In rotary drilling, the rock is not hammered by the drill as in percussive drilling. Instead, the rock fails by a combination of pull-down pressure on the bit and rotation. Cuttings from the drilling are flushed up the annulus of the hole using compressed air. Air pressure for rotary drill rigs are usually in the range of 50-100 psi. A comparison of rotary drills from various suppliers is provided here. Optimizing overall performance of rotary drills will involve a combination of individual factors namely: bit load, bit rotation, bit selection, and operator performance. Bit Selection For blasthole rotary drilling, bits are usually of two different types: 1. Drag or Blade Bits 2. Roller-Cone Bits
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Drag bits have hardened blades that act to penetrate soft rock and gouge it out. For harder rock, the blades can be tipped with tungsten carbide to increase the wear life. Drag bits are usually used in rock with compressive strength less than about 20,000 psi (140 MPa) and in small diameters (89-152 mm). Roller cone or tricone bits are the most common bit used for rotary blasthole drilling. The bit consists of three rotating cones, usually containing carbide inserts. Bearings within the roller cone bit must be kept clean and cool by flushing with compressed air. Bit Load An empirical formula for estimating the maximum bit load is a follows: Max. Bit Load (pounds) = 810(D2 ) where D = Bit Diameter Optimum Bit Load (pounds) = UCSxD/5 where UCS = uniaxial compressive strength of rock (psi) and D = bit diameter
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The optimum bit load calculated using the above formula can be expected to produce the optimum load plus or minus 10%. A comparison of features of rotary blasthole rigs is provided here. |
