Why Do Gears Fail — Pitting, Scuffing, Wear and Tooth Breakage? Stress Data and Countermeasures

Core answer: Gear damage falls into four modes — pitting, scuffing, abrasive wear and tooth breakage. Pitting happens when contact stress exceeds the allowable value (about 650–720 MPa for quenched-and-tempered 40Cr, 1400–1500 MPa for carburized 20CrMnTi; table reference values); scuffing is weld-and-tear damage from excess flash temperature; wear is mostly contaminated oil; breakage is bending overload or a hidden grinding burn. Countermeasures in three steps: check strength per GB/T 3480, control heat treatment and grinding, manage lubrication and filtration. Most early failures are avoidable.

1. Four Failure Modes: Identify the Location First

Pitting is pockmark flaking on the tooth flank under cyclic contact stress. Scuffing is weld-and-tear damage from high pressure and temperature. Abrasive wear is hard particles scraping the flank. Tooth breakage is a whole or partial tooth fracture. Each has its own location, mechanism and fix. The overview:

Failure mode	Location	Mechanism	Typical trigger	Primary fix
Pitting	Near the pitch line	Contact stress above allowable, film breakdown	Undersized gear, low hardness	Bigger module, harder surface
Scuffing	Sliding zone of flank	Flash temperature too high, weld & tear	Heavy/high-speed load, poor lubrication	EP oil, lower roughness
Abrasive wear	Whole flank	Hard particles scraping	Dirty oil, seal failure	Filtration, scheduled oil change
Tooth breakage	Tooth root	Bending overload or shock	Overload, grinding burn	Check bending strength, shot peen

2. Pitting and Scuffing: Two Kinds of Flank Damage

2.1 Pitting: The Cumulative Result of Excess Contact Stress

Contact stress is the instantaneous compressive pressure between meshing teeth, checked per GB/T 3480 (identical to ISO 6336) with the Hertz formula. Pitting usually starts near the pitch line, where sliding speed is low and the oil film is hardest to build.

Data for steel gears: allowable contact stress depends on material and treatment — about 650–720 MPa for Q&T 40Cr, 700–780 MPa for Q&T 42CrMo, 1400–1500 MPa for carburized 20CrMnTi (HRC 58–62) (table values, pulsating loading). The same gear pair at 750 MPa contact stress has nearly double the margin in 20CrMnTi and sits near the limit in 40Cr. Same duty, different life — that is where the difference comes from.

Three fix directions: enlarge size or change material to push actual stress below allowable; keep lubrication sound, since film condition decides life when stress sits near the limit; raise surface hardness, which moves the allowable stress up. Target safety factor SH: 1.0–1.3 for general machinery, 1.3–1.6 for critical drives.

2.2 Scuffing: Weld, Then Tear

Scuffing is damage where the film breaks under high pressure and speed, metal touches metal, welds, and tears apart leaving grooves. Slow heavy-duty gears and high-speed gears both suffer it; the indicator is flash temperature. Mineral oils typically fail above about 120–180°C flash temperature; synthetics can go higher.

Machining and selection countermeasures: use oils with sulfur/phosphorus EP additives; bring flank roughness below Ra 0.4 to cut micro-asperity contact; higher surface hardness also helps. Nitrided gears have a low surface friction coefficient and show up in these duties too.

3. Wear: Half of It Is Lubrication Management

Abrasive wear is hard particles (dust, swarf) carried into the mesh by the oil, scraping the flank. New gears have a run-in period of roughly 0.005–0.02 mm of wear before settling into steady state. When oil cleanliness is poor, the wear rate climbs sharply, tooth profiles drift, and noise and backlash both worsen.

Oil cleanliness is rated per ISO 4406: 17/15/12, for instance, means 1300–2500 particles larger than 4 µm per milliliter. The controls are simple: matched filter rating, sealed housing, scheduled oil changes. Lubrication method matters too — splash for medium/low speed, forced jet for high speed and heavy load.

4. Tooth Breakage: Bending Stress, Shock and Grinding Burn

Root bending stress is the tensile stress at the tooth root under load; when it exceeds the limit, the tooth fractures from the root. Breakage comes in two forms: fatigue fracture from accumulated cycles, and overload fracture from a single shock. Shock duty takes an application factor KA up to 1.5–2.0; do not design it as smooth load.

Two hidden factors in machining. First, the root fillet: standard rack-type cutters leave a radius of about 0.38m; a small radius means higher stress concentration. Switching to full-radius cutters cuts the form factor YF by about 5%, and bending stress follows. Second, grinding burn: too aggressive infeed or poor cooling during gear grinding can temper or even re-harden the surface, leaving residual tensile stress on the order of hundreds of MPa — bending fatigue life drops noticeably. Detection: Barkhausen noise (magnetoelastic method) and nital etch. Finish-grind infeed is typically held at 0.005–0.01 mm per pass.

Shot peening is a common remedy: it puts roughly 300–600 MPa of compressive residual stress into the root surface layer, offsetting part of the working tensile stress. Carburized gears respond especially well because the root is hard.

5. Selection Points: Six Steps

Check strength: contact and bending safety factors per GB/T 3480, SH/SF 1.0–1.3 (general) or 1.3–1.6 (critical);
Set the application factor: KA 1.0–1.25 uniform, 1.25–1.5 moderate shock, 1.5–2.0 heavy shock;
Pick material by failure risk: harder surface (carburize, nitride) for pitting, check root strength and shot peening for breakage, EP oil for scuffing;
Choose lubrication: splash for medium/low speed, forced jet for high speed and heavy load, oil grade by ISO VG viscosity;
Control oil cleanliness: set filtration targets per ISO 4406, test on a schedule;
Set accuracy grade: grinding reaches DIN 5–6, hobbing and shaving DIN 7–8; accuracy affects load distribution and life.

Run the strength check before discussing process; spend money where it counts. When margin is short, enlarge module or face width first, then change material.

6. What Gyan Drive Can Do

Gyan Drive machines spur and helical gears from drawings in modules M0.5–M12, up to Φ600 mm OD, with tooth accuracy to DIN 5. Common materials — 20CrMnTi, 40Cr, 42CrMo, 38CrMoAl — are all in house, backed by carburizing, gas nitriding and full inspection (profile/helix/pitch testers, hardness testers, metallography), with grinding burn under process control.

Send your duty data — power, speed, shock level, lubrication conditions — and the engineering team usually replies within 24 hours with a free strength check and quotation. Identify the failure mode first; the fix direction follows.

👉 Visit the Gyan Drive website or the quote page with your duty data for a free strength calculation and process proposal.