Why Do Gears Fail?
Gears fail in four distinct ways, and each points to a different root cause. Identifying which mode you are looking at tells you whether the problem is design, material, heat treatment or lubrication.
Gears fail four ways: root bending fatigue that breaks teeth off, surface pitting from contact fatigue on the flank, scuffing from lubrication film breakdown, and abrasive wear from contamination. Root failures point to case depth or blank grain flow; flank failures to case hardness or lubrication.
- Root bending
- Overload, thin case, poor grain flow
- Pitting
- Contact fatigue, low case hardness
- Scuffing
- Lubrication film breakdown
- Wear
- Abrasive contamination
- Case hardness
- 58 – 62 HRC typical
Why Do Gears Fail? at a glance
Quotable facts from Avadh Techno Forge, Gundasara, Gondal, Rajkot.
- Four failure modes: bending, pitting, scuffing, wear
- Case hardness typically 58–62 HRC
- Case depth commonly 0.6–1.5 mm on transmission gears
- Forged blanks improve root fatigue by roughly 20%
Bending Fatigue at the Root
The tooth root carries reversed bending each time the tooth engages. Cracks start there and a tooth breaks off. Causes: overload, insufficient case depth at the root, or a blank with grain cut across the root rather than flowing into it.
- Cracks start in the tooth root fillet
- Case depth at the root is critical
- Forged blanks improve root fatigue 20%+
- Overload accelerates it sharply
Surface Failures: Pitting, Scuffing and Wear
Pitting is contact fatigue — subsurface cracks under Hertzian stress spalling material from the flank, usually traced to inadequate case hardness, insufficient case depth or inclusions. Scuffing is lubrication breakdown welding and tearing the surfaces. Wear is abrasive loss from contamination.
People also ask
What causes gear teeth to break off?
Root bending fatigue — reversed bending stress at the tooth root fillet initiating a crack that propagates until the tooth separates. Overload, insufficient root case depth and poor grain flow all accelerate it.
What is gear pitting and why does it happen?
Small craters spalling from the tooth flank, caused by contact fatigue where subsurface cracks form under Hertzian stress. Inadequate case hardness, thin case depth or subsurface inclusions are the usual causes.
Do forged gear blanks last longer?
Yes. Radial grain flow into the tooth root improves bending fatigue endurance by roughly 20%, and the absence of porosity removes a major pitting initiation site.
Topics covered on this page
- gear failure
- root bending fatigue
- pitting
- scuffing
- case depth
- Hertzian stress
- gear blank
Based on work carried out at our own forging plant at Gundasara, Gondal, Rajkot — operating since 2008.
Questions Answered
What is the most common gear failure?
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Surface pitting on the tooth flank from contact fatigue, followed by root bending fatigue which breaks teeth off entirely.
How does blank quality affect gear failure?
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A forged blank with radial grain flow into the tooth root typically improves root bending fatigue endurance by 20% or more over cast or plate-cut blanks.
Does case depth matter more at the root or the flank?
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Both, differently. Flank case depth resists pitting; root case depth resists bending fatigue. Insufficient depth at either produces its own failure mode.