Common Safety Analysis of the AIR Wheelset

The paper describes the process and the results of a Common Safety Assessment (CSA) applied to a highly-innovative wheelset design with apparently independently rotating (AIR) wheels. All the components of the AIR Wheelset, which has been designed to improve maintenance and to remove the risk associated with axle failures, are evaluated in order to investigate their failure modes, the associated hazards, risk levels and possible mitigations. The results are critically compared with the conventional wheelset architecture, showing that the AIR Wheelset is superior from all points of view. Safety of wheelsets remains a central issue in modern railways. The progresses in non-destructive testing (NDT) and in maintenance organization and procedures help minimizing the risk associated to the harder operation profiles and the scarcity of resources. Nevertheless, axle failures remain a spectre for railway engineers, due to their sudden growth and their catastrophic consequences. As only off-line non-destructive testing are possible today, the risk may be mitigated with a proper definition of the inspection interval but it cannot be completely eliminated. As an attempt to drastically eliminate all the risks associated to the use of axles, a novel wheelset design was developed and patented. Although the concept was described in a number of references, all mentioned in this paper, the central issue of safety was not addressed yet. The goal of this paper is to evaluate safety of this novel design showing its advantages with respect to conventional wheelsets. 2. THE AIR WHEELSET 2.1. General concept The AIR Wheelset was conceived with the twofold goal of reducing the risks associated to rotating bending fatigue of axles and to make maintenance easier. The idea was first presented in 2015 [1], after having filed on 22.06.2015 the application for a patent that was eventually released on 16.01.2019 [2]. It consists of a wheelset made of a non-rotating bridge on which two wheels rotating on roller bearings are mounted, similarly to the well-known principle of the Independently Rotating Wheels (IRW) architecture. In order to avoid the “erratic” dynamics of IRW wheelsets, that prevents their use in conventional vehicles limiting their application to low-floor trams, a torsional link connecting the two wheels, restoring the correct behavior of a rigid wheelset, is used. Furthermore, different solutions for a torque limiting device were developed, giving the AIRW (Apparently Independently Rotating Wheelset) previously unreachable and outstanding performances in terms of negotiation of sharp curves and track friendliness. Academics can get more information on running dynamics of AIR Wheelset in [3]; contact mechanics advantages are discussed in [4]; the design of the specific bearings arrangement is described in [5]; torque limiter validation on a mixed line is described in [6]. Railway engineers can get information about maintenance in [7]; integration on existing vehicles is covered in [8]; wheelset specialists can get a comparison of different wheelset innovative design in paper [9], given at the previous edition of this series of Conferences. In the following paragraphs a short description of the main components is made with reference to Fig. 1. The description is limited to the target of this research, i.e. the correct allocation of risks and hazards linked to the adoption of the AIR Wheelset.