Over the centuries it's been used in a wide variety of machines. Following traditional construction techniques, early piston engine crankshafts were built up from multiple parts, but when a longer crankshaft was needed to accommodate more cylinders, speed and durability were severely limited. The one-piece cast iron crankshaft was a major improvement.
In modern production engines, the crankshaft is either cast or forged. Cast cranks are easier and less expensive to make, so most engines have a cast iron crankshaft. The raw casting is close to the final dimensions, so less machining is needed to finish the part. Cast iron becomes work-hardened during machining, reducing the need for additional hardening of the bearing journals. Because cast iron is less dense than forged steel, the lower mass means more net engine power. However, the lower density limits the strength of the finished part, limiting the maximum load and rpm of the engine. Cast crankshafts are best suited for "everyday" engines.
Forged cranks are made from a steel bar that's heated and pressed into shape. The press dies are flat, so the result is a flat blank with all the rod journals offset by 180 degrees. To make a different offset, the hot blank is then twisted. The pressing and twisting processes distort and stress the grain in the metal, and a fair amount of machining is needed to create a finished part.
However, forged steel is denser and better able to withstand bending and twisting loads. Forged steel cranks are used in high-performance engines for street and track.
Some racing crankshafts are machined from a solid billet of high-grade steel, and there are three-dimensional presses for forging crankshafts without twisting. These are expensive ways to make a crankshaft, so they're typically used for low-volume production of high-performance cranks.
During machining, the bearing journals are ground and the oil holes are drilled and chamfered to relieve stress and improve oil flow. Once machining is completed, the bearing journals need to be hardened.Two different techniques are common. Nitriding is a process in which the crankshaft is heated to about 1000°F in an oven filled with nitrogen, then allowed to cool. The even heating and slow cooling relieve stress in the metal, but it's a slow process and the surface is hardened only to a depth of about 0.025 inch.
For production engines, induction hardening is more common. Each bearing journal is surrounded by an extremely strong magnetic field. As the crankshaft rotates, the alternating magnetic field heats the metal locally as opposed to heating the whole part, and each journal is quenched.
This quick process hardens the metal to a depth of about 0.060 inch, but the uneven heating creates stress that reduces the metal's strength. Still, the finished part is stronger than a cast iron crank, and the deep hardening means the crankshaft can be reconditioned.
Every crankshaft has a speed limit. At the end of the piston's stroke, when it stops and reverses direction, all that inertia is absorbed by the crankshaft. As piston speed increases, its inertia increases proportional to the square of its speed, so going from 4,500 rpm to 5,000 rpm increases the piston's inertia by 25 percent. That inertia is trying to bend the crankshaft at the "cheek" where the rod journal joins the counterweight.
A longer stroke means higher piston speed, more inertia and greater bending stress. In addition to bending, each piston's inertia acts on the crankshaft in a different direction, creating twisting forces that concentrate in the same area. So along with the material and manufacturing process, piston stroke is a major factor in a crankshaft's safe speed limit. The crank in a four-cylinder 2.4L engine with a stroke of 96mm is typically redlined at 6,000 RPM, while a 2.4L Formula 1 V8 engine with a stroke of only 40mm is routinely run at 17,000 RPM.
These are just some of the considerations that go into developing a crankshaft. Balance, vibration damping, lubrication, firing sequence, bore-to-stroke ratio and a dozen other parameters must all be factored into the design. It may have started out as an almost primitive iron casting, but today the crankshaft is one of the most highly developed components in a modern piston engine.
