Page 80 - Air and gas Drilling Field Guide 3rd Edition

4.2 Drill Bits 71

FIGURE 4-10. Single cone roller cone drill bit (courtesy of Rock Bit International Incorporated).

The use of percussion air hammers (or down-the-hole air hammers) is an
acceptable option to using rotating tricone or single cone drill bits for air and
gas drilling operations. The air hammer utilizes an internal piston (or hammer)
that is actuated by the compressed air (or other gas) flow inside the drill string.
The internal piston moves up and down in a chamber under the action of air pres-
sure applied either below or above the piston through ports in the inside of the air
hammer. In the downward stroke, the hammer strikes the bottom of the upper
end of the drill bit shaft (via a coupling shaft) and imparts an impact load to the drill
bit. The drill bit in turn transfers this impact load to the rock face of the bit.
This impact load creates a crushing action on the rock face very similar to that
discussed earlier at the beginning of Section 4.2.2. However, in this situation, the
crushing action is dynamic and is more effective than the quasi-static crushing
action of tricone and single cone drill bits. Therefore, air hammer drilling operations
require far less WOB than comparable drilling operations using tricone or single cone
drill bits.