At first glance, engine bearings are just simple shells that hold the engine’s lubricant against either the crankshaft main journals, the rod journals, or the cam journals. But we all know that there is much more to them than that.
Dick Amacher of Daido Engine Bearings recently presented at the Automotive Engineering Technology Conference (AETC) and shared some very interesting science about bearings and how they can both help your engine longer and make more power.
We run simple shell bearings in an engine and not a more complex roller bearing for many reasons beyond cost (and the fact that a roller bearing can’t easily be split like you see in main and rod bearings). A journal bearing, however, has it’s quirks, which affects performance and how you build an engine.
For example, a journal bearing with proper oil supply will gain efficiency (less drag) as the engine speeds up. But that’s only to a point. After that, additional rpms will decrease efficiency and increase drag and heat produced. A journal bearing is also better able to absorb impact (when the journal touches the bearing for whatever reason) than a roller bearing.
Oil Film and Inconsistent Pressure
An internal combustion engine is a chaotic environment. Every time the spark plug fires and lights off the air/fuel charge in the combustion chamber a pressure spike is created that is transferred from the piston through the connecting rod and into the crankshaft. It is these spikes that try to smash the journal into the bearing which must be resisted by the thin film of oil that is pumped between the bearing and journal.
The oil film’s ability to resist pressure from the journal depends on several factors. You can pump oil into the gaps between the bearings and the journals at very high pressure, but that wastes horsepower and shortens the life of the oil pump, so it’s not a very good option. A narrow gap between the journal and bearing can help improve the oil’s resistance to pressure. That’s why many racing and performance engines minimize bearing clearance. Increase the resistance to pressure by closing up the gaps often means you can run lighter oil and free up some horsepower by reducing pumping losses.
Meanwhile, increasing bearing thickness (or width) also creates a greater area for the oil film to be created, A wider oil film zone means the pressure spike is spread over a greater area, which reduces the overall load. That, however, comes with a friction trade-off, which is why performance engine builders often try to run narrower bearings, and even cut them down in a lathe sometimes, whenever they can get away with it.
Finding the right bearing for your application is all about finding the right balance. A hard bearing will exhibit greater fatigue strength and reduced wear. But a softer bearing will have a greater ability to absorb any debris that could scratch the metal journal.
If you are looking for increased strength and wear resistance, go with a harder bearing. Generally, a copper bearing will be harder than aluminum. The hardness of a copper bearing increases further if the manufacturer includes some tin in the alloy or heat treats the bearing.
If you are looking for a bearing that can absorb small particles into the bearing overlay so that they won’t damage the bearing, go with a softer bearing material. This is usually done with an aluminum bearing, or a copper bearing with increased amounts of tin in the alloy. A softer bearing is also better able to conform to inconsistencies in the machine work.