Crankshaft Failures

All Caterpillar Engines

There 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.

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.

NOTE: 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 exceeds the specifications for “bend” (total indicator reading). See Guideline for Reusable Parts, “Procedure to Measure and Straighten Bent Crankshafts,” Form No. SEBF8054.
8. Misalignment of the main bearing bores in the block. This can be caused by incorrect machining or by distortions from external forces, such as a bent engine base or improper alignment with the driven member. This driven member could be a marine transmission or a generator.

Additional information about the installation and alignment of Caterpillar engines, marine transmissions and generator sets is available in several Application and Installation Guides and Special Instructions. These publications are shown in the chart.

Illustration 3. Failure caused by bending fatigue.

Longitudinal Cracking

Crankshaft failures due to longitudinal cracking start with a momentary seizure of the bearing. This momentary seizure of the bearing is usually caused by a loss of oil film in a small area resulting in metal-to-metal contact with the journal. The metal-to-metal contact produces localized heat which causes small cracks (usually longitudinal) to form on the surface of the journal. These cracks eventually grow larger until the journal is weakened enough to permit a torsional fatigue failure. This torsional break often moves from the crack in opposite directions and at a 45° angle from each end of the crack. See Illustration 4.

One of the more common causes of momentary bearing seizures is excessively barrel shaped journals. Such journals carry a large portion of the load in the center of the journal and only a small portion on the ends. This high load condition in the center of the journal can result in metal-to-metal contact. Barrel shaped journals are usually produced during the grinding operation. The cause is normally a grinding wheel that has not been kept to the proper flatness. Such journals can also be produced by improper polishing.

Additional information is available in the Guidelines For Reusable Parts, “Crankshaft Measurements,” Form SEBF8041-01, “Procedure To Polish Crankshafts,” Form SEBF8042-01 and “Procedure To Grind Crankshafts,” Form SEBF8047.

Illustration 4. Longitudinal cracks.

Bearing Seizures

Crankshaft failures caused by a bearing seizure are different from longitudinal cracking failures because the oil film is lost over the entire journal. This causes the bearing to seize to the journal. High temperatures are generated during the bearing seizure. This tempers (softens) the journal and weakens the fillet. The result is a bending fatigue failure. The fracture usually starts in the fillet and progresses through the web to the adjacent main journal. See Illustration 5. In a failure of this type, there are usually several small cracks in the fillet or journal. These small cracks are an indication of a seizure in a large area.

Crankshaft failures due to bearing seizures can be caused by:

1. Plugged oil passages because of dirty oil or sludge.
2. Not enough bearing clearance. It is recommended that the bearing bores and the crankshaft journals be measured to determine if the bearing clearance is within specifications.
3. Not enough oil to the rod bearings because of momentary seizure of the main bearings. The momentary seizure can be caused by excessive main bearing clearance or a dry start after an overhaul. If there isn’t any oil in the oil cooler, oil filter or oil passages, the bearings can be damaged in the time it takes for these components to fill with oil. A shortage of oil to the bearings can also be the result of operating the engine at too high an rpm when the oil is still cold.After an engine overhaul, always pressure lubricate the engine before starting.

After an oil change, always crank the engine, without fuel, until the engine has some oil pressure.

4. Lubrication problems such as fuel dilution of the oil, anti-freeze contamination of the oil, excessive oil temperatures or improper oil cooling.
5. By not changing the oil at the correct interval.
6. An improper surface finish on the journals. See the Guideline For Reusable Parts “Crankshaft Measurement,” Form SEBF8041-01 and “Procedure To Polish Crankshafts,” Form SEBF8042-01.
7. Improper journal profile. An improper journal profile can cause a loss of oil film where the clearance is the smallest. For more information, see Engine Bearings and Crankshafts, Section 3, Form SEBD0531.
8. Engine overspeed.
9. No oil.
10. Improper assembly.

Illustration 5. Failure caused by a bearing seizure.

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