# Impact Characteristics of a Bidirectional Pneumatic DTH Hammer for Unconsolidated Formations
## 2 Configuration of the Bidirectional DTH Hammer  
Figure 1 illustrates the bidirectional pneumatic DTH hammer. It is composed of a drill bit, a bit joint, an anvil, a gas distribution sleeve, an outer cylinder, a piston, an inner tube, an inner cylinder, a gas distribution valve, an upper shaft sleeve, a gas distribution shaft, a check valve, an inner joint, a pipe joint, and other components. The components that achieve bidirectional impact are installed in the outer cylinder. The main parameters for the DTH hammer are listed in Table 1.  
The component for achieving bidirectional impact is the floating gas distribution mechanism, which is composed of a pipe joint, an inner joint, an O-ring seal, a check valve, a check valve spring, a gas distribution shaft, carbide teeth, two shaft keys, an upper shaft sleeve, a gas distribution valve, an inner tube, and an outer cylinder, as shown in Figure 2. The upper end of the floating gas distribution mechanism is connected to dual-wall drill pipes via its pipe joint using trapezoidal threads, where driving torque and axial force (pressure or tension) can be obtained from drilling rigs. The lower end of the pipe joint is connected to the gas distribution shaft with threads. There are two key grooves on the gas distribution shaft, where two flat keys are mounted. The upper shaft sleeve is connected with the outer cylinder of the DTH hammer via threads, and there are also two key grooves \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Diameter of the drill bit & 135 mm \\ Diameter of the outer cylinder & 130 mm \\ Length of the DTH hammer & 1861 mm \\ Impact energy of single stroke & Forward: 500–900 J \\ Backward: 300–450 J \\ Impact frequency & 10–16 Hz \\ Piston mass & 16–20 kg \\ Impact stroke & 200–230 mm \\ Nominal pressure of the gas source & 1.0–1.4 MPa \\ Nominal flow rate of the gas source & 10–20 m\({}^{3}\)/min \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main parameters of the DTH hammer. Figure 1: Configuration of the bidirectional DTH hammer. 1. drill bit, 2. bit joint, 3. anvil, 4. gas distribution sleeve, 5. outer cylinder, 6. piston, 7. inner tube, 8. inner cylinder, 9. gas distribution valve, 10. flat key, 11. upper shaft sleeve, 12. gas distribution shaft, 13. check valve, 14. Inner joint, and 15. pipe joint. on the upper shaft sleeve. Two flat keys are mounted between the gas distribution shaft and the upper shaft sleeve to achieve torque transmission. The length of the key grooves on the gas distribution shaft is the sum of the length of the flat keys and the designed floating stroke, while the length of the key grooves on the upper shaft sleeve is consistent with the length of the flat keys. The piston of the DTH hammer is driven to reciprocate using compressed air. When the flat keys are in a different position of the key grooves on the gas distribution shaft, the impact stroke of the piston will change. Different impact strokes for the piston will be applied when the DTH hammer is used for drilling boreholes using forward impact and releasing sticking drilling tools using backward impact. In drilling mode, the flat keys are located at the upper end of the key grooves on the gas distribution shaft and the piston has a larger impact stroke. It means that the piston can descend with a sufficient acceleration process and then impact the anvil at high speeds to drive the bit to break rocks. Afterwards, the piston runs upward in the direction opposite to its initial speed. Since there is also a sufficient deceleration stroke, when the piston ascends to the gas distribution valve, its end velocity is very small or even zero, ensuring that the piston only has an impact on the drill bit to achieve impact drilling and does not impact drill pipes to protect the reliability of drill pipe connections. When drilling tools become stuck, by lifting drill pipes, the gas distribution shaft moves upward relative to the upper shaft sleeve and then the inner tube moves upward.