## 6 Discussion
### Precision Issues  
Machining tolerances and bearing frictions are the two potential precision issues affecting the experimental results the most.  
The larger the machine part is, the harder to guarantee the machining accuracy. The largest machine part of the scaled IRW experimental system is the roller, which also can input irregularity excitations on the IRW's wheels and affect the dynamic processes. Thus, the machining tolerances of the roller, especially of the surface profile near the wheel-roller contact patches, should be reduced. We collaborated with a machining company to build the experimental system. They have a large CNC lathe capable of machining a cylinder up to 60 cm in diameter, with a machining process accuracy of \(\pm 0.01\) mm. The geometry information of the surface profile was coded into the CNC lathe so the nonlinear  
Figure 32: Linear regression results of the experiment data during IRW’s offset running at average rotational speed 575 r/min.  
surface profile could be well processed. We used a dial gauge to check several points of the surface profile to check the machining tolerances, as shown in Figure 33. Because of the nonuniformity of materials, the roller will slightly deform with time. When the heavy scaled IRW with vertical loads runs on the roller, it will also slightly deform. In addition, the assembling clearances in the whole mechanical system deteriorate the machining working accuracy. Overall, the machining tolerances cannot be decreased to \(\pm\)0.01 mm. We have made three versions of the experimental systems and can guarantee the machining tolerances within \(\pm\)0.1 mm of the best one.  
The three most important points for decreasing bearing frictions are guaranteeing installation accuracy, adopting high-quality bearings, and lubricating bearings before experiments. The carrier plate was processed in a CNC drilling and milling machine with a machining process accuracy of \(\pm\)0.01 mm. The thread holes can be located accurately to guarantee installation accuracy. As shown in Figure 34, there are two types of relative rotations on the scaled IRW. One is the wheels' rotational motion to the carrier, and the other is the IRW's yaw motion to the traction plate. The ball bearings and thrust bearing were bought from an online professional machine parts supplier, MiSUMi, to reduce rotational frictions. The bearings' accuracy satisfied the Japanese standard JJS B 1514 Class 0 [24], meaning the rotational frictions of the bearings themselves are small enough. Moreover, we lubricated these bearings before experiments to decrease the bearing frictions.  
Due to machining precision issues and the unavoidable bearing frictions, it is not reasonable to compare the experimental results of the scaled IRW with the one built in SIMPACK 2018 point by point in the time domain. Although we have made three sets of the experimental systems and chose the best one, the least machining tolerance of the roller is \(\pm\)0.1 mm, which can still cause unwanted irregularity input and influence the results. As shown in Figures 26 and 29, the high-frequency oscillations in the steady convergent process demon  
Figure 34: Bearings’ installation on the scaled IRW.  
Figure 33: The machining tolerance test of the roller’s surface profile near the wheel–roller contact patches. (**a**) The test points; (**b**) a dial gauge for checking the machining tolerances.  
strate the impact. The amplitude of the high-frequency oscillations on the differential rotational speed curve in Figure 29 is about 0.5 r/min. According to Equation (47), the corresponding lateral displacement \(y\) is 0.162 mm, which is very close to the machining tolerances of the roller.  
However, the demonstrations of the three types of dynamic motions and the steady convergent process are not affected. The derailment, hunting, and offset running are still observable and recognizable. The experimental results are still in accordance with the theoretical analysis and the phenomena found in simulations.