TURBOCHARGER GP 1955995 - Caterpillar

All Caterpillar EnginesThere are two forces that act upon every crankshaft. Torsional forces are those forces which try to twist the crankshaft. These forces are the result of the rotating power that goes into the crankshaft when there is a power impulse (power stroke of the piston). Bending forces are those forces that act perpendicular to the centerline of the crankshaft and cause it to move towards the main bearing caps. This force is the result of the downward motion of the connecting rod when there is a power impulse.
Power impulses, which are the primary cause of both torsional and bending forces, occur at different times in the rotation of the crankshaft. For example, six cylinder engines will have a power impulse at every 120° of rotation. Eight cylinder engines will have a power impulse every 90° and twelve cylinder engines every 60°.
The presence of these forces, when combined with torsional fatigue, bending fatigue, longitudinal cracking or bearing seizure can cause a crankshaft failure.
Torsional Fatigue
Torsional fatigue failures are usually the result of fatigue of the crankshaft which is caused by excessive torsional vibration. Under normal conditions, the vibration damper will keep this torsional vibration from becoming excessive. If the vibration damper has failed, this excessive vibration can cause a torsional fatigue failure in the crankshaft.
The failure usually begins on a journal at a point of stress concentration, such as a flaw, crack, oil hole or lightening hole. It then progresses into a torsional fatigue failure. The fracture is generally at a 45° angle through the oil hole. It usually occurs about 2/3 of the way down the crankshaft from the damper.
Illustration 1 shows two examples of a torsional fatigue failure. The fracture usually goes in one of two directions as shown by the lower illustration in Illustration 1.
Illustration 1. Failure caused by torsional fatigue.Few crankshafts fail from true torsional fatigue, primarily because the vibration damper keeps the torsional vibrations within a safe limit. If the vibration damper does fail, normally the timing gears will fail before the crankshaft is damaged.
The following are some of the causes of crankshaft failures due to true torsional fatigue:
1. Defective or failed vibration damper.2. Failed torsional coupling.3. Improper applications of the engine. All engines in Caterpillar vehicles and packaged generator sets can be operated at approved loads and speeds without a torsional fatigue failure. Caterpillar engines in other applications will give acceptable operation if a torsional vibration analysis is made on the complete system before installation. This analysis can be performed by Caterpillar. For more information, see the Special Additions section of the Engines Price List.4. A large inclusion or groups of inclusions (foreign material) in or under the journal surface. These inclusions are generally characterized by a short longitudinal fracture with 45° fractures at each end as shown in Illustration 2. Many inclusions are not harmful. For more information, see Guideline For Reusable Parts, "Inspection Of Crankshafts For Cracks," Form No. SEBF8039.5. Nicks, large scratches and cracks in the oil hole. See