CNC wheel profiling refit gives Zurich trams a smoother ride

Swiss engineering company Emotec has upgraded a tram wheel profiling machine used by the Zurich public transport organisation VBZ, by adding an advanced CNC system and a lathing capability.

Nearly all tram wheels are fitted with flanged metal tyres, which require regular re-profiling to compensate for wear - failure to do this would lead to impaired performance and a less comfortable ride. The upgraded machine can re-profile all four wheels on a bogie within a couple of hours, a task which previously took up to a day.

VBZ performs wheel re-profiling operations at its maintenance hangar adjacent to the Zurich-Oerlikon station, using a machine supplied by the Kellenberger company in 1991. Until recently, the re-profiling machine used a form grinding wheel to bring the tyres back into shape.

However, this process proved very time-consuming - especially when compensating for heavier degrees of wear, when it was sometimes necessary to remove several millimetres of tyre in order to bring the wheels back to a uniform diameter. Furthermore, the form grinding wheel had to be changed for different diameter tram wheels, which added delays, and the machine’s 20 year-old control unitwas becoming increasingly unreliable and expensive to maintain.

When VBZ decided to bring several new types of tram into service in Zurich, it sought the advice of Emotec, a company which specialises in surface finishing technology and the application of CNC. The new trams have lower floors to make it easier for passengers to board and disembark, and are equipped with fewer wheelsets and smaller wheels than their predecessors, which means that they are subjected to higher loads and therefore greater wear.

Emotec determined that instead of VBZ purchasing an entirely new machine, the existing re-profiling machine could be adapted by adding a lathing capability and a modern CNC system. By turning the tram wheels on a lathe prior to grinding them, the tyre re-profiling operation could be accelerated significantly. Interestingly, lathing the type of extremely hard metals used on tram tyres has only recently become practical, with the advent of specialised turning tools and powerful CNC systems.

To develop a control unit capable of handling both form grinding and lathing, Emotec worked with CNC specialist NUM. The final system comprises two NUM Axium CNC units and NUM’s MDLU3 drive technology, controlling a total of 16 axes. NUM also developed all the control and visualisation software.

A further requirement was that since the two machining processes are completely different, the machine operator would need very clear guidance about the procedures involved. To achieve this, the machine is equipped with two NUM FSi-151 control panels, providing an intuitive and easy-to-use human-machine interface (HMI).

The retrofitted wheel re-profiling machine occupies the same physical space as its predecessor. The tram is driven into position in VBZ’s maintenance hangar and the machine is raised hydraulically until its rubber drive wheels are in contact with the tram wheels. The machine can now re-profile the tyres of two tram wheels simultaneously, effectively doubling its throughput, and because the tyres are now either lathe-turned prior to grinding or are only lathe-turned, depending on wear, the entire process is much faster.

As well as much shorter machining times - especially with heavily worn tyres - turning also brings the ability to re-profile the flange and the back of the wheel. Other advantages include the option of machining different tyre profiles without having to adapt the tools, and optimised turning processes for roughing and finishing.

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