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Three historical aerospace examples illustrate the common theme of hi/σi. They also reveal many important lessons learned.


The mechanical components of recent high efficiency geared-fan gas turbine engines are lubricated with low viscosity synthetic polyol ester oils designed to withstand the thermal environment created by the engine core. The limited hi from low viscosity oils for film thickness is offset by high speeds and superfinished surfaces (σi). This is particularly true for the high-speed planetary gears. High gear and bearing efficiencies are achieved through low interface EHD film traction (hi) enabled by high interface film temperature. The design and implementation success through the technology supply chain was a 25-year venture.


Rotorcraft drive systems, which operate with the same or similar polyol ester oils, cover both high and low speeds. While component engineering design is focused on stress, interface tribology mechanisms are mostly motion-driven. The tribology challenge of rotorcraft drive systems is frequently on the low speed, high torque end, where the bearing/gear components are large. Low viscosity qualified oils operate with extremely thin films (h) relative to surface roughness (σ). While interface component temperatures are not high, the precision manufacturing of large bearing and gear surfaces is challenging for thin-film operation. Through extensive trial and error exploration, operational success for on-condition overhaul is achieved by developing run-in polishing (RIP) attributes of the contacting materials, i.e. the fulfillment of a tribology functional requirement through σi. While a successful hi/σi outcome was achieved, it consumed considerable time and cost. Both time and cost were investments by the OEMs and suppliers.


The third example, Space Shuttle Main Engine (SSME) turbopump bearings, utilises liquid oxygen (or hydrogen) as the working fluid. For the first fifteen years of the SSME history, the turbopump bearings had to be replaced after every launch due to excessive wear. More than a decade of NASA funded research was not able to solve the tribology interface problem (i.e. hi/σi). Out of desperation, and the reliance on a hint of tribology cryogenic testing success, the bearing material pair was changed to a hybrid using silicon nitride (Si3


N4 ) balls. The combination of


precision finishing (σi), thermal management to keep the cryogenic fluids liquid and at low temperature


18 LUBE MAGAZINE NO.172 DECEMBER 2022


Figure 2: Lubricated Hertzian contact illustration identifying four ingredients operating with entraining motion Ue


= ½(U1 +U2 ) and sliding motion Us to achieve interface hydrodynamics (hi) and surface integrity (si) = U1 - U2


for some degree of working rheology, along with exceptionally high speeds, created sufficient hi for operational success. The tribology outcome resulted in the ability to conduct ten missions before bearing removal and inspection. Subsequent inspections revealed essentially no measurable wear. While hi/ σi is the common theme here, the most significant lesson learned is that success was achieved in a matter of weeks. This was accomplished though special testing capabilities, manufacturing specialties, OEM leadership, and complete engagement of the entire technology supply chain.


Tribology crisis and Technology Readiness Level (TRL) From the early days of gas turbine development through the 1990s significant advances were made in bearing/gear steels and high temperature synthetic oils for engines and transmissions. Steel manufacturing to produce metallurgically “clean” bearing steels, like through-hardened M50 and case hardened M50 NiL, provided bearing life and fracture toughness for high-speed operation. Phosphate based anti-wear additives and additive concentrations provided good boundary lubrication. Interface tribology success was achieved through hi/σi. Subsequent performance demands for higher temperatures, improved bearing material rolling element fatigue life, and to some degree corrosion resistance, soon resulted in parallel and independent initiatives for alternate steels and higher thermal stability oils. With insufficient test and analysis tools at that time, new oil formulations had to be pulled out of service and new engine designs, based on expectations of advanced bearing steels, had to be redesigned.


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