Information About Crankshafts - CHP How It Works

Crankshaft Experts Talk Metallurgy, Race Balancing, Counterweight Profiling, Heat Treating, Production Techniques, And Much More.

Stephen Kim May 28, 2010 0 Comment(s)
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Counterweight Shape
Dwayne Boes: "Knife-edging the counterweights doesn't really make much of a difference in a street motor. In fact, a knife edge isn't even the most efficient design for a leading edge anyways. You want oil to land on the nose of counterweight and flow off smoothly to the sides. With a knife edge, oil gets thrown around all over the place. In truth, knife-edging was developed more for ease of balancing than for horsepower. A bullnose-rounded leading edge is actually the most efficient. Just like the bow of a ship moves through the water, it allows the counterweights to cut through the oil."

1007chp_13_o Information_about_crankshafts Machine 2/8

Alan Davis: "How you drill oiling holes into a crank is the subject of lots of controversy. Factory cranks feature standard oiling, which means that the holes are drilled straight across the main and rod journals. The other method is cross-drilling, and there are two different ways of doing it. The first method is to drill across from the main journal to the rod throw, making a '7-shaped' passage where the hole intersects the crankshaft centerline. This works well at low rpm, but not at high rpm, as centrifugal force pushes oil away from the rod throws. The second, more effective method of cross-drilling is to drill a hole from the rod journal and stopping at the main journal. This T-shaped passage provides more consistent oil flow to the rods at high rpm. Cross-drilling isn't necessarily bad, but it depends on how you do it. Today at Eagle, we do standard, straight-shot oiling. Cross-drilled cranks have gotten such a bad rep, that we don't offer it anymore, but it does have benefits if done the right way."

Hardness vs. Ductility
Dwayne Boes: "Designing a durable crank is an exercise in striking a balance between hardness and ductility. Increased hardness can lead to a stronger crank, but it still has to have some give in it so it can bend without cracking, which is referred to as ductility. A good way to explain ductility is to compare glass to rubber. Glass is hard, very hard, but it cracks easily, so it's not ductile. Rubber bends easily so it is very ductile, but not hard. Like a fishing pole, you want the crank to give a bit under load, but snap back into shape without being permanently deformed. Cranks do in fact flex under load, and in a motor with an aluminum block, they can bend as much as 0.200-inch. Where premium forged cranks shine is in their ability to be extremely hard while still maintaining ductility. The ideal crank is one that can be very hard and maintain its shape to spread bearing loads evenly throughout the crank while still having enough ductility to prevent cracking. Generally, as a crank's hardness increases, so does its tensile strength. Having higher carbon content in steel increases hardness, but sacrifices ductility in the process. That's why you don't want too much carbon content in a crank. Nodular iron is the least ductile material used to build cranks, as you go up the scale, you can increase hardness without sacrificing ductility."


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