Feasibility Study On The Application Of NdFeB Permanent Magnet To DTH Impactor

Rotary percussive drilling technology is one of the effective methods to solve hard rock drilling,and down-the-hole impactor is its core component.The traditional impactor converts the energy of a pressurized liquid or gas into a cyclic impact force,the impactor applying these methods has been relatively mature in the field of drilling.But there are some deficiencies:requirements for mud pumps/air compressors,limited by liquid/gas,high sealing requirements and so on.Permanent magnets have the properties of repulsing the same poles and attracting the opposite poles,designing it as an impactor has a significant advantage.Now,the third generation of rare-earth magnets--NdFeB permanent magnet is widely used due to high remanence,high coercivity and high magnetic energy product,has the reputation of ‘Magnetic King',and has not been used in the DTH impactor.NdFeB permanent magnet does not need any medium to transmit strong repulsive force.When it is applied to DTH impactor,it can supplement the blind area of traditional hydro pneumatic impactor.The feasibility of the application of NdFeB permanent magnet in DTH impactor is studied.The specific work is as follows:1.The overall structure design of NdFeB permanent magnet DTH impactor.No matter how strong the permanent magnet is,it can't work at a long distance.In this paper,NdFeB is designed as a magnetic force conversion structure with radial opposite sector magnets.The impact of a strong magnetic force through multi-section overlay.Design a braking device on the shell.When encountering hard rock,the casing is braked by shaft lining,phase difference between internal magnetic rotor and external magnetic stator.And then the magnetic conversion structure generates axial magnetic force.2.Theoretical feasibility analysis of NdFeB permanent magnet DTH impactor.Combining the principle of magnetic common energy and virtual work,Taking the arrangement of six pairs of sector magnets as an example,the theoretical calculation formula of axial magnetic force of magnetic conversion structure is deduced.Matlab is used to calculate the impact force of the magnetic conversion structure with the periodic change of the phase difference between the internal and external magnetic fans.And the maximum axial force of the one-piece conversion structure can reach 168 N,and the magnetic force change cycle is 120°,that is,the drill rod can be impacted up to 3 times per revolution.3.Feasibility of Simulation Calculation for NdFeB Permanent Magnetic Down-the-Hole Impactor.Use software Maxwell to simulate the magnet part in the magnetic force conversion structure,and there is a certain deviation between the result and the theoretical calculation.By analyzing the cross-sectional magnetic field distribution and the distribution of magnetic field lines,it is concluded that the reason is caused by the magnetic leakage of the structure composed of magnetic fans.Use the simulation results to revise the theoretical calculation formula,and the revised calculation results are consistent with the simulation results.4.The feasibility of circulating magnetic force was verified by designing experiment frame.Using Solid Works to design the experimental frame,CAD drawing,production parts assembly experimental frame,the experimental results show that the impact force can change periodically.It is calculated that the single-section magnetic impact conversion structure singleshot maximum impact energy is not less than 8.319 J.The average impact force is up to 380.22 Newtons.From theoretical calculations,simulation results and experimental results,it can be known that the NdFeB down-the-hole impactor is feasible.