| Gear is a key transmission part widely used in mechanical structures,the operating quality of which is directly affected by the meshing contact state.The traction gear pair of high-power electric locomotive is one of the key subsystems that affect the service behavior of locomotive transmission system.It is the basis for smooth transmission of traction torque,reasonable matching of motor and wheelset speed,effective application of traction/braking force and other functions.In operation,traction gear is in wide frequency vibration with the wheelset,tooth surface wear and contact fatigue damage will inevitably occur,which will lead to deterioration of the smoothness of the transmission system,increase the risk of gear failure and even threaten driving safety.In view of this,it is necessary to systematically study the transient engagement process of gears under different service conditions to provide reference for dynamic design and timely maintenance of gear pairs.To achieve this goal,one of the keys is to establish an analytical model which can accurately predict the transient meshing contact behavior of gears under structural vibration environment and the related analysis methods for the transient contact behavior of tooth surfaces,which is the research content of this thesis.Firstly,the development and status of gear transmission research methods and contents are investigated in thesis.It is found that the main research methods used at present are lumped mass method and finite element method,in which the hypothesis of quasi-static is usually implicit and other structures including gear shafts are ignored.The related researches focus on load distribution of tooth surface,prediction of tooth surface wear,influence of torque excitation and tooth surface fatigue on gear contact,etc.,but does not take the transient contact behavior of tooth surface into account.In view of this,the research objectives and contents of this thesis are proposed.Taking the traction gear of a high-power electric locomotive in China as an example,the transient meshing contact time domain model of the gear is established under the ANSYS/LS-DYNA environment,and the modeling and analysis of the first stage is completed.The real 3D geometry of the teeth is considered in the model,and axle(bull gear shaft)and other components are ignored.The meshing contact of tooth surfaces is solved by the surface-to-surface contact algorithm considering Coulomb friction model,and non-Newtonian elastohydrodynamic lubrication friction is introduced.The model is solved by an explicit time integration algorithm,and the extremely small time step determined by its conditional stability characteristics ensures that the transient contact and dynamic behaviors are captured.Based on the results of each time step,an analysis method for transient meshing contact behavior of gear tooth surface is proposed and a numerical program is developed.The program can obtain the total contact force of gear pair,the shape and size of contact patch,as well as the normal/tangential contact stress,relative slip and tooth surface wear in the contact patch with time,which provides an accurate means for the analysis of gear transient meshing contact behavior.By analyzing the transient meshing contact behavior of tooth surface of locomotive in quasi-steady condition,it is found that the time from initial meshing to steady meshing of gear pair is positively correlated with the locomotive running speed.During steady meshing,the total contact force of gear pair fluctuates up and down in the theoretical solution with the teeth in/out meshing.The increase of traction coefficient will increase the gear pair contact force,tooth surface contact stress and tooth surface wear.The increase of speed will reduce the wear depth.With a traction coefficient of 0.15,the maximum wear depth per meshing at120 km/h is 48% lower than that at 65 km/h.On the basis of the above model,other structures such as sprung mass,primary suspension and axle are introduced,to establish the gear transient meshing contact model under structural vibration environment.This model can analyze the dynamic meshing process of gears more precisely and complete the second stage of modeling and analysis.The results show that the initial precompression makes the gear pair reach the approximate steady meshing at the beginning of contact.The bending deformation of axle and the vibration of related structure aggravate the dynamic action of the gear pair in the end face,and deteriorate the matching state between the gears.When the locomotive operates at full power at 120 km/h speed,the average fluctuation amplitude of the total contact force of the gear pair,the maximum normal contact stress of the tooth surface and the maximum tooth surface wear depth are 2.45,2.1 and 2.83 times of the first stage model results respectively.Considering that the output torque of traction motor is not constant in actual operation,the traction torque excitation with different frequencies and amplitudes is introduced in the second stage model and its influence on gear transient meshing contact behavior is analyzed.The results show that traction torque excitation causes significant contact force fluctuation of gear pair.The fluctuation frequency is the same as that of torque excitation frequency.When the excitation is low,the meshing time of single tooth is much less than that of the excitation cycle.Therefore,the change of traction torque during the engagement cycle is not obvious.At this time,the influence of torque excitation on the transient contact behavior of tooth surface is similar to that of traction coefficient.The results of different excitation frequencies show that serious contact behavior deteriorates when the excitation frequency coincides with gear meshing frequency.Taking the operating condition of 120 km/h locomotive as an example,when the excitation frequency is equal to gear meshing frequency 899 Hz at this speed and the ratio of fluctuation amplitude to preset value is 35%,the maximum peak-valley value of gear pair contact total force is about 3.6 times that of the quasi-steady results.The maximum wear depth and the maximum normal contact stress at the mid-meshing time increased by 20.9% and 15.7% respectively compared with the quasi-steady results.All the above analyses are based on the designed tooth profile.The tooth surface deformations caused by spalling and wear in service also have an important influence on the transient meshing contact behavior of gears.Therefore,the transient meshing contact behavior under the tooth surface deformations is analyzed by using the second stage model.The predicted results considering the spalling of the tooth surface show that the contact stress and the wear depth of the point on the edge line of the spalling area both increase significantly,and the increase under the calculated conditions is positively related to the length and width of the spalling area.The predicted results considering the tooth surface wear show that the contact force of the gear pair increases with the increase of the tooth surface wear,and the contact area of the tooth surface changes significantly,and the tooth surface contact stress and the overall wear depth decrease under the simulated conditions.Ultimately,the main work of the thesis is summarized and a prospect is made. |
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