The crankshaft should convert the movement of the piston along the middle axis of the cylinder into a rotating force. With it bending forces evolve that must be absorbed by sufficient main bearings, firmness and counterbalances. In addition, the longer the engine, the more torsion forces (twist) have to be countered by the crankshaft. Also the piston does not deliver its force steadily, but impulse-like. At the beginning of the power stroke the forces are stronger.

The crankshaft is connected to the engine case through its main bearings (5). The connecting rods enclose the connecting rod bearings or stroke bearings (2) with their connecting-rod big end and move with it the crankshaft into a rotary movement. Half a rotation of the crankshaft generates the full stroke of the piston. The centre distance of the main bearings and the cylinders is the same. Width and diameter of the main bearings and connecting rod bearings depend on the load of the crankshaft. This in turn depends on:
- large single cubic capacities,
- high combustion pressures,
- increased numbers of revolutions.

One of the main bearings has a collar as flanged bearing. This is an axial sliding bearing which fits exactly to the thrust strip (1) of the crankshaft and guides it in axial direction. It is actuated, e.g., whenever the clutch is operated. Sometimes there is instead of a flanged bearing a normal one with four sideways attached half shells.The big-end bearing (2) and the crank webs (3) determine the offset. In-line engines and opposed cylinder engines feature as many offsets as numbers of cylinder. V engines (see picture) often have two offsets integrated into one. Opposite the crank webs there are the counterbalances (4) whose size and shape also depends on the connecting rod length and the form of the piston shaft.

Counterbalances in the crankshaft raise the flywheel mass, resulting in a steadier run of the engine, helping it to get over pressure differences, dead centers and perhaps idle strokes (e.g., in case of a three-cylinder engine). In addition, kinetic energy is stored for a sudden load rise. However, for sporty engines this is undesirable as it restrains the quick rev increase. As the name indicates, the weights counteract the forces in the crankshaft originating from the piston. The threat of offset sag is thereby decreased. In addition, oscillations of the whole engine are reduced which develop from the transmission of the violent pistons and connecting rod movements onto the crankshaft. The counterweights are especially helpful for torque-strong diesel engines whose turning force may alter considerably also during the power stroke. 06/07