PSI - Issue 42

Omar D. Mohammed et al. / Procedia Structural Integrity 42 (2022) 1607–1618 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3.2. Tooth microgeometry asymmetry The gear tooth has two opposite flanks. The first flank, which is called ‘drive flank’, works in the normal load transfer mode and endures a higher load for longer periods than the opposite flank. The second flank on the opposite side, which is called ‘coast flank’, works when the driven gear acts as a driver, e.g in case of releasing the car fuel pedal or driving downhill. The coast flank is then either unloaded or can be loaded with a relatively lighter load and shorter period than the drive flank [2]. The generated misalignment can be also different depending on the engaged gears delivering the load for each of the drive and the coast driving modes. Due to the different applied load conditions between the drive and the coast flanks, the microgeometry design of the coast flank is commonly different from that of the drive flank. This effectively affects tooth durability and PPTE. The different tooth flanks in the micro-scale can entail the benefits of obtaining the best possible results in terms of durability and NVH. In the studied model, the maximum applied load and the generated misalignment are (700 Nm, -0.135 mm) for the coast engagement mode of Gear 1-Ring engagement, while it is (1000 Nm, -0.0062 mm) for the drive engagement mode. Due to this difference, the generated stresses and PPTE show different results, and then a different microgeometry set should be designed for the opposite flanks to fulfil the design requirements for each driving mode. In Figs. 5 and 6b, design A_G1-C involves the same microgeometry set used in the drive flank, while design B_G1-C represents the optimized coast microgeometry set which is designed differently from the drive microgeometry set. Having a special microgeometry design for the coast flank, as presented by design B, will obtain much better results. 3.3. Peak-peak transmission error (PPTE) The target of minimising the NVH to the lowest possible level is one of the most important requirements in gear design. The PPTE acts as an excitation to the NVH of the gear system, so that the gear PPTE must be reduced to meet the NVH requirements. Reducing the PPTE below the target limit is not an easy task to achieve for different torque levels. Furthermore, the trends can be different for reducing the acting stresses and the PPTE. To ensure fulfilling the predefined PPTE requirements, usually another load engagement of different torque and misalignment values is applied to examine the PPTE. In the current research, the load (L2) of 400 Nm maximum torque was applied. In Fig.3, the designs A_G1-D and B_G1-D were studied. For each design two different engagement loads (L1) and (L2) were applied, as stated in Table 3. Every load was divided into arbitrary torque levels, starting from 10% of the applied torque to the full applied torque at that gear engagement load or driving mode. In the current research, 10 torque levels were studied to cover the driving torque span accurately. From Figs.3 and 6a, design A_G1-D shows a good contact pattern of a lower maximum contact stress and lower PPTE at the low torque levels (L2) than design B_G1-D. However, design B_G1-D shows results of PPTE at the high torque levels, are lower than those of design A_G1-D. Designing a microgeometry set for obtaining the lowest possible contact pattern does not necessarily fulfil other requirements of having acceptable root bending stresses and PPTE over various torque levels. Hence, a compromise design should be obtained to meet all requirements.

Fig.2 Microgeometry modifications on a tooth flank