Gear Tooth Breakage — Failure Analysis & 8D Corrective

What Is Gear Tooth Breakage?

Gear tooth breakage is the fracture of a tooth, usually at the root fillet, under bending fatigue or a single overload event. Fatigue breakage starts as a micro-crack at a stress concentration (a small fillet, a grinding mark or an inclusion) and grows until the remaining section cannot carry the load. Because it leaves characteristic beach marks on the fracture surface, breakage analysis can usually distinguish fatigue from overload. This is a D4 investigation where metallurgy, tooling and process control all come together.

Symptoms

• Fracture at the root fillet, or partial spalling of the contact surface
• Metallic debris in the gearbox oil, captured by the magnetic plug
• Knocking or grinding noise during operation
• Gear mesh frequency (GMF) sidebands in the vibration spectrum
• Reduced gearbox efficiency and torque fluctuation at the output

Affected Components

• Spur, helical, bevel and worm gears in automotive transmissions and industrial gearboxes
• Gear materials: case-hardening steels (20MnCr5, 8620), quenched and tempered steels, powder metallurgy
• Mating gears, shafts, bearings and the gearbox housing
• Lubrication system (oil pump, filter and oil grade)

Detection Methods

• Vibration spectrum: increasing GMF sidebands and harmonics
• Oil debris monitoring: magnetic plug inspection and spectrometric Fe analysis
• Magnetic particle inspection (MPI): surface crack detection at the root fillet
• Metallography: microstructure evaluation (retained austenite, martensite grade, non-metallic inclusions)
• Hardness profile: verifies case depth, surface hardness and core hardness against specification
• SEM fractography: fatigue striations indicate fatigue, cleavage indicates overload, intergranular indicates a material defect

Possible Causes

• Bending fatigue: cyclic stress at the root fillet exceeds the material fatigue limit; cracks start at machining marks or inclusions and propagate.
• Overload: a single excessive torque event, such as a sudden jam or impact.
• Pitting to spalling to breakage: contact fatigue grows, pits merge and reduce the effective section.
• Case-core interface failure: wrong case depth — too shallow puts the peak subsurface shear stress in the core; too deep makes the case brittle.
• Grinding burn: excessive grinding temperature tempers or re-hardens the surface and leaves residual tensile stress, causing micro-cracks.
• Misalignment: axial or angular misalignment concentrates load at the tooth end.
• Lubrication failure: metal-to-metal contact, scuffing and adhesive wear create scratches that concentrate stress.

5-Why Example

Problem: 3rd gear helical tooth broke at the root fillet after 15,000 km.

1. Why did the tooth break? → High-cycle bending fatigue — beach marks visible on the fracture
2. Why did fatigue occur? → Cyclic bending stress at the root fillet exceeded the material fatigue limit
3. Why was the stress high? → The fillet radius was 0.3 mm against a 0.5 mm specification, raising stress by about 30%
4. Why was the fillet undersized? → The hob tool had worn past its allowable limit, reducing the tip radius in production
5. Why was the tool not replaced? → Tool life was tracked by parts count, not by measuring the tool profile

TRC: Stress concentration at an undersized root fillet caused bending fatigue. MRC: Hob life management relied on parts count instead of tool geometry verification.

Fishbone (6M)

• Man: Increased feed rate to raise output accelerated tool wear
• Machine: The hob sharpening machine produced an inconsistent cutter tip radius
• Method: Tool change every 2,000 pieces regardless of measured wear
• Material: Steel lot variation — core hardness 28 HRC against the 32 HRC specification
• Measurement: Gear inspection every 200 pieces allowed many bad gears between checks
• Environment: Coolant concentration drifted to 5% against the 8% specification

D3 Containment

• 100% gear inspection of stock and in-transit parts using CMM or a gear tester
• Replace the hob immediately and verify the root fillet on the first five pieces
• Notify customers to inspect affected vehicles and replace the gearbox if abnormal noise is found

D5 Permanent Fix

• Automatic tool profile measurement every 200 pieces, including the root fillet
• Reduce the maximum parts per hob from 2,000 to 1,500
• Make the root fillet a mandatory CMM check with SPC
• Automatic coolant concentration control at 8% ± 0.5%

D7 Prevention

• PFMEA: raise detection for the root fillet with in-line measurement
• Tool wear prediction using cutting force monitoring
• Add root shot peening to introduce compressive residual stress and extend fatigue life

Key Takeaways

1. Fractography tells you fatigue versus overload before you change anything.
2. The root fillet radius is the single most important geometric control on a gear tooth.
3. Track tool life by geometry, not by parts count.
4. Case depth, core hardness and grinding burn must all be verified.
5. The management root cause is usually a tool-life management gap.