Marvelous things. Simple at first sight but a great bit of engineering on closer look. They are the soldiers on the front line – overcoming many destructive forces deep in the heart of the engine. I am worryingly talking with admiration about a small piece of metal. But what a piece of metal it is.
The connecting rod (con rod) is a bit of metal that connects the crankshaft to the pistons. Forces from the combustion in the cylinder push the pistons down and rotate the crankshaft via the con rods. The rotation of the crankshaft is eventually transferred to motion in your wheels. A rather important function of a car when you think about it. Yes, quite right.
RPM in your car is a measure of how many times the crankshaft completes a full cycle in one minute. At an average redline of 7,000rpm the crankshaft is spinning 117 times per second! In other words, hauling ass. As the con rods need to be connected to move around with the hauling crankshaft the points of contact have to be something special. The job is given to your con rod bearings (also called insert bearings).
If the con rods themselves were designed to fit over the crank journals, after some wear or damage you might find yourself with an expensive bill to replace either the con rods or the crankshaft itself. As a precaution, the con rods are bored slightly larger than the crank journal it attaches to and in between you will find a thin bit of metal that can take the abuse and damage – our con rod bearings.
Manufactured in two halves, they have an outer steel shell that gives it shape and strength. The inside is then coated with either babbitt lining or a blend of copper-lead-tin matrix and then a very thin coat of pure tin. Some use an aluminium coating. In the middle they have an oil hole for lubrication and a shallow grove to spread the oil.
The con rod bearing leads a vicious life. In order to survive they had to learn some pretty neat tricks from their forefathers.
Strength – A con rod bearing must be very strong to survive the stress from the power stroke. It gets pounded, squeezed and pummelled as the crank pushes up and then down. It must not crack, distort or fatigue from the constant forces.
Conductivity – With the friction involved all bearings create lots of heat. It must conduct well enough to pass this heat to the con rod while still maintaining a good fit over the crank journal.
Temperature Change – The bearing must maintain strength and function when cold and hot.
Anti-scuffing – Should the surface of oil lubricating the bearing be lost, the material must not damage the crank journal by scuffing or scratching.
Corrosion Resistance – It must resist corrosion from acids and vapours in the crankcase.
Conformation – It must be soft and ductile enough to allow the crank journal to bed in to the bearing.
Embedability – Should a small piece of abrasive particle enter the bearing area it must allow the particle to embed itself into the bearing to keep the crank journal damage free.
Can you believe the demands set on the con rod bearings? They truly are a marvellous bit of engineering. Next time you’re at the traffic lights about to take off in your car, give thanks to this cool bit of metal that takes a pounding and keeps your more expensive parts running efficiently and damage free.