Study On Mechanism And Prevention Of Wheel Rolling Contact Fatigue Of High-Power AC Locomotives Running In Complicated Environments

In recent years,severe rolling contact fatigue(RCF)has been frequently found on the wheels of HXD high-power AC locomotives running in complicated environments such as sharp curves and steep slopes in China.Cracks,pitting and indentations were all visible on the wheel tread surfaces,increasing the roughness level of the wheel contact surfaces and further deteriorating the dynamic wheel-rail interactions.Under extreme conditions,some RCF cracks can propagate down to 14.5 mm deep,resulting in large cutting amounts on turning lathes to restore the wheel profile,great reduction of wheel service lives and significant increase of operation costs.Therefore,it is urgent to study the mechanism of RCF occurring on wheels of high-power AC locomotives running in complicated environments,as well as the countermeasures.Aiming the above-mentioned wheel RCF,this dissertation first reviews the studies related wheel RCF,through which the importance of this research is clarified.This dissertation mainly includes the following work.(1)Field observations are made on wheel RCF occurring on two types of high-power AC locomotives(7200 k W)with the design speed of 120 km/h and 160 km/h,respectively.Besides the two types of locomotives of Co-Co axle configuration,hereinafter referred to as Types A1and A2,respectively,another type of locomotive with 2(Bo-Bo)axle configuration is also observed.The macroscopic morphology and crack propagation law of locomotive wheel RCF are investigated,and the related influencing factors are analyzed.It is found that RCF cracks occur on the flange root,the center zone and the field side of the wheel tread with a continuous pattern around the circumference,and Type 3 RCF(RCF3 for short hereinafter)occurring on the center zone with approximately lateral cracks is the most dangerous because its cracks may develop downward into a depth of up to 14.5 mm.Among the 15 Type A1 and 17 Type A2locomotives observed,most of the deep cracks are 3?12 mm deep,and the reprofiling interval was 71,000 and 156,000 km on average,respectively.The occurrence rate of RCF3 is higher among wheels on Axles 1,3,4,and 6 for Types A1 and A2 locomotives,but is obviously higher on Axles 3 and 4 for Type A2 locomotives.Compared to Co-Co locomotives,the crack growth rates of RCF3 on 2(Bo-Bo)locomotives are lower,and wheels with RCF3 are more evenly distributed among the axles.RCF3 is found to be unrelated to hollow wear of the tread and material defects of wheels,but influenced by running lines,seasons,hardness of tread and sand.The damage on wheel surfaces of Type A1 locomotives is mainly dominated by the lateral cracks and pittings.After changing the unqualified sand with high quality one,the pitting has been greatly reduced and the wheel reprofiling has been dominated by the remaining lateral cracks,but the reprofiling interval is prolonged from 50,000-120,000 km to100,000-280,000 km.The practice has shown that a reduction of the maximum regenerative braking force of Type A2 locomotives by 15%could not reduce the occurrence rate of RCF3.(2)A wheel RCF prediction model for high-power AC locomotives is developed.It is consisted of a train multibody dynamic model and a wheel damage model,in which the locomotive and trailers,the coupler model,and the traction control are all included.The train multibody dynamic model was established in mutibody dynamics software SIMPACK combined with MATLAB/Simulink.Due to the relatively high traction/braking forces applied in locomotives,the creep forces may saturate(i.e.,the maximum friction force),and the adhesion coefficient decreases with the slip velocity between wheel and rail.Thus,the Polach creep force model considered the change of adhesion coefficient with the creepage is used to treat the tangential wheel-rail contact.The damage function,developed by British rail safety and standards board(RSSB)and considered the competition between RCF and wear,is applied to assess the wheel damage,for which the wear number within the wheel/rail contact patch is required to predict the wheel damage.(3)The influences of complicated running environments on the wheel RCF are revealed.With the model developed above,the effects of the tangent tracks,curves,and slopes on wheel RCF3 are studied for the two types of axle configuration locomotives.The results have shown that RCF3 may not initiate on wheels running on the uphill slopes or tangent tracks because the longitudinal creep forces are along the travelling direction.On tangent tracks with downhill slopes,RCF3 may occur on all braking wheels,and the initiation possibility increases with the slope.When running on curves with or without downhill slopes,RCF3 only occurs on wheels of high rail side of the last wheelset of bogies,and both curves and slopes play a significant role.Under the same downhill slopes,RCF3 initiation caused by curves is 2-10 times about that by tangent tracks.Considering locomotives operating on left or right curves and turning around,RCF3 wheels are more located on axles 1,3,4 and 6 of Co-Co locomotives,but are similar for all the axles of 2(Bo-Bo)locomotives.Under the same downhill slopes,RCF3initiation for the wet condition(relatively low adhesion coefficient,?₀=0.3)is 2-5 times about that for the dry condition(?₀=0.55)due to the greater longitudinal creepages.When the downhill slopes are larger than 20‰,RCF3 initiation decreases with the slope due to the competition of the wear.Under the dry condition the fixed wheelbase of Co-Co locomotives is larger than that of 2(Bo-Bo)locomotives,resulting in RCF3 initiation on Co-Co locomotive wheel is 1.2-2.5 times about that of 2(Bo-Bo)locomotive wheel under the same braking force at per axle when curving.High rail with the severe side wear on curves can significantly enlarge RCF3 initiation due to the reduction of the area of the contact patch.(4)The effects of axle load transfer on wheel RCF3 initiation are investigated.When the rear end of the guided bogie and the front end of the non-guided bogie are hinged with the middle of the locomotive body respectively through the oblique tow bar,the force is generated at the hinge joint of the bogie along the vertical direction when the oblique tow bar transmits the coupler force.The additional bending moment can result in the significant increase for axle load transfer of the locomotive.When it is improperly matched with the electrical compensation,which will significantly increase RCF3 initiation difference of wheels on different axles under the wet condition.For example,when Type A2 locomotives(the traction rod with a slope of 10:1)with regenerative braking force of 112-162 k N are running on tangent track with the downhill slopes,RCF3 initiation of wheels of Axle 4 is about 1.2-1.5 times that of other wheels due to low axle load and high torque when Axle 1 leading.If Axle 6 leads the train under the similar conditions,RCF3 initiation on Axle 3 is higher.It can explain that wheels of Axles 3 and 4 are more susceptible to RCF3 than the others from the field investigations when considering Type A2 locomotives run turning around.(5)The effect of low adhesion on wheel RCF3 is studied.When there is a low adhesion zone(LAZ,the friction coefficient is less than the adhesion requirement)on the rail surfaces,the longitudinal creepage of the left and right wheels of the same wheelset increases rapidly.When there is LAZ on only one side rail surface,the reduction of the longitudinal creep force of the wheel in the LAZ leads to the increase of the longitudinal creep force of the wheel on the other wheel,while the longitudinal creep force decreases when both sides of the rail surface have LAZ.When the locomotive is running on tangent tracks with downhill slopes,only LAZ existing in one side rail surface increases RCF3 initiation of the left and right wheels of a wheelset,but RCF3 initiation of the wheel on the non-LAZ is greater.When locomotive is running on the curves with downhill slopes,LAZ on the low rail only increases RCF3 initiation of the wheel on the high rail side,and LAZ on the high rail reduces RCF3 of the wheel on the high rail side,but significantly increases RCF3 of the wheel on the low rail side.When there is LAZ on both sides of the rail,RCF3 initiation first increases and then decreases with the increased the length of LAZ.After the creep control system intervenes,the wheel damage is dominated by the wear due to the high longtidutinal creepage.When the length of LAZ is less than the wheel circumference,RCF3 will initiate at the local wheel surface or RCF3 initiation distribution will be uneven along the wheel circumference.(6)The counter measures against wheel RCF3 of locomotive using regenerative brake are researched.Based on the above field investigations and numerical simulations,prevention measures against locomotive wheel RCF3 are recommended,mainly including:the application of qualified sand for adhesion enhancement to reduce the deep cracks caused by the pitting on wheel surface,automatically applying more braking forces to the trailers when braking a train to reduce the braking force requirement from the locomotives,arranging the locomotives with 2(Bo-Bo)axle configuration as far as possible for the lines with many complicated environments such as sharp curves with large slopes,grinding the profile of the rail with severe side wear on curves,the optimizations for the traction rods and the electrical compensation to reduce the axle load transfer,and the optimization for the creep control of the locomotives to reduce the high wheel creep.