The tribology science behind this painful crisis is associated with the incompatibility and risks associated with the introduction of alternate bearing/ gear steels and oil formulations. This can be illustrated with a Hertzian contact model and its interface ingredients under motion.
The hydrodynamics (hi) for oil film generation and its internal friction (and total interface temperature, Tc
)
are derived from the fluid rheology. Film thickness (h) is easily calculated from EHD theory. With new traction models, the interface oil film and surface temperatures can be calculated. To maintain hi under thin film conditions, requires compatibility of the no.1 ingredient (the fluid rheology and chemistry) with three other ingredients of the interface: no. 2 bounding materials and their surface chemistry, no. 3 boundary lubricating films, and no. 4 surface topography (geometry, roughness, and texture). What makes engineering design and predictability of the contact interface difficult is that the critical ingredient no. 3 for surface integrity is derived from ingredients no. 1 and no. 2 during operation. Interface engineering design must be done “on-the-fly”. Interface tribology performance is all about harmonisation of the interface ingredients for surface integrity (σi) so that hi mechanisms can be enabled to operate appropriately.
Around the year 2000, the realisation of the risks associated with the introduction of independently formulated oils and the separate development of
substrate material ingredients for the contact interface resulted in the introduction of the NASA Technology Readiness Level (TRL) approach for the development of bearing/gear components with high demands for tribology performance. The nine-step TRL approach, which provides successive maturity levels of development, proved to be successful in reducing risk. While technology management felt comfortable with risk reduction, particularly after painful experiences, the impact on cost and time was not helpful. More importantly, time and cost inhibit innovation.
Lessons learned for new horizons in aerospace tribology To rapidly respond to future aerospace tribology demands within the TRL framework in a more cost-effective and timely manner, we can build a foundation based on lessons learned. We have learned that success derived from capturing hydrodynamic mechanisms (hi), which are enabled by capturing the boundary lubricating and substrate shear strength mechanisms (σi). Of particular significance is realising the mechanistic importance of the fluid rheological dynamics while being entrained within and transported through a Hertzian contact interface. The outcome is remarkable film generation to carry loads and accommodate shear to provide low friction (traction). The fluid becomes the lifeblood, and traction the heartbeat. Also, of particular significance is the required mechanistic harmony needed among the interface ingredients
Figure 3: Tribology-by-Design (T/D) roadmap illustrating the theory (MST-m-Tm development of MST-m-Tm
), test/analysis tools, and methodology to enable virtual interface engineering at TRL 4 to rapidly respond to the technology requirements of targeted components at TRL 8. Continued on page 20 LUBE MAGAZINE NO.172 DECEMBER 2022 19
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