Page 573 - Air and Gas Drilling Manual

Chapter 11: Specialized Drilling Equipment 11-11
lubricant injected into the injected air during the drilling operation. This lubricant is
needed to lubricate the piston surfaces as it moves in the hammer housing. Air
hammers are used exclusively for vertical drilling operations.
This piston cycle is repeated as the drill string (and thus the drill bit) is rotated
and in this manner continuous layers of rock on the rock face are crushed and the
cuttings removed. The air flow through the air hammer is not continuous. When
the piston is being lifted in the hammer, air is not being exhausted through the drill
bit. For example, at a piston impact rate of 600 strikes per minute the air is shut
down for about 0.050 seconds per cycle. This is so short a time that the air flow
rate through the annulus can be assumed as a continuous flow.

Figure 11-9: Schematic cutaway of a typical air hammer (courtesy of Diamond Air
Drilling Services, Incorporated).
Downhole air hammers are available in housing outside diameters from 3 inches
to 16 inches. The 3 inch housing outside diameter hammer can drill a borehole as