The crankshaft should forward the force which is transferred to the con-rod(s) by the piston in the shape of torque to the flywheel or to the clutch. It is a part of the crank mechanism, to which belong also the:
Piston with the gudgeon pin,
Con-rod with the bearing bush on the small end and the bearing cup on the big end.
The main- and con-rod bearings, together with the crank-webs and and the counter balances belong to the crankshaft.
Importance of the crankshaft for the engine capacity
The big-end of the con-rod is clamped around the con-rod bearing of the crankshaft. The con-rod bearings are staggered by a half a stroke in relation to the main bearings. Thus, the
crankshaft is part of the calculation of the cubic capacity which is determined by the bore of the cylinder and the stroke. Opposite to the con-rod bearing there are often
counterweights installed, which compensate for the imbalance vibrations at high RPM. In larger engines they may also be bolted on. In the past, there were also counterweights which were riveted to the main shaft.
Should the crankshaft still be out of balance after the finishing process, the excess material is mostly drilled off. On the right the camshaft drive-gear for the timing belt can be seen.
Lubrication through transverse drillings in the shaft and splash-oil
The lubrication of the con-rod bearings takes place from the main bearings, and then through slanted borings (picture 4 in red) inside the crankshaft from one bearing to the other.
Through the crankshaft rotation the the oil escaping from the con-rod bearings is splashed onto the cylinder sleeves, thus lubricating the piston and possibly, also the gudgeon pins.
One crankpin for two con-rods and layered bearings
In V-engines two conrods can be mounted on only one crankpin. These crankpins are not only wider, they also have two lubrication borings for the con-rod bearings (see
picture 2 above). In four-stroke-engines there are generally two-piece, triple-layer plain bearings made of leaded bronze. The layers of leaded bronze, nickle-barrier and babbit are
made softer from the outside inwards. One of the bearings has a collar and serves as a fitting-bearing. Here, the crankshaft, with 0,2 - 0,3 mm play is situated. In mixture-lubricated two-
stroke-engines roller bearings are used, perhaps in the future, also more often in four-stroke-engines.
Cast-iron or steel with a surface finishing
Crankshafts are made of nodular graphite cast-iron, tempered-steel or nitrided-steel. High-stress crankshafts can also be forged (see clip below). When nitriding steel, during the selective heat treatment, nitrogen is
diffused into the steel at a depth of approx. 1 millimeter. Not only does this make the surface harder, it also gives the shaft more stability. Finally, the tension is reduced by annealing. 06/07