Over the years there have been numerous articles concerning engine bearings. The articles focus on details ranging from the manufacturing process, composition, and materials. While the technical data from these articles is awesome, the lowly engine bearing is still commonly overlooked.
Too often the reason for the oversight is related to a price point. I say this because when most people are given a choice between multiple options, they will often choose the cheaper of the lot. But, one thing that we have to keep in mind is that engine bearings, like most other internal engine parts, are usually designed for a specific application. Enter King Engine Bearings’ new GPC bearing, which has been designed for a very narrow application window.
Two vs. Three — What’s For Me?
A colleague of mine, Jeff Smith, wrote an article several years ago called “A Quick Guide to Engine Bearings.” In the article he basically stated that there are two kinds of engine bearings available — bi-metal and tri-metal bearings. To give you a general recap of the article, Jeff lays out the facts that bearing inserts start out with a steel shell and then softer materials are bonded to that shell to create the bearing surface. As the name implies bi-metal usually use two different metals, and tri-metals use three.
For most OE applications, a steel shell is bonded with an aluminum overlay to create a traditional bi-metal bearing. Sometimes referred to as “aluminum bearings,” they are strong and have great wear resistance. The downside is that because of their hardness they do not like cyclic loads from the crankshaft and they have poor embeddability.
For a typical OE engine, the cyclic load of the crankshaft does not pose any potential problems to the aluminum overlay. But, as the horsepower increases, so does the cylinder pressure. This in turn creates more cyclic load on the crankshaft. The low embeddability of the aluminum means that the surface of the bearing cannot trap small particles of dirt and debris, and instead, they wear the bearing and journal.
In high-performance applications, the tri-metal bearing is preferred. The tri-metal bearing consists of a steel backing with a copper/lead alloy overlay. The softer material used in these bearings is more conforming to the cyclic loads of the crankshaft. In other words, the bearing material can take a beating without coming apart like an aluminum bearing.
This does not mean that they will not wear, because depending on the application, over a period of time they will. The softer overlay of a tri-metal bearing also has great embeddability characteristics. Dirt and debris can be mashed into the bearing surface which can ultimately prevent damage to the rotating components.
Supporting The Load
An important thing to keep in mind is that the bearing is designed to support the load of the rotating shaft via an oil wedge between the rotating components and the engine bearing. The oil pump supplies lubrication to the engine bearings and a small oil film layer forms between the bearing and component. The journals do not contact the bearing unless the surface is rough or the engine load overcomes the film strength. Additionally, by keeping a film of oil between the two components, heat can be transferred from the bearing and dissipated.
In a perfect world, the bearing should only contact the crankshaft journal on start-up. If the bearing load were constant and unchanging, with the correct oil pressure, there would never be any bearing wear. But because in the real world the crankshaft is constantly loading and unloading, there will be bearing fatigue.
There was an example given several years ago about cylinder pressures and how they affected the loads to the crankshaft. In a typical everyday commuter car, cylinder pressures are around 1,200 psi. For a racing engine making around 900 horsepower, cylinder pressures will be around 2,200 psi. An engine making 3,000 horsepower will have cylinder pressures in the 6,000 psi range, and a Top Fuel engine can see cylinder pressures as high as 10,000 psi.
From the cylinder pressure examples, we can infer that higher-compression engines will have higher bearing wear and fatigue. Dr. Dmitri Kopeliovich, the director of research and development for King Bearings and inventor of the new bearing materials used in the GPC bearings, explains the issues at hand. “A higher compression ratio results in greater pressure in the cylinders during the power stroke. That results in greater load applied to the upper rod bearings and lower main bearings.”
Dr. Kopeliovich continues, “The load is not only high but also alternating because of the cyclic character of the combustion process with a peak value in each direction happening every two rotations (or 720 degrees) of the crankshaft. An alternating load may cause material fatigue (resulting in cracking) if the peak load exceeds the fatigue limit of the material.”
The peak of the alternating load would be most severe during the compression stroke and the power stroke of the cylinder. So the issue isn’t just one of peak material strength, but the ability to withstand those pressures thousands of times per minute. The softer materials used in bearings come with a lower fatigue limit, and can’t be used in applications with higher loads. Under these conditions, a stronger material is required.
The traditional drawback of using a stronger material is reduced anti-friction properties such as conformability, embeddability, and wear-resistance. If you use a stronger material you fix the fatigue issues, but you have now increased friction with a bearing that isn’t forgiving enough to not hurt components, which goes directly against the point of having a good bearing.
Enter the GPC Bearings
In order to combat these issues, King Bearings is using a different approach in its designs, aiming to design a bearing capable of performing well at both ends of the spectrum. The GPC bearing is a combination of several components that make up the overlay of the bearing. “King’s new GPC tri-metal lead-free silvery-matrix-overlay bearing is designed for applications where there is extreme loading due to very high cylinder pressure,” explains Ron Sledge, King Bearings’ performance and technical manager.
The first layer of high-load-capacity silver alloy is just the start, and is followed by a layer of lead-free bronze with additional tin mixed in. “On top of the unique heavy duty intermediate bronze and silver overlay layers is a specially formulated polymer coating that resists wear and fatigue during periods of oil starvation and possible metal to metal contact,” says Sledge.
The polymer coating contains additives that offer low friction, high wear resistance, and — especially important in a severe-duty bearing — high-impact resistance. It is capable of withstanding extreme conditions of high-load and low-oil-film conditions which exist in high-load engines.
The result of the new bearing construction is a bearing rated to 17,000 psi of load — or 30-percent more load-carrying ability than King’s premiere line of XP-series tri-metal race bearings. While this might sound like an all around breakthrough, applicable to all tri-metal bearings, the GPC bearing is a laser-guided smart bomb, in that it solves a problem really only found in a very narrow set of applications.
“The new GPC bearing material is only available for limited applications that require extreme fatigue resistance,” Sledge says. These include sprint car engines, high-horsepower direct-injection engines, and performance diesel applications. So while the new GPC bearings are a leap forward in bearing capabilities, as of now, the technology will be confined to the applications which truly require additional high-reciprocating load handling capabilities.