# Development of a New Test-Rig for Wheel-Rail Contact Experiments under Large Slip Conditions
## 2 Overall Design
### Category Selection  
The experimental device is the basis of wheel-rail contact experimental investigations. Through the evaluation of the existing test-rigs for wheel-rail contact experiments, three main categories are defined for structure: wheel-on-roller, wheel-on-rail and wheel-on-ring. The test-rigs can also be generally classified into three categories for scale: single wheel, wheel set and bogie/vehicle. All the above are classified and summarized in Table 1.  
\begin{table}
\begin{tabular}{c c c} \hline
**Categories** & **Wheel-On-Roller** & **Wheel-On-Rail** & **Wheel-On-Ring** \\ \hline single wheel & U & \\ \end{tabular}
\end{table}
Table 1: A list of categories of test-rigs and example of each for wheel–rail contact experiments.  
Wheel-on-roller test-rigs are widely used for wheel-rail contact and adhesion research because of high operating speed and convenience in vertical loading. The most common application of rollers is in a single-wheel-on-roller rig, also known as the twin-disc rig, with the advantages of limited size and lower cost. A bogie/vehicle roller, such as the Full Scale Roller Rig of the State Key Laboratory of Traction Power in SWJTU and Full-scale Stand Test in NTSEL [29; 30], is used for railway vehicle dynamics investigation on account of the suspension system. However, the finite roller diameter causes inevitable errors compared with real wheel-rail contact [31]. Furthermore, due to the high-speed rotation of the roller, the fluid layer thickness between wheel and rail can neither be measured nor quantitatively controlled when investigating a contaminated contact interface. Wheel-on-rail test-rigs reproduce the actual track structure and are usually used to investigate wear, rolling contact fatigue, fastening system and other track components. However, the operating speed of these rigs is quite low and the straight rail occupies a lot of space. There are no such problems in a wheel-on-ring test-rig, which is applied to the study of noise and vibration as well, allowing faster running speed and limited space at the same time. As the ring track is fixed, the fluid distribution on the contact interface is kept as close as possible to the actual situation. Thus, the wheel-on-ring test-rig shows potential in adhesion research under wet and contaminated surface conditions. Its disadvantage is the complex structure of the mechanism.  
Since the research object is adhesion recovery under large slip conditions, the creep force may change greatly when creepage increases and decreases on the test-rig. Therefore, in order to precisely control the slip, it is necessary to avoid the possible influence of sudden changes in adhesion on the creepage. The wheel-on-roller test-rig simulates the speed of a vehicle by the rotation of a roller, making the speed of the vehicle and wheel coupled with adhesion. It is obvious that at the contact interface of wheel and roller, the direction of speed is the same while the direction of creep force is the opposite. Consequently, in the braking process, the motor driving the wheel operates in a generator condition, as shown in Figure 2, which is unfavorable for precise speed control. The motors driving the vehicle and wheel may decouple on a wheel-on-ring test-rig, hence the sudden change in adhesion will hardly affect the creepage.  
Based on the above discussion, considering the requirements for quantitative control of fluid layer thickness and precise control of the slip between wheel and rail in this research, with faster running speed and limited space in the laboratory, the wheel-on-ring test-rig is the qualified category for wheel-rail contact experiments under large slip conditions.