1、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 the
2、rmal 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 co
3、mmon 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 br
4、ake, 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
5、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 therma
6、lly 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 R
7、ail 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 character
8、istics. * 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 c
9、oncerning 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 p
10、er 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 t
11、he wheels 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
12、must undergo 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 detaile
13、d maintenance 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
14、 achievable 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 te
15、sted under the 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. Requirements As a result of the re
16、search work, 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
17、 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 100 MPa (mean value) and 80 MPa (minimum value) given in EN 13262. Regarding the wheel design requirements the UIC published the new UIC l
18、eaflet 510-58 which was prepared by the ERRI B 169 specialists committee. This document is also the basis for the development of a new Draft European standard prEN 13979-1 which is in preparation now. The new standards are built up as a specification giving more freedom to the designer. According to
19、 these specifications four aspects of a new wheel design have to be considered: * Geometrical aspect: to allow interchangeability. * Thermo mechanical aspect: to manage wheel deformation and to ensure that braking do not induce wheel failure. * Mechanical aspect: to ensure that no fatigue crack in t
20、he web will occur. * Acoustical aspect: to ensure that the solution is better or equal compared with a reference wheel. Concerning the interchangeability requirements in three ways are necessary depending on the customer1: * Functional requirements, e.g. wheel diameter, tread profile, asymmetry of t
21、he hub with regard to the rim. * Fitting requirements, for example, length of the hub, bore diameter. * Maintenance requirements, e.g. clamping conditions of the wheelset reprofiling lathes. The designer has full freedom regarding the design of the wheel web. For railway vehicles with block brakes t
22、he brake power has to be considered. Tests with freight trains running on long mountainous lines through the Alps received an average brake power level of 50kW for a wheel with 920mm diameter. For smaller wheels the brake power is on a corresponding lower level. Therefore wheels for freight wagons h
23、ave to resist these brake loads. For vehicles with different brake systems, such as disc brakes, an assessment of the thermal behaviour is not necessary. For combined brake systems (block brake and others) modified loads shall be agreed between customer and supplier. The brake loads are reproduced o
24、n a brake test bench. In order to check the thermal behaviour the wheel is loaded with a number of brake cycles. For the assessment unified criteria are defined in UIC 510-5 and prEN 13979-1 respectively. For the level of residual tensile stresses in the rim the following criteria are valid: For a w
25、heel with its nominal diameter a stress level of maximum 200MPa (mean value) and maximum 250MPa (for each cross section) is acceptable. For a wheel with its diameter near the wear limit a stress level of maximum 275 MPa (mean value) and maximum 300 MPa is acceptable. Regarding the lateral displaceme
26、nt the analysis of maintenance rules, of the service experience and of the dimension of crossings and points led to allowable values between -1 mm and +3 mm (during braking) and between -0.5 mm and +1mm (in cold condition). For the mechanical aspect8 determine a relative conventional procedure. Firs
27、t step is a stress calculation using the finite element method. Three conventional load cases are to be considered representing straight track full curves and points and crossings. Based on these loads the normal stresses for each node of the FE mesh is calculated. Comparing the stresses for the dif
28、ferent load cases a stress range or a stress amplitude can be calculated. The stress of the most stressed node shall be compared with the decision criteria, which are 180 MPa for wheels with fully machined web and 145 MPa for wheels with unmachined web. In addition to the calculation fatigue tests c
29、an be required. This depends on the results of the calculation and on the validity of the conventional loads. Two methods for fatigue tests are possible, either a random fatigue test or a one-stage fatigue test8. For both methods the test loads are derived from measured loads during field tests. Con
30、cerning the acoustical aspect it is, of course, not a target that new developed wheels have higher sound radiation compared with existing designs. Therefore a sound level is described which is comparable with the former ORE standard wheels8. The sound level can be determined by a calculation. The acoustical requirements are informative only. Product development and verification The stress ranges for the various designs are calculated as follows: * Wheel 21.061.00 (BA 004)/21.061.10 (BA 304) 240.9 MPa (25 t axle load), * Wheel 21.431.01 (BA 378) 175.9 MPa,
