Information About Crankshafts - CHP How It Works

Even so, there are some very basic questions that need to be addressed when shopping for a new crankshaft. Everyone knows that forgings are stronger than castings, but how much stronger are they, and how much power can they take? Where do billet cranks fall into the hierarchy? What differentiates the various grades of alloys? What are the different heat treating procedures, and how effective are they? How advantageous are lightweight cranks? To find some answers, we contacted the foremost authorities in the crankshaft business including Alan Davis of Eagle, Tom Lieb of Scat, Dwayne Boes of Callies, Tim Langley of Lunati, and Shawn Mendenhall of Coast High Performance. Here's what they had to say.

"On the other hand, manufacturing a forged crank is a much more involved process. With a forging, you start with a large ingot of steel alloy, and then pound it into shape with 200-ton press and dies. The heavy-duty presses needed to forge a crank cost at least $100,000, so you have to commit to building a ton of cranks before you can even recoup your investment in equipment. Additionally, the forging process isn't as precise as the casting process as far as the shape of the crank is concerned, so it requires more extensive machine work. That's why forged cranks cost three to four times more than cast cranks, but the trade-off is a dramatic increase in strength. A typical small-block cast crank is good up to 500 hp, while a 4340 forging can take 1,500 hp.

"Billet cranks are similar to forged cranks in that they also start out as a big ingot of steel. The difference is that the billet blanks are already forged, and they're machined into shape instead of being pounded into a die by a hydraulic press. However, the benefit of billet cranks is that they're highly adaptable to short production runs since you don't have to invest in expensive presses and dies. It might cost $3,000, but you can order up a billet crank with any length stroke and any journal diameter you want. You don't have that flexibility with a forged crank. Even if you manufactured a batch of 1,000 cranks, you still wouldn't be able to recover the costs of the dies and presses needed to make it."

"Billet cranks are the strong ones, which is why they're used extensively in NASCAR Sprint Cup and Top Fuel. Visually, billet cranks are indistinguishable from forged cranks, so the difference between them is in the grain's structure and metal. If you're making a forged 4.000-inch-stroke small-block Chevy crank, you start with a round bar of metal that's 4.75 inches in diameter. After the forging process, the total width of the crank ends up being 6.75 inches. That means grain that was flowing parallel to the crank has been forced to negotiate a series of 90-degree turns to make the rod journals and the mains. What was once the centerline of the crank has now been offset by stretching, sharing, and weakening the grain. On the other hand, a billet crank has uniform grain structure than runs parallel throughout the length of the crank. Instead of being pressed into shape, a billet crank is formed from a round bar of highly refined forged steel that's much larger in diameter. Compared to a forged crank, the metal blanks used in a billet crank weigh more than twice as much. To make a 4.000-inch-stroke crank, you'd start with an 8-inch diameter piece of steel, then whittle it down into the shape of a crank. This method eliminates all stress risers, yielding a stronger end product than a forged crank."

Shawn Mendenhall: "It's hard to generalize how much power a crank can handle, because there are other variables besides whether a crank is cast or forged. Bigger main and rod journals will obviously take more abuse in a cast crank. For small-blocks, a safe limit for a cast crank is 500 hp and a maximum of 6,500 rpm. With big-blocks, their bigger mains will handle 650 hp. We've seen cast cranks in turbo motors take over 2,000 hp, but we don't recommend it. These days, 4340 is the standard for forged cranks. Just a few years ago, 5140 and 4130 cranks were sold alongside 4340 cranks. However, they aren't much cheaper, so it makes no sense to use them over a 4340 forging. For 99 percent of people, a 4340 crank is more than they will ever need. If you can break one somehow, then something else was probably wrong with the motor. Once you get into very high rpm and tons of power, then billet may be the best option."

Dwayne Boes: "Nitriding is a chemical process where nitrogen is absorbed by the surface of crank at high temperature, which hardens the crank. It also improves fatigue strength. By heat treating the bearing surfaces of the crank, you create an extra layer of protection without affecting the rest of the crank. The older method of heat treating is induction hardening, in which the journals are heated up and plunged into water. Induction hardening can be done with cheaper equipment, but if the rate of cooling isn't carefully controlled, it can create stress risers. That's why nitriding is much more common in aftermarket crank manufacturing."

Forging Process
Tim Langley: "A lot of manufacturers market their cranks as being non-twist forgings. What that refers to is how the crank is forged in the die. With a twist forging, after one crank throw is forged, the crank is twisted before the next throw is forged. This lets you get away with using a simpler die. In a non-twist crank, all four throws are forged at the same time. This requires a much more complex die. The non-twist process reduces internal stresses during the forging process, but it also requires running the procedure at a higher temperature, which can increase grain growth and reduce strength. However, if done properly, then there isn't any difference in strength between one and the other."

Grinding and Polishing
Alan Davis: "The surface finish on a crank's main and rod journals is very important. It should be smooth, and polished to a mirror finish to reduce friction. The last thing you want are big variations from the high points to the low points. These peaks and valleys will create stress risers, and that's where cracks will form. However, there's a point of diminishing returns, since you can spend an extra five hours finishing a crank and it won't strengthen it at all. When machining a crank, all that's needed is some grinding, shot-peening, and finishing and polishing of the bearing surfaces."

Cross-Drilling
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."

Balancing
Alan Davis: "When balancing a rotating assembly for a street motor, the goal is to equalize the rotating mass and the reciprocating mass. However, in race motors it's not uncommon to overbalance the crank. A balancer generally spins a crank 500-750 rpm, and for obvious safety reason, you can't replicate the actual rpm the crank will experience in a running engine. However, if you spin a motor at very high rpm, say 7,000-8,000, parts can stretch and move around. Aluminum rods might stretch as much as 0.030-inch. This stretch increases load on the crank and bends it, making the pistons and rods behave as if they're heavier than they really are due to dynamic inertial effects. To the crank, the pistons feel heavier. So if you have a rotating assembly that calls for a bobweight of 1,800 grams, a motor may run more smoothly if you overbalance the crank by two percent. In other ways, you'd compensate for the inertial loads the crank endures at high rpm by balancing the rotating assembly to a bobweight of 1,836 grams instead of 1,800 grams. The balancer will indicate that the crank isn't balanced, but the bearings will actually look better when you tear the motor down. On a 6,500 rpm street motor there's no need to overbalance. It's more for race engines than run 7,000 to 8,000 rpm all day."

Overlap
Dwayne Boes: "Crank overlap is something you should always pay attention to. If you were to stand a crank up vertically, overlap can easily be seen as the portions of the main and rod journals that overlap each other. The more overlap you have, the stronger the crank. To keep production costs down back in the '60s, the OEs used cheap casting materials in their cranks, but compensated for it by making the main and rod journals very large, which increased overlap. With stroker cranks, however, moving the rod throws farther away from the mains inherently weakens the crank by reducing overlap. Likewise, high-end race motors often reduce the rod journal diameters of a crank to reduce bearing speed and friction. This also reduces crank overlap as well. Fortunately, with today's quality aftermarket forgings overlap isn't as big of an issue as it used to be."