Transport companies have traditionally attached less importance to innovative technologies when it comes to drive systems for Electric Multiple Units (EMU): Criteria such as low acquisition and running costs as well as a long service life still rank much higher than maximum efficiency. However, these requirements are going to change with steadily increasing electricity prices.
As the world's first EMU multispeed transmission, the Get2 concept combines the advantages of previous ZF applications and an energy saving potential of up to five percent – and this compared to one of the most advanced current metro drive systems.
"With our Get2 concept, we clearly demonstrate that we develop our products based on decades of experience not only for current customer requirements, but with a view to the requirements of the future, which means we are aiming at maximum efficiency," says Lars Kräft, Head of ZF's Rail Drive Systems product line. "This first multispeed transmission for Electric Multiple Units is in all respects a trendsetter – from the hardware and actuators to the sensors and the intelligent control electronics enabling completely new functions."
All good things come in pairs
Having conducted extensive simulative tests, the ZF engineers were certain that the ambitious efficiency targets cannot be achieved with conventional EMU drives and their fixed gear ratio. Instead, a new drive concept with two gears – more precisely, with an additional starting gear – proved ideal for the task. The resulting advantages of Get2 are particularly noticeable in all setting off processes: The multispeed transmission continuously moves the load point in such a way that the electric motor and converter always work in a favorable efficiency range and, thus, consume less energy. At the same time, this provides greater tractive power while offering the same drive power. Therefore, the ZF innovation is particularly well suited for metro applications. Here, the distances between the stops are short – usually less than a kilometer – and the train has to decelerate and set off quite frequently along the route. Thanks to its two gears alone, the transmission allows for energy savings of up to five percent compared to one of the most advanced current metro drive systems. What is more, the optimized velocity profile realized in conjunction with the additional starting gear can result in significant additional energy savings.
Additional potential – also for applications with a large share of driving at constant speed – is offered by the so-called multiple traction system made by ZF. It comprises an intelligent shifting strategy which optimally exploits the full potential of the drive and its actuators, which are either controlled by compressed air or electromechanically. Here, it was possible to draw on existing ZF competence from other transmission applications. That way, each unit at all of the vehicle's driven axles can be shifted automatically. Thanks to the smart serial sequence, there are no noticeable tractive force interruptions – the train accelerates without any jolts. For that reason, and because Get2 changes gears very quickly, it does not only offer increased efficiency, but also the very high level of comfort known from other EMU drives made by ZF.
Thanks to additional functions, the energy consumption can be reduced even further: The new EMU drive system does not only feature two gears, but also a neutral position. The latter is activated for one or several drive units when driving at constant speed to enable coasting: All drives that are not required for maintaining the velocity are decoupled and, therefore, do not cause any losses. On the other hand, when decelerating, the multi-speed concept allows for the use of the respective optimal recuperation point of the traction machine. As a result, it is easier than before to recuperate energy. The ZF engineers have even optimized the transmission housing: It is manufactured using an aluminum casting process, resulting in reduced weight, while at the same time it features high load capacity and unchanged compact dimensions. This forms the basis for the future integration of the innovation prototype – with modular adjustments – into a great number of current bogies.
Advantages carried over
In addition, the Get2 concept includes the advanced technologies which are already successfully used in day-to-day operations in the latest generations of ZF's EMU drives. Thrust cones, for instance, reduce the load on the bearings and housing compared to conventional solutions. More specifically, these are rings that are located on the left and right sides of the drive pinion and slide on an oil film at the output gear. This lateral guidance guarantees that axial forces from the gearing do no longer take effect on the bearings and the housing. This in turn allows for the use of cylindrical roller bearings instead of tapered roller bearings for the input and output shafts. The former are setting-free, very temperature-resistant, and make assembly especially easy for service personnel, for instance in case of general inspections, thus leading to excellent cost-effectiveness for transport companies.
In the world's first multispeed transmission concept for EMU applications, ZF has pooled its entire driveline technology know-how for rail vehicles, thus demonstrating the huge potential for significantly reduced consumption and low life cycle costs without having to cut back on other features: Get2 also offers comfort, reliability, a long service life, and easy maintainability.