Are there different types of crankshafts?

An engine crankshaft on a workbench

Apart from a few specialised applications that do not use metal bearing inserts, but roller bearings to reduce friction, the only meaningful difference between mass-produced crankshafts intended for use on standard vehicles of all types and weight classes involves the way they are made. Let us look at the three main crankshaft-manufacturing processes in turn-

Cast crankshafts

This is both the most common and most cost-effective way of mass-producing crankshafts for light vehicles. The process is relatively simple; foundries make moulds from sand or other materials and then pour molten metal into the moulds under controlled conditions to-

  • prevent air or gas bubbles from forming in the casting, or
  • to prevent the casting from cooling down unevenly or too fast, which could cause the casting to crack and/or fracture under even moderate loads

However, the steel alloys used for crankshaft castings by reputable foundries/manufacturers are fairly advanced, and they typically contain a variety of minerals that increase the hardness, wear resistance, and tensile strength of the finished product.

Once the castings pass an initial but fairly rigorous quality check, the castings are sent to machine shops where they are machined, ground, polished, and balanced to produce crankshafts that conform to OEM specifications in terms of form, fit, and function.

Forged crankshafts

The word "forged" refers to a process in which a piece of hot metal is hammered into the desired shape by hugely powerful hydraulic or mechanical hammers. Since forging effectively compacts the metal by forcing the molecules that make up the metal closer together, this process produces products that are stronger than similar cast products by several orders of magnitude.

However, since the forging process is both expensive and time-consuming because of the large number of machining steps required to produce a usable crankshaft, this method of making crankshafts is almost exclusively used to fabricate extremely strong crankshafts for use in heavy truck engines, diesel locomotive engines, and in extremely large marine (ship) engines.

Billet crankshafts

In this method, a crankshaft is machined out of a single piece of forged steel, aka, billet steel, by computer-controlled lathes. While this method is more expensive and time-consuming than both casting and forging processes, the advantage of this method is that it produces crankshafts that are uniformly strong and rigid throughout their structure.

Since the steel billet is never heated during the machining process, a billet crankshaft is not affected by the uneven stresses and mass distributions in the base material that result from uneven heating and/or cooling. As a result, it is possible to balance a billet crankshaft to a far higher degree of accuracy than either a forged or cast crankshaft, which is why manufacturers of high-performance super/hypercars use billet crankshafts in their car's engines.

Note that regardless of how any given crankshaft is made, all crankshafts undergo several rounds of heat treatment at different stages during the manufacturing process to increase the hardness and wear resistance of the outer layer of the bearing journals.

While the type, duration, and method of heat treatment depend on the steel alloy used in the crankshaft, the fact is that if heat treatments are not performed, or if they are performed incorrectly, even the best-made crankshaft's useful life could be measured in minutes due to the high rate at which the bearing journals would wear away.      

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