Main Bearings 101

March 14, 2022

Some things we take for granted. The human heart, for example, beats more than a billion times during an average lifetime -- but it only takes one problem to get our full attention. So it is with the quiet workhorse of the engine, the humble main bearing. It does a demanding job for years on end and barely even gets noticed.

What you may not realize is that these little marvels are some of the most scientifically advanced components in the entire engine. The metallurgy and precision needed to make them is the stuff of clean rooms and science journals. Yet, they weigh only a few ounces, aren't that expensive, and endure more than half a billion engine revolutions. Surprisingly, the alloy in use today was invented nearly 200 years ago and has changed very little since.

Let’s look at what they do, their history, composition, and what they need to work their best.

The Basics

Simply put, a bearing is “a part of a machine that bears friction, especially between a rotating part and its housing.” The main bearings are the interface between the engine block and the rotating crankshaft in a piston engine. It has two jobs: let the crank rotate freely with minimal friction, and hold it tightly in place under extreme forces and heat.

Therein lies the challenge as those two functions work at cross purposes. We’ll cover this in detail in a bit, but a bearing is made of a hard metal -- usually steel -- with a softer alloy that is in contact with the crankshaft. The specific type of bearing is called a journal, plain, or sliding bearing.

Main bearings are circular but are split into halves, which are called shells. The shells are held in position with a tab, tang, or sometimes a pin. The shells fit into a bore in the engine block and are made slightly larger than the circumference of the housing. They are slightly crushed in this position, which helps keep it in place and also more efficiently transfer heat to the housing. They are also ever-so-slightly oval-shaped, to promote lubrication as well as the ability to handle loads that typically deform the housing.

The lower shell is housed in the engine block and the upper shell is housed in the main bearing cap. Once the crankshaft is installed, the semicircular pieces are bolted to the engine block. This completes the housing to supports the crankshaft. There are exceptions, such as a boxer engine, which uses a split-block, so both shells are housed in the engine block.

Finally, the main bearings are only able to endure because of constant lubrication. Engine oil is pumped into the crankcase at high pressure. This provides a constant fluid layer just a few atoms thick between the crankshaft and the main bearings. Some main bearings have grooves and passages for pumping oil directly into the bearing.

Inside of the crankcase, there are two other types of complementary bearings. The connecting rod bearings are the main bearings’ counterparts. These bear the friction at the point where the kinetic energy generated by engine combustion attaches to the crankshaft. The thrust bearings control the crankshaft’s fore and aft “play” and keep it correctly positioned.

The number of main bearings vary based on the engine’s design, specifically how many cylinders and in which alignment. For example an inline six-cylinder engine may have seven main bearings, while a V-8 might only have five.

The amount of precision required to make all of this work is hard to overstate. The machining and material tolerances are measured in microns, and accurate torque and bolt stretch can make all the difference between success and failure. That’s why engine builders get paid the big bucks!

Bearing Beginnings

The concept of bearings goes back to the Ancient World of Egypt and the Roman Empire. However, the first practical concept of a bearing was sketched by none other than Leonardo Da Vinci.

The key development for plain bearings happened in 1839 when Isaac Babbitt, a Massachusetts goldsmith, invented a metal that resisted galling. Galling occurs when two metal surfaces stick or scrape when coming in contact with one another. The soft, or anti-friction material is now referred to as the “babbitt” (note lower case) in his honor.

The original babbitt metal is a soft alloy made of copper, antimony, lead, arsenic, and tin, all in varying degrees based on need. The babbitt is then mated to a harder metal, with steel being the de facto standard.

Mr. Babbitt also figured out an interesting property: the thinner the softer material, the longer it would last. It would take more than a century to figure out why this is the case (revealed below).

The technique for making the plain bearings was to pour the molten babbitt alloy into a shell around the actual crankshaft so as to ensure a perfect fit. In fact, this technique was used right up to the 1950s, but wasn’t optimal for mass production. Some specialists still keep this technique alive to preserve the originality of prized collector cars.

Most early cars adopted plain bearings, but there were some oddballs that made a go with ball bearings for the main bearings. Most notably, the Peugeot L45 that we covered in this article.

Babbitt’s Alchemy

The original formula that Babbitt invented nearly two centuries ago is still the basis for what is used to this day. The secret to the alloy is that it has very hard metal crystals suspended in a softer metal, referred to as a metal matrix composite.

Babbitt microstructure (Credit Edward Pleshakov)

The crystals are super hard, like tiny diamonds, and the soft material wears away creating a microscopic void between the crystals. This is called the oil or bearing clearance. This “space” is where the oil resides. It provides a fluid boundary layer to minimize friction -- similar to how a golf ball has dimples to create a boundary layer of air to help it fly farther through the air. The babbitt metal can be as thin as 5/10,000ths of an inch thick and, as Babbitt discovered, less is more.

The common construction of a main bearing includes a steel backing, an intermediate layer of softer material like copper/lead, a boundary layer of nickel, and finally the babbitt alloy. This is called a "tri-metal" bearing, but less demanding applications can get by with just the steel and babbitt.

Material science has made numerous advances over time so more modern materials are now being employed, especially in high-performance engines. These are nano-polymers that further reduce friction, especially where oil delivery may be compromised due to extremely tight tolerances. Some examples would be graphite/molybdenum (or “moly”), silicon, and other exotic materials.  

Diagram showing the layers of the main bearing. (Credit: DSport Magazine)


What To Know

With a working knowledge of how main bearings function, let’s now take a look at the practical information. The trade-offs are controlled by the “recipe” of the alloy that is tweaked to favor certain properties over others and dictated by the engine’s RPM range, operating temperatures and peak cylinder loads, among other factors.

The properties of bearings, as described by leading producer King Bearings, are as follows: 

  • Load Capacity (fatigue strength) is the maximum value of cycling stress that the bearing can withstand without developing fatigue cracks.
  • Wear Resistance is the ability of the bearing material to maintain its dimensional stability (oil clearance).
  • Compatibility (seizure resistance) is the ability of the bearing material to resist physical joining with the journal material.
  • Conformability is the ability of the bearing material to accommodate imperfections of the journal, housing, or bearing itself.
  • Embeddability is the ability of the bearing material to absorb small foreign particles circulating in the lubricating oil.
  • Corrosion Resistance is the ability of the bearing materials to resist chemical attack of the lubricants.
  • Cavitation Resistance is the ability of the bearing material to withstand impact stresses caused by collapsing cavitation bubbles, which form as a result of sharp drops of pressure in the flowing lubricant.

Choosing the right lubrication is essential for the bearings to operate properly. If the engine is built with tight clearance, such as with a racing engine, a low viscosity (i.e. more watery) oil is needed whereas a higher-viscosity oil is applicable when larger bearing clearances are used. Knowing how the oil is circulated (and how much), how clean it will be, and whether it is cooled are critical to selecting the right bearing for the job.

Bearings operate at extremely tight tolerances, so a high level of precision and expertise is essential for proper installation. They are manufactured in standard, oversized, and undersized dimensions in increments of in 5/10,000th of inch. Getting the crush height exactly right is another factor. Too tight and the bearing will buckle and starve it of oil, too little and the bearing will chatter and overheat.

The good news is that properly installed bearings usually outlast most other engine components, and rarely the cause of engine failure. Replacing them requires an engine rebuild.

Hopefully, you have a better appreciation for what these little guys do and what goes into making them. Since they are static parts, they are underappreciated, but they perform an essential function using technology developed long before cars were even a distant thought.

References


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