Oil-out conditions, or conditions
in which an aircraft is
operating without any oil in its
gearbox or transmission, are
devastating for an aircraft's
hardware. Even the sturdiest gears
usually can't last 30 minutes under such
conditions before they catastrophically
fail, and the whole system usually follows shortly after. That doesn't leave pilots with a whole lot of time to find a suitable location to land in the case of an oil-out emergency.
This paper addresses the lubrication of helical gears - especially
those factors influencing lubricant film thickness and pressure.
Contact between gear teeth is protected by the elastohydrodynamic
lubrication (EHL) mechanism that occurs between nonconforming
contact when pressure is high enough to cause large
increases in lubricant viscosity due to the pressure-viscosity
effect, and changes of component shape due to elastic deflection.
Acting together, these effects lead to oil films that are stiff
enough to separate the contacting surfaces and thus prevent
significant metal-to-metal contact occurring in a well-designed
gear pair.