# Development of a New Test-Rig for Wheel-Rail Contact Experiments under Large Slip Conditions
## 5 Conclusions  
In this paper, an innovative new test-rig for wheel-rail contact experiments under large slip conditions was developed. According to the research described above, the following conclusions are drawn:  
1. A wide range of existing test devices for wheel-rail contact experiments were classified and comparatively reviewed. Three categories of test-rig were distinguished and functionally evaluated: (1) wheel-on-roller, (2) wheel-on-rail, (3) wheel-on-ring. With the requirements for quantitative control of fluid layer thickness and precise control of the slip between wheel and rail, the wheel-on-ring test-rig was finally selected;
2. The scale of the test-rig determines the wheel diameter as well as the required power of the test-rig. The circling diameter of the test-rig affects the similarity of longitudinal creepage and creep force on the contact patch to that of the actual vehicle. Considering the power limit in the laboratory and wheel-rail contact similarity, a scale factor of 1/4 and a circling diameter of 2 m were adopted for the new test-rig;
3. The downscale test-rig consists of wheel assemblies, ring track, gearbox, motors and sprinkler system in the main structure. The creepage was the specific value of two torque meters while the adhesion coefficient was calculated from the torque meter and the load sensor. By selecting an appropriate gear ratio of the gearbox, the two motors of the test-rig were decoupled. Precise slip control was realized in MATLAB/Simulink simulation under PID control;
4. Validations of the test-rig were conducted and showed little impact on creepage due to a sudden change in adhesion, meanwhile the error between measured and target creepage was not greater than 0.005. Adhesion experiments under large slip conditions with different speeds, axle loads and surface contaminants were performed. Positive correlation with running speed and negative correlation with axle load of adhesion recovery were observed. Oil and leaf contaminants caused no adhesion recovery under large slip conditions;
5. The experimental results of wheel-rail adhesion indicate the accuracy of the test-rig. The phenomenon of adhesion recovery reproduced on the test-rig suggests that changes in wheel-rail interface conditions during large creepage contribute to restoring adhesion, including the effects of water evaporation caused by temperature rise and the removal of the surface layer. In accordance with other experimental observations, the test-rig is capable of performing further adhesion experiments under various surface conditions.  
In summary, the new test-rig was specially designed for wheel-rail contact experiments under large slip conditions. The fixed ring track ensured fluid distribution on the contact interface close to the reality. Furthermore, the accuracy of slip control was verified by simulation and experimental validation. Afterwards, experimental observations showed the potential of the new test-rig for exploring adhesion recovery under various surface conditions. Further detailed experimental investigations will be carried out utilizing the newly developed test-rig.