numerical simulation designed by computer image technology can visually
demonstrate the changes of various parameters (shape, stress, strain, etc.) in the
whole process of rock bursting by impact. Using the software program, different
physical and geometric parameters can be selected to calculate, and the influence
degree of different parameters on the results of the interaction between teeth and
rock can be obtained, so as to give a more comprehensive result.
The cylindrical teeth of DTH hammer bits are made of hard metal, which greatly
improves the service life of DTH hammer bits. However, in the use process, there
are still some phenomena such as broken teeth, dropping teeth, broken waist, split
trousers, and so on. How to effectively reduce the fracture and breakage of cylindrical teeth has become a key factor to effectively improve the life of DTH hammer
bits. In practice, it is found that the fracture and broken of the side teeth of DTH bits
are the most common failure forms in the damage of the total cylindrical teeth. For
example, when the bob-weight rock drill with Swedish /40 mm spherical teeth is
used in a mine, the drop rate of the edge teeth reaches 23%, and the breakage rate of
the edge teeth reaches 30%. The domestic /40 mm ball-tooth bit of pneumatic rock
drill has an edge teeth drop rate of over 30% and an edge teeth crushing rate of
20%. However, other failure modes include broken middle teeth, broken waist and
split trousers. The failure rates of falling middle teeth, capping, and inverted cone
are all below 5%.
The optimum structure parameters, high strength materials, and reasonable fixing technology are three effective means to improve the life of cylindrical teeth.
However, the selection of high strength materials is not only limited by the existing
level of material research but also by the cost funds of production and construction
units. Simply depending on improving the strength of materials without optimizing
the structural parameters cannot achieve the desired results. Starting from the force
analysis of the edge teeth, the dangerous points on the edge teeth of the cylindrical
bit are found and checked by the maximum shear stress criterion. The relationship
between the stress and the inclination angle of the edge teeth, the height of the
exposed teeth, the friction coefficient and the radius of the cylindrical teeth at each
dangerous point is analyzed, so as to find a reasonable inclination angle and the
exposed teeth height to reduce the stress at each point under the same working
conditions, so as to reduce the failure probability of the edge teeth.
11.5.2 Mechanical Model of Side Tooth of Large Diameter
Pneumatic DTH Hammer Bit
As can be seen from Fig. 11.28, the spherical teeth of large diameter DTH bit are
composed of solid hemisphere and cylinder. A part of the cylinder is embedded into
the bit body through cold-pressed fixed teeth. The angle a with the axis of the bit is
the inclination angle of the edge teeth, the radius of the spherical teeth is R, and the
height of the exposed part of the cylinder is h. There are two main forces on
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11 Pneumatic Down-the-Hole Hammer
demonstrate the changes of various parameters (shape, stress, strain, etc.) in the
whole process of rock bursting by impact. Using the software program, different
physical and geometric parameters can be selected to calculate, and the influence
degree of different parameters on the results of the interaction between teeth and
rock can be obtained, so as to give a more comprehensive result.
The cylindrical teeth of DTH hammer bits are made of hard metal, which greatly
improves the service life of DTH hammer bits. However, in the use process, there
are still some phenomena such as broken teeth, dropping teeth, broken waist, split
trousers, and so on. How to effectively reduce the fracture and breakage of cylindrical teeth has become a key factor to effectively improve the life of DTH hammer
bits. In practice, it is found that the fracture and broken of the side teeth of DTH bits
are the most common failure forms in the damage of the total cylindrical teeth. For
example, when the bob-weight rock drill with Swedish /40 mm spherical teeth is
used in a mine, the drop rate of the edge teeth reaches 23%, and the breakage rate of
the edge teeth reaches 30%. The domestic /40 mm ball-tooth bit of pneumatic rock
drill has an edge teeth drop rate of over 30% and an edge teeth crushing rate of
20%. However, other failure modes include broken middle teeth, broken waist and
split trousers. The failure rates of falling middle teeth, capping, and inverted cone
are all below 5%.
The optimum structure parameters, high strength materials, and reasonable fixing technology are three effective means to improve the life of cylindrical teeth.
However, the selection of high strength materials is not only limited by the existing
level of material research but also by the cost funds of production and construction
units. Simply depending on improving the strength of materials without optimizing
the structural parameters cannot achieve the desired results. Starting from the force
analysis of the edge teeth, the dangerous points on the edge teeth of the cylindrical
bit are found and checked by the maximum shear stress criterion. The relationship
between the stress and the inclination angle of the edge teeth, the height of the
exposed teeth, the friction coefficient and the radius of the cylindrical teeth at each
dangerous point is analyzed, so as to find a reasonable inclination angle and the
exposed teeth height to reduce the stress at each point under the same working
conditions, so as to reduce the failure probability of the edge teeth.
11.5.2 Mechanical Model of Side Tooth of Large Diameter
Pneumatic DTH Hammer Bit
As can be seen from Fig. 11.28, the spherical teeth of large diameter DTH bit are
composed of solid hemisphere and cylinder. A part of the cylinder is embedded into
the bit body through cold-pressed fixed teeth. The angle a with the axis of the bit is
the inclination angle of the edge teeth, the radius of the spherical teeth is R, and the
height of the exposed part of the cylinder is h. There are two main forces on
242
11 Pneumatic Down-the-Hole Hammer