impactor are used. By substituting Eqs. (11.71) and (11.72), an estimate of the axial
force P is obtained P ¼ 1:82 Â 10
5 N.
The bits are made of hard metal YG11 with a compressive strength of 4600 MPa
and shear strength of 80% of the compressive strength. Here, 3680 MPa is taken.
The parameters of the original bit are a ¼ 40
; h ¼ 0:009 m; R ¼ 0:01 m, and the
friction coefficient f varies with different rock, soil and working conditions, with a
range of 0:2 $ 0:6. From the equations deduced above, it can be clearly seen that
the equivalent stress at each dangerous point is related to load P, exposed tooth
height h, inclination angle a, friction coefficient f of rotary cutting and radius R of
spherical teeth. Obviously, with the increase of P, the stress at each point increases;
with the increase of R, the stress at each point decreases. Next, the effects of the
exposed tooth height h, the inclination a; and the friction coefficient f of rotary
cutting on the stress at each point are discussed.
11.5.3 Layout Principle of Large Diameter Pneumatic DTH
Hammer Bit
The principle of DTH hammer drilling is percussive rotary drilling. The gas generated by the air compressor rushes into the piston and cylinder space to make the
piston move. The drill bit is percussed periodically. The energy is transmitted to the
drill bit in the form of stress wave, and the drill bit is percussed to destroy the rock.
In the process of drilling, the drilling rig will also exert certain static pressure to
prevent the efficiency reduction due to rebound from impact. So DTH hammer
drilling is the result of combined action of static pressure, impact force, and rotating
force. In the process of drilling hard rock with DTH hammer bit, impact plays a
major role, with emphasis on rock breaking by impact.
11.5.3.1 Spherical Teeth Hydrostatic Rock Breaking
Figure 11.34 shows the principle diagram of spherical tooth hydrostatic rock
crushing. The final force acting on the spherical teeth of DTH hammer piston is P,
based on the model of spherical pressing into brittle rock, the process of rock
fragmentation can be divided into the following stages.
(1) Elastic deformation stage
As shown in Fig. 11.34a, when P is small, a pressure surface is formed. Cracks a; b
appear at the edge. When the force P is removed, the rock surface restores to its
original state and the cracks at a; b disappear.
11.5 Design of Large Diameter DTH Hammer Bit and Spherical Tooth Layout
253
force P is obtained P ¼ 1:82 Â 10
5 N.
The bits are made of hard metal YG11 with a compressive strength of 4600 MPa
and shear strength of 80% of the compressive strength. Here, 3680 MPa is taken.
The parameters of the original bit are a ¼ 40
; h ¼ 0:009 m; R ¼ 0:01 m, and the
friction coefficient f varies with different rock, soil and working conditions, with a
range of 0:2 $ 0:6. From the equations deduced above, it can be clearly seen that
the equivalent stress at each dangerous point is related to load P, exposed tooth
height h, inclination angle a, friction coefficient f of rotary cutting and radius R of
spherical teeth. Obviously, with the increase of P, the stress at each point increases;
with the increase of R, the stress at each point decreases. Next, the effects of the
exposed tooth height h, the inclination a; and the friction coefficient f of rotary
cutting on the stress at each point are discussed.
11.5.3 Layout Principle of Large Diameter Pneumatic DTH
Hammer Bit
The principle of DTH hammer drilling is percussive rotary drilling. The gas generated by the air compressor rushes into the piston and cylinder space to make the
piston move. The drill bit is percussed periodically. The energy is transmitted to the
drill bit in the form of stress wave, and the drill bit is percussed to destroy the rock.
In the process of drilling, the drilling rig will also exert certain static pressure to
prevent the efficiency reduction due to rebound from impact. So DTH hammer
drilling is the result of combined action of static pressure, impact force, and rotating
force. In the process of drilling hard rock with DTH hammer bit, impact plays a
major role, with emphasis on rock breaking by impact.
11.5.3.1 Spherical Teeth Hydrostatic Rock Breaking
Figure 11.34 shows the principle diagram of spherical tooth hydrostatic rock
crushing. The final force acting on the spherical teeth of DTH hammer piston is P,
based on the model of spherical pressing into brittle rock, the process of rock
fragmentation can be divided into the following stages.
(1) Elastic deformation stage
As shown in Fig. 11.34a, when P is small, a pressure surface is formed. Cracks a; b
appear at the edge. When the force P is removed, the rock surface restores to its
original state and the cracks at a; b disappear.
11.5 Design of Large Diameter DTH Hammer Bit and Spherical Tooth Layout
253