Crankshaft

Crankshafts are affected by compressive, bending, shearing, stretching and twisting forces. In addition, these parts are subject to stresses caused by elastic deformations as a result of vibrations.

Currently, crankshafts are usually forged. Nevertheless, the use of casting technology has its advantages, the greatest of which is ease of obtaining desired shape and simplification of machining process. Cast shafts also have sufficient hardness, so there is no need for additional hardening of the surface.

Forged crankshafts are made mainly of non-alloy steel of higher quality and sometimes also of alloy steel. After forging, they undergo further thermo-chemical treatment. Surface hardening is used in order to obtain appropriate surface hardness of both main and crank pins. In case of shafts made of alloy steel, carburizing is used.

In the crankshaft one can distinguish: the main pins, which define axis of rotation of the shaft, the crank pins where the connecting rods are attached, and the arms connecting the main and crank pins. Shape of shaft is influenced by many factors, including engine design, number and layout of cylinders, order of ignitions, number of shaft pins, etc.

For in-line engines, number of crank pins is equal to number of cylinders. For fork engines this is reduced by half because usually feet of two connecting rods are attached to one crank pin.


Typical crankshaft- components

In modern engines, crankshafts are supported on main pins at each cylinder. In older designs that are not heavily worn, this might be the case at every second cylinder. Thanks to the use of bearing at every cylinder, the distance between support points is reduced and overall rigidity of the entire system increases.

Because of the use of a greater number of bearings, there are also more mechanical losses and consequently efficiency of such engine decreases. However, these are small values, because most losses are generated by friction between the piston and the cylinder wall. Hence, greater precision of production and machining is required in order to preserve coaxiality of seats for the main pins.

Rear part of the crankshaft usually ends with a flange for mounting the flywheel. It can be preceded by another flange, which acts as an oil dump and centrifuges oil flowing down by rear wall of the cylinder block.

Front end of the crankshaft generally serves for mounting the camshaft drive, belt pulleys of auxiliary devices (alternator, air conditioning compressor, power steering pump), centrifugal oil filter or torsion damper.

Extended crankshaft arms act as a counterweight, designed to balance the system. Inside the crankshaft arms and pins, holes and channels are drilled, through which pressurized oil flows to all places that require lubrication, e.g. bearings of pins.

Bearings of the main and crankshaft pins are plain bearings. They are characterized by higher load capacity and greater load-bearing resistance. They are lubricated in the above-mentioned manner through pressurized oil. Such plain bearing consists mainly of two half shells. Plain bearing in the form of one non-split shell, can be found in the first and last main pin. In this case, they can be made in the form of a sleeve.

It turns out that automotive bearing designs are not always standardized. Changes come as a result of more specific customer policy and unification of designs rather than adjusting to loads.