function
The connecting rod connects the piston to the crankshaft; it consists of the small and large connecting rod eyes and the shaft.
The rotation of the crankshaft causes a rotating movement of the large connecting rod eye. The small connecting rod eye follows the axial stroke movement of the piston in the cylinder. The connecting rod is therefore a component through which the axial movement of the piston is converted into a rotation of the crankshaft.
stresses
As an element that transmits forces and movements between the piston and the crankshaft, the connecting rod is subjected to large, changing loads. The connecting rod is subjected to compression and tension from the piston. The connecting rod is also subjected to bending stress due to its pivoting movement. It should be as light as possible but still sufficiently rigid. In addition, sufficient component and structural strength must be guaranteed.
Requirements
In principle, moving masses should be small in order to improve efficiency and stimulate vibrations. Weight can be minimized by reducing the length of the component. However, the changed lateral forces on the piston skirt and in the connecting rod must be taken into account.
connecting rod guide
The axial guidance of the connecting rod is usually carried out via the large connecting rod eye on the crank web. This is referred to as "bottom-guided". Less common is the guidance via the piston and the small connecting rod eye "top-guided". This requires a contact surface on the piston and the play must be reduced.
The advantages of the "top-mounted" connecting rod are: The thrust washers on the crankshaft are eliminated and the length of the piston pin can be reduced. This leads to a reduction in weight. Due to the smaller thrust surfaces, there are lower friction losses.
Disadvantages of the "top-mounted" connecting rod are: Vibrations and movements from the crankshaft are transmitted directly to the piston, which reduces the service life of the piston. Furthermore, the lubrication conditions in the small eye deteriorate because less oil gets through the narrow gap between the connecting rod and the piston.
materials
The sinter forging process has become the standard for use in series connecting rods. A forging process is used to reduce porosity. The material used, 3Cu6c, has a carbon content of 0.5-0.6%. Strength can be increased by adding up to 3% copper. Steel connecting rods manufactured using the drop forging process have become the standard for engines subject to high loads.
