Understanding What a Crankshaft Is

Crankshaft Image
Often we take calls from customers who were advised by an automotive technician that their vehicle engine’s crankshaft is bad, has spun a bearing or is knocking. Some of these customers do not understand what a crankshaft is (pictured to the left), where it’s located and why it is important. Because of this, we wanted to spend a little time educating those who are new to the inner workings of an internal combustion engine.

An understanding of the crankshaft is not just important for auto mechanics and automotive machinists, but vehicle owners should have a general idea of how their engine operates. An educated vehicle owner is better prepared to make important decisions about their car or truck, which in the end helps them to save money.

It is our hope that the information below will help you better understand the purpose of a crankshaft, how it operates and aspects of this internal engine part.

Purpose of Crankshafts and Their Operation

When you open your vehicle's hood, a good portion of the engine block is visible. Inside the engine block is where the crankshaft is located. Therefore, it is not an easy engine part to access and most often requires that the complete engine be removed to extract the crankshaft for repairs. A harmonic balancer is normally visible inside the engine compartment, which bolts onto the front of the crankshaft. A flywheel or flexplate is affixed to the rear of the crankshaft, which has gears that allow the starter to crank the engine when the ignition switch is actuated. The flywheel or flexplate also transfers the power produced from an internal engine to the vehicle’s transmission.

Now that you understand where the crankshaft is located, let’s look at why it is needed. A crankshaft is where an engine’s connecting rods and pistons mount to. These three components, being a crankshaft, connecting rods and pistons, are known collectively as a rotating assembly. As an internal combustion engine fires a cylinder, the piston is driven down the engine block’s cylinder as another piston is brought to the top of a different cylinder and will fire as that cylinder’s spark plug ignites the fuel within. The distance between the up and down movement of the pistons is known as the stroke. The connecting rods have a reciprocating motion during this process and the crankshaft’s main journals translate that into a rotating motion so that the engine’s power can be transferred to the transmission. The transmission then delivers this power to the axles so that the vehicle’s wheels are powered. As you can see, a crankshaft is a very important part of the internal combustion engine and must accept a great deal of stress.

A crankshaft has some very critical areas that must maintain close tolerances, including the radius for strength. The reciprocating portion of the crankshaft, which are the connecting rod journals, are machined to precise diameters that are often within specification limits of .001” or less. The main journals, which are bolted into the engine block, also must maintain close tolerances. These tolerances allow the use of engine bearings to be used, which creates an oil barrier to separate the crankshaft's main and rod journal steel from the steel on the engine block’s main caps and connecting rods. In most automotive engines, the crankshaft also has what is called a thrust. The thrust is also protected with a bearing and is designed to limit the crankshaft’s forward and backward movement. When a bearing wears, due to improper clearances or improper oil supply, the bearing will wear to the point where the journal’s steel can come into contact with a connecting rod or main cap’s steel. This is what has occurred when the engine is knocking or as some like to call it a spun bearing.

As you can imagine, a crankshaft needs to rotate rather quickly to produce enough horsepower to move a vehicle on city streets and highways. Therefore, the crankshaft must be balanced so that it does not vibrate. To assist in maintaining its balance, crankshafts have counterweights that help to compensate for the weight of the connecting rods, pistons and piston rings. Although the counterweights do help to balance a crankshaft, whenever connect rods or pistons are changed the entire rotating assembly should be professionally balanced so that the engine runs properly at all RPM ranges.

Crankshafts are manufactured with a variety of metals, which is largely dependent on their application. For example, many OEM engines have cast iron crankshafts while high performance engines may have 4340 forged steel crankshafts. While cast iron is more brittle and prone to cracking, 4340 forged steel crankshafts can handle higher RPMs and more horsepower without failing.

Now that you understand what a crankshaft is, we invite you to visit our crankshaft terminology page to view an image that describes all the specific areas of the crankshaft. Taken with the context of this page, you will develop a thorough understanding of the crankshaft that is powering your vehicle’s engine.

Please note that if your crankshaft does need to be repaired, we do offer affordable welding and grinding services. Those outside of our service area can quickly locate a nearby automotive machine shop in our directory.
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