Impact and crushing effects of cone drills
During drilling, the drill bit and its cone rotate around the axis of the drill bit. The teeth of the cone rotate around their own axis under the resistance of the formation rock, and the teeth alternately contact the bottom of the well with a single or double tooth.
When the tooth wheel contacts the bottom of the well from a single tooth to a double tooth, the center of the tooth wheel drops from the highest position to the lowest position; When the tooth wheel contacts the bottom of the well again from double teeth to single teeth, the center of the tooth wheel rises from the lowest position to the highest position. This repeated movement periodically raises and lowers the axis of the cone, causing the drill bit to vibrate longitudinally, with the amplitude being the vertical displacement of the wheel center. During each longitudinal vibration process, the upward movement of the axis compresses the accumulated elastic performance of the lower drill string, while the downward movement of the axis causes the elastic elongation of the lower drill string to release the elastic performance. Therefore, when a cone drill bit crushes rocks at the bottom of the well, the force exerted by the teeth on the rock not only includes the static load generated by the drilling pressure, but also the dynamic load generated by the teeth rushing towards the rock at a high speed due to longitudinal vibration. The former causes the teeth to crush the rock, which is called crushing effect; The latter causes teeth to impact and break rocks, known as impact action.
The impact load of the drill bit is beneficial for breaking rocks, but it can also cause premature failure of the drill bit bearings, causing teeth, especially hard alloy teeth, to collapse and cause fatigue failure of the drill string. Therefore, in drilling wells, especially in hard formations, shock absorbers should be used.
The shear effect of cone bit
The shear effect of a cone bit is achieved by the rolling of the cone at the bottom of the well and the sliding of the teeth against the rock at the bottom of the well. There are three factors that cause sliding: overtop, complex cone, and axis shift.
Sliding caused by overtopping (sliding direction perpendicular to the tooth axis, i.e. along the tangent direction)
Sliding caused by axis shift (sliding direction along the axial direction of the tooth)
Sliding caused by complex cones
The compound cone cone includes the main cone and the auxiliary cone. If the main cone top coincides with the center of the drill bit, the auxiliary cone top must be the top of the super top, causing sliding in the tangential direction.
The axial sliding caused by shifting the axis can shear off the rock between the gear rings, while the tangential sliding caused by the super top and complex cone can shear off the rock between the adjacent tooth fracture pits of the same gear ring. Sliding increases tooth wear.
For drill bits in extremely soft to medium hard formations, there are generally both axial displacement, superelevation, and compound cone; Drill bits in medium or hard formations have super top and double cone. In extremely hard formations, the selected drill bit is pure rolling (single cone, no overtop, no axis shift).
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