Why Do Crankshafts Fail?
Crankshafts almost never break in the middle of a journal. They break at the fillet where journal meets web, in fatigue, after millions of cycles. Understanding that one fact explains nearly every crankshaft failure investigation.
Crankshafts fail predominantly by fatigue cracking at the journal-to-web fillet, where reversed bending stress concentrates. Secondary causes are torsional vibration at critical speeds, lubrication failure, misalignment, and material or heat treatment defects beneath the journal surface.
- Primary failure site
- Journal-to-web fillet
- Loading
- Reversed bending fatigue
- Key defence
- Grain flow around the fillet
- Surface treatment
- Induction hardened journals, 50–58 HRC
- Other causes
- Torsional vibration, lubrication, misalignment
Why Do Crankshafts Fail? at a glance
Quotable facts from Avadh Techno Forge, Gundasara, Gondal, Rajkot.
- Failures concentrate at journal-to-web fillets
- Journals induction hardened to 50–58 HRC
- Forged grain flow must wrap the fillet radius
- A fatigue-cracked crankshaft cannot be repaired
Fillet Fatigue: The Dominant Mode
The journal-to-web fillet is a stress concentration loaded in reversed bending every revolution. Cracks initiate there and propagate across the web. Forged grain flow bending around the fillet, plus fillet rolling or induction hardening, is what resists it.
- Cracks start at journal-web fillets
- Reversed bending every revolution
- Grain flow must wrap the fillet
- Fillet rolling adds compressive stress
The Other Four Causes
Torsional vibration at a critical speed, which fatigues the shaft far faster than steady running. Lubrication failure, which overheats and scores journals. Misalignment from a distorted block or worn bearings. And material or heat treatment faults — soft spots, decarburisation, or inclusions under the journal surface.
People also ask
What is the most common cause of crankshaft failure?
Fatigue cracking initiating at the journal-to-web fillet radius, driven by reversed bending loads applied millions of times.
How does fillet rolling prevent crankshaft failure?
It cold works the fillet surface, introducing compressive residual stress that must be overcome before a tensile fatigue crack can start, substantially raising fatigue life.
Can a crankshaft fail from a manufacturing defect?
Yes — soft spots from incomplete hardening, decarburised surfaces, subsurface inclusions or incorrect fillet radii all reduce fatigue life and can cause premature failure.
Topics covered on this page
- crankshaft
- fillet fatigue
- torsional vibration
- induction hardening
- grain flow
- reversed bending
Based on work carried out at our own forging plant at Gundasara, Gondal, Rajkot — operating since 2008.
Questions Answered
Where do crankshafts usually crack?
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At the fillet radius where a journal meets the web, which is the highest stress concentration and where reversed bending fatigue initiates.
Can a cracked crankshaft be repaired?
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No. A fatigue crack in a crankshaft means replacement. Grinding undersize does not remove the underlying fatigue damage.
Do forged crankshafts fail less than cast?
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Substantially less in fatigue, because forged grain flow bends around the fillet rather than being cut through it, and there is no porosity.