PSI - Issue 4

Ivo Černý / Procedia Structural Integrity 4 (2017) 35– 41 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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1. Introduction

Railway wheelsets, crucial components of railway bogies and vehicles from the viewpoint of function, reliability and operation safety, are compound of two subcomponents, wheels and axle, connected to each other by press fitting or shrink fitting. Wheelsets are components extremely dynamically loaded in service, by a very high number of cycles, usually more than 10 9 during the life. That is why fatigue resistance of these components have to be adequately proved during the certification process. Such proves are strictly requested. A system of European standards have been elaborated (EN 13260-2, 13103-4) and is being systematically improved according to needs of the railway transport, its increasing demands on service safety and reliability in connection with increasing train speeds, both freight and passenger, with increasing demands on payload and other aspects – Cervello (2016), Foletti, Beretta and Gurer (2016). Though strict standard criteria for the press or shrink fitting methods exist like overlapping dimensions of the axle seat diameter and wheel hole, dependence of press forces during the press fitting process, press fits still remain one of the critical areas where possible cracking occur – Kubota et al. (2003), Hirakawa et al. (1998), Marshall et al. (2011). It is necessary to evaluate not only fatigue strength in terms of endurance limit of axle bodies and wheels separately, but also fatigue resistance of press fits. As regards axles are concerned, two or three characteristics are defined in the above mentioned EN standards, which have to be proved, namely (i) fatigue resistance of the smooth part of the axle body on its free surface, so called characteristics F1 and (ii) fatigue resistance under press fit (or shrink fit) - characteristics F3. Another characteristics – F2 concerns internal surface of hollow axle body. One of the test methods, how to generate the rotating bending moment, is to attach the half wheelset using the wheel to the horizontal ground base and to rotate an eccentric mass on the axle top. Such principle, incorporated into the standards, is used in the majority of test rigs available on the market. In the past, typical load frequency was 15 – 17 Hz. Under such conditions, only dynamic forces of the eccentric mass could be considered. However, recent design changes of the Sincotec facilities have resulted in increase of test frequencies to more than 25 – 30 Hz, which makes the tests significantly shorter in comparison with the previous test conditions, when one axle standard test to 10 7 cycles without any interruption took more than one week. One of the reasons, why the last, very sophisticated version of the Sincotec facility has been designed to reach high test frequencies, was the test costs and time demands. Full-scale fatigue tests are quite expensive, including the specimens – actual axles, and it is usually necessary to know the results as soon as possible. On the other hand, high test precision is an essential demand, together with further requirements of the load uniformity along the whole circumference, ensured in case of new Sincotec machines by very special and massive, patented round basis – Fig. 1. Careful, exact static measurement and machine calibration is the first essential step followed by dynamic load evaluation during the whole fatigue test. All these aspects are crucial for providing both exact research results and correct results of the fatigue tests, which are at SVÚM a.s. laboratory fully accredited according to the EN ISO 17025 Standard and can be therefore fully accepted by any European railway certification body. As already mentioned, the main advantages of the innovated facility in comparison with the previous generation are shorter time needed to complete the test – around 100 hours, i.e. four days only, and higher precision of the tests. On the other hand, there are some disadvantages connected with the higher test frequency, namely dynamic effects, which may negatively affect particularly dynamic loading and test results in the press fit and its vicinity, characteristic by high static stress resulting from the press fit – Madia, Beretta and Zerbst (2008), Hannemann and Sander (2016), Lunden, Vernersson and Ekberg (2009), Luke et al. (2011), Zerbst, Schödel and Beier (2011). Using a concrete example, these aspects are discussed in this paper.

2. Experiments

The works were performed within a special project aimed at evaluation of different types of press fitting on fatigue resistance. Therefore, the tested axles did not have a shape typical for standard railway axles, but had a constant diameter throughout its whole length. The full scale model of a 230 mm diameter axle of approximately 2  m length, made of a C45 steel, press fitted in a hub, was fixed by the hub to the base of the rotating bending machine. The shape and dimensions of the axle in the hub is in Fig. 2.