Engine Crankshafts

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The crankshaft is the component in the engine that translates reciprocating linear piston motion,into rotary motion, which the engine can more freely use. To convert this reciprocating motion, the crankshaft uses crankpins which are bearing surfaces that are offset from the crank. Connecting rods from each cylinder attach to the crankpins to complete the transfer of motion which will create the rotary motion that the engine can use. The crankshaft will usually connect to a flywheel which will help reduce the pulsation characteristics of the engine. Dampers can be used at the opposite end of the crankshaft to reduce torsion vibrations which are caused by the cylinders farthest from the output end acting on the torsional elasticity of the metal.

Crankshafts are used in all different varieties of engines ranging from a complex engine with multiple cylinders and pistons, to an engine that has one cylinder and a single piston. The configuration of the engine and number of pistons in relation to each other and the crankshaft leads to straight, V or flat engines. The firing order differs in engines depending on its size, which is why a variety of crankshafts are produced.

Since the crankshaft has a linear axis which it rotates on, several bearings are used to provide for a smoother spin. These bearings also help protect the crankshaft from the huge amounts of sideways load that it receives from each cylinder in a multicylinder engine. Higher performance engines are more likely to use a greater amount of bearings than a lower performance engine to prevent flex when using high compression ratios and higher rotational speeds.

Crankshafts play a key role when taking into account engine balance. Since rotational motion provides more of a balance issue, counterweights may be used to control the reciprocating mass of each piston and connecting rod. These counterweights are typically cast as part of the crankshaft, but sometimes they will come as bolt on pieces. The heavy counterweights are usually made out of tungsten alloy or depleted uranium which can both stand up to the environment and stresses put on the crankshaft. Even though counter weights add a considerable amount of weight to the crankshaft, it helps provide a smoother running engine which allows higher RPM’s to be achieved.

Most crankshafts are monolithic which means that they are made out of a solid piece of material. Some very small and very large crankshafts are assembled using multiple pieces which can allow for easier transportation or delivery. The most common method when producing a crankshaft in modern day vehicles is forging. Forging the steel to make the crankshaft provides an advantage because it provides for a lighter weight, more compact dimensions, and better inherent dampening. Machining a crankshaft out of billet steel is also a popular production method. These machined crankshafts tend to be more expensive because more material is needed to be removed when shaping the crankshaft. Machining mat also leave fiber flow which is usually undesirable but not a problem with the higher quality steels that are used when machining.

Fatigue of the metal can also be a problem on crankshafts since they are put under such heavy loads. To prevent fatigue, a radius at the end of each main and crankpin bearing is used. The radius itself will help reduce stress in these critical areas, while leaving compression residue from its rolled surface to prevent cracks from forming. Almost all production crankshafts use induction hardened bearing surfaces which increase strength and provide good results at a low price.

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