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The perfect rotor shaftthyssenkrupp takes an innovative approach
As the heart of the electric machine, it converts electrical energy into kinetic energy and transmits it to the drive train. Its design determines the speeds and torques at which an electric motor can be operated. In developing the multi-part rotor shaft, the experts were able to draw on valuable expertise from a completely different drive technology: their experience with assembled camshafts for internal combustion engines.
From camshaft to rotor shaft
The camshaft specialists in Eschen, Liechtenstein decided at an early stage to transfer their expertise to rotor shafts for electric motors. Ten years ago, it was still difficult to predict the dynamics that the e-mobility market would develop from the 2020s onwards. But, as with camshafts, the focus for rotor shafts was on weight optimization and maximum cost efficiency. The key difference was that rotor shafts had to withstand significantly higher torques and speeds. While a camshaft rotates at only half the speed of the crankshaft – around 3,500 rpm on average – rotor shafts were already expected to reach speeds of 20,000 rpm at that time. This represents an enormous load, and it is by no means the end of the development targets: today’s trend is already moving towards 25,000 rpm.
Ten years ago, rotor shafts were predominantly designed as solid components. They transmit the kinetic energy generated by the alternating electromagnetic field between the rotor and stator to the transmission. At speeds of up to 25,000 rpm, the torques transmitted are three to five times greater than those of conventional passenger car camshafts. To cope with these loads, load-optimized design and tight manufacturing tolerances are essential – for example, when it comes to the runout of the spline connection.
The thyssenkrupp assembled camshaft
The development of the rotor shaft began around a decade ago. At thyssenkrupp Dynamic Components, the assembled camshaft had already established itself as an alternative to both cast and forged camshafts. Its key advantage: different materials can be combined within the complex component according to the loads they are required to withstand. This means that the material used for the entire shaft does not have to be selected based on the component subject to the highest loads.In assembled camshafts, only highly stressed components such as the cams are made of high-strength steel. For other areas, completely different material properties can be prioritized. This increases flexibility in material selection, resulting in cost and weight benefits as well as more flexible manufacturing options.However, mastering this technology – particularly the joining of different materials and components – requires specialized expertise. Here, the engineers were able to draw on thyssenkrupp Dynamic Components’ extensive in-house expertise and established manufacturing processes.
We quickly came to the conclusion that the requirements for a rotor shaft could be met well with a multi-part design and that this approach would offer a number of advantages. Our experience with assembled camshafts proved invaluable in developing a multi-part rotor shaft for series production. In both cases, the key expertise lies in joining the different materials and components.
Using different materials also offers considerable potential for rotor shafts. Since higher torques occur at the drive end of the shaft around the splines, these loads can be effectively counterbalanced by selecting materials with a higher alloy content. The tube and bearing flange at the opposite end can be made from materials with a lower alloy content.
Hollow design enables additional functions
Another key advantage of the multi-part design is that the shaft can be made hollow. In particularly powerful electric motors, the hollow space can be used for additional functions. The engineers at thyssenkrupp use it for cooling: injected coolant is forced against the inside of the shaft wall by the centrifugal force generated as the shaft rotates. Heat is transferred by convection from the rotor shaft to the coolant, thereby cooling the electric motor. The coolant is then discharged from the shaft and cooled down in a circuit via a heat exchanger.
One-piece hollow rotor shafts can also harness centrifugal force for cooling. However, the same principle applies to this design as to one-piece camshafts in internal combustion engines: their design is inevitably determined by the most highly stressed area. This dictates the material grade for the entire shaft, leaving no room for flexible, application-specific material selection. Multi-part shafts, by contrast, allow materials to be selected according to the requirements of individual components – creating cost advantages.
From pilot project to series production
The series development was driven forward through close collaboration between the sites in Eschen (Liechtenstein), Ilsenburg and Chemnitz, together with component and equipment suppliers and in line with the OEM’s specifications. Following delivery of the required prototypes and qualification of the product, the production lines were set up and series production was launched first at the Chemnitz site. Follow-up projects have since been launched in Ilsenburg. In 2021, the company group was renamed thyssenkrupp Dynamic Components as part of the expansion of its product portfolio.
The successful market launch of the first multi-part rotor shaft produced using the thyssenkrupp Presta process has given rise to two further initiatives. First, an increasing number of inquiries from OEMs have led to new rotor shaft projects. These projects are aimed at further expanding our expertise and experience while investigating and implementing innovative manufacturing technologies.
From shaft to rotor
In addition, the research and development team at thyssenkrupp Dynamic Components has expanded its focus from optimizing the rotor shaft to developing the assembled rotor. Particular challenges include securing the lamination stacks to the rotor shaft and the magnets within the lamination stack.
Another challenge in the production of assembled rotors arises from customer requirements regarding permissible residual unbalance and balancing quality. These requirements are driven, on the one hand, by the high rotor speeds, which affect bearing service life and, consequently, the service life of the electric motor. On the other hand, unbalance causes the electric motor’s dynamic overall system to vibrate. The intensity of these vibrations and their propagation as airborne and structure-borne noise have a significant impact on driving comfort. With the elimination of the internal combustion engine as a source of masking noise, it can be expected that the permissible limits for airborne and structure-borne noise will become even more stringent.
Martial Danthois: “Our ambition is to further optimize our existing series production processes. At the same time, we are also working on completely new manufacturing processes that will enable us to further increase productivity while reducing scrap and rework.”