# Impact Characteristics of a Bidirectional Pneumatic DTH Hammer for Unconsolidated Formations
## 4 Result Analysis and Discussion
### Pressure of the Gas Source  
One of the paramount factors influencing the drilling efficiency of the DTH hammer is pressure of the gas source. For the DTH hammer with a piston mass of 18.66 kg and a total weight of 128 kg, setting the gas source pressures to 2.0 MPa, 2.2 MPa, and 2.4 MPa, the piston velocity is shown in Figure 11. When the gas source pressure is 2.0 MPa, the impact frequency is 13.25 Hz. In the first impact cycle, the peak of impact velocity can reach up to 9.37 m/s. The peak impact velocity of the piston shows a declining trend, with a 2.45 m/s difference between the highest and lowest peaks within a one-second interval. When the gas source pressure is set at 2.2 MPa, the impact frequency measures 14.25 Hz. Within the first impact cycle, the peak impact velocity can reach up to 9.95 m/s, and there is a decreasing trend in the peak impact velocity, with a decreasing amplitude between the maximum peak and minimum peak of 2.61 m/s. When the gas source pressure is 2.4 MPa, the impact frequency is 14.25 Hz. Within the first impact cycle, the peak impact velocity can reach up to 9.95 m/s, and there is a decreasing trend in the peak impact velocity, with a decreasing amplitude of 2.61 m/s. From the analysis results, it is evident that as the gas source pressure rises, the piston's impact frequency, peak of impact velocity, and velocity reduction amplitude all increase. To enhance the piston's peak impact velocity during the drilling process, it is advisable to adjust the gas source pressure accordingly. In Figure 12, the displacement curve between the piston and cylinder of the DTH hammer is depicted. This is illustrated for three different scenarios: when the piston mass is 18.66 kg, the total DTH hammer weight is 128 kg, and the gas source pressures are set at 2.0 MPa, 2.2 MPa, and 2.4 MPa, respectively. Observing the curve, it becomes evident that at a gas source pressure of 2.0 MPa, the piston descends 227 mm from the initial position to impact its anvil and then returns upward to the upper endpoint with a distance of 25 mm from the initial position. Within 1 s of simulation, the above distance can reach up to 72 mm. When the gas source pressure is 2.2 MPa, the piston descends 227 mm from the initial position to impact its anvil and then returns upward to the upper endpoint with a distance of 31 mm from the initial position. Within 1 s of simulation, the above distance can reach up to 77 mm. When the gas source pressure is 2.4 MPa, the piston descends 227 mm from the initial position to impact its anvil and then returns upward to the upper endpoint with a distance of 38 mm from the initial position. Within 1 s of simulation, the above distance can reach up to 77 mm. As indicated by the curve, it is apparent that the gas source pressure has very little influence on the displacement of the piston, which is determined using the structure of the DTH hammer. As the pressure of the gas source increases, the upper endpoint of the piston increases. This is because the upward movement of the piston changes the gas distribution direction of the high pressure  
Figure 11: Piston velocity at different gas source pressure. gas inside the DTH hammer. When the piston ascends to the position where high pressure gas is supplied to the upper chamber, increasing the gas source pressure means causing resistance to the upward movement of the piston. As the piston decelerates and reaches the point of zero velocity during its upward movement, it will be further away from the initial position. Figure 13 shows the piston's impact energy when the gas source pressures are 2.0 MPa, 2.2 MPa, and 2.4 MPa, respectively. When the air source pressure are 2.0 MPa, 2.2 MPa, and 2.4 MPa, the first impact of the piston can generate impact energy with a value of 818 J, 923 J, and 1029 J. In other words, when the piston mass is constant, an increase in gas source pressure can increase the piston peak impact velocity, thereby increasing the piston impact energy.