the load reaches the rock's fatigue strength but is less than its crushing strength, increasing the impact frequency can improve the rock breaking efficiency.