The MFL-NDT Method And Equipment For Micro-cracks In Hub Bearings

As an efficient NDT technology,the Magnetic Flux Leakage(MFL)has been widely used for elongated ferromagnetic objects like steel pipes,wire ropes,oil tanks and other ferromagnetic objects.However,the MFL has little been used for detecting cracks in bearings,and most bearing parts are tested by the Magnetic Particle Inspection(MPI)method within manual operations.The main problem is detecting micron cracks by the MFL method instead of the MPI method.So,it's of great importance to research the micron MFL method,and it has broad application prospects.The paper contrasts the difference between the MPI method and the MFL method.The Brownian movement model is applied to calculate the testing capability of the MPI.It shows that the MFL method can detect micro-cracks instead of the MPI method.On the other hand,cracks in MPI standard blocks and MPI sensitivity testing pieces are all detected by the MFL method in this paper by the experimental verification.The testing result means the MFL method can replace the MPI method with the same testing capability.Natural cracks in hub bearing rings are of far smaller sizes than the usual MFL applications,and its leakage magnetic field will change with sizes of defects.It's very important to improve the frequency response characteristics of MFL probes in the spatial spectrum domain.In this paper,the magnetic dipole theory is applied to calculate the magnetic leakage field caused by a slot in the two dimensional infinite plate.Furthermore,the Fourier Transform(FT)method is applied to calculate the spatial spectrum functions of the magnetic leakage field.Defects in spinning surface of hub bearings must be detected by the NDT method.In this paper,a novel MFL method and a new MFL probe based on the loop-measurement technology which works in near-zero lift-off value is proposed.The result shows this probe can work in the best state only when the gap width value of this MFL probe is same with the width value of defect both form theoretical analysis and experimental verification.For different defects in the spinning surface always have a large scale of spatial spectrum values,a novel MFL probe with different gap values arealso proposed to meet the demanded of detecting cracks in spinning surface of large size scales.Testing equipment for testing cracks in spinning surface of hub bearings is developed.The surface roughness of channel surfaces in hub bearing parts is just 0.16?m.It means the probe cannot touch with the tested channel surface in avoid of making any scratch in those surface,and it's a difficult problem to detect the leakage magnetic field when there is a lift-off value between the probe and the tested surface.In this paper,the relationship between the lift-off values and the best gap width values is researched.A new method and a new probe are proposed to detect cracks with different sizes.Finally,automated MFL testing equipment is developed for detecting cracks in parts of hub bearings like larger inner circles and outer circles.