Crankshaft

Only both pictures can be enlarged!

Function

The crankshaft should transfer the force from the piston and piston rod, as torque to the flywheel and/or the clutch. Together with its main bearing and connecting rod bearing it is part of the crank mechanism. Other parts belonging to the crank mechanism are:
- the piston and the piston pin,
- the connecting rod with the bearing bush at the small, and the bearing
  shell at the large connecting rod.

How it works

The piston rod encloses the connecting rod bearing of the crankshaft. The connecting rod bearings (here 4) are staggered by half a stroke in relation to the main bearings. Thus the crankshaft contributes its capacity, which results from cylinder bore and stroke. Opposite the connecting rod bearings frequently there are counterweights arranged, which adjust their out-of-balances at higher numbers of revolutions. If the crankshaft still exhibits imbalance after the manufacturing process, material is bored-away here. To the right the drive wheel for the camshaft drive via cambelts can be recognized.The lubricating of the connecting rod bearings takes place via (in fig. 4 above red marked) cross holes within the crankshaft from the side of the main bearings. The oil withdrawing at the connecting rod bearings is hurled by the rotating motion of the crankshaft against the cylinder walls and lubricates the piston and possibly also the gudgeon pin.

Special designs

Older V-engines might have two piston rods on a crankpin. These are not only broader, but they also have two drillings for oil lubrication of the connecting rod bearings (2nd picture above). Four-stroke engines usually have two-piece three-layer sliding bearings. The materials of the layers from inside (more soft) to outside (more hard): lead bronze, nickel and white metal. One of the bearings serves as thrust bearing. The crankshaft with a play of max. 0,2 - 0.3 mm is axially stored. Mixture-lubricated two-stroke engines have antifriction bearings.

Material

Crankshafts are made from nodular graphite iron, quenched and tempered or nitriding steel. Heavily used crankshafts can be forged (see video).