# Impact Characteristics of a Bidirectional Pneumatic DTH Hammer for Unconsolidated Formations
## 3 Bidirectional Impact Model for the Pneumatic DTH Hammer  
### Theoretical Model  
Due to structure of the piston and outer cylinder, the DTH hammer is divided into several different gas chambers, as shown in Figure 1, which is the chamber above the piston (Q1), the chamber between the inner cylinder and the piston (Q2), the chamber between the inner spacer ring and the piston (Q3), the chamber between the gas distribution sleeve and the piston (Q4), and the chamber below the piston (Q5). The reciprocating motion of DTH hammer piston directs the compressed gas into different chambers. The piston serves as more than just a power component for producing impact energy; it also functions as a control element for regulating its own reciprocating motion. This reciprocating motion is an intricate and dynamic process. During a complete impact process of the piston, the chamber above the piston (Q1) undergoes two gas admissions, two gas emissions, one expansion process, and one compression process; the chamber between the inner cylinder and piston (Q2) maintains a constant connection with the high-pressure air source; the chamber between the inner spacer ring and piston (Q3) and the chamber between the gas distribution sleeve and piston (Q4) connect constantly, which undergoes two gas admissions, two gas emissions, one expansion process, and one compression process too; and the chamber below the piston (Q5) is constantly connected to the low pressure air in boreholes.  
Taking gas in the chamber above the piston (Q1) for example, at the beginning of the piston downward impact, the chamber Q1 is connected to the high pressure air source, which is in a compressed air admission state. Afterwards, the connection between chamber Q1 and the high-pressure gas source gradually closes with the DTH hammer piston moving forward. When chamber Q1 is disconnected from the high-pressure air source, the volume of chamber Q1 gradually increases due to the forward motion of the piston, and thus, compressed air expands to do work. The piston proceeds to descend and impacts the anvil ultimately to achieve impact drilling, during which process, chamber Q1 will be connected to the low pressure gas in boreholes and the gas in chamber Q1 will be discharged. After the piston impacts the anvil, the direction of the piston movement changes from forward to backward. In the initial stage of backward movement, chamber Q1 is still connected to low pressure gas in the borehole, and it is still in the state of exhaust. After being disconnected from the low pressure gas in the borehole, chamber Q1 is in a sealed state. As the piston moves backwards, the volume of chamber Q1 decreases and the air inside the chamber is compressed. With the piston moving backwards continuously, chamber Q1 gradually connects with the high-pressure air source again, and this is the second time high pressure air enters chamber Q1, causing a deceleration motion with the increasing acceleration in the piston's motion.  
According to the working principle of a bidirectional DTH hammer, it is essentially a pneumatic transmission system. The reciprocating motion of the DTH hammer piston inside the DTH hammer is exactly the same as a piston operating in an air cylinder. However, the switching of the gas direction in the DTH hammer is achieved by changing the relative position between the piston and the DTH hammer cylinder. For a pneumatic transmission system, directional valves are used to change the intake and exhaust status of air cylinders. Based on the structure of the bidirectional pneumatic DTH hammer, a physical model was constructed, as shown in Figure 3. There were also five chambers from the left to right, which are Q1, Q2, Q3, Q4, and Q5, respectively. For the DTH hammer, the opening and closing of each chamber is determined using the piston position. In the pneumatic transmission circuit model, the connection between the above chambers and high pressure air source or low pressure air in boreholes can be changed by controlling the opening and closing of valves.