ELBOW 9Y3948 - Caterpillar

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.
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 Engine Bearings And Crankshafts, Form SEBD0531, Section 5, for more information and illustrations on crankshafts with nicks, large scratches and cracks.
Illustration 1. Failure caused by torsional fatigue.
Illustration 2. Torsional failure because of large inclusions.Bending Fatigue
Crankshaft failures that are the result of bending fatigue usually start at a point of stress concentration in the fillet. The bending forces on the crankshaft cause a small crack to form which moves through the web between the rod journal and the adjacent main journal. See Illustration 3.
The normal causes of a stress concentration in the fillet are:
1. Sharp nick, notch, or scratch.2. Inclusion.3. Grinder burn.4. Wrong fillet (radius too sharp, incorrect finish).5. Wrong or no shot peen operation after grinding. See Guideline for Reusable Parts, "Procedure to Grind Crankshafts," Form SEBF8047.6. Main bearing failure. The main journal is no longer supported correctly and the bending forces increase across the unsupported journal. The result is a bending fatigue failure.7. Misalignment of the crankshaft in the block. This results when a crankshaft is used that