The drilling speed of a percussive drill may be predicted from a drillability laboratory test as a function of the drill percussion power, the hole diameter and the bit cutting edge length.
La vitesse reelle de forage d'un marteau peut être determinee à partir de test de forage en laboratoire, comme une relation entre la puissance de percution, Le diamètre de forage et la longueur du fil du taillant.
Die Bohrgeschwindigkeit eines durchschlagshammers ist durch einen laborversuch vorawszusehen der auf die durchschlagskraft (-leistung), den durchnesser des bohrers und die Laenge der bohrerkante (-klinge) basiert ist.
This report describes a method to estimate penetration rate in percussive drilling based on a simple laboratory test carried out on small rock samples. - The test is a reduced scale reproduction of the actual percussive drilling phenomenon. The small rock samples must be representative of the rocks existing at the site. Therefore it is recommended to do a geological study previously to the sample collection.
These factors can be classified in two groups: - Factors depending on the drill rig - Factors depending on the drilling rods and bits. Among the first ones, the most important are: percussion power, rotation and feed force. From those depending on the drilling accessories, the diameter of the hole, the type of bit and the number of rods have to be mentioned. Percussion power The percussion is the main energy source in percussive drilling and hence it has the biggest influence on penetration rate. Percussion power is usually expressed as the product of the number of blows per minute by the kinetic energy of the piston at the - instant when it hits the drill string. Rotation The basic purpose of the rotation in percussive drilling is to provide indexed blows so that each - crater breaks into the preceding one, taking advantage of the additional free face created and increasing the drilling efficiency. Theoretically, for each application, there is an ideal indexing angle and, consequently, and optimum rotary speed, which provides biggest cutting sizes and maximun efficiency. Nevertheless, rotation in percussive drilling is just an auxiliary operation, which only means 10–15% of the power output. Feed force This force is necessary to manta in the bit in contact with the rock in order to make the energy transfer between them possible. The increase of this force beyond the value -- which assures a continuous interface contact, does not mean any substancial improvement in the penetration rate, but rather increases the wearing rate of the cutting tool. Hole diameter The diameter of the hole is one of the most important factors affecting the penetration rate, be-. cause the percussion energy is distributed over a -- larger area as the diameter increases. Type of bit The type of bit has also a certain influence on penetration rate. A button bit, for example, does not penetrate so much into the rock in each blow. However, the number and distribution of the buttons is - such that fewer blows are necessary to cover the full surface of the hole. Therefore, the penetration rate achieved with this type of bits is a little higher that with cross bits. For a similar reason, single chisel bits of integral steels provide lower penetration rates than cross bits. Number of drillings rods In drifter drilling the hole depth has also an influence on the penetration rate. In each meter of rod, 0' 2–0 '4% of the shock wave energy is converted into heat as a result of friction forces. In the same way, in each coupling, about 3% of the energy is reflected backwards and an additional 5' 5% is lost due to friction. Therefore, it can be concluded that in each rod 9–10% of the available energy is dissipated and consequently the penetration rate will be reduced in the same proportion.
The horizontal projection of the area of contact between the cutting blade and the rock in the case of a cross bit, is function of the penetration achieved and can be expressed (see Fig. 1) by the following formula: a = 2hl tg /2· where:
- –
"a" is the projected area
- –
"h" is the penetration
- –
"1" is the length of the cutting edge
- –
" " is the wedge angle of the cutting tool, which is approximately the same for all bits.
