# Impact Characteristics of a Bidirectional Pneumatic DTH Hammer for Unconsolidated Formations
## 4 Result Analysis and Discussion
### Backward Impact Stroke  
During drilling process, the piston backward movement stroke can be changed by lifting the inner tube assembly of the DTH hammer, thereby achieving a backward impact. In order to rescue drilling tools successfully in the case of sticking or burying accidents, the piston must have sufficient upward impact energy in the upward impact process. For a DTH hammer with its piston mass of 18.66 kg and total weight of 128 kg, when the lifting distance of its inner tube assembly is set to 40 mm, 50 mm, and 60 mm, respectively, the piston velocity is shown in Figure 14. When the lifting distance is 40 mm, the impact  
Figure 12: Piston displacement at different gas source pressure.  
Figure 13: Impact energy at different gas source pressure.  
frequency is 14.50 Hz and the peak impact velocity during the first impact cycle can reach up to 9.89 m/s. When the lifting distance is 50 mm, the impact frequency is 14.25 Hz and the peak impact velocity during the first impact cycle can reach up to 10.00 m/s. When the lifting distance is 60 mm, the impact frequency is 14.75 Hz and the peak impact velocity during the first impact cycle can reach 10.11 m/s. From the analysis results, it can be seen that as lifting distance increases, peak impact velocity increases, which is due to the forward distance of the piston increasing, thereby increasing the acceleration distance. When switching to the backward impact by lifting the inner tube, the piston still has relatively high downward end velocities, which is determined using the mechanism of the DTH hammer. Currently, the structure of the DTH hemmer cannot achieve zero forward impact end velocity.  
Figure 15 shows the displacement differences curve between the piston and outer cylinder when the lifting distances are 40 mm, 50 mm, and 60 mm, respectively. From the curve, it can be seen that when the lifting distance is 40 mm, 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 52 mm from the initial position. Within 1 s of simulation, the above distance can reach up to 82 mm. When the lifting distance is 50 mm, 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 62 mm from the initial position. Within 1 s of simulation, the above distance can reach up to 87 mm. When the lifting distance is 60 mm, 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 73 mm from the initial position. Within 1 s of simulation, the above distance can reach up to 112 mm. As the lifting distance of the inner tube assembly increases, during the piston's deceleration and ascent to the point where its velocity reaches zero, it will progressively move farther away from its initial position.  
Figure 16 shows the piston impact energy when the inner tube assembly is lifted upward with a different distance. As the lifting distances are 40 mm, 50 mm, and 60 mm, the first impact of the piston can generate impact energy with a value of 912 J, 935 J and 955 J. According to the results, when the piston mass is constant, an increase in the backward distance for the inner tube can increase the piston peak impact velocity, thereby increasing the piston impact energy. Based on the above analysis results of the impact frequency, peak of impact velocity, and impact energy, the recommended lifting distance for the backward impact is 60 mm.  
Figure 14: Piston velocity at different inner tube assembly lifting distance.