Gears and bearings: how a tooth survives pressure that should squeeze out any oil film
Contact pressures of 0.5 to 3 GPa and more. Elastohydrodynamic lubrication is why the film survives, and why heat and water still matter.
The problem
A rolling bearing or a meshing gear tooth does not get the generous converging oil wedge of a plain bearing. The contact patch is tiny and the load sits on a sliver of surface, so the pressure is enormous, commonly 0.5 to 3 GPa and more. By ordinary intuition that should squeeze the oil out completely. It does not, because of a different regime: elastohydrodynamic (EHD) lubrication.
Two things happen
- The metal is not perfectly rigid. It deforms elastically, flattening the contact into a small flat patch and spreading the load over more area than classical contact theory alone suggests.
- The oil's viscosity does not stay constant under pressure. It rises sharply, following the Barus relationship known since 1893. Around 1 GPa (about 145,000 psi) that is a millionfold to billionfold increase for the fraction of a millisecond the oil spends in the contact. At 2 to 3 GPa (290,000 to 435,000 psi) the oil behaves like a glassy, solid-like material.
How the theory developed
A.N. Grubin gave the first approximate solution combining elastic deformation with hydrodynamic theory in 1949. D. Dowson and G.R. Higginson built the fuller numerical theory through the 1960s and published their book in 1966, including a minimum film thickness formula that is still used.
A counterintuitive result
EHD film thickness depends only weakly on load, scaling roughly with load to the power of minus 0.13. Doubling the load thins the film by only about 9 percent. Speed and viscosity matter far more than load. That is the opposite of a plain bearing, where load has a much larger direct effect.
Not bulletproof
The film is thin enough to be breached at individual asperity tips. That is why gear and bearing lubricants still carry anti-wear (AW) and extreme-pressure (EP) additives: not to replace the EHD film, but to back it up at the microscopic points where it is occasionally not enough.
What happens to the heat
The oil as a whole may run only 20 to 60 degrees F (about 11 to 33 degrees C) above normal in a gearbox. But the instant two high points actually touch, friction creates a flash of heat at that single point, hundreds of degrees hotter for a fraction of a second. That is a big part of why gear teeth scuff and scar.
And a stray drop of water
Water barely compresses, so it does not flash-boil. What seems to happen is worse: the water reacts with the bare metal exposed by the rubbing and releases hydrogen, which works into the steel and makes it brittle from the inside. This is a leading suspect behind a failure mode called white etching cracks, common in wind turbine gearboxes but not limited to them. It is not fully settled science, but it is documented well enough that keeping water out of the oil is worth taking seriously.