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Three main concerns are in the focus of the development of geared transmissions nowadays: load carrying capacity, noise-vibration-harshness (NVH) behavior, and efficiency. Increasing the efficiency of modern gearboxes contributes significantly to the reduction of energy consumption and the saving of resources. Gearboxes are frequently designed conservatively with an oversupply of oil to guarantee operational reliability. An oversupply of oil results in an unnecessarily high amount of oil kept in motion and to high no-load losses. Detailed information on the oil distribution in the early design stages of gearboxes would help to optimize the lubrication and to increase the efficiency. Thereby, CFD (computational fluid dynamics) methods offer a very flexible way to visualize the oil flow inside gearboxes with much fewer restrictions compared to measurements with transparent gearbox designs. In this study, a verified CFD model based on the finite volume method is used to investigate the oil flow in a single-stage gearbox. Different oil viscosities and circumferential speeds are considered. The investigations focus on the oil flow. The gear churning loss, as part of the no-load loss, is additionally considered. Experimental validation is obtained by high-speed camera recordings and measurements at the FZG no-load power loss test rig. The results show very strong agreement between simulation and measurement. The results show that CFD simulations can visualize the oil flow behavior with a very high degree of detail.

Sufficient oil supply of all machine elements in gearboxes is usually required to avoid damage during operation. Quite frequently, transmissions are conservatively designed with an oversupply of oil to guarantee operational reliability. An oversupply of oil results in an unnecessarily high amount of oil being kept in motion, which in turn leads to excessive hydraulic gear power losses. In high-speed gearboxes in particular, churning losses can contribute greatly to the total power losses. Further detailed information on the oil distribution in gearboxes is needed in order to increase the efficiency and operational reliability of gearboxes. Computational Fluid Dynamics methods provide a flexible way of investigating oil behaviour in transmissions with almost no restrictions regarding geometry and operating conditions. Generally, there are two main methods of computational fluid dynamics simulation in gearboxes: the traditional finite-volume based method (Eulerian approach) and the mesh-free particle-based method (Lagrangian approach). In this work, a computational fluid dynamics model based on the particle-based smoothed particle hydrodynamics method is built to investigate the oil distribution and churning losses of a dip-lubricated single stage gearbox on an efficiency gear test rig. Results are shown and discussed for different rotational speeds and oil temperatures. The smoothed particle hydrodynamics method provides a high potential of predicting the oil distribution of modern dip-lubricated transmission systems. Comparisons with high-speed camera recordings show good agreement. However, the method shows a need for improvement in churning loss prediction.

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The topic of this study was subject to the efficiency calculation of gearboxes. Additional analyses were performed with regard to an existing calculation approach, to improve the calculation of load-dependent gear losses by so far disregarded influences. On the one hand, the influence of the driving direction was investigated. During this, the driving and driven test gears considered showed high differences of the specific sliding at tooth root and tooth tip. On the other hand, the influence of the change of contact ratio under load was studied. Both influences showed considerable influence on the load-dependent gear losses. An equation for an optimized calculation of the load-dependent gear losses is given.