Bearing the Load

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/)

The crankshafts in nearly all modern motorcycle engines now spin in durable and inexpensive plain journal bearings. This is possible because today’s engines have excellent supplies of filtered, and often cooled, oil circulating under pressure. Journal bearings are rugged because the thin film of oil supporting the crank or connecting rod is extremely rigid in the radial direction, much more so than are ball or roller bearings, whose elastic rolling elements flatten measurably under load.

The earliest motorcycle engines, built at the end of the 19th century, had journal bearings lined with brass bushings. Lubrication was accidental; the user saw to it that there were several ounces of oil in the crankcase before starting up, and it was the motion of crank and rod that splattered that oil onto every internal surface, including the piston, its sealing rings, and the con-rod.

Brass bushings, being fairly hard, were rapidly abraded by any contaminants in the oil. (Sometimes the engine accommodatingly supplied its own sand and grit, courtesy of the original crankcase casting process.) To address this, an improved kind of bushing was developed, one made of quite soft “white metal,” sometimes referred to as Babbitt metal, after its inventor Isaac Babbitt. It had important advantages.

One, the material consisted of harder crystals of tin surrounded by softer metal. As a shaft spun in such material, the soft material wore away to expose the harder tin, whose greater strength carried the load. This is analogous to our modern Nikasil cylinder-wall plating, which consists of about 5 percent silicon carbide particles in a matrix of electroless nickel. The nickel wears away in service and leaves the silicon carbide, smooth from diamond honing and extremely hard, to carry the load.

Plain bearings are used in connecting rods and crankshaft journals instead of roller bearings because the oil film supporting those parts is extremely rigid in the radial direction.

Plain bearings are used in connecting rods and crankshaft journals instead of roller bearings because the oil film supporting those parts is extremely rigid in the radial direction. (Ducati/)

White metal’s second useful property was that it was “embeddable.” Hard contaminants carried in the lubricating oil, instead of grinding away the journal, were quickly pounded into the soft bearing material, deeply enough to become harmless.

The third useful quality was surface texture. The white-metal surface, consisting of slightly protruding tin crystals above a lower surface of softer metal, was oil-wettable.

The Right Bearing for Each Oil System

Builders of auto engines quickly saw the durability value of pumped circulating oil systems with white-metal bearings, but low cost was the main appeal of motorcycles in those days. Rather than adopt the more complex automotive oiling system, bike makers switched to a bearing that could get along on very little oil: the rolling element.

Things were happening fast in 1895. Just a dozen years earlier, Friedrich Fischer had conceived a ball-grinding machine, and a year later was making 10 million balls a week. Demand exploded with widespread mechanization and the bicycle craze. Rolling bearings were durable because they were made of through-hardened high-carbon chromium steel. In general, such bearings locate and align their rolling elements with a cage or separator. This prevents rollers or balls from “rubbing each other the wrong way.”

In these early motorcycle engines, oil was steadily lost past piston rings and intake valve guides, where oil loss is especially fast because there is a partial vacuum in the intake system. As those few ounces of oil in the crankcase dwindled away, journal bearings would overheat and then seize. Ball and roller bearings could survive longer, long enough that the rider, looking back, saw that the engine had stopped smoking, and realized the crankcase needed a shot or two of oil from the on-board tank and hand pump. Ah, good for another few miles.

But rolling-element bearings had their downsides as well. They were subject to fatigue failure because the very small contact areas of the rolling elements against their raceways generated high subsurface shear stresses. As stress cycles accumulated, tiny imperfections in the metal became cracks, and failure eventually came about when the normally polished surfaces of balls, rollers, or raceways broke up.

Why Harleys Use Roller Bearings

Like nearly every other motorcycle maker of that time, Harley-Davidson rationally chose rolling bearings. Its Big Twin cranks still spin on rolling bearings. With the 61-inch EL of 1936, the company adopted pumped recirculating oil, which made rolling bearings viable. When highly defect-free vacuum-remelted bearing steels went commercial in the mid-1960s, this greatly extended the fatigue lives of balls and rollers. When The Motor Company bought itself back from AMF in June of 1981, a new and highly successful sales approach made venerated icons of rolling bearings, air-cooling, and pushrods.

As engines with white-metal journal bearings began to make more power and spin faster, fatigue failures also began to appear. These were sometimes called “picking-out,” as cracks in the bearing surface allowed material to flake off.

Old-World Craftsmanship

White-metal bearings were originally made by pouring the melted metal into the space between the crankshaft journal and the bearing housing (be sure to “smoke” the journal with soot before to prevent sticking). Once all was cooled, a skilled worker with hand scraping tools would fit the shaft to the bearings at a proper clearance. Apply a thin layer of Prussian blue to crank journals, set the crank into its bearings, rotate it, then remove it and see where blue areas—the “high spots”—appear on the white metal. Repeat this process until full contact is achieved. As you may imagine, this was slow and expensive because it required a skilled and experienced operator. As romantic as the old-time craftsperson appears from a century later (leather apron, wire-framed specs, kindly eyes) this work was too slow to support a mass market.

To eliminate this bottleneck in World War I, bearings were machined as semi-circular inserts of brass with the white metal already cast onto their inner surface and precision machined to dimension. When bearings wore out it was an easy matter to pop out the old inserts and set new ones in their place. Interchangeable parts! Even so, the V-12 Liberty aircraft engine would need new crank and rod bearings after just 200 hours of operation, and it was the limited fatigue life of its bearings that set the limit.

A Better Plain Bearing

How could such failures be prevented without reverting to the harder bearing materials of the past? American ingenuity (remember that?) got busy and showed that when the layer of white metal was made thinner, the bearing could carry more load without breaking up from fatigue. Starting with a much stronger steel insert, in 1923 the Allison Co. of Indianapolis patented a new plain bearing that is the direct ancestor of the bearings in today’s auto, truck, and motorcycle engines. A quite thin layer of white metal was applied to this higher strength steel insert, and then an additional layer of very soft lead-indium over that, to prevent the white metal from corroding or “hen-tracking” due to combustion-generated acids in the oil.

Oil circulating under pressure to the bearing journals make it possible for plain bearings to have a long service life.

Oil circulating under pressure to the bearing journals make it possible for plain bearings to have a long service life. (KTM/)

This new kind of plain bearing tripled the MTBO (Mean Time Before Overhaul) of the Liberty V-12. During WWII similar bearings, but with stronger silver or copper-lead base layers, were widely used in Allied in-line aircraft piston engines such as the Rolls-Royce Merlin and the Allison V-1710. After the war the record of durability demonstrated by such bearings made them the rational choice for production automotive engines.

Since that time other combinations of materials have been used in plain bearings. Lead is gone because of environmental concerns, and aluminum has become a frequent choice in place of white metal. Unlike their predecessors, when high-mileage engines finally come to rest in junkyards today it is seldom because of crank- or con-rod-bearing failure. Progress!

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