# Impact Characteristics of a Bidirectional Pneumatic DTH Hammer for Unconsolidated Formations
## 4 Result Analysis and Discussion  
### Impact Characteristics under Different Piston Mass  
Setting the mass of the piston to 16 kg, 18 kg, and 20 kg respectively, when the piston stroke is 230 mm, the piston velocity is obtained as shown in Figure 5 using the simulation model established in Section 3. With a piston mass of 16 kg, an impact frequency of 15 Hz is generated. In the first impact cycle, the peak of impact velocity can reach up to 10.15 m/s. Due to friction occurring between the piston and the cylinder during the reciprocating movement, the peak velocity of the piston tends to diminish. As a result, there is a 2.76 m/s difference between the highest and lowest velocity peaks within a one-second timeframe. When the piston mass is 18 kg, the impact frequency is 14 Hz. In the first impact cycle, the peak of impact velocity can reach up to 9.54 m/s. There is also a decreasing trend in the piston velocity peak and its decrease within 1 s is 2.19 m/s. When the piston mass is 20 kg, the impact frequency is 13 Hz. In the first impact cycle, the peak of impact velocity can reach up to 9.02 m/s. The peak velocity decrease within 1 s is 2.21 m/s. According to the above results, when the piston's mass increases, both the impact frequency and the peak velocity decrease. Figure 4: Simulation model of the DTH hammer based on SimulationX. Area I. DTH control part, Area II. the dynamic testing part, Area III. the hammer functional part, Area IV. the rock interaction part. Figure 5: Piston velocity at different piston mass. When the DTH hammer acts on rocks, there are displacements for cylinders and pistons in the simulation calculation. To analyze the displacement variation in the piston, displacement difference between pistons and cylinders is taken as the target parameter. The piston's initial position in the simulation is at the top of its impact stroke, close to the gas distribution valve and farthest from the anvil. Displacement difference between the piston and the outer cylinder is displayed in Figure 6, when the piston masses are set to 16 kg, 18 kg, and 20 kg, respectively. From the curve, it can be seen that for the piston with a mass of 16 kg, it descends 227 mm from its initial position to impact the anvil and then returns upwards. The upper endpoint is 29 mm away from the initial position. Afterwards, the next impact process begins. With the separation between the upper endpoint and the initial position, it can avoid impacting on the DTH hammer pipe joint, thereby protecting the reliability of the threaded connection between the DTH hammer and drill pipes. It is worth noting that due to friction between the pistons and cylinders, there is an increasing tendency in the separation between the upper endpoint and the piston's initial position. Within 1 s, the separation can be up to 84 mm. For the piston with a mass of 18 kg, it descends 227 mm from its initial position to impact the anvil and then returns upwards. The upper endpoint is 26 mm away from the initial position. There is also an increasing trend in the separation between the upper endpoint and the initial position, and the separation can be up to 71 mm within 1 s. In the simulation for the piston with a mass of 20 kg, the piston descends 227 mm from its initial position and then returns upwards. The upper endpoint is 24 mm away from the initial position. There is also an increasing trend in the distance between the upper endpoint and the initial position. Within 1 s, the distance can be up to 72 mm. Drawing from the aforementioned findings, it can be concluded that as the piston mass increases, the separation between the upper endpoint and the piston's initial position decreases. Figure 7 shows the piston impact energy when the piston mass of the DTH hammer is set to 16 kg, 18 kg, and 20 kg. respectively. To facilitate the analysis of the piston impact energy, it is defined that when the piston velocity is positive, the impact energy is also positive; and conversely, as the piston velocity is negative, the impact energy is negative.