ELECTRIFICATION TRENDS: As a result of public regulations concerning the output of carbon dioxide, e-Drive applications are becoming increasingly important. However, electrified transmissions are also subject to several challenges and requirements. What is true is that the number of gear wheels is significantly reduced in electric vehicles due to the use of one or two-speed transmissions instead of the classic manual or twin-clutch gearboxes, Fig. 1. In return, these are loaded with torque and rpm not previously found in high-volume production. 14.000 to 16.000 rpm will be normal; 30.000 rpm are under discussion, also. Even if the gear box complexity is reduced with an increasing degree of electrification, current e-Drive concepts show up with stronger performance and lightweight requirements. Additionally, leaving the combustion engine as primary noise source requires a detailed look on the NVH (Noise Vibration Harshness) behavior of such transmissions and gears.
Figure 1: Electrification will change transmission design
SELECTING OF PM TECHNOLOGY AND DEMONSTRATOR: We at GKN Powder Metallurgy strongly believe that surface densified transmission gears can make the difference in the future mobile world and that PM technology is able to provide significant benefits on performance and NVH (Noise Vibration Harshness). While a ß = 34° surface densified helical transmission gear for a 6-speed manual transmission has recently been fully validated, the motivation of our engineers was high to apply this technology also for e-Drive applications. Together with our colleagues from GKN Driveline an intermediate gear of a generic e-Drive rear-axle gear box was selected, which is in terms of excitation one of the key “noise-drivers” of the system. Tasks the team gave themselves for the “PM inside” e-Drive transmission within a phased approach were first a proof of concept for the NVH improvement and afterwards a full system validation in terms of performance and NVH.
PROOF OF CONCEPT: After the project kick-off, a significant influence on the transmissions noise-behavior could be figured out - just by replacing the steel intermediate gear with a not densified PM gear at a density of 6.8 g/cm³, as shown by the Campbell diagrams in Fig. 2. Additionally, for single rpm/loading conditions up to 3dB lower structure borne noise could be evaluated.
Figure 2: NVH potential of Steel vs. PM Gear (FFT analysis)
Motivated by these results and having the task to meet the performance of the challenging application, GKN’s cross-divisional team has developed an intermediate e-Drive gear, compacted off-tool and surface densified (rolled). Based on our experience on surface densified manual transmission gears, the core density for the off-tool gears was moved from 6.8 g/cm³ to 7.15 g/cm³. After case hardening and hard finishing, the gears have been applied to a system based durability test with a typical customer oriented load-collective, Fig. 3.
Figure 3: Durability tests of PM e-Drive Gear, GKN Driveline Lohmar
We can state today that the endurance performance of the PM gear is comparable to a steel gear, which will allow us to go for the first potential product application. Knowing that these initial system tests were made with gears having a not yet optimized micro-geometry, it was an even bigger success to reach with this first shot directly the lower NVH or noise tolerance of the steel gears equivalent.
THE FUTURE OF PM GEARS STARTED ALREADY: Overall, it is clear that a PM gear design approach needs to be holistic, starting from understanding the products NVH requirements and merging the benefits of PM technology with modeling and testing support on all levels, Fig. 4. Knowing that the performance requirements for this type of transmission gears requires surface densification and PM technology allows to apply individual design features of the gear body, further investigations on evaluating the NVH characteristics and optimizing the NVH behavior of sintered and surface densified gears are ongoing. For example, in 2016 we started to develop a new FEA based approach to design surface densified gears, which allows to tailor the micro geometry of a gear by considering the density gradient and applying tooth contact analysis for dedicated loading conditions. Validating and quantifying the NVH improvements is done either on the newly installed NVH-component test bench at GKN Sinter Metals Innovationcenter or on system level, as shown exemplary below.
Figure 4: Strong alignment between gear design and system testing
So, please keep informed, “The best is yet to come”.
Björn Leupold, Manager Advanced Gear Programs, GKN Sinter Metals Engineering GmbH, Radevormwald
Vitali Janzen, - Manager Advanced NVH Development, GKN Sinter Metals Engineering GmbH, Radevormwald
Gerd Kotthoff , Director Advanced Gear Technology, GKN Sinter Metals Engineering GmbH, Radevormwald