Percussive Drilling

Percussive drilling breaks the rock by hammering impacts transferred from the rock drill to the drillbit at the hole bottom. The energy required to break the rock is generated by a pneumatic or hydraulic rock drill. A pressure is built up, which, when released, drives the piston forwards. Figure 1 illustrates the principle of top hammer percussive drilling.

The piston strikes on the shank adapter, and the kinetic energy of the piston is converted into a stress wave travelling through the drill string to the hole bottom. In order to obtain the best drilling economy, the entire system, rock drill to drillsteel to rock, must harmonize.

Stress Wave

Theoretically, the stress wave has a rectangular shape, the length of which is twice that of the piston, while the height 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 drillsteel.

The total energy that the wave contains is indicated diagramatically in Figure 2. To calculate the output power obtained from a rock drill, the wave energy is multiplied by the impact frequency of the piston, and is usually stated in kW. Rock drill designers seek to find the best combinations of various parameters, such as the piston geometry, the impact rate and the frequency. Two rock drills having the same nominal power rating might therefore have quite different properties.

The shock waves that are generated by hydraulic (Figure 3) and pneumatic (Figure 4) rock drills are significantly different in shape. Drillrods used with hydraulic rock drills will normally show substantially longer service life, compared with pneumatic rock drills, because of the higher stress level obtained with the pneumatic driven piston.

The reason is the larger cross-section needed when operating at substantially lower pressure, which is 6-8 bars, compared to the 150-250 bars used with hydraulic systems. The slimmer the piston shape, the lower the stress level.

Figure 5 compares the stress level generated by three different pistons having the same weight, but with different shapes and working at different pressures. The lowest stress, or shock wave amplitude, is obtained with the long slender piston working at high pressure.

Date: 2015-12-24; view: 902