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.

The complexity of thermal elastohydrodynamic lubrication (TEHL) problems has led to a variety of specialised numerical approaches ranging from finite difference based direct and inverse iterative methods such as Multilevel Multi-Integration solvers, via differential deflection methods, to finite element based full-system approaches. Hence, not only knowledge of the physical and technical relationships but also knowledge of the numerical procedures and solvers is necessary to perform TEHL simulations. Considering the state of the art of multiphysics software, the authors note the absence of a commercial software package for solving TEHL problems embedded in larger multiphysics software. By providing guidelines on how to implement a TEHL simulation model in commercial multiphysics software, the authors want to stimulate the research in computational tribology, so that, hopefully, the research focus can be shifted even more on physical modelling instead of numerical modelling. Validations, as well as result examples of the suggested TEHL model by means of simulated coefficients of friction, coated surfaces, and nonsmooth surfaces, highlight the flexibility and simplicity of the presented approach.

Fluid friction in elastohydrodynamically lubricated (EHL) contacts depends strongly on the lubricant considered. Synthetic oils can have significantly lower fluid friction than mineral oils. Water-containing fluids have the potential to significantly reduce fluid friction further. The aim of this study is to investigate the film formation and frictional behavior of highly-loaded EHL contacts with water-containing fluids. Comparisons are made with mineral and polyalphaolefin oils. Measurements at an optical EHL tribometer show good lubricant film formation of the considered water-containing gear fluids. Measurements at a twin-disk test rig show coefficients of friction smaller than 0.01, which is referred to as superlubricity, for all considered operating conditions.