The Functions of Crankshafts in Engines

Cranks and crankshafts have existed since the ancient Roman Empire, in a variety of agricultural and industrial applications. With the dawn of the Industrial Revolution and the invention of the internal combustion engine, however, crankshafts became an integral and vital part of motors, without which vehicles like automobiles and aircraft could not function.

A crankshaft is a shaft with one or more cranks, obviously, and in the context of an internal combustion engine is the primary shaft to which connecting rods pair it with the pistons of the engine. As the cylinders of the engine fire and push the pistons, the connecting rods transfer this motion to the crankshaft, which transmits this energy to the wheels of an automobile, the propeller of an aircraft, or whatever means of propulsion the vehicle has. Essentially, the crankshaft transforms the linear motion of the piston into a rotational motion that provides energy for the vehicle to move.

Crankshafts must endure tremendous stresses in their operation, so they must be extremely durable and well-engineered. The crankshaft is connected to the fly-wheel, the engine block, and to the pistons via their respective connecting rods. The crankshaft has a linear axis that it rotates around, with several bearing journals riding on replaceable bearings in the engine block. As the crankshaft undergoes sideways load from each cylinder in the engine, it has to be supported by several bearings, not just one at each end. Higher performance engines tend to have more main bearings than low-performance engines to provide more support to the crankshaft.

Some engines must mount counterweights for the reciprocating mass of each piston and connecting rod to maintain engine balance. These are usually cast as part of the crankshaft, but some are bolt-on pieces. These add weight, obviously, but provide a smoother-running engine and allow higher RPM levels to be achieved. In some configurations, the crankshaft contains direct links between adjacent crank pins without an intermediate main bearing. These links are called flying arms, and are sometimes used in V6 and V8 engines, and allow the engine to be designed with different angles between valves than what would otherwise be required to create an even firing interval. This arrangement reduces weight and total engine length, but also reduces the crankshaft’s rigidity.


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