Design optimization and feasibility analysis of pneumatic DTH Hammer with self-rotation bit

2.1. Working principle

The rotation assembly includes a helical rod, a rotary joint, a connector, a piston fitted with a nut at the tail, and a ratchet mechanism. It combines the ratchet and the air distributor into one part (see Fig. 2). The head of the helical rod is equipped with pawls, while the tail ties in with the nut. The tail is milled with spiral teeth and forms a spiral pair with the nut, and pawls rest on the internal teeth of the ratchet under the action of tower springs. The ratchet mechanism has the characteristic of intermittent circular motion in one direction so that the ratchet can only rotate in one direction. The ratchet threaded with the hammer casing can drive the chuck and the drill bit to revolve together. The return stroke of the impact piston is used for anticlockwise rotation, and the working principle of the novel hammer is as follows.

At the beginning of the impact strike process, the piston and the check valve are extremely upper. The compressed air enters the annular space and the rear chamber through the radial hole of the inner cylinder and the air distributor, as shown in Fig. 3. Then the piston begins to move downward at high speed to impact the drill bit under pressure difference, and the air in the front chamber is discharged into the hole bottom. Since the nut fixed in the rear end of the piston is in a splined joint with the spiral rod, the latter will rotate clockwise to a certain degree with the ratchet mechanism while the nut is going down with the piston. At this moment, the pawls are turning along with the ratchet, and the spiral rod is in idling condition. The hammer casing and the drill bit are in the non-rotating stage.

Fig. 2Mechanism of self-rotation: 1 – piston; 2 – nut; 3 – helical rod; 4 – air distributor; 5 – ratchet wheel; 6 – tower spring; 7 – pawls

Fig. 3Impact stroke process. Notes: Red region is air inflow channel connected to air source and is high-pressure zone; Blue region is gas outflow channel connected to the hole bottom and is low-pressure zone

The central hole of the drill bit will be closed when the piston goes down, and the pressure in the front chamber starts to rise. The radial holes of the helical rod leave the piston, the rear chamber may communicate with the hole bottom. Meanwhile, the pressure reduces to the atmosphere. The piston strikes the drill bit at the end of the impact stroke, and the compressed air filled with the front chamber through the annular space between cylinder and casing and the air distributor, and then the piston starts to move upward, and the return stroke begins (see Fig. 4). In this process, the spiral rod will rotate counterclockwise to a certain degree with the ratchet mechanism, and the pawls hold the internal teeth of the ratchet. When the helical moment of the spiral rod is sufficient enough to overcome the resistance moment of the hammer case and rocks, pawls on the spiral rod drive the ratchet to move and make it rotate with the spiral rod to a certain degree. The ratchet is in a threaded joint with the hammer casing and is easy to achieve, and the hammer casing can drive the drill bit to rotate simultaneously and realize self-rotation motion.

Fig. 4Return stroke process

2.2. The design of the cutting removal mechanism

The cutting removal mechanism consists of an air distributor, a helical rod, a piston, a bit, and a tailpipe. The pressured gas is divided into two paths when it gets into the DTH hammer. One channel between the rotation joint and the check valve flows to the hole bottom directly through the central hole of the helical rod, the piston’s central hole, and the drill bit's vent hole. The other vents from the front chamber and gets through the vent hole of the drill bit, or vents from the rear chamber and gets through the vent hole of the helical rod, the central hole of the helical rod, the central hole of the piston, the vent hole of the drill bit. The function of this mechanism is to carry the rock cutting out of the hole and cooling the drill bit. The venting paths are shown in Fig. 5.

3.1. Primary hypothesis and governing equation

This paper focuses on the generation, transmission, and dissipation of stress waves in the two critical components of piston and drill bit and the stress change at each variable section through ANSYS software, regardless of the drilling effect of the drill bit. Therefore, the basic assumptions are as follows:

(1) Ignoring the process of shear rock breaking between bit and rock, and only consider the axial impact process;

(2) Ignoring the influence of dynamic load generated by the whole drill string system on the impact process of piston and bit is ignored;

(3) Remove the carbide tooth of the drill bit and simplify the chamfer and fillet of the piston and the drill bit.

The algorithm of the ANSYS program adopts the Lagrange formulation. When the piston collides with the bit in the DTH hammer, their contact is assumed to have no friction, and the governing equations are presented as follows [21]. if the element coordinates at the initial time are $X_i$ and the element coordinate is $x_i$ any time, we can get Then the motion equation of the particle are as follows:

When $t=$0, the initial condition is:

where $V_i$ denotes the initial velocity.

Equation of mass conservation:

where $\rho$ represents the current density; ${\rho }_0$ represents the reference density; $J$ represents the Jacobian matrix.

Equation of momentum conservation:

where ${\ddot{u}}_i$ denotes the acceleration; ${\sigma }_{ij,j}$ denotes the Cauchy stress; $f_i$ denotes the body force density.

Equation of energy conservation used to calculate the total energy balance:

where $V$ presents the current volume; ${\dot{\epsilon }}_{ij}$ presents the strain rate tensor; $p$ and $q$ present the hydrostatic pressure and bulk viscosity, respectively, the deviatoric stresses $S_{ij}$ is given by:

where ${\delta }_{ij}$ denotes the Kronecker delta (${\delta }_{ij}=$1, if $i=j$; otherwise ${\delta }_{ij}=$0):

Based on the virtual work principle, Eq. (4) can express as the weak form of equilibrium equation [14]:

where $\delta u_i$ fulfills all boundary conditions, and the integration is over the current geometry.

If the finite element technique is interconnected using a matrix form [14, 21], Eq. (8) becomes:

where $\mathbf{N}$ is an interpolation matrix; $\mathbf{\sigma }$ is the stress vector; $\mathbf{B}$ is the strain-displacement matrix; $\mathbf{a}$ is the nodal acceleration vector; $\mathbf{b}$ is the body force load vector, and $\mathbf{t}$ is applied traction load.

This equation is integrated over time and used to calculate the state and global energy balance equations.

3.2. Finite element model and parameters

The analysis model of the novel DTH hammer is composed of piston and bit, and the piston is applied an initial velocity $v_0$ and the force down to strike the drill bit, as shown in Fig. 6. The SOLID164 unit was adopted to mesh the analysis model and define their contact area and critical parts during the calculating process.

Fig. 6Analysis model of the piston and the drill bit

The piston and the drill bit were meshed by tetrahedron elements. The clearance between the piston and the drill bit is 2 mm, and this value does not affect the propagation effect of stress waves in the impact process. This analysis adopted the Johnson Holmquist model, which has a good performance in rock impact simulation analysis under high strain rates and large deformation. In addition, the physical properties of the piston and the drill bit are listed in Table 1 [22].

4.1. Propagation process of stress wave

When the piston strikes the drill bit, the impact energy propagates between the piston, the drill bit, and the rock in the form of stress wave. Part of the energy used in broken rock, another part of the energy in the form of stress wave transmitted back and forth between piston, drill bits, and rock. The incident wave and reflected wave easily cause stress concentration in some parts. In the long-term work, fatigue failure is easy to occur in some regions, so we selected four planes or variable cross-sections in the position and the drill bit as monitoring objects (see Fig. 6).

Fig. 7 shows the equivalent stress diagram of the piston and the bit at 175 μs. At this time, the reflected wave is transmitted to the piston's big end and the drill bit's tail and superimposed with the subsequent stress wave. The stress of the big end face of the piston reaches the maximum pressure of 305MPa in the whole collision process, and the stress at the tail of the drill bit comes to the maximum stress of 519 MPa.

Fig. 7Equivalent stress diagram of the piston and drill bit

4.2. Energy transfer analysis

Fig. 8 shows the velocity changed the curve of the piston during the process of impacting. When the piston strikes the drill bit at 6.5 m/s, the piston contacts with the drill bit at 75 μs, and the piston starts to slow down. The velocity of the piston reduces to 0 at 216 μs, and the piston begins to rebound. The maximum rebound velocity is around 3.5 m/s. The rebound coefficient is the ratio of piston rebound velocity to the last impacting speed of the piston, and its value is 0.54.

Fig. 8The velocity changed the curve of the piston

When the rebound coefficient is 0, all the energy of the impact is transmitted to the drill bit to break the rock, which is ideal. In actual working conditions, the rebound phenomenon of the piston is common, and the rebound coefficient is about 0~0.64. Generally, the incident wave is the critical cause of rebound.

Fig. 9 shows the equivalent stress curve during the collision between the piston and the drill bit. The analysis indicates that the stress on the contact surface between the piston and the drill bit reaches the extremum in the whole collision process. The piston stress gets the maximum value of 304 MPa at 175 μs. The fatigue failure of the contact surface between the piston and drill bit will be the leading cause of piston failure.

In actual working, the repeated impact will form fatigue concave on the contact surface of the piston, which will further aggravate the fatigue damage until the piston is invalid. Therefore, during machining and heat treatment, it is necessary to ensure that the contact surface of the piston has high flatness and roughness as far as possible.

In addition, the stress value of the piston in variable cross-section 1 is also very high, which is easy to cause piston fracture. Therefore, the difference in diameter should be approximate, and the fillet shall be added to make the diameter transition smoothly when designing the piston. The Von-Mises stress acted on the piston will reduce to 56 MPa at 300 μs.

Fig. 9Equivalent stress curve of the piston in plane 2

Fig. 10 shows the equivalent stress curve during the collision process. It can be seen that the stress of the drill bit reaches a maximum value of 519 MPa at 175 μs, and the bit's stress reduces to 227 MPa at 300 μs. The variation trend of equivalent stress on each variable section of the bit is the same, but there are also minor differences in tress value.

To sum up, the safety factor of the piston is 2.8, and that of the drill bit is 2.35, which can meet actual security requirements. In addition, by comparing Fig. 9 and Fig. 10, the stress of the drill bit is greater than that of the piston at the same point in time, so the strength of the drill bit material should be higher when selecting the materials.

Fig. 10Equivalent stress curve of the drill bit in plane 2

4.3. Analysis of final impact velocity

When the piston mass is constant, the higher the compressed gas enters the hammer, the faster the piston impacts the drill bit and the greater the energy transmitted to the rock. However, at this time, the amplitude of the stress wave will increase, which may cause the maximum stress on the piston or drill bit to exceed the fatigue strength of the material, resulting in deformation or fracture of the piston or drill bit, and reduce the service life of piston or drill bit. Therefore, it is necessary to limit the maximum impact final velocity of the piston. The maximum stresses of piston and bit at different final impacting speeds are analyzed and shown in Table 2. The maximum stress of the piston and bit increase with the impacting velocity. When the final impact velocity of the piston is larger than 10.5 m/s, the safety factor of both piston and bit are lower than 1.5. Thus, the Final impact velocity should be less than 10.5 m/s.