While the quantity of gears within drive trains for e-mobility has drastically decreased compared to drive trains for vehicles with combustion engines, their design has changed significantly and alongside the geometrical tolerances were massively sharpened. The reasons for this are twofold: With the omission of the noise emission of the motor, the gearboxes’ acoustic behavior moved into focus. Furthermore, the range of electric vehicles became an important selling argument, which is why the transmission efficiency of drive trains considerably gained in importance.
New requirements and challenges
There are a number of changes in requirements with regard to the drive components of electric vehicles:
Reduction of noise emissions
Due to the elimination of masking noise from an internal combustion engine, noise emissions from the transmission can dominate in the low-speed range.
Higher Loads
The gears in the transmission of electric vehicles are under different and potentially higher loads, as they generally only have one gear (and two gear stages). In addition, electric motors have a different speed/torque curve (already high torques at low speeds).
Higher efficiency
The efficiency of an electric motor is significantly higher than that of an internal combustion engine. This contributes to the great relevance of the transmission’s efficiency.
Complex contact mechanisms
In order to meet the above-mentioned requirements, the different mechanisms occurring during the gear-tooth contact within the transmission must be considered. These include the material fatigue, excitation, and tribological behavior of the gear pair. These contact mechanisms are influenced by the shape deviations of different orders (First order: form deviation, second order: waviness, third and fourth order: roughness, fifth order: microstructure).
Lightweight construction
In order to be able to map the high-speed ranges of transmissions for e-mobility drive trains, lightweight construction of gears becomes more common. This results in challenges with regard to heat treatment related distortions as well as workpiece clamping.
New measuring values
In roughness measurement, “new” characteristic values are increasingly being used, some of which are only defined by factory standards.
Achievable quality by different finishing technologies
The consequent increase in demand for smoother surfaces on tooth flanks leads to a wider use of surface finishing technologies such as fine grinding or polish grinding. To maximize the productivity, these finishing tools are usually used within a hybrid tool concept that comprise different zones for roughing and finishing enabling a successive and complete machining by tangential shifting. While surfaces manufactured by fine grinding still exhibit a structure comparable to conventionally ground surfaces but with a much lower amplitude, polish ground surfaces almost entirely lack roughness peaks.
Challenges of polish grinding technology
The distinctive structure of polish ground surfaces is caused by the elastic nature of the often-used polyurethane- bound grinding tools. While the structure of the surfaces manufactured with these tools seems to be preferable in comparison to fine grinding, the elastic behavior of the tools poses great challenges for meeting the tighter tolerances regarding form deviations. Geometrical effects such as dents, rounding of the tooth tip, constrictions around the tooth root, or waviness are typical for polish grinding.
Adapting the polishing pressure can improve the surface quality and reduce geometric anomalies. While a reduction of the center distance between tool and workpiece leads to a global increase in polishing pressure, it is often necessary to modify the polishing pressure in a local manner to avoid the mentioned geometric anomalies. Furthermore, the quality of the ground tooth flanks, which represents the initial condition of the polishing process, must never be left unconsidered in the design of the polish grinding process or when analyzing geometric deviations.
Challenges of measurement technology
Achieving new levels of surface quality while also meeting the requirements regarding form deviations does not only challenge manufacturing technology but is also demanding in regard to measurement technology. In this context, conventional values for describing surface roughness such as Ra and Rz as well as form deviations such as ffα and ffβ lose their significance. Therefore, values derived from the Abbott-Firestone curve for surface description and waviness analyses come to the fore when characterizing the quality of gearboxes for electromobility.
Optimization strategies
In the past years, KAPP NILES has developed several strategies as to how machine, software, tool, and process design must be adapted to reach an optimal balance between productivity, surface quality, as well as form deviations within e-mobility applications. Prerequisites for achieving the high requirements include a highly precise machine tool and a good optimization of the dynamic behavior of its drives and axes as well as a high-quality dressing tool design.
Furthermore, in order to meet the challenges described above, software solutions for process optimization have been developed.
To account for deformation effects of the elastic tools, different correction options contribute to achieving a uniform polishing pressure. First, the manipulation of the axis distance can increase or decrease the global polishing pressure. As a result of large axis distance corrections, the polishing pressure at the root section can exceed the critical limit leading to constrictions within the root section. A reduction of tooth height helps avoiding this pressure excess at the tooth root and thus, minimize form deviations. To equalize polishing pressure and produce an equal surface roughness on both flanks, a stock division function is available. Furthermore, pre-loading correction as well as a zone-dependent adaption of an fHα-correction can contribute to a consistent grinding process along the tooth profile.
Not only corrections during grinding are essential for an optimal grinding process, but also the dressing process of the grinding worm. Corrections, such as an fHα-correction, can be set zone-specifically between the conventional and polished part. The dressing conditions (speed ratio and feed rate) can also be set zone-specifically to ensure optimal machining conditions. When developing these functions, it was always a priority to provide intuitive input options so that end users and machine operators achieve the desired quality as easy and fast as possible.
