Perfection taken to the extreme
Maybe it’s the nearness of the mountains, but colleagues at the Siemens Mobility global competence center for developing and manufacturing bogies in Graz-Eggenberg, Austria, are used to taking things to the extreme in order to deliver extraordinary performance on rails. And that’s just what they do, because this is the place where MoComp bogies for all types of rail vehicle are developed. Not surprisingly, for Dr. Stefan Erlach, head of the Global Competence Center for Bogies in Siemens Mobility’s Rolling Stock Business Unit, the bogie is the center of the universe, or at least of the successful rail vehicle. We spoke to him to find out more.
Dr. Erlach, you see the bogie as crucial for the rail vehicle. Why is that?
Maybe we should start by looking at the functions a bogie serves in a rail vehicle’s overall system. What makes the bogie so critical is that all the forces to which the vehicle is subject start here. The bogie’s wheels are the interface between the vehicle and the track. Every acceleration, every braking operation, every lateral wind, and every load that affect the vehicle come into play here. That’s why the bogie’s properties are crucial for a train’s safety and riding comfort, as well as its noise levels.
In other words, there are definite mechanical standards that have to be met. But are there also other areas where you can optimize the bogie’s performance?
Yes, there are, and from the customers point of view, the most important is energy efficiency, because it has a direct effect on the vehicle’s operating costs. There are definite requirements that an energy-efficient bogie has to meet, including weighing as little as possible and having minimal rolling resistance with an optimal coefficient of friction between the wheel and track. But first let’s take the mechanical properties. MoComp bogies have always been developed and optimized with the overall rail vehicle in mind – but always on the basis of clearly defined requirements for safety against derailment, stability, acoustics, riding comfort, and serviceability. Whatever else we optimize mustn´t interfere with these mandatory properties and, in the best case, should also affect them positively.
What makes the bogie so important is that it not only has to meet safety-relevant requirements, but also significantly affects passenger comfort.Dr. Stefan Erlach, head of the Competence Center for Bogies of Siemens Mobility Austria GmbH Graz
That sounds especially challenging.
These are of course tremendous challenges, but we welcome them. One of our guiding principles is: “When you stop getting better, you stop being good.” An excellent example is in the area of lightweight construction, where we’ve always been able to guarantee identical or superior properties with a significantly reduced bogie mass. In recent years, we’ve managed to design a bogie frame – which is the core of the bogie – that’s up to 40 percent lighter by using new materials and applying new design principles. This not only reduces our customers’ operating costs, but the reduced axle loads also allow them to save money on track access charges, or simply to increase their carrying capacity.
But isn’t the bogie frame a simple steel construction?
On the contrary, the bogie frame is a complex construction made of sheet steel coupled with cast or forged components with tolerances within a tenth of a millimeter. These components are also subject to extreme dynamic stress and have to function safely for a service period of 30 to 40 years or an operating life of up to 15 million kilometers. That’s just one of the reasons we place such highest demands on the quality of our design methods, on our manufacturing processes, and on employee training. In addition to highly qualified employees, in recent years we’ve been especially focused on increasing the level of automation in production using welding robots and, in this way, exploiting the possibilities of the digital transformation.
Deployment for a period of 30 years or more sounds like extremely high maintenance expenditures.
Yes, you’re right, the bogie is the one subsystem in a rail vehicle that requires especially high-maintenance and accounts for 30 to 40 percent of the total maintenance costs. This is also due to the fact that maintenance continues to be performed on the basis of a specific time period or mileage instead of being based on the condition of the bogie and its components. Bogies and their components are also subject to different levels of stress, depending on how the vehicle is deployed (lines, load, etc.). But because they’re designed with the worst case in mind and are equipped with safety factors, there’s tremendous potential for exploiting the components’ service life more fully. That’s why we developed a diagnostic system that uses sensors, algorithms, and on-board computers to diagnose the condition of the bogie and its individual components and classify them on a scale from 1 to 10. By connecting to the track side (vehicle depot) and accessing additional data (data from other vehicles in the fleet, service data, etc.), it’s possible to determine the actual service life or operating life remaining and greatly reduce maintenance costs. In combination with a consistent service schedule, these diagnostic systems permit a highly cost-optimized maintenance process.
One topic that’s always being discussed is noise emissions from trains.
The acoustic behavior of a rail vehicle depends on a number of factors, but especially on the properties of the bogie, and of the wheel discs and brakes in particular. The mechanical brakes are certainly responsible for the rail vehicle’s most noticeable noise, but rolling and curve noise also has to be taken into account. Together, they make up a vehicle’s noise emissions, which affect both passengers and the people living alongside the tracks.
And what exactly do you do to reduce noise?
We have to dampen the propagation of sound vibrations in the structure and design our components to be “quieter.” In the case of our wheels, we do this by acoustically optimizing the design of the wheel webs so as to reduce rolling noise by 3 dB, which corresponds to cutting the sound energy in half. In the case of our wheels, we do this by acoustically optimizing the design of the wheel webs so as to reduce rolling noise by 3 dB, which corresponds to cutting the sound energy in half. By using wheel damping devices and noise absorbers, we’re able to reduce rolling noise by an additional 3 dB, meaning that the total sound energy is then a quarter of what it was originally. And to reduce structure-borne sound, we use rubber inserts for sound decoupling in order to reduce transmission paths in the bogie as much as possible. At the same time, intelligent braking control ensures that mechanical braking is avoided whenever possible, and resilient wheels and noise absorbers significantly reduce curve noise.
Are there other areas that you can optimize?
Acoustic optimization is closely linked to another area for which our bogies are responsible. On-board comfort isn’t just a matter of a comfortable level of background noise, but also depends on the vehicle’s smooth running. The two together have a significant effect on passenger comfort, and MoComp bogies are exceptional for their extraordinary performance. The level of performance is primarily based on spring and damper stiffnesses that are optimally tuned to the overall system and are determined using complex methods of dynamic multi-body simulation. The result is maximum passenger comfort in many European high-speed trains like the Velaro in Germany and the UK, as well as in hotel coaches, couchette, and sleeping cars like those of ÖBB’s Night Jet.