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What Is The Function Of The Piston Pin On The Piston?

2021-12-24

The function of the piston pin is to connect the piston and the small end of the connecting rod, and transmit the force between the piston and the connecting rod.

The piston pin bears periodic shock load, and it swings itself, and it works under poor lubrication conditions. Therefore, the piston pin is required to have sufficient strength and rigidity, good surface toughness, good wear resistance, and light weight. Piston pins are generally made of hollow cylinders, made of low-carbon steel and low-carbon alloy steel. The outer surface is carburized and quenched to increase the hardness. After finishing, it is polished to have high dimensional accuracy and surface finish.

The basic structure of the piston pin is a thick-walled tubular body, and some have variable cross-section structures according to the requirements of equal strength
There are two ways to connect the piston pin to the piston pin seat hole and the connecting rod small end bushing hole: full-floating connection and semi-floating connection.

Full-floating connection means that when the engine is at normal operating temperature, the piston pin can rotate freely in the piston pin seat hole and the connecting rod small head sleeve hole. As shown in the figure, with a full-floating connection, the piston pin can withstand greater impact load, wear more uniformly, and have a long service life, but the assembly process is more complicated. When assembling the aluminum alloy piston and the piston pin, the piston should be heated in water or oil at a temperature of 70-90℃ to expand the piston pin seat hole before installing the piston pin. In order to prevent the axial rotation of the piston pin, snap rings are also installed at both ends of the pin seat.

Semi-floating connection means that one place is fixed between the piston pin and the piston pin seat hole and the connecting rod small end hole. The general piston pin is fixed to the connecting rod small end.The bolts shown are also fixed with the piston pin and the small end of the connecting rod by interference fit. Since there is no relative movement between the piston pin and the connecting rod, there is no need for a snap ring to restrict the axial position of the piston pin, that is, the semi-floating connection piston pin has no snap ring, and there is no bushing at the small end of the connecting rod. The semi-floating piston pin can reduce engine noise and eliminate possible accidents caused by the snap ring. It is mostly used in light high-speed engines, such as Cherokee and Cheetah, etc. cars adopt this form.
A-Full-floating
B-Semi-floating