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Getting Your Bearings
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Barkhaus says that on their bi-metal bearings the IDs are bored so the debris is flushed out instead of being embedded. “That’s one of the real benefits of it,” he says. “The other benefit of bi-metal is the wearability. They will last virtually forever, as long as you keep the oil changed and the engine is run in a fairly normal operating condition.”
Aluminum bearings can last much longer in mild load applications because of the wearability and the hardness of the silicon composite that actually polishes the crankshaft and wipes any nodes or debris out of the way or flushes it out, Barkhaus continues.
“OEMs have moved to bi-metal bearings because they are lead-free, economical to manufacture, and are extremely hard, which allows them to perform a very long time in passenger car applications,” says MAHLE Clevite’s Dayringer. “As we’ve all seen, it is not uncommon for modern engines to run 150,000 miles or more on the original bearings.”
Virtually all performance bearings today are constructed using a tri-metal construction. Tri-metal bearings have the same steel back as a bi-metal bearing, a copper-lead intermediate layer, and a surface layer of soft material like Babbitt.
“Tri-metal bearings work very well on race and performance engines because they are more forgiving, which allows for better embeddability than a harder bi-metal bearing,” says Dayringer.
Bearing materials are, in some ways, the hidden secret of bearing design, according to our experts. The “hidden” aspect of bearing material capability is in the design of the metallurgical properties embodied in the alloys. So, while all tri-metals and all bi-metals might look alike their properties and abilities can be significantly different.
“While proper fitment provides the basis for bearing system operation the materials provide the basis for system durability,” says ACL’s Scott. “To operate successfully, bearings require a range of attributes that, in terms of material properties, require a degree of compromise. For instance, the strength required to transmit high cyclic loads has to be moderated in order to provide good seizure resistance. How this balance is achieved is different for each bearing’s material/alloy combination. The clearest example is the comparison of properties between aluminum base ‘bi-metals’ and copper-lead based ‘tri-metals.’”
What are the primary differences in hardness, strength and embeddability between today’s aluminum and tri-metal bearings?
“Generally aluminum bearings offer greater embeddability and conformability while tri-metal bearings offer greater fatigue strength and hardness,” says Sledge. “The aluminum alloy layer on top of a steel back is a minimum .012˝ thick and can embed particles larger than .0005˝ in dia. The overlay thickness on tri-metal bearings range from .0005˝-.0008˝ therefore allowing less embeddability.”
Federal-Mogul’s Sturk says there are basic differences in the hardnesses of the two bearings as well as some other differences in strength and embeddability. “In my view, embeddability isn’t really that big of a problem. I think today’s aluminum bi-metal bearings can handle a reasonable amount of debris. But if there are excessive amounts, that’s where the tri-metal bearings are better. The over-plate embeddability allows them to handle debris a bit better. If it’s a reasonable amount, if oil has been properly maintained, I don’t really worry about the aluminum’s embeddability.”
If you are looking to extract more power, experts say the copper/lead bearings are likely the optimal choice. Sturk explains that bearing engineers are primarily concerned with unit load as opposed to horsepower numbers in order to rate the strength of a specific bearing or material. Unit loads come from the cylinder pressures that are developed above the bearings in the combustion chamber.
It’s hard for an engine builder to estimate combustion pressure without a lot of expensive testing equipment that an OEM or bearing manufacturer would have, so Sturk says it’s probably best to go with a tri-metal bearing for any upgrade, just to be safe.
There is just a big difference in the wear rates of the copper/lead trimetals and aluminum bimetal bearings, say experts.
The copper lead tri-metal bearings are still the strongest materials available today, says Sturk. He says if you are upgrading or adding a supercharger or using other methods to extract more horsepower, it usually puts higher loads on the bearings. “In that case,” he says, “the tri-metal bearings will handle much higher loads. Our H14 materials (copper/lead) will handle unit loads up around 15,000 psi. The aluminum will handle much less, around 8,000-9,000 psi. It’s a big difference in strength characteristics,” he notes.
According to Dayringer, race and performance engines see more crankshaft deflection, rod housing bore distortion, and thinner oil film thickness compared to passenger car engines and therefore require a different material and construction than bi-metal aluminum bearings. “The softer Babbitt facing allows the bearing face to ‘move’ slightly with the crankshaft to avoid seizure during high load and rpm situations, which makes tri-metal bearings the preferred choice for performance and race engine builds,” he explains.
The attributes that a bearing needs to have can be compared to a tightrope because on the one hand you need hard fatigue strength components for durability and load carrying capacity, but on the other hand you need soft phase characteristics for conformability (when things are out of alignment), embeddability and sliding properties. When manufacturers select a bearing material, they look at the engine application and try to determine the most important thing the engine needs and then design the bearing around that.
Today’s bimetals contain a mix of other materials such as silicon, which is a hard particle that is added for increased durability. They have tin which is considered a soft face metal that is for the embeddability and conformability aspect, so today’s aluminum bi-metals have alloying elements in them to address those issues. But still, not all alloys are the same.
While all tri-metals and all bi-metals might look alike their properties and abilities can be significantly different. Bearing material capability is in the “hidden” part of the design of the metallurgical properties embodied in the alloys.
“The differences get down to alloy composition, casting conditions, bonding methods and heat treatments for bi-metals,” says Scott. “And for tri-metals, it’s the alloy composition, sinter/casting conditions and electroplating/coating technologies that account for any differences.”
All these factors give the bearing manufacturers a great deal of freedom to tailor materials, and it can be confusing for engine builders trying to understand how appropriate the materials are for their specific application. Because the quality of the material may not be clearly apparent it is also hard for engine builders to make a judgment between competing products.
Different suppliers materials do behave differently, and within each family of materials there is a substantial range of capability. It is nearly impossible for a rebuilder to judge, on face value, how any brand of bearing is going to perform across a broad range of characteristics, so experts say it is best to rely on bearings from reputable, established brands.
When bearings are found to be damaged or show unusual or uneven wear, it typically indicates other problems that need correcting, problems that if left uncorrected may cause the replacement bearings to suffer the same fate.
Special thanks to MAHLE Clevite, King Engine Bearings, ACL and Federal-Mogul for their input into this article.
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