How Do Plain Journal Bearings Work Within A Motorcycle?

Описание: Kevin Cameron explains why plain journal bearings are generally not the cause of an engine’s demise.

Plain journal bearings are at the heart of nearly all modern engines. A one-piece forged crankshaft must have bearings made in halves for the bearings to be assembled over the crank. The nature of those bearings is determined by the fact that when people tried to have rotating parts supported by a film of oil rather than by balls or rollers they had to deal with the problem of oil contamination by wear particles or dirt.

Engines for years were sand cast with a rough surface finish. You guessed it: There might be sand in that finish. What happens when a piece of grit gets between the crankshaft journal, which is highly polished and truly cylindrical, and the bearing? If there is no place for that piece of grit to go, it cuts the daylights out of both of them and might cause the bearing to seize. So it was learned very early that the bearing had to be made out of material that was softer than the journal or another rotating part.

In the early days, bearings were poured. Liquid Babbitt metal was poured into the bearing in a thin layer and then finished after it was cast in place to make a cylindrical seat for the journal. But because that was a very slow way to build things and incurred a lot of costs, the idea of having the soft bearing metal on a steel backing ready to simply drop into place had a certain appeal.

A process was developed in Indianapolis and the manufacture of this type of bearing was licensed by an outfit in England that made them for aircraft engines. If a piece of grit got between the hard journal on the crankshaft and the bearing that supported it, the grit was simply pounded into the soft material and ceased to do harm.

To supply these bearings with a steady supply of oil, a wedge must be formed. Viscosity—the internal resistance of the oil—causes the oil to be swept into the open end of that wedge and dragged toward the loaded zone, which is where the journal is closest to the bearing surface. In an engine that is working hard, the distance of closest approach between the journal and the bearing surface is measured in microns, which are millionths of a meter.

In a dragster, the pressure in the loaded zone can be as high as 12,000 psi. Oil pumps typically produce 40 to 60 psi so it’s clear that oil-pump pressure cannot support the load. The combination of oil viscosity and the rotation of the journal supports the load so a stable condition is reached in which the bearing is floating on a film of oil. Because the clearance in a bearing is extremely small and the oil film is present, such a bearing has great damping properties.

Plain bearings are now a mature technology. When an engine is finally worn out and goes to the scrapyard, crankshaft bearings are generally not the cause of its demise. Plain bearings are very durable as long as they get oil that is clean and of an appropriate temperature because oil is constantly keeping the two parts separate.

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