Ensuring optimal gearbox lubrication

As critical transmission element, gearbox must be designed to ensure optimal lubrication of the engaged gears. To design innovative solutions, for which neither empirical expertise nor experimental testing can provide worthwhile results, numerical simulation is greatly needed.

Your application

A gearbox adapts the power generated by the engine before delivering it to the wheels. It can, for instance, increase the torque by reducing the rotational speed. This power is conveyed by the gears, which are subject to high speeds and considerable strains. Without proper lubrication, friction would cause mechanical losses and, above all, rapid and premature wear.

Gearbox designers must ensure optimal lubrication of each of the numerous gears. The challenge is especially complex when considering the wide range of operating points – involving shaft speed, engaged gear, oil temperature, vehicle inclination, etc. – and the multiple design constraints – such as maintenance, cost or durability.

To develop suitable designs, traditional tools consist of empirical expertise on conventional configurations and of visualizations through the transparent walls of an experimental prototype. The former is not sufficient to design innovative gearboxes. The latter, despite its cost and complexity, is only partially satisfactory: oil sprayed on the gearbox casing is often so scattered that no relevant lubricating flow can be observed.

CFD has been the much-needed solution for designing various sophisticated applications. However, standard solvers struggle with gearbox lubrication. The problem involves simultaneous phenomena which can be complex to simulate, such as atomized sprays, viscous film, complex moving gears.

Our solution

Splash lubrication

The particle-based Lagrangian SPH-flow solver offers key features and characteristics that address the challenge of simulating gearbox splash lubrication. Beyond its ability to account for viscosity, its ability to inherently track free surfaces makes all the difference. It can properly describe the essential scattering of the fluid into multiple droplets, pulled off from the lubricant pool by spinning gears. The possible coalescence of those droplets into jets and their behavior during splash-landing is also well captured.

We have gained long-term experience in the simulation of gearbox lubrication. For years, half a dozen gearbox designs have been studied. Starting from elemental geometry of simplified splash lubrification, the key physical phenomena have been addressed using advanced SPH-flow numerical models.

For example, the influence of the air flow on the oil has been investigated, mono-fluid and bi-fluid simulations were compared and analyzed on an elementary 2-dimensional single-gear circular configuration (only the oil phase was computed for the mono-fluid case, while both the oil and air phases were accounted for in the bi-fluid simulation). The corresponding algorithms have been improved, and their suitability was validated

Based on such solid foundations, we have tackled more realistic designs, including industrial 6-speed manual transmission. A numerical probe, measuring the mass flow rate, was used to assess the relevancy of the simulation result. The versatility of SPH-flow allowed a fast 2-hour setup. Moreover, this easy configuration also allowed for flexible methodologies: for instance, the flow rate registered by the probe was re-used, as an input parameter, to set an equivalent inlet for a new computation. The downstream part of the flow was thus specifically studied without the need for full domain computation.

Spray lubrication

Beside splash lubrication, spray lubrication was also addressed. Although not applied to a gearbox but to an electric engine, the lubrication issue is similar in many aspects. The wettability of the solid can be monitored, time after time, as can other near-wall quantities like pressure, temperature or shear-stress.

We have addressed many other gearbox lubrication issues, including sensitivity to leakage, thermal influence, lubrication in planetary gear trains, etc.

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