- Poor Bit Life - Large diameter cuttings continually fall back to the bottom of the hole and are reground to smaller sizes by the bit until the air velocity is sufficient to lift them to the top. This can cause premature wear.
- Reduced Drilling Speed - Exhaust air at the bit face does not have enough velocity to move the cuttings before the piston strikes the bit again. Therefore, the bit is redrilling the cuttings rather than breaking fresh rock.
- Lost Tools - Cuttings will sometimes flow part of the way up the hole and form what is called a collar. This will block any cuttings from exhausting from the hole and eventually cause the tools to become stuck below the collar.
Annular velocity above 7,000 FPM (2135 MPM) will result in excessive erosion of the down hole hammer, drill bit and drill rod.