Pull down load is required to adequately push teeth into the rock to break in efficiently. It is the primary factor in rate of penetration. The unconfined compressive strength of the rock determines how much pull down is to be applied.
High UCS rocks may need time for the indenter to break it. Lower RPM is often required for efficient rock breakage.
Rotation is required to move the cutting teeth to the next rock cutting position. The faster you move the teeth to the next position, the faster you will drill. If the rock resists indentation by the teeth, there will be minimal rock breakage, and rate of penetration will not increase proportionally with higher rotation.
For softer rock high RPM is required as Soft rock responds to frequency and for harder rock low RPM is required as Hard rock responds to time.
Rotation is required to move the cutting teeth to the next rock cutting position. The faster you move the teeth to the next position, the faster you will drill. If the rock resists indentation by the teeth, there will be minimal rock breakage, and rate of penetration will not increase proportionally with higher rotation.
For softer rock high RPM is required as Soft rock responds to frequency and for harder rock low RPM is required as Hard rock responds to time.
In rotary blasthole drilling, delivery of air in sufficient volume and at proper pressure is very essential to assure optimum bit performance.
Right amount of air ensures efficient cuttings removal from the hole bottom to the surface. It also reduces the cutting structure wear and bearing erosive wear by means of efficient bottom hole cleaning.
The right amount of back pressure is also required to cool the bearing and to keep the bearing clean from drilling debris.
The air volume provided must be good enough to produce a bailing velocity of 5,000-7,000 ft./ min. for dry cuttings; and 7,000-10,000 ft./min. for wet cuttings.
To determine volumetric requirements, and bailing velocity the simple equation may be used.
Q = V/183.35 (D2 – d2)
Q = cubic feet per minute of free air. V = Annular Velocity feet / min d = drill pipe outside diameter, inches. D = hole diameter, inches. 183.35 = Factor for conversion
Hence to determine the bailing velocity the equations is
V = Q x 183.35 / (D²- d2) feet per min
Nozzles should be selected so that the pressure inside the bit is 40-45 psi. The cab operating pressure inside the drills cabin could be higher, depending on the type of drill and CFM of air circulated. Typically, on compressors rated with 80-100 psi bit pressures can be 10 – 25 psi lower than the cab gauge reading.