Gear Failure Analysis

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Gear Failure Analysis usually requires a close examination of the gear teeth, which have a difficult life. Load is applied to them each time they mesh and unmesh with another gear. This means that a gear rotating at 1,000 RPM will experience 1,000 fatigue cycles every minute. Because of this, it is necessary to design most gears so that they have infinite fatigue life. There is also a small amount of sliding between gear teeth as they mesh, which can lead to fretting damage, as shown in the left photo. Factors such as misalignment, poor lubrication, debris between the gear teeth & corrosive environments such as salt water can degrade life substantially. To deal with the sliding contact, Gears often fail because of surface damage which becomes a stress concentration factor that initiates a crack. After initiation, the crack grows into the interior of the tooth, as shown in the photo on the right, eventually resulting in failure.

Gear Failure Analysis - Fretting
Gear Tooth Fretting Damage
Gear Failure Analysis - Crack Propagation
Gear Tooth Crack Emanating from Fretting Damage

Gears are case hardened, often by carburizing, which results in a different microstructure on the surface than the interior. This produces a component where the hardness varies with surface depth, as shown in the graph on the left. It also produces a high level of residual stress near the surface, as shown in the graph on the right. The very high compressive stress in the first 0.001" below the surface are due to machining of the gear. However, it is so shallow that it has little effect on fatigue life. The compressive stresses in the 20-40 KSI range that extend 0.010" into the gear are due to carburizing, and has a greater effect on fatigue life.

Gear Failure Analysis - Hardness
Hardness Versus Depth
Residual Stress
Residual Stress Versus Depth

Gear Failure Analysis Articles


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Failure Analysis & Durability Improvement

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Ed Pope
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Ed@Failure-Analysis-Durability.com


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