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Everything about the Crankshaft

Function | Manufacturing | Different types | Forged | Billet

All internal combustion engine crankshafts have main journals and rod journals. Here are the main journals, these are what the crankshaft itself rotates on and are held in the engine block by the main bearing caps.

Rod journals, a.k.a. crank pins or big end journals are where the big ends of the conrods are connected to. Rod journals are connected to main journals via crankshaft webs. Now, the distance between the main journal center-line, and the rod journal centerline is called the crank throw, a.k.a. crankshaft radius. And this measure determines the stroke of the engine.

The stroke of an engine will be 2 times the crank throw. At the end of the crankshaft we are going to find a flywheel flange, this is where the flywheel is bolted onto. The flywheel with it’s heavy round mass smooths out the pulsation of the combustion inside the engine occurring at different times.

On the other end of the crankshaft is the nose. This is where the crankshaft pulley is attached. These are the counterweights. The operation of an internal combustion engine generates strong rotational forces, and the mass of the piston, piston pin and rings and the connecting rod moving up and down at high speeds generates a very significant force that is exerted onto the crankshaft. The counterweights have the task of balancing out these forces.

We will talk about counterweights in more detail later in the video. The holes you can find in the rod and main journals are oiling holes. Oil coming thorough the engine block into the crankshaft and out these. Another very important design element of the crankshaft is the radius fillet.

Engineers take great care when designing this, because a proper radius fillet is key to a crankshaft not breaking apart. The radius fillet is key because it spreads the load and relieves the stress in what would otherwise be an extremely common point of stress fracture on any crankshaft.

Crankshaft manufacturing process

There are 3 main manufacturing processes for crankshafts are Casting, forging and CNC machining. Casting is the most cost effective processes and in general results in the weakest art. Cast parts are often more brittle, that is to say they have a lower tensile strength and lower ductility compared to forged and machined billet parts.

Forged crankshafts –

the forging process of a crankshaft involves a large crankshaft sized billet being heated up to about 2.500 -2.700 degrees Fahrenheit and then put into giant presses with dies in them that apply anywhere from 150 to 250 tonnes of pressure to shape the heated up billet into a rough forging. The rough forging is then machined and heat treated to create the finished crankshaft. You can tell a crankshaft has been forged by looking for wide parting lines and signs of grinding on those lines. The main difference when it comes to forged crankshaft vs cast is that the forging process compresses the grain structure of the metal into a much more confirm one compared to a cast part which results in greater strength and ductility.

Billet crankshafts –

when it comes to billet crankshafts there’s no casting, or forging or anything. You take a big billet and machine away material until you’re left with a crankshaft, that’s it. This takes a lot of time, and a lot of machining is needed to make a billet crankshaft which is why billet crankshafts are often very expensive and often reserved for racing and other extreme applications. The great thing when it comes to billet crankshafts vs forged is that there are infinite design possibilities for billet! Crankshafts also often undergo heat and surface treatments such as induction hardening, tufftriding (tuftriding). When it comes to lubrication we have two different types. Cross drilled crankshafts and straight shot oiling crankshafts. To combat drag created by crankcase windage, crankshaft counterweights are sometimes knife-edged.

Here’s a summary of crossplane vs flatplane crankshafts:

Crossplane cranks are usually larger and heavier so they have a lower max rpm, but they make the engine run smoother, generate more torque and sound different.

Flatplane crankshafts engines are more prone to vibration, but are also more compact and capable of higher max rpms.

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