Conrod Basics

The connecting rod connects the piston to the crankshaft and consists of the small end and big end bores as well as the shank. The rotation of the crankshaft induces a rotational motion of the big end bore, which has a bearing eccentric to the axis of the crankshaft. The small end bore follows the axial stroke motion of the piston in the cylinder (See below figure).

The connecting rod is thus a machine element that transforms the axial motion of the piston into the rotation of the crankshaft.

The space covered by the connecting rod during one revolution of the crankshaft, also known as the conrod sweep, must be considered in collision studies for the crankcase and engine block.

While the small end bore is always closed, the big end bore is normally designed to come apart for assembly. Below table provides information about the different design details of connecting rods, but not about the interrelationship of individual details. The task of the designer is to determine the correct configuration associated with the requirements profile.

Below figure shows the important terminology and dimensions of a connecting rod.

Types of connecting rods and design parameters:

Stress of conrod: As the element that transfers forces and motions between the piston and the crankshaft, the connecting rod is subjected to large, alternating loads. The connecting rod is loaded by the piston in compression (under prevailing gas force) and in tension (primarily because of inertia force).

The connecting rod is also stressed in bending as a result of its pivoting motion. As a moving engine component, it should be as light as possible and sufficiently stiff in shape in terms of interacting with the piston pin and the crankshaft pin. Sufficient component and structural strength must also be ensured.