Gear Failure Analysis: Pitting, Scuffing, Wear and Tooth Breakage — Stress Data and Corrective Measures

Contents

Four damage mechanisms account for the majority of gearbox failures: pitting, scuffing, abrasive wear and tooth breakage. Each one leaves a signature at a specific location on the tooth, and each one responds to a different corrective measure. Diagnosis therefore comes before redesign. Once the mode is confirmed, the remedy path is usually one of three: re-verify load capacity against GB/T 3480, tighten heat treatment and grinding control, or upgrade lubrication and filtration practice. Most failures that appear early in service trace back to one of these three areas rather than to an unavoidable material limit.

1. Damage Location Map: Four Modes Side by Side

Before any measurement, the position of the damage on the tooth narrows the cause considerably.

Pitting presents as clustered flaking pits on the working flank. Scuffing shows as welded-and-torn grooves along the sliding direction. Abrasive wear removes material uniformly across the flank. Breakage is a complete or partial fracture originating at the root fillet.

2. Flank Damage: Pitting and Scuffing

2.1 Pitting: Contact Stress Above the Limit

The instantaneous compressive pressure where two teeth touch is the contact stress. It is evaluated with the Hertz formula as set out in GB/T 3480, which is technically equivalent to ISO 6336. Pitting usually starts near the pitch line because sliding velocity there is lowest and building a separating oil film is most difficult.

For steel gears the permissible contact stress is governed by the material and its heat treatment. Under pulsating load, published table values are roughly:

Take a pair running at 750 MPa. With 20CrMnTi the remaining margin is close to double; with 40Cr the stress sits near the ceiling. Same transmitted torque, very different service life.

Corrective work runs along three lines:

• Resize or reselect the material so that the calculated stress drops below the permissible figure.
• Keep the lubrication system healthy, because when stress approaches the limit the film condition decides life.
• Increase surface hardness, which directly raises the permissible stress.

Safety factors to aim for on contact stress, SH: 1.0–1.3 for ordinary machinery, 1.3–1.6 for drives where downtime is costly.

2.2 Scuffing: Welding Followed by Tearing

When pressure and speed overcome the film, asperities touch, weld together and are then ripped apart, leaving scored grooves. This is scuffing. It threatens both low-speed heavily loaded units and high-speed units. Flash temperature is the controlling variable. Mineral oils lose effectiveness once flash temperature passes roughly 120–180°C; synthetic fluids withstand higher values.

Practical countermeasures span oil choice and geometry: select lubricants formulated with sulfur- and phosphorus-based EP additives; hold flank roughness below Ra 0.4 so that fewer micro-asperities make contact; increase surface hardness. Nitrided gears, which exhibit a low coefficient of friction at the surface, also find use in these applications.

3. Root Breakage: Bending Stress, Shock and Grinding Burn

Bending stress at the root is the tensile stress carried by the tooth under load. Exceed the material limit and the tooth snaps off at the root. Two distinct forms exist: fatigue fracture, which accumulates over many cycles, and overload fracture, which follows a single severe event. Where shock is present the application factor KA reaches 1.5–2.0, and such duties must not be treated as smooth-running.

Two factors hide inside the machining process. The first is the root fillet radius. A standard rack-type cutter produces a radius near 0.38m; a tighter radius concentrates stress. Moving to a full-radius cutter lowers the form factor YF by roughly 5%, and bending stress falls with it. The second is grinding burn. Too much infeed or insufficient cooling during gear grinding can temper, or in severe cases re-harden, the surface layer, leaving residual tensile stress measured in hundreds of MPa; bending fatigue life then drops noticeably. Barkhausen noise testing (the magnetoelastic method) and nital etching are the accepted detection routes. For finish grinding, infeed is normally limited to 0.005–0.01 mm per pass.

Shot peening is the standard remedy. It places compressive residual stress of approximately 300–600 MPa into the surface layer at the root, which offsets part of the tensile stress the tooth sees in service. Carburized gears benefit most, since their roots are already hard.

4. Abrasive Wear: Mostly an Oil Cleanliness Problem

Hard particles such as dust or machining swarf enter the mesh carried by the lubricant and cut into the flank. New gears go through a run-in stage during which roughly 0.005–0.02 mm of material is worn away before steady conditions are reached. If oil cleanliness is poor, the wear rate rises steeply, the tooth profile shifts, and both noise and backlash deteriorate.

Cleanliness is classified under ISO 4406. A code of 17/15/12, for instance, corresponds to 1300–2500 particles above 4 µm in each milliliter. The countermeasures are uncomplicated: match the filter rating to the requirement, seal the housing, and change oil on schedule. Lubrication method matters as well — splash lubrication suits medium and low speeds, while high speed and heavy load call for forced jet supply.

5. Selection Workflow in Six Steps

1. Verify strength. Calculate contact and bending safety factors per GB/T 3480, targeting SH/SF of 1.0–1.3 for general machinery or 1.3–1.6 for critical drives.
2. Assign the application factor. KA 1.0–1.25 for uniform loading, 1.25–1.5 for moderate shock, 1.5–2.0 for heavy shock.
3. Select material against the dominant failure risk. For pitting, a harder surface via carburizing or nitriding; for breakage, verify root strength and consider shot peening; for scuffing, specify EP oil.
4. Choose the lubrication approach. Splash for medium and low speed, forced jet for high speed and heavy load, with the oil grade set by ISO VG viscosity.
5. Set cleanliness targets per ISO 4406 and test the oil at defined intervals.
6. Fix the accuracy grade. Ground teeth reach DIN 5–6; hobbing and shaving reach DIN 7–8. Accuracy governs load distribution and, in turn, service life.

Complete the strength check before debating process details, and direct spending toward the limiting factor. When margin is tight, enlarge module or face width first; change material second.

6. What PinyiGear Manufactures

PinyiGear produces spur and helical gears to drawing; module range, outside diameter and tooth accuracy are per the values stated on the customer drawing. The frequently specified grades — 20CrMnTi, 40Cr, 42CrMo and 38CrMoAl — are all handled in house, supported by carburizing, gas nitriding and inspection using profile, helix and pitch testers, hardness testers and metallographic equipment, with grinding burn kept under process control.

• Spur gear products
• Helical gear products
• Manufacturing capabilities

Supply your duty data — power, speed, shock level and lubrication conditions — and the engineering team will return a strength check and a quotation once that data has been reviewed. Establish which failure mode is at work first; the corrective direction follows from it.

Send your duty data through the PinyiGear website or use the engineering review page to request a free strength calculation and process proposal.