Study On DTH With Self-propelled Round Bit And Its Simulation Analysis

The percussive rotary drilling is the most effective method of hard rock, pebbles gravel and broken strata drilling. The percussive rotary drilling is used very widely because of its characteristics, such as high efficiency, excellent quality, low cost and security. Combining with other advanced drilling methods, multi-technique percussion and rotation drilling technology is formed. Its application fields are expanding, and it is playing a more important role in every fields of drilling engineering. At present, the percussive rotary drilling is used widely in different geological explorations and engineering constructions in different strata. Under some conditions, there are various problems of whole-hole rotation in trenchless drilling, tunnel tubular shed construction, directional drilling and seafloor sampling drilling construction.In allusion to problems above, through general and detailed researches to DTH hammer and pneumatic rock drill, this paper uses the former people's research thought for reference, exploringly applies the theory of pneumatic rock drill to DTH hammer. Combining DTH hammer with pneumatic rock drill, use the high-pressure air of air-compressor to drive DTH hammer and make it rotate. This paper designs a new type DTH Hammer with self-propelled round bit. Such combination uses the respective advantages of DTH hammer and pneumatic rock drill and conquers their disadvantages. It makes a great significance to geological explorations and engineering constructions.This new type DTH hammer can adequately utilize the energy of returning stroke of the piston and achieve self-propelled round of the bit. It fully utilizes the energy of compressive air to efficiently break rocks by percussive rotary drilling. Comparing to the traditional DTH hammer, the structure of this new type DTH hammer changed greatly. It uses high-pressure air of the air-compressor to drive the piston of DTH hammer, and then transfers the linear movement of the piston to the rotation movement of the bit by a special structure. It realizes percussive and rotary movements of the bit. The operational principle and structural characteristics of this new type DTH hammer are introduced in this paper. The improved design of DTH hammer and design of key parts and suits are elaborated.Original working pattern of the DTH hammer is changed, and the bit rotates when the piston returns. The bit rotates when the friction between the DTH hammer and the bottom of the hole is small with the aid of the bounce force. The rotation of the bit uses the energy of return motion of the piston efficiently. So the high impacting work and less energy are obtained.Creative design of upper rotation joint, long air distribution seat, screw, and special-purpose piston of the new type DTH hammer is done in this paper. The upper rotation joint is used to separate rotation parts and other parts of DTH hammer. In the return stroke of the piston, upper rotation joint rotates, and external pipe rotates. So the bit rotates with the external pipe. Because the upper rotation joint mates with upper joint by hole, the upper rotation joint and other parts on it do not revolute. So it separates rotation parts and other parts.Long air distribution seat and notch wheel are made into one part. The primary function of it is used to be an air distribution seat and high-pressure air come into back and forth air chamber of DTH hammer through it. High-pressure air pulls the piston to reciprocate. The second function of long air distribution seat is used as notch wheel, which mates with pawl to come true interrupted movement of the notch wheel mechanism.Screw rod is the key part to transfer linear movement of the piston to rotation of DTH hammer. The screw rod has two engagement parts. The lower part of it has spiral teeth, which make the screw rod mate with nut in the piston. The linear movement of piston pulls spiral teeth of screw rod, and then the screw rod gets torque and rotates. The other part of screw rod is the seat of pawls. The pawls and tower springs are all fixed in it. The pawls in it mate with the notch wheel.The nut is mounted in special-purpose piston and mates with screw rod. The mate of them transfers linear movement of the piston to rotation of screw rod. Because of the single way movement of notch wheel and pawls, the screw rod rotates with load in the return stroke of the piston, but rotates without load in the impacting stroke of the piston, and doesn't make other parts rotate.Relative theories of the DTH hammer with self-propelled round bit are studied by using some equations, such as air state equation, acceleration differential equation, and velocity differential equation, displace differential equation of piston, air flow equation, impact frequency equation, resistance equation of piston return, impact energy equation, and air consumption equation, etc. Then set up the moving and dynamic models of the piston and the rotation mechanism. So it lays the foundation of simulation analysis.According to the mathematical models, Runge-Kutta equation and advanced software MATLAB are used to simulate and calculate the DTH hammer, and then respective parameters of the DTH hammer are obtained. From the results of simulation, we get:a) When the piston finish its impact stroke, the last impact velocity of the piston exceeds 6m/s, and the impact work reach 76N·m. Impact frequency is 28.09Hz, and the largest air consumption is 3.18m3/min. The largest velocity of the piston in return stroke is 4.6m/s.b) The simulation curves clearly show the change of pressure in back and forth air chamber, air consumption, velocity and acceleration of the piston in it's impact and return stroke.c) Compared with conventional DTH hammer, the pressure of two chambers, acceleration and velocity of the piston changed slowly. Impact frequency of the new type DTH hammer is a little higher than conventional DTH hammer's, and other parameters are similar to conventional DTH hammer's. Because the piston drives the screw rod, notch wheel, external pipe of DTH hammer, and bit, it needs more energy, the new type DTH hammer needs an air-compressor with higher pressure. The output energy of the new type DTH hammer approaches to general DTH hammer's. So it doesn't influence the selection of DTH hammer. The new type DTH hammer only has more requirements of air-compressor.Using software ANSYS, the strength and rigidity of the key or throwaway parts (the notch wheel and pawl mechanism) of the rotational units are analyzed and calculated. From the results of analysis, the conclusions are as follows:a) When the notch wheel and pawls are acted a load of 165 N·m, stress of the notch wheel and pawls are very small, the value of the stress is much smaller than their yield stress. Strain of the notch wheel and pawls is much smaller than their acceptability limit also.b) Contact stress of the notch wheel and pawl is 9.811N/m2, it is much smaller than the permissible stress. So the notch wheel and pawl mechanism of the DTH hammer is reasonable, and it can meet the requirements of use.The new type of DTH hammer uses the energy of the compressive air of the air-compressor to realize the percussion and self-rotation, so it expands the application fields of traditional DTH hammer. The principle of the DTH hammer with self-propelled round bit is feasible, and the structure design is reasonable. The simulation analysis and the strength and rigidity calculation prove the respective parameters of the DTH hammer are correct. So it can be used in trenchless drilling, large-caliber shallow-hole rock-socketed engineering drilling, large-caliber curtain engineering drilling, tunnel tubular shed construction, directional drilling and seafloor sampling drilling engineering. It provides new techniques to enhance the efficiency of large-caliber curtain engineering, water conservancy and electricity engineering, and trenchless engineering. It makes a great significance to geological explorations and engineering constructions.