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A drag bit is used for soft rocks, like sand and clay. The drill stem is rotated, and teeth on the bit tear up the rock.
A rock bit (also called a roller bit) consists of teeth on wheels which turn as the drill stem is rotated. These teeth apply a shearing pressure to the rock, breaking it up into small pieces.
The original patent for the rotary rock bit was issued to Howard Hughes Sr in 1909. It consisted of two interlocking wheels. The success of this bit lead to the founding of the Sharp-Hughes Tool Company.
In 1933 two Hughes engineers invented the tricone bit. This bit has three wheels and is still the dominant bit in the market today. The Hughes patent for the tricone bit lasted until 1951, after which time other companies started making similar bits. However, the Hughes’s market share is still 40% of the worlds drill bit market.