What Causes A Crankshaft To Fail?

A crankshaft is the heartbeat of an engine, converting the linear motion of pistons into rotational power. When it fails, the result can be catastrophic—costly repairs, extensive downtime, or even full engine replacement. Understanding the root causes of crankshaft failure is critical for proactive maintenance and prolonging engine life. Below, we break down the most common factors behind crankshaft failure, supported by technical insights and practical prevention strategies.

1. Fatigue Failure: The Silent Culprit

Why it happens: Fatigue failure is the leading cause of crankshaft damage, accounting for up to 68% of cases in heavy-duty diesel engines (Journal of Mechanical Engineering, 2023). It begins with tiny cracks forming at stress concentration points—such as journal fillets, oil hole edges, or manufacturing flaws—under repeated cyclic loads (e.g., bending, torsional, or shear stress). Over time, these cracks propagate, weakening the shaft until it fractures.

• High-risk zones:

• Journal fillets: Where the journal meets the crank web, a common spot for stress concentration.

• Oil hole edges: Sharp edges here act as stress raisers under torsional loads.

• Manufacturing defects: Imperfections from forging or casting create weak points.

• Load types & failure locations:

  Load Type	Impact on Crankshaft	Typical Failure Location
  Bending Loads	Max stress at center/ends	Journal fillets
  Shear Loads	Cyclic twisting forces	Crankpin radii
  Torsional Loads	Rotational stress	Oil hole edges

2. Insufficient Lubrication: The Engine’s Lifeline Lost

Why it matters: Proper lubrication is non-negotiable. The crankshaft operates at high speeds (up to 6,500 RPM in some engines) and withstands immense pressure from cylinder explosions. Inadequate oil flow or degraded lubricant leads to:

• Bearing wear: Metal-to-metal contact between journals and bearings, causing scoring, seizures, or heat buildup.

• Thermal stress: Friction generates excessive heat, weakening material integrity and accelerating fatigue.

• Lubrication system red flags:

• Straight-shot oiling systems: Prioritize main bearings but may neglect rod bearings if passages are clogged.

• Cross-drilled passages: Improve rod bearing lubrication but are prone to debris blockage.

• Prevention: Regularly check oil viscosity, pressure (ideally 25–65 PSI at operating temp), and replace filters to ensure clean, consistent oil delivery.

3. Mechanical Overload & Sudden Jamming: Pushing Beyond Limits

Overload scenarios:

• Continuous overloading: Running the engine beyond its rated torque or RPM (e.g., exceeding 6,500 RPM in a design-limited system) causes gradual bending or twisting.

• Sudden jamming: Gearbox failures, piston seizure, or foreign object intrusion create instantaneous shock loads, increasing fracture risk by 30% (Smith et al., 2022).

Abnormal combustion effects:

• Pre-ignition: Early fuel-air mixture ignition increases compression pressure, straining the crankshaft.

• Detonation (Engine Knock): High-pressure waves vibrate cylinder walls and shaft, causing metal fatigue and thermal overload. Combustion temperatures can spike by 200°C, accelerating wear.

4. Vibration & Dynamic Imbalance: The Hidden Stress Multipliers

Sources of vibration:

• Torsional vibration (Diesel engines): Pulsating torque from cylinder firing causes the shaft to twist cyclically, leading to cracks at crankpin radii.

• Dynamic imbalance: Unbalanced pistons, connecting rods, or a loose flywheel (misalignment > 0.05 mm) create vertical/side-to-side vibrations, stressing bearings and journals.

Danger of resonance: When vibration frequency matches the crankshaft’s natural frequency, resonance amplifies stress, leading to rapid fatigue and catastrophic failure.

5. Bearing Issues: The Foundation of Stability

Bearing failure pathways:

• Clearance problems: Too tight (0.025 mm < clearance < 0.075 mm target) causes metal rubbing; too loose reduces oil pressure and increases windage.

• Misassembly: Reversed bearing caps, misaligned oil holes, or improper torque (below 120 Nm or above 150 Nm for cap bolts) create uneven load distribution.

• Contamination: Dirt or debris in oil scratches bearing surfaces, breaking the oil film and causing scoring.

Visual warning signs: Deep grooves on journals, copper-layered bearings, or debris in oil pan—indicators of advanced wear requiring immediate attention.

6. Corrosion & Manufacturing Flaws: Weakness from Within

• Corrosion: Moisture or corrosive contaminants (e.g., engine coolant leaks) degrade material strength, leading to rust pits and stress corrosion cracking.

• Forging/casting defects: Incomplete heat treatment, porosity, or uneven grain structure weaken the shaft, making it susceptible to failure under normal loads.

7. Improper Installation: A Recipe for Disaster

Critical installation errors:

• Incorrect torque specs: Under-tightened cap bolts (e.g., <120 Nm) cause bearing movement; over-tightening leads to excessive crush and bearing deformation.

• Misaligned flywheel: Tolerance > 0.05 mm creates imbalance, increasing vibration and stress.

• Neglected lubrication priming: Running the engine without pre-lubricating bearings increases startup friction by 40%.

Case Study: Lessons from a V-16 Diesel Engine Failure

A 20,000-hour-old V-16 engine failed due to a lethal combination of:

1. Abnormal combustion (detonation-induced thermal stress).

2. Fatigue cracks at journal fillets from prolonged high-load operation.

3. Bearing wear from inadequate oil pressure, accelerating stress concentration.
   The failure could have been avoided with regular torsional vibration analysis and strict lubrication system maintenance.

Prevention Strategies: Keep Your Crankshaft Running Strong

1. Proactive Maintenance:

• Perform regular NDT (non-destructive testing, e.g., magnetic particle inspection) to detect early cracks.

• Monitor oil quality and pressure, replacing filters every 500 hours or as manufacturer-recommended.

2. Precision Installation:

• Use torque wrenches and follow OEM specs for bearing clearances (0.025–0.075 mm) and flywheel alignment (<0.05 mm).

• Dynamic balance all rotating components (pistons, rods, flywheel) to minimize vibration.

3. Load Management:

• Never exceed rated RPM/torque; install overload protection systems for high-risk applications.

• Address detonation promptly by checking spark timing and fuel quality.

Conclusion: Prioritize Prevention Over Repair

Crankshaft failure is rarely sudden—it’s a result of ignored warning signs and neglected maintenance. By understanding the key causes—fatigue, lubrication failure, overload, and installation errors—you can implement targeted strategies to protect your engine’s most vital component. Regular inspections, precise installations, and proactive care will save you from costly downtime and ensure your engine operates at peak performance for years to come.

Remember: A crankshaft in good health is the foundation of a reliable, efficient engine. Don’t wait for failure to strike—build a maintenance plan that keeps stress, wear, and corrosion in check.

As a leading foundry with decades of expertise in crankshaft manufacturing, TIEGU provides end-to-end solutions for industrial equipment and engine manufacturers. From precision casting and non-destructive testing to customized repair and preventive maintenance, our technical excellence ensures your crankshafts withstand the toughest conditions. Trust TIEGU to safeguard your engine’s core performance and keep your operations running smoothly.