# Key Factors of the Initiation and Development of Polygonal Wear in the Wheels of a High-Speed Train
## 2 Characteristics of Polygonal Wear of High-Speed Wheels  
Polygonal wear occurs on the wheels of many high-speed trains. The census test of polygonal wear was conducted over a long time in this paper. The tested polygonal wheels comprised about 48,000 sets and the test mileage was \(>\)200,000 km after re-profiling. Figure 1a shows the Muller-BBM wheel roughness test instrument which was used for measuring polygonal wear, and Figure 1b shows a typical polygonal CL60 steel wheelset, which was provided by CSR Tangshan Rolling Stock Co., Ltd. During the test, the measured wheelsets were jacked up using hydraulic equipment so that the wheel could rotate around its axis line freely. The displacement sensor of the roughness test instrument contacts the wheel tread to measure the radial change in the wheel. Figure 2 shows the proportion of the polygonal wheel with a diameter change. The proportion of the polygonal wheel represents the quotient, which is the number of polygonal wheels divided by the number of all measured wheels with diameters of 830-840 mm, 840-850 mm, and 910-920 mm. A diameter of 920 mm is the nominal diameter for new high-speed wheels. Figure 1: (a) Test instrument; (b) Polygonal wheel. In the analysis, shown in Figure 2, the threshold of \"high-speed\" rail roughness (ISO-3095 Third edition 1 August 2013) was used as the threshold for high-speed wheels because there are no official partition criteria in China railways so far. Temporary (suggested) partition criteria for polygonal wear and non-polygonal wheels are designed by rolling stock companies and railway transport departments in China based on their requirements. The temporary partition criteria consider the rail roughness threshold line specified in ISO-3095 for the maintenance of high-speed wheels with severe polygonal wear. If the depths (peak to valley) of the polygonal wear of some dominant orders exceed the high-speed wheel roughness threshold line, the occurrence of the polygonal wear phenomenon is confirmed. As shown by the black line in Figure 3, the peak of 18 th-order exceeds the roughness threshold line; thus, the wheel is regarded as a polygonal wheel. For the blue line in Figure 3, there is no peak exceeding the roughness threshold; thus, the wheel is regarded as a normal wheel. It is noted that ISO-3095 shows that the threshold line of rail roughness includes the wavelengths of 0.3 cm to 40 cm only. The polygonal wear of high-speed wheels covers the wavelengths of 12 cm to the whole wheel circumference. However, the wavelengths of the polygonal wear, which are much more concerned with the point view of controlling dynamic behaviour and noise, are in the range determined by the ISO-3095. Figure 3: Sketch map: Amplitude spectrum of polygonal wear (site measurement) and threshold line of high-speed wheel roughness (referring to the ISO-3095). Figure 2: Statistical results of polygonal wheels. Through the statistical analysis of the test data, the characteristics of polygonal wear can be concluded as follows:  
1. Wheels with different diameters have different proportions of polygonal wheels and different dominant orders (main wavelength) of polygonal wear (Figure 2). Furthermore, it seems that the smaller the diameter, the higher the proportion of the polygonal wheels. This depends on the depth of the decarburised layer on the surface of the wheel. 2. The proportion of polygonal wheels exhibits a non-monotonic variation as a function of the wheel diameter, including three local maxima around 830-840 mm, 870-880 mm, and 910-920 mm, corresponding to three periods of high-rate development. Figure 4 shows the roughness spectra of polygonal wear, measured from different cars for a typical high-speed train. The subgraphs show that different cars have different polygonal wear orders. The polygonal wear of the 20 th order occurred on Car 1; 19 th order occurred on Cars 2, 4, and 7; 18 th order occurred on Cars 3, 6, and 8.