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1、Cut wheel fracture problems and maintenance costsJorg Villmann looks at the problems of wheel fracture and the development of new designs to reduce failure problems and maintenance costs.In the late 1960s and 1970s axel loads and speeds of railway vehicles increased rapidly. This led to higher therm
2、al an mechanical loads of the wheels. Tiered wheels showed loose types after strong heating during runs on mountainous lines or following to brake irregularities. Maintenance costs for type changing increased more and more. In order to solve these problems solid wheels were introduced. The most comm
3、on used wheel type was the so-called ORE wheel developed by the European railways under the roof of the ORE (today European Rail Research Institute ERRI) as the research institute of the UIC (International Union of Railways). Following the extended use of solid wheels in connection with a block brak
4、e, the unforeseen problem of wheel fracture occurred. Investigation of failured wheels showed that two principal forms of wheel fracture occurred - radial fracture from the wheel rim straight through the web down to the hub or beginning in the rim, running straight in to the web and shared in two br
5、anches. It was also found that the fracture was initiated from half-elliptical or fourth-elliptical fatigue cracks, which started on the tread, around the chamfer or due to sharp notches from clamping devices of reprofiling lathes. Detailed investigation showed that all failured wheels were thermall
6、y damaged and had high residual tensile stresses in the rim of about 300 MPa. Though the number of failed wheels was relatively small each failure could lead to devastating consequences. Therefore intensive research work was carriued out to improve this situation.Research programme The European Rail
7、 Research Institute (ERRI), which is part of the UIC, was selected to lead the project work. The committee responsible for the work was the B 169 specialists committee. Three major problems were considered work programme :* Monitoring of the wheels in service.* Improvement of material characteristic
8、s.* Improvement of the residual stress level and the displacement behaviour .With the first problem it was important to summarise the experience of the different railways and to get more detailed knowledge of the condition of the wheels in service. These investigations confirmed the results concerni
9、ng the residual stresses. Approximately 10 per cent of the wheels had residual stresses of about 300 MPa. On the other hand, the fracture toughness KIC or KQ of the wheels investigated was between 40 and 70 MPa. From fracture mechanics calculation it could be concluded that approximately 10 per cent
10、 of the wheels had a potential risk of failure . The analysis also brought up some cases of fatigue cracks in the wheel web and many cases of unacceptable lateral displacements of the wheel rim leading to high maintenance costs. Therefore the first step was to set-up rules for monitoring of the whee
11、ls in service including acceptance criteria. The B169 specialists committee developed four characteristics for visual inspection to identify potential wheels thermally overloaded . Criteria for the assessment of the wheels undergoing maintenance were also defined. Wheels with thermal damages must un
12、dergo residual stress measuring and, if required, crack detection. The whole procedure is defined in 4. Following to the implementation of the in-service rules and the continuous monitoring an essential reduction of wheel fracture was reached in Europe.In order to be independent from detailed mainte
13、nance rules and in-service monitoring, research then focused on the improvement of the wheel material. The results can be summarized as follows5:* Normally no KIC values were found, that are KQ values.* KQ is suitable to describe the material characteristics,.* KQ between 70 and 85 MPa is achievable
14、 for steel grade R7T.The third step focused on the reduction of the residual tensile stress level in the rim and on the lateral displacement of the rim. In this regard the shape of the wheel web is essential. Therefore different proposals were developed by the wheel producers and were tested under t
15、he roof of the ERRI research programme . Generally it can be stated that a more flexible wheel web is suitable to reduce the residual tensile stresses in the rim. On the other hand it is also possible to hold the displacements in a small tolerance band. RequirementsAs a result of the research work,
16、a number of new requirements for wheel material and wheel design were defined. These requirements led to new or revised international specifications. The material requirements are defined in UIC-leaflet 812-36 and recently in the European standard EN 132627. For R7T steel grade (or ER7T according to EN 13262) a fracture toughness KIC or KQ of 80 MPa (mean value) and 70 MPa (minimum value) is required. For ER6T the corresponding requirements are 10
