Innovative Retrofit To Wheel Profiling Machine Ensures Zurich’s New Trams Provide Smooth Ride

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. VBZ performs wheel re-profiling operations at its maintenance hangar adjacent to the Zurich-Oerlikon station, using a machine that was originally supplied by the Kellenberger company back 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 whenever it was necessary to accommodate a different diameter tram wheel, which again contributed to delays and lack of throughput. In addition to this, the machine’s control unit, which was nearly 20 years old, was 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 AG, a company which specialises in surface finishing technology and the application of CNC. The new trams feature lower floor heights – to make it easier for passengers to board and disembark – and are equipped with less wheel-sets and smaller wheels than their predecessors, which means that they are subjected to higher loads and thus greater wear.

After thoroughly investigating the process, Emotec determined that it would be possible to adapt the existing re-profiling machine by adding a lathing capability and a modern CNC system, instead of requiring that VBZ purchase an entirely new machine. 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 tenable, with the advent of specialised turning tools and powerful CNC systems.

The demands placed on the control unit of a machine capable of handling both form grinding and lathing are considerable, so Emotec worked closely with NUM to develop a high performance replacement for the existing unit. The final system configuration 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 key requirement was that since the two machining processes are completely different, the machine operator would need very clear guidance about the various procedures involved. To achieve this, the machine is equipped with two NUM FSi-151 control panels, which provide an exceptionally intuitive and easy-to-use HMI (human-machine interface).

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 – which is effectively located on a lower level in the building – is then raised by hydraulic actuators 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, depending on wear, the tyres are now either lathe-turned prior to grinding, or are only lathe-turned, the entire process is much faster.

The advantages of turning are, above all, the much shorter machining times – especially with heavily worn tyres – and 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, as and optimised turning processes for roughing and finishing. Emotec and NUM have clearly demonstrated that the retrofitting of an existing machine using novel ideas can indeed compete with the purchase of a new machine. So the next time you take a ride on a Zurich tram, spare a thought for the efforts that go into ensuring you enjoy a smooth ride!