When connecting-rod bearings develop black streaks—not metal-to-metal contact but the next closest thing to it—engineers place a groove around the crankshaft bearing shells. If that doesn’t work, what next? Cycle World Technical Editor Kevin Cameron has the answer.
This Honda CBR600RR crankshaft is another example of how inanimate, unvocal engine parts can tell a story. With respect to lubrication, oil comes up from the gallery below the crankshaft and enters each main bearing through a drilling.
You’ll notice four of the main bearings have grooves in the middle; one bearing has no groove. If you had a crankshaft from the predecessor of this engine, the CBR600F, you would see all of its main bearings were groove-less. Why did Honda make this change?
This engine—and most car engines, as well—gets oil to the crankpins by bleeding it off the main bearings. A hole is drilled at an angle down through the crankpin into the bearing, and a little plug is staked in place.
That drill way conducts oil from this bearing through the flywheel cheek into the hollow crankpin, and the oil comes out through two holes to lubricate the rod bearing. That sounds like it makes excellent sense.
If this crankshaft is turning 14,000 rpm, oil has to be pumped into the crankshaft against so-called centrifugal force to get it into the drilling that carries it to the crankpin. Every manufacturer that has made high-rpm engines has run into this same problem.
They start seeing little problems with their connecting-rod bearings. They develop black streaks. That’s not metal-to-metal contact, but it’s the next thing to it. When you see those black streaks, you think to yourself, “Not much margin there.”
First the engineers put a groove around the bearing shells, all the way around each main bearing that fed a crankpin. Here, just grooving the bearing shells evidently didn’t work because they went a step further and grooved the crankshaft itself.
Engineers don’t like to cut into crankshafts because they have already made the bearings as small as they can. They know bearing friction is proportional to the diameter cubed; double the diameter, get 2 x 2 x 2 = 8 times more friction.
Engineers don’t want eight times more friction, so they make these bearings as small as they can get away with. And they don’t like cutting grooves in them either. But they have clearly cut grooves here.
Evidently, in order to get enough oil to the connecting rods to keep them looking as bright and shiny as they do in this engine, the engineers needed not only to groove the bearing shells but also to groove the journals themselves.
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