Study On Axial Magnetically Excited Fretting Wear Of Large Transformer Flange Bolts

Because of the advantages of simple structure,convenient disassembly and low cost,the screw connection is widely used in various mechanical structures,as well as large transformer flange joints.In practice,the vibration for a long time may cause the bolt connection to become loose.Once the thread becomes loose,it will bring unpredictable consequences.At present,scholars at home and abroad have no unified understanding of the mechanism of bolt loosening.In order to clarify the mechanism of the decline of bolt preload,this paper studies the influence of the axial alternating load of different frequencies and amplitudes on the decline of bolt preload,bolt deformation and thread wear by means of the experimental method to simulate the fretting condition.Through the analysis of the experimental data and phenomena,some instructive conclusions are obtained.The main conclusions are as follows:(1)The decline of Bolt clamping force increases with the increase of axial alternating load amplitude.In addition,the decline of bolt pre-tightening force is divided into two stages,the first stage of rapid decline,the second stage of slow decline.The study shows that the rapid decrease of preload in the first stage is due to the plastic deformation of the bolt.When the plastic deformation reaches the second stage,the fretting wear between the contact surfaces becomes the main reason for the decline of the pretension.Because wear and tear need time to accumulate,so the pre-tightening force recession slowed.(2)It was found in the test that the preload appeared to rise and fall suddenly.This is caused by the tiny motion between the contact surfaces.As a result of micro-movement,the contact surface of the thread will be temporarily stuck,so that the friction coefficient between the threads will be temporarily increased,resulting in the decline of the pre-tightening force fluctuations.However,with the continuous use of micro action,the phenomenon of death is restored,and the pre-tightening force also has a steady decline.(3)Bolt deformation increases with the increase of axial alternating load amplitude and decreases with the increase of frequency.This is because under low frequency loading,compared with the high frequency condition,the single loading duration is longer,that is,the bolt keeps the deformation state for the same long time.In addition,the variation trend of bolt deformation with the number of cycles was rapid increase in the early stage,and slowed downin the later stage.This indicates that when the bolt deformation increases rapidly in the early stage,due to the plastic deformation of the material,the decline of bolt pre-tightening force also increases rapidly.When the plastic deformation reaches the stable stage,the increase of bolt deformation also slows down.At this time,the main reason for the decline of the preload is the fretting wear of the contact surface of the thread,and the decline of the preload also slows down.(4)According to the thread wear test,the reason for the slow decline of the preload is mainly caused by the fretting wear between the thread surfaces.The wear of thread surface is typical linear contact fatigue wear and adhesive wear.The width of thread wear mark increases with the increase of axial load amplitude and decreases with the increase of frequency.This is due to the higher frequency thread contact surface under a single load of short time.In addition,as the thread load is not uniform,the working load load in turn weakened.For this purpose,the first working thread wear mark width is greater than the second working thread wear mark width.