Mysteries of Lubrication

[Admin]

The viscosity of the oil—its internal friction—causes it to be dragged by the rotation of the bearing into the very thin film in the loaded zone. There, the pressure generated by this dragging process can be thousands of pounds per square inch, and the minimum oil-film thickness is as little as 1.5 microns (0.00006 inch). (Jim Hatch/)

A mature mathematical description exists for such things as oil-lubricated plain-journal bearings. It relates oil viscosity, shaft rpm, dimensions, and load. There is generally good agreement between this model and measured results. And yet, every so often, we are still amazed by something unexpected.

One example: the general reduction in new-car engine-oil viscosities, from the 20w-50s and 10w-40s of the past to recent surprises such as 0w-15. Back when Rob Muzzy was still managing a roadrace team he expressed natural astonishment at this change.

Related: The Wonderful Weirdness of Oil Viscosity

Many years earlier, stock-car builder Junior Johnson wondered what he could do to cut down on the amount of oil flying around inside NASCAR V-8s. Just reducing bearing clearances was clearly a risk; when the predicted minimum oil-film thickness approaches the surface-roughness height of a shaft’s journal and its bearings, peaks will hit peaks and the resulting metal-to-metal contact will produce a shower of damaging wear particles. Failure soon follows.

A Better Way to Break In an Engine

What if we could reduce the journal’s roughness height? It turned out that previous researchers had tried to do just this. During World War II, US companies produced roughly half a million large aircraft piston engines, all of which had to be broken in before use. That required a huge amount of high-performance-number gasoline that was urgently needed elsewhere. Wasn’t there a better way to break in engines than to just run them and hope they wore their bearing surfaces smooth?

There was, and for journal bearings the resulting process was called Chrysler Superfinish. Its basic tooling consisted of fine, curved abrasive laps, floating on an oil film and enclosing the rotating journal. The oil film between lap and journal was controlled by varying the temperature of the oil being supplied—since oils lose viscosity as they warm, the warmer the oil, the closer the shaped abrasive laps came to the journal’s surface. At a certain temperature they would begin to remove just the peaks of journal surface roughness. Continuing in this way allowed engine builders to produce highly finished and truly cylindrical (which is very different from just shiny!) journals. Journals that were ready to carry full load as-manufactured, without break-in.

Related: The Lubrication of Gears—50 Ways to Get it Wrong

Trouble was, once the war ended and demand for peacetime goods took priority, industry forgot all about Chrysler Superfinishing and it fell out of use in the US. Johnson then found a German outfit that could Superfinish his crank journals. With roughness height thus reduced, it became possible to save some horsepower by switching to oils of lower viscosity, operating at closer clearances. When the EPA mandated better fuel mileage in the vehicle fleet, new low-viscosity lubricants, operating at smaller minimum oil-film thicknesses, soon became routine.

Gear-Tooth Lubrication

Now we change our focus from journals to gears. When accepted lubrication theory was applied to the very high pressures and low sliding velocities between gear teeth, the results seemed impossible. These pressures may be 100,000 psi and higher in truck transmissions, but the calculated minimum oil-film thicknesses were less than any attainable average surface-roughness height.

Yet gears not only worked well in the real world, they lasted for thousands of hours. And even then, after their service was over, the fine pattern of precision grinding that had originally produced their tooth surfaces could often still be seen. While the math predicted damaging metal-to-metal contact, that was clearly not happening.

What was wrong with the lubrication theory? Elasticity of materials predicted that tooth surfaces would slightly flatten in contact with each other as they transmitted power, but not enough to make any sense of the oil viscosities that had long been widely and successfully used.

In the end, interested parties concluded that under extreme pressure, the viscosity of the lubricants themselves somehow increases enough to keep a protective film in place between the tooth surfaces even under extreme pressure, a characteristic now known as Elastohydrodynamic Lubrication (EHL). How much is this increase? According to surface scientist Jacob Israelachvili, working at UC Santa Barbara, about five orders of magnitude, or 100,000 times.

It appears that lubricants—consisting of long-chain molecules—assume a layered form when film thickness becomes of the order of a few molecular dimensions. Molecules align lengthwise as well, and begin to act as if they are taking crystalline form. They become solidlike. Once the pressure is relieved, the oil resumes its normal liquid form.

Another group modeled this behavior and found that the only way the aligned molecules can escape from between confining surfaces is endwise, a process they called “reptating”—(moving “snakewise”).

That greatly increased viscosity—a liquid’s frictional resistance to movement—keeps the aligned oil molecules in place between surfaces that are pressed together at extreme pressure. And that’s what’s keeping those gears alive.

View the full article

[fastbob]

I've got a roller bearing crank .

[Bender]

9 minutes ago, fastbob said:

I've got a roller bearing crank .

That's not really a selling point, there is a reason journal bearings are used in high performance applications as I'm sure your aware