Motor Drill

a portable drilling outfit for driving prospecting and mapping, geological, and blast holes 50–100 mm in diameter to depths of 10 m in friable and soft rock. It consists of a gasoline engine with a power of 2.2–3.7 kilowatts (3–5 hp), a reduction gear, and a screw-conveyor column with a bit at the lower end. Single-speed motor drills can rotate the screw conveyor at 150–250 revolutions per minute (rpm), with speed adjustment through a change in the fuel feed to the engine. In two-speed motor drills the engine power can be used more efficiently by using the high (500–600 rpm) or low (150–300 rpm) rate of rotation of the screw conveyors, depending on the geological cross section. The weight of a motor drill is 12–17 kg. The average drilling rate in soft rock is 150–250 m/hr.