The crankshaft is the main rotating mechanism of the engine, which is responsible for transforming the reciprocating motion of the piston into its circular motion. In general, the engine speed is the crankshaft speed.

The crankshaft bears the force from the connecting rod and converts it into torque, which is output through the crankshaft and drives other accessories on the engine to work. It is subject to the combined action of centrifugal force of rotating mass, periodic gas inertial force, and reciprocating inertial force, which makes the crankshaft bear bending and torsional load. Therefore, the crankshaft is required to have sufficient strength and stiffness, and the journal surface needs to be wear-resistant, work evenly, and have good balance.

Composition of crankshaft

The crankshaft is mainly composed of the front end of the crankshaft (or called the free end), the connecting rod journal, the main journal, the crank, the balance weight, and the rear flange of the crankshaft (or called the power output end). The main journal is installed on the cylinder block, the connecting rod journal is connected with the large end hole of the connecting rod, and the small end hole of the connecting rod is connected with the cylinder piston, which is a typical crank slider mechanism.

Function of crankshaft

The function of the crankshaft is to convert the reciprocating linear motion of the piston into its rotary motion and output power.

In the process of engine operation, the piston is driven to move in a straight line by the combustion and explosion of the mixed compressed gas, and the force is transmitted to the crankshaft through the connecting rod. The crankshaft transforms the straight-line movement into rotary movement.

Therefore, the rotation of the crankshaft is the power source of the engine and the power source of the whole mechanical system.

The crankshaft is an important part of an automobile engine. Its role is to convert the gas force from the piston and connecting rod into torque and convert the linear motion of the piston into rotary motion, which is used to drive the automobile transmission system, engine valve train, and other auxiliary devices. To put it bluntly, it is the power output component of the car.

The force on the crankshaft is extremely complex. It works under the combined action of periodically changing gas force, inertia force, and torque, and bears the alternating load of bending and torsion. Therefore, the crankshaft is required to have sufficient bending and torsion resistance fatigue strength and stiffness; The journal shall have sufficient pressure-bearing surface and wear resistance.

The crankshaft is generally forged from 45, 40Cr, 35Mn2, and other medium carbon steel and medium carbon alloy steel. The journal surface is subject to high-frequency quenching or nitriding treatment and then finish machining. To improve the fatigue strength of the crankshaft and eliminate stress concentration, the journal surface shall be shot blasted and the fillet shall be rolled. It should be noted that the nitrided crankshaft must be nitrided again after grinding, otherwise, the crankshaft may break.

The crankshaft is composed of several unit cranks. A crank pin, left and right crank arms, and left and right main journals form a unit crank. The relative position or arrangement of each crank depends on the number of cylinders, cylinder arrangement, and engine working order.

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