| 摘要 |
The invention provides an excavation bit (10) which is constructed from either a single or double carrier. If two carriers (14 and 15) are present the carriers (14, 15) are contra-rotating. By the off setting of the axes of rotation (17 and 18) of single or dual carriers from a longitudinal axis (22) of the bit (10) and by driving the carriers to rotate, a ground engaging thrust is produced, as well as the rotation of the excavation bit (10) in the ground as a consequence of the rotation of the carriers, and not vice versa as is the case with prior art. By the invention, there can result sufficient thrust on the bit (10) by the rotation of the carriers (14 and 15) so that the need to apply thrust down the bore via the drill rod is reduced or eliminated. As a result of the invention the number and/or size of the ground engaging tools (25) are not a function of the bore diameter to be drilled. Thus as the excavation bit is scaled up for larger diameter bores more ground engaging tools (25) and/or an increase in their size is not required. By the invention, thrust applied (either via the drill rod or from the rotation of the carriers) is thought to be, through a quasi lever system, multiplied at some of the ground engaging tools in the radial direction. That is the total thrust in the longitudinal axis direction (whether externally applied or resultant from the contra-rotation of the carriers), is multiplied so that the outward forces exerted (by the cutters onto the rock surface in the region approaching perpendicular to the longitudinal axis of the bore) is thought to be significantly higher than the magnitude of the total thrust.
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