ABSTRACT
This chapter focuses on the large rigs that are capable of giving high feed force on the bit, considering that the units that work with drag bits are of a more simple design and smaller in size. Rotary rock drills essentially consist of a power source, a string of pipes, individual or connected in series that transmit the load, rotation and flushing air to a bit with steel teeth or tungsten carbide inserts that acts upon the rock. The feed force applied to the bit should be sufficient to overcome the compressive strength of the rock, but not excessive to avoid premature or abnormal failure of the tricone bit. In hard formations, a high thrust force on the bit can produce breakage of the inserts before the bit becomes stuck or the hole cleaning is defective. Also the bearing life is diminished, but not necessarily the length drilled by the rolling cone bit.
