Cut wheel fracture problems and maintenance costs
Jorg 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 thermal 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 common 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 brake, 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 branches. 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 thermally 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 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 characteristics.
* 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 concerning 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 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 wheels in service including acceptance criteria. The B169 specialists committee developed four
