Crush Zone Mean?
The crush zone can be defined as the crushed rock just below the tooth of a rock drill bit. The drill bit exerts high compressive stress on the rock causing it to fail. The rock in the crush zone should be effectively created and removed in order for the drill bit to function efficiently. Rocks that haven't been broken properly in the crush zone and removed effectively will be equivalent to drilling the hole twice.
Roller-cone bit is one such tool designed for the crush zone to crush rock effectively while minimizing wear on the cutting surface.
Trenchlesspedia Explains Crush Zone
The roller-cone bit was designed by Howard Hughes, and it consists of conical cutters with spiked teeth around them. When the drill string is rotated, the cones roll along in a circle at the bottom of the hole. The rolling causes new teeth to come in contact with the bottom crushing the rock below and around the teeth of the roller bit. As the tooth lifts up from the bottom, a fluid jet at high velocity hits the rock chips in the crush zone and forces it into the annulus.
The next tooth then contacts the hole bottom and creates more crushed rock that is removed by the fluid jet. This continuous process ensures that the crush zone at the bottom of the hole is effectively cleared.