DRILLING
DRILLING THEORY
Principles of percussive drilling
In percussive drilling, the energy required to break the rock is generated by a pneumatic or hydraulic rock drill. Machines of this kind build up a pressure which, on being released, drives the piston forward (Fig. 1).
The piston strikes on the shank adaptor. The kinetic energy of the piston is converted into a stress wave which travels along the drill string to the rock.
In order to obtain the best possible drilling economy the whole system, i.e. rock drill, drill steel and the rock, must harmonize.
Part One - Wave shape and output power
Theoretically, the wave has a rectangular shape.
Its length is twice that of the piston, while its height (Fig. 2) depends on the speed of the piston at the moment of impact and on the relationship between the cross-sectional area of the piston and that of the drill steel. Fig 2
The total energy that the wave contains is indicated diagramatically by the grey field in fig. 2.
To calculate the output power obtained from a rock drill we multiply the wave energy by the impact frequency of the piston, and it is usually stated in kW.
The designer seeking to evolve rock drills with special characteristics will combine several variables, such as the piston geometry, the impact rate and the frequency..
The shock waves that are generated by hydraulic (Fig. 3) and pneumatic (Fig. 4) rock drills are different in shape. A drill steel used with hydraulic machines will normally show a substantially longer service life than one used with pneumatic machines, the reason being that the stress level is higher with a pneumatic piston. Fig 3 Fig 4
This is due to the fact that the pneumatic piston has a larger cross-section, and this in its turn is explained by the circumstance that a considerably greater cross-section is needed, since the pneumatic drill operates at a substantially lower working pressure, 6-8 bars as compared with the 150-250 bars found
Principles of percussive drilling
In percussive drilling, the energy required to break the rock is generated by a pneumatic or hydraulic rock drill. Machines of this kind build up a pressure which, on being released, drives the piston forward (Fig. 1).
The piston strikes on the shank adaptor. The kinetic energy of the piston is converted into a stress wave which travels along the drill string to the rock.
In order to obtain the best possible drilling economy the whole system, i.e. rock drill, drill steel and the rock, must harmonize.
Part One - Wave shape and output power
Theoretically, the wave has a rectangular shape.
Its length is twice that of the piston, while its height (Fig. 2) depends on the speed of the piston at the moment of impact and on the relationship between the cross-sectional area of the piston and that of the drill steel. Fig 2
The total energy that the wave contains is indicated diagramatically by the grey field in fig. 2.
To calculate the output power obtained from a rock drill we multiply the wave energy by the impact frequency of the piston, and it is usually stated in kW.
The designer seeking to evolve rock drills with special characteristics will combine several variables, such as the piston geometry, the impact rate and the frequency..
The shock waves that are generated by hydraulic (Fig. 3) and pneumatic (Fig. 4) rock drills are different in shape. A drill steel used with hydraulic machines will normally show a substantially longer service life than one used with pneumatic machines, the reason being that the stress level is higher with a pneumatic piston. Fig 3 Fig 4
This is due to the fact that the pneumatic piston has a larger cross-section, and this in its turn is explained by the circumstance that a considerably greater cross-section is needed, since the pneumatic drill operates at a substantially lower working pressure, 6-8 bars as compared with the 150-250 bars found