Dovetail Tyred Wheels – Application to a DMU

Tyred wheels are perceived as “out-of-fashion” and are not even considered by current European standards. Nevertheless many railway enterprises still have in their assets vehicles or entire fleets equipped with tyred wheels. The maintenance of this component drains a large amount of time and money. The paper describes the results of a joint project in which a University, a workshop and a train operating company developed, implemented and successfully tested a new maintenance methodology that looks very promising. Design and assembly criteria are discussed, as well as the impact on existing vehicles. Results were so promising that plans exist at the moment to extend this approach to other vehicles and possibly also to new rolling stock Although tyred vehicles are residual in modern fleets, the economic resources that the overhaul of their wheelsets drain are noticeable. Two of the authors recently publish a set of papers reinterpreting the role and the importance of tyred wheels. Very briefly, the authors reached the conclusion that tyred wheels were designed in the past according to practice without having the possibility of conducting detailed numerical analyses that on the contrary are widely available nowadays. This resulted in a quite inefficient design, leading to wheels that were heavy and not optimized from a structural point of view. The presence of tread braking, a standard 50 years ago, is today much less important and, in practice, confined to freight wagons. The use of disc braking and the availability of Finite Element codes with solvers able to simulate non-linearities, allowed to show that many wheel centres were subjected to plastic strain during tyre fitting [1]. The thickness of the tyre in the fully worn condition, dictated by the need to guarantee a proper coupling (avoiding spinning) between the tyre and the wheel centre during both stop and drag tread braking, proved to be largely in excess of the one needed when the thermal input given by tread braking is avoided [2]. But the authors realized that the death of tyred wheels was dictated by other factors. First, high-speed trains cannot be equipped with such wheels for clear centrifugal forces reasons; second, tread braking was recognized as deleterious for tyred wheels already in the ‘60s, forcing all freight wagons to be equipped with monobloc wheels; third, maintenance practice for tyre wheels was a real nightmare for the reasons discussed in detail in [3]. While the problem of unsprung masses and spinning can be easily solved with a proper design [2], maintenance had to be faced taking into account workshop practice that include machine tools, checking tools and allowable deviations (tolerances). During the analysis of all these factors, it was readily realized that modern workshop practice is extremely different from the one available at the beginning of the railway era. Today, all machine tools are equipped with a CNC (continuous numerical control) and the availability of CMM (coordinate measuring machines) is common in nearly all workshops. Together with a re-analysis of the machining, assembly and checking phases, the idea of dramatically simplifying the architecture of the tyre-wheel centre mating emerged. A minor modification to the shape of wheel centres and tyres that was believed to significantly reduce maintenance costs was then proposed [3]. The basic idea was at that time to machine on the mating parts a dovetail self-locking shape capable to eliminate the need of the retaining ring. This was moreover compatible with the possibility of purchasing fully finished tyres, similarly to what happens in the case of “elastic wheels” for light rail vehicles. The outcome of the research indicated that with a preliminary FEM calculation and a carefully studied maintenance cycle, no machining of any kind was probably needed on wheelsets after fully-machined tyres were fitted on fully-machined wheel centres. An extremely important “side effect” of the introduction of this process was the possibility for wheel centres and axles to exploit really infinite life as they are not replaced as in the case of monobloc wheels (scrapped when fully worn) and of axles (whose wheel seats need to be machined from time to time limiting their useful life).