When is an Oil Not an Oil?

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Engineers were entranced with the physics of full-fluid-film lubrication of the kind that makes the plain bearings in modern engines workable. They weren’t so happy when they tried to apply the same physics to the lubrication of power gearing. Their tried-and-true equations indicated that such gearing, operating at very high tooth-to-tooth pressures, couldn’t possibly work. Metal-to-metal contact should occur, causing rapid wear followed by early failure.

But gears do work. Well-made, surface-hardened steel power gearing survives over long periods of operation at tooth-to-tooth pressures as high as 100,000 psi. In any modern motorcycle engine, power passes through a pair of primary gears from crankshaft to gearbox input shaft, and then through any of the six pairs of gears in the usual 6-speed gearbox to the output sprocket or shaft. I’ve never even seen a primary gear failure (other than one caused by loss of lubricant).

A 1949 paper by Russian scientist A.N. Grubin described what he called an “elastohydrodynamic” effect (EHD), with a very thin layer of lubricant somehow trapped between the elastically deforming surfaces of hard gear teeth. What could possibly cause a liquid to remain in such a place, rather than be instantly squirted out?

Pressures between transmission gear teeth can reach 100,000 psi.

Pressures between transmission gear teeth can reach 100,000 psi. (Jeff Allen /)

The colors we see when oil forms a film on the surface of water come from interference, as the varying thickness of the film cause some light frequencies to interfere with each other as they are reflected from the top and bottom of the oil film. The same effect, but caused by transparent surface oxide layers, produces the color bands seen on welded titanium exhaust pipes. This same effect was used at Imperial College, London, to measure the elastohydrodynamic oil film between surfaces heavily loaded against each other. Researchers found film thicknesses of 0.1 to 1.0 micron.

How thick is a mono-molecular layer of oil? A classic experiment attributed to the ever-curious Benjamin Franklin provides the answer. Dusting a non-wettable powder over the water surface allows the experimenter to easily see the clear area covered by a spreading oil film after placing a drop on the water’s surface. Once it has spread as far as it can and has a uniform appearance it is just arithmetic to compare the volume of the original oil drop (measure its diameter just before putting it on the water) with the volume of the film, which is just its area multiplied times what we want to know, which is its thickness. In one case, the answer was .017 micron.

Comparing that with the EHD film thickness range measured at Imperial College, we find this to be between 6 and 60 molecular thicknesses.

Next question: what can possibly allow that oil to remain between two surfaces that are exerting such tremendous pressure to squeeze it out? One way to look at this would be to say that the viscosity of the liquid is somehow hugely increased by such great pressure. The calculation showed that the oil’s viscosity had increased by roughly five orders of magnitude (that is, by a factor of 100,000, or ten to the 5th power).

We know that hydrocarbon molecules take the form of long chains of carbon atoms, each one bonded to hydrogen atoms. For hydrocarbons in the lubricating oil range, there are roughly between 20 and 70 carbon atoms, making the chains quite long. What has been discovered is that under great pressure, oil molecules self-align to become parallel with each other. As this happens in oil trapped between hard surfaces, their interaction with each other increasingly assumes the nature of a crystalline solid. In a crystal, the atoms form highly regular ranks and rows, each bonded by electrical forces to its neighbors. Something similar happens when rubber is greatly deformed – its long chain molecules align with one another and may be attracted to each other by short-range forces: the rubber now assumes the properties of a crystalline solid.

Computer simulations of molecular-level oil behavior in such strange conditions indicate that the only way any individual molecule can escape the zone of high pressure is end-wise – a difficult process. A witty researcher has given this the name “reptation” (snaking).

This molecular alignment and confinement seems to be the mechanism that allows heavily loaded gears to survive through long and productive lives.

On the two occasions when I’ve ridden in helicopters (It was a John Wayne/LAX commuter service) I’ve found myself thinking about the roughly 100-to-one rpm reduction from the fast spinning turbines to the main rotor, beating its way through the air at 300 or so rpm. Helicopter gearboxes are highly refined pieces of machinery.

Good thing oils just happen to approach a solid crystalline state as they get pinched between the huge number of gear teeth that serve the human project.

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