AEROSPACE
New horizons for aerospace tribology
Lavern Wedeven, Pres., Wedeven Associates, Inc., Edgmont PA, USA
The aerospace community has made significant achievements over more than five decades of turbine powered mechanical component developments that rely on tribology technology. These achievements are represented in mechanical system power density, life, and reliability. Powerful lubrication mechanisms working within the confines of micro-scale bearing and gear contact interfaces enable the heavy-lifting and long-haul travel for contacting surfaces to carry loads and transmit power. The journey to this outcome has not been easy. Tribology mechanisms are mysterious. The engineering integration of lubricating fluids, substrate materials, and precision manufacturing has not been harmonious. The journey has been risky, costly, and time-consuming. There are important lessons for the science and technology communities to be learned relating to capturing powerful lubrication mechanisms and avoiding dysfunctional decisions leading to failure mechanisms.
Lessons learned from major mechanical systems in aerospace
Intimate engagement in the development of major mechanical systems in aerospace covering a wide range of applications reveals two common science and technology themes for contact interface
engineering. No matter the application, performance is derived from: (1) the generation pressure within a thin fluid film to accommodate shear with low friction, and (2) the enabling of the hydrodynamic mechanisms in (1) is derived from the surface integrity mechanisms associated with the boundary contact surfaces that carry the load and accommodate the shear. To represent the themes (1) and (2) above, the traditional design parameter for (1) is the calculated elastohydrodynamic (EHD) film thickness, h, and the design parameter for (2) is the combined surface roughness of the contacting bodies, σ. For the future, the traditional lambda ratio (h/σ) used for design is not adequate to represent all the controlling mechanisms. Accordingly, we capture all the hydrodynamic and internal fluid friction (traction) mechanisms with hi. Likewise, the surface roughness σ is expanded to include all the surface integrity mechanisms with σi. While interface surface integrity (σi) includes roughness, it also includes boundary lubrication mechanisms created by the interaction between fluid chemistry and substrate surface chemistry. The engineering design of component interfaces is all about capturing hydrodynamic mechanisms (hi) and surface integrity mechanisms (σi), i.e. hi/σi.
Figure 1: Three aerospace examples of mechanical systems requiring extensive development time and cost to overcome challenging tribology component problems to achieve successful contact interface hydrodynamics (hi) and surface integrity (si).
Continued on page 18 LUBE MAGAZINE NO.172 DECEMBER 2022 17
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