The system was redesigned with a lathing capability for turning the tram wheels prior to grinding, to speed up processing; plus two NUM Axium CNC units and NUM MDLU3 drive technology to control a total of 16 axes.
Emotec is a Swiss engineering company that recently completed a retrofit of a tram wheel profiling machine used by the Zurich public transport organization VBZ, adding an innovative lathing capability and an advanced CNC system from NUM. Since the upgrade, the machine is capable of re-profiling all four wheels on a bogie within a couple of hours – a task that previously took as much as a full day to complete.
Nearly all tram wheels are fitted with flanged metal tires that require regular re-profiling to compensate for wear. Without reconditioning the wheels, their performance soon becomes impaired performance and the tram passengers are subjected to an uncomfortable ride. VBZ performs wheel re-profiling at its maintenance garage adjacent to the Zurich-Oerlikon station, using a machine that was originally supplied by Kellenberger in 1991.
Until recently, the re-profiling machine used a form-grinding wheel to bring the tires back into shape. However, this process proved time-consuming, especially when compensating for heavier degrees of wear, when it was sometimes necessary to remove several millimeters of tire 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 prolonged the delays and contributed to lack of throughput. In addition, the machine’s control unit was nearly 20-years-old and becoming increasingly unreliable and expensive to maintain.
When VBZ decided to bring several new designs of trams into service, it consulted Emotec AG about a machine update. Emotec is a custom engineering group that specializes in surface finishing technology, and produces a range of CNC punching and drilling machines.
NUM is one of the firms that originated CNC control technology, and now in addition to CNC hardware and software it develops and manufactures the related CNC drives and motors.
The new VBZ trams are designed with lower floors to make it easier for passengers to board and disembark at the curb, and they are equipped with fewer wheel-sets and smaller wheels than their predecessors. All these details mean that they are subjected to higher loads and thus greater wear.
After investigating the process, Emotec determined that it would be possible to adapt the existing re-profiling machine by adding lathing capability and a modern CNC system, instead of requiring VBZ to invest an entirely new machine. By turning the tram wheels on a lathe prior to grinding them, the tire re-profiling operation could be accelerated significantly. Interestingly, lathing the type of extremely hard metals used on tram tires has only recently become tenable, with the advent of specialized 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.
The new Zurich trams are designed with lower floors to make it easier for passengers to board and disembark at the curb, and they are equipped with fewer wheel-sets and smaller wheels than their predecessors. All these details mean that they are subjected to higher loads, and thus greater wear.
NUM also developed all the control and visualization software. A further, critical requirement was that because 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 human-machine interface (HMI.)
The retrofitted wheel re-profiling machine occupies the same 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 raised by hydraulic actuators until its rubber drive wheels are in contact with the tram wheels. The machine can re-profile the tires of two tram wheels simultaneously, effectively doubling its throughput. Also, the entire process is much faster because, depending on wear, the tires are now either lathe-turned prior to grinding, or are only lathe-turned.
The advantages of turning start with the much shorter machining times, especially with heavily worn tires. Another is the ability to re-profile the flange and the back of the wheel. Futher advantages include the option of machining different tire profiles without having to adapt the tools, as and optimized turning processes for roughing and finishing.
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