Tophammer drilling is a percussive-drilling method, the percussion mechanism is located on the drill rig, i.e. outside of the hole and conventional tophammers dominate the 28 mm to 140 mm hole-range.
To drill holes of greater depth, extension rods are added in front of the rock drill until the final hole-depth is reached.
Since each extension-rod must be joined with the next, a threaded union is needed between each rod. In each such joint in the drill-string, impact energy is lost. The loss per joint is equal to between 4 and 6 per cent of the value of the impact energy before the joint.
The exact value of the energy loss depends on the design of the joint. The greater the mass of the joint, the greater the energy loss. If the joint is not properly tightened, the energy loss can be as much as 10 per cent.
In long-hole drilling underground, where short (and therefore many) rods are used, about half the impact energy can be lost.
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