The crank shaft ( see photo at the end of the article) converts the up and down motion of the pistons to rotary motion. The polished bearing surfaces are called journals. The journals that support the crank shaft as it turns are called main bearing journals. Rod bearing journals transfer up and down motion between the crankshaft and connecting rod. The crankshaft has oil passages drilled from the main journals to the rod journals. Each crankshaft journal has a radius at its edge for added strength.
Opposite each rod journal is a counterweight that precisely balances the combined rotating mass of the offset rod journals and the rod. The counterweight is much heavier than the rod journal to compensate for the weight of the connecting rods, bearings, and piston assembly. Crankshafts are either cast or forged. Forged crankshafts are stronger but more costly. Cast cranks are of high enough quality to do an adequate job. The crankshaft can be pushed forward by pressure in the torque converter or by the release spring pressure of th clutch. This is called end thrust. One of the crankshaft main bearing has precision bearing surfaces ground on its sides, called thrust surfaces. A flanged thrust bearing fits between the crankshaft thrust surfaces, controlling back and forth movement (end thrust).
A forged crankshaft that is bent can be straightened while a bent cast one is typically replaced. During combustion, the force on the piston twists the crankshaft and the crankshaft tries to straighten itself. It overcorrects, twisting back in the other direction, these oscillations occur for several cycles before fading out. When oscillations from several cylinders are occurring at the same time, torsional vibrations can cause the crankshaft to break. The part that dampens torsional vibration at the front of the crankshaft is called the vibration damper ( see photo).
It has a heavy outer inertia ring and an inner hub separated by a synthetic rubber strip. The two parts stretching against the rubber strip absorb vibrations. Six and eight cylinder engines almost always have dampers. If a crankshaft is broken, the vibration damper should be checked for obvious signs of damage.
The crankshaft sometimes experiences excessive wear because of abrasives in the oil. Journals can wear out of round and tapered.
- Out of round wear: Crankshaft main bearing journals wear out of round. When the engine is first cranked after sitting for a period of time, there is little or no lubrication between the crankshaft and the lower main bearings. Hence the lower main bearing wears excessively and the main journals wear out of round.
- Tapered wear: Rod journals sometimes suffer taper wear. The presence of uneven rod bearing wear, and sometimes piston skirt wear, usually indicates taper. Connecting rods should be checked for misalignment whenever uneven wear is found.
When there is a nick in the crankshaft journal, it could be a high spot or a low spot. A low spot does not usually present a problem. A nick that results in a high spot must be lowered or it will wear a groove in the bearing. Another kind of wear to the crankshaft journal results from the void left by a bearing oil groove. The area that corresponds to the groove will be unworn, whereas the bearing support areas on both sides exhibit wear.
Shot blasting a crankshaft strengthens it and relieves stress. Certain precautions are necessary during the operation. Grinding the journal surfaces after blasting will result in a smooth surface, but the oil seal and thrust bearing surfaces must be protected from the shot. There is a lot to say about the crankshaft, we will leave that to one of the next articles.