# Numerical and Experimental Analysis of the Oil Flow in a Planetary Gearbox
## 3 Results and Discussion
### Details of the Oil Flow in the Planetary Gearbox  
To gain deeper insight into the oil flow inside the gearbox, 3D isometric views are reported for the analyzed operating conditions. The gearbox housing was made transparent to enable access to the internal fluid domain region. In Figure 12, the oil distribution is shown for the analyzed operating conditions. In the top and in the bottom row of the figure, the low and the high filling level cases are shown, respectively. Near each result, the corresponding operating condition is indicated in a box on the bottom right. Figure 11: Experimental (**left**) and CFD (**right**) comparison of the oil flow for SIM-id2v3. For the low filling level cases (top row), the oil is dragged from the sum and delivered to the other parts of the gearbox. As the rotational velocity increases, more oil participates to the lubrication and the ring gears receive a greater amount of oil. For the high filling levels (bottom row), oil rings also originate around the planet gears, as illustrated in Figure 12. These are generated by the roto-translation of the planets which are completely submerged when they are below the centerline of the gearbox. As they move above the centerline, the oil remains entrapped between the planets and the planet carrier, and the roto-translation of the wheels promotes the formation of these additional oil rings. The faces of the gears remain wet, and the cylindrical shape of the gear shafts contribute to the rotatory movement of the oil around the wheels. The squeezing effect that can be seen in all cases derives from the mutual interaction between the planets and the ring gears. The squeezing of the oil from the planet-ring gear meshing zone was also observed by [PERSON] et al. [8]. The CFD simulations provide a physical explanation for these axial fluxes. Indeed, while the gears engage, the gap between the teeth reduces and increases continuously during the gear mesh. The sudden contraction of this volume implies an overpressure in the gap that gives origin to axial fluxes. After reaching the minimum value for the volume, the volume of the gap suddenly increases again causing a lower pressure in the gap (it should be considered that the pressure drop is significantly smaller than environment pressure of 1 bar, and, therefore, this pressure gradient will not lead to phase exchanges, namely, cavitation). It should be highlighted that the calculation of the pressure gradients due to the volume variation during the gear meshing with the present model is not representative of the pressures on the mating flanks near the contact area. These considerations are confirmed in literature [37; 38; 39; 40; 41]. In Figure 13, the pressure contour in the mating region that justifies the oil squeezing is reported. The 2D representation shows the slice taken in the center of the middle gear. In Figure 14, the evolution of the oil flow at different timesteps during the planet carrier rotation is illustrated for SIM-id2v3. From the standstill position (A), the planets gradually enter in contact with the oil in the sum (B, C, D). The oil is then distributed circumferentially along the ring gears (E, F). At this velocity, the planet carrier rotation discards the oil radially towards the housing. The oil that remains in the sum is recovered by the wheel's passage during the motion and is continuously brought to the other parts of the gearbox. Figure 12: Oil distribution inside the gearbox for the analyzed operating conditions. Top row: low filling level cases. Bottom row: high filling level cases. Axial flows (squeezing effect) and oil rings around the planets are highlighted. In this section, a better insight into the lubrication in the planetary gearbox was provided. The oil squeezed between the planets and the ring gears creates axial flows that meet the oil coming from the planet carrier rotation, confirming what was observed by [PERSON] et al. [8]. Additional oil rings around the planets were detected.