A Comprehensive Review of Gear Failure Modes (28 Types in 6 Categories)

A Comprehensive Review of Gear Failure Modes (28 Types in 6 Categories)
Gears are core transmission components in mechanical systems, widely applied in aerospace, automotive, wind power, and heavy machinery industries. Their failure will lead to equipment downtime, efficiency reduction, and even catastrophic accidents. This paper systematically summarizes 28 gear failure modes divided into 6 major categories, including their formation mechanisms, typical characteristics, and preventive measures, which is an essential industry knowledge for mechanical design, failure analysis, and maintenance personnel.
I. Bending Fatigue
Bending fatigue occurs when the repeated bending stress on gear teeth exceeds the material's fatigue strength, with the maximum stress concentrated at the tooth root fillet, initiating and propagating cracks. Ratchet marks are observed at the crack initiation site, and beach patterns in the propagation area. The key influencing factors are cycle number and load level, and reasonable load control and structural optimization are the core of prevention.
1.1 Low-Cycle Bending Fatigue
It occurs when the cyclic load is less than 10,000 cycles. Ductile materials with low hardness show rough, fibrous fracture surfaces with tear marks, while hard and brittle materials present smooth and sleek fracture characteristics.
1.2 High-Cycle Bending Fatigue
It happens when the cyclic load exceeds 10,000 cycles, with the material in a low-strain state and generally within the elastic deformation range.
Preventive Measures
Select materials with higher fatigue strength and optimize gear design to match load and fatigue limit.
Adopt shot peening (introduce compressive stress at tooth root fillet) or polishing (reduce surface roughness) processes.
Implement rational heat treatment to minimize residual stress in gear teeth.
II. Hertzian Fatigue
Hertzian fatigue failure is caused by cracks initiated by repeated Hertzian forces on the surface or subsurface, leading to material loss with crack propagation. Reasonable material selection and lubrication are the main ways to mitigate such failure, which is a common surface fatigue form of gears under contact load.
2.1 Macropitting
Cracks form on the surface or at a certain subsurface depth and propagate a short distance almost parallel to the surface, forming pitting craters with sharp and angular features. It is classified into non-progressive (diameter < 1 mm), progressive (diameter > 1 mm) and flaky types. Flaky macropitting is relatively shallow but covers a larger area, and fatigue cracks expand from the fan-shaped initiation area of the tooth surface to form triangular pits.
2.2 Micropitting
It appears in areas with dull, gray spot-like stains and wear signs on the surface, which is common in surface-hardened gears and may also occur in fully hardened gears with improper design.
2.3 Subsurface Fatigue
Cracks initiate in the transition zone between the tooth core and the hardened layer under the tooth surface, mainly caused by surface hardening processes (carburizing, nitriding, induction hardening, flame hardening).
2.4 Spalling
A severe form of surface fatigue, characterized by large-area shallow pits on the tooth surface formed by the expansion and merging of multiple small pits. It significantly increases gear noise and vibration, induced by high contact stress and material defects.
Preventive Measures
Reduce contact stress, use clean steel, and add grinding or honing processes in tooth surface manufacturing.
Adopt high-viscosity lubricants; avoid shot peening on tooth flanks for micropitting prevention.
Select hardenable steel, prevent overheating, and control stress below subsurface strength for subsurface fatigue.
Ensure correct gear alignment and use high-quality materials to prevent spalling.
III. Wear
Wear refers to the surface degradation of gear teeth caused by material removal or displacement under electrical, mechanical and chemical stresses over time, leading to abnormal noise, poor NVH performance and even potential failure. It is divided into mild, moderate and severe grades, among which mild wear is normal in many applications.
3.1 Adhesive Wear
Material transfers from one tooth surface to another due to micro-welding and tearing, limited to the oxide layer and surface film of the tooth surface. Mild adhesion may self-disappear as the surface smooths, while moderate adhesion erases part or all of the original machining marks.
3.2 Abrasive Wear
Caused by hard particles (external contaminants such as dust in lubricant, or self-generated hard metal debris from other failures), mostly occurring at the tooth tip or root due to high local sliding speed. Severe wear leads to complete disappearance of machining marks and sharp reduction of tooth thickness.
3.3 Polishing Wear
A slight abrasive wear that makes the tooth surface gradually smooth due to contamination by chemically active lubricants with high-efficiency anti-scuffing additives, more likely to occur when hard and soft surfaces contact.
3.4 Corrosive Wear
A severe surface degradation caused by the combined action of mechanical wear and chemical erosion, with the tooth surface showing stains, rust and reddish-brown appearance, and the whole tooth surface may be affected.
3.5 Fretting Corrosion
Occurs when contact surfaces bear small-amplitude reciprocating relative motion under pressure, with lubricant squeezed out leading to metal-to-metal contact and adhesion. The fractured rough peaks generate cocoa powder-like iron oxide powder, which hinders lubricant replenishment and aggravates wear.
3.6 Scuffing
A severe adhesive wear where metal debris from one tooth surface transfers to another and welds/tears on the surface. The damaged area is rough and dull, distributed as thin/broad bands along the sliding direction, and may occur instantaneously rather than due to fatigue. Severe scuffing leads to plastic distortion of surface material.
3.7 Electrical Discharge Machining (EDM) Wear
High temperature caused by electric spark breakdown of the oil film between gear teeth leads to local melting of the tooth surface, forming smooth and round hemispherical pits surrounded by ablated/melted steel, induced by static electricity, shaft current or poor grounding.
Preventive Measures
Increase lubricant film thickness (smooth surface, high speed, low temperature lubricant) for adhesive wear; install high-efficiency oil filters to remove hard particles for abrasive wear.
Use borate-based anti-scuffing additives and remove abrasive particles from lubricant for polishing wear.
Select corrosion-resistant materials (e.g., stainless steel), reduce environmental humidity and use high-quality lubricants for corrosive wear.
Adopt materials with similar electrochemical properties, apply coatings and add corrosion inhibitors for fretting corrosion.
Optimize gear geometric parameters, use nitrided steel and high-viscosity anti-scuffing lubricants for scuffing.
Design rational grounding systems, use insulated bearings and conduct regular maintenance for EDM wear.
IV. Cracking
Cracking refers to the formation of various cracks in gears during manufacturing, heat treatment or operation, which is a dangerous failure form that may lead to sudden fracture of gears, mainly related to process defects, stress concentration and material defects.
4.1 Quenching Cracks
Caused by heat treatment processes (carburizing, nitriding, induction hardening) due to thermal stress, material defects or improper cooling, characterized by intergranular linear cracks expanding from the surface to the tooth center.
4.2 Grinding Cracks
Surface or subsurface cracks generated during tooth grinding, induced by excessive heat generation and insufficient cooling, leading to residual tensile stress and even grinding burn (softening or rehardening of local areas). Overheated areas can be identified by acid etching, with brown/black tempering zones and white untempered martensite spots.
4.3 Rim and Web Cracks
Rim cracks usually fracture between adjacent teeth, expanding radially through the rim and web; web cracks are caused by high cyclic stress or stress concentration (e.g., holes), and resonance of gear blanks will aggravate such cracks. High centrifugal force may lead to catastrophic failure at high speed.
4.4 Tooth Surface/Core Separation
The hardened tooth surface separates from the soft tooth core due to internal cracks at the interface, leading to fracture of tooth corners, edges or the entire tooth tip. Cracks may appear immediately after heat treatment, during transportation/storage or in service.
Preventive Measures
Ensure symmetrical gear blank structure, uniform wall thickness, optimize heat treatment process and temper immediately after quenching for quenching cracks.
Select appropriate grinding wheels, control feeding speed and coolant supply, limit surface hardness below 60 HRC and retained austenite content below 20% for grinding cracks; use magnetic particle inspection for crack detection.
Design rim thickness as twice the tooth depth, reduce stress concentration and avoid resonance for rim and web cracks; conduct magnetic particle inspection regularly.
Control carburizing depth at the tooth tip, select clean steel with high fracture toughness, avoid shot peening on the tooth surface and temper immediately after quenching for tooth surface/core separation; use ultrasonic testing for defect identification.
V. Plastic Deformation
Plastic deformation refers to the permanent change of gear tooth profile, leading to high vibration, abnormal noise and poor meshing performance, mainly caused by excessive contact stress, high temperature and poor lubrication, with the material undergoing irreversible shape change.
5.1 Cold Flow
Occurs below the recrystallization temperature, with material pushed/dragged along the sliding direction under high contact pressure, leading to surface indentation and severe rounding of tooth tips; cold work hardening of surface and subsurface materials occurs under high load.
5.2 Hot Flow
Occurs above the recrystallization temperature, with plastic flow of gear material under the combined action of high temperature and stress, leading to distortion of gear size and shape, mostly caused by overloading or insufficient lubrication (frictional heat accumulation).
5.3 Indentation
Hard foreign bodies (metal or debris) on meshing tooth surfaces form depressions or pitting on the driving tooth surface, leading to increased stress, reduced efficiency and abnormal vibration.
5.4 Rolling Deformation
High contact stress caused by the combined rolling and sliding motion during gear meshing leads to plastic deformation, which may induce surface cracks and material displacement with crack propagation.
5.5 Waviness
Periodic wavy deformation of the driving tooth surface with wave crests perpendicular to the sliding direction and fish-scale appearance along the tooth length, mostly occurring at low speed due to insufficient elastohydrodynamic oil film, related to plastic flow under high contact stress and boundary lubrication.
5.6 Ridging
Formed by surface/subsurface wear and plastic flow, induced by poor lubrication, lubricant contamination, misalignment and overloading, leading to increased noise, reduced efficiency and even severe damage.
5.7 Tooth Root Fillet Yielding
Permanent bending of gear teeth when the bending stress at the tooth root fillet exceeds the material's yield strength, leading to significant pitch error and destructive interference between meshing teeth, and increased noise and vibration.
5.8 Tip-Root Interference
Plastic deformation, adhesion and abrasion occur at the tooth tip of one gear and the tooth root of the mating gear, caused by insufficient tip/root modification, geometric/pitch error or improper center distance; overloading will aggravate the defect by reducing the meshing clearance, and the damaged area may induce Hertzian fatigue.
Preventive Measures
Reduce contact stress, improve surface/subsurface hardness, and increase pitch accuracy for cold flow; select high-temperature resistant materials and ensure sufficient lubrication for hot flow.
Use clean high-quality lubricant, install efficient filters and seal gear boxes reasonably for indentation; adopt rational lubrication, material selection and surface treatment for rolling deformation.
Ensure proper lubrication, correct alignment and surface hardening of soft materials for waviness; guarantee sufficient lubrication, optimize gear design and correct alignment regularly for ridging.
Select materials with higher yield strength, increase tooth root fillet radius and adopt surface hardening (carburizing/nitriding) for tooth root fillet yielding; avoid overloading.
Increase pressure angle, conduct root cutting, add tooth number or enlarge center distance for tip-root interference; optimize gear geometric parameters and control machining accuracy.
VI. Fracture
Fracture is the ultimate failure form of gears, with the tooth structure completely damaged and losing transmission function, mainly divided into brittle fracture, ductile fracture and impact fracture, different in fracture characteristics and inducing factors.
6.1 Brittle Fracture
Characterized by rapid crack propagation without obvious plastic deformation, with bright and granular macroscopic fracture surface, flat and perpendicular to the stress axis. It is usually caused by high impact load or stress concentration, and occurs at a lower stress level than ductile fracture.
6.2 Ductile Fracture
Significant plastic deformation occurs before material fracture, with obvious necking or elongation, and the fracture surface showing cup-cone shape, fibrous and gray appearance. A shear lip may form on the non-working side of gear teeth, and the material absorbs a large amount of energy during fracture.
6.3 Impact Fracture
Caused by sudden high-stress load or impact (overloading or accidents), leading to sudden fracture of gear teeth without obvious precursor. The fracture surface is flat and perpendicular to the main tensile stress direction, similar to brittle fracture, induced by excessive load, accidental impact and internal material defects.
Preventive Measures
Implement proper heat treatment to improve material toughness and select materials with high fracture toughness and ductility for brittle fracture.
Use high-strength materials with minimal defects for ductile fracture.
Select high-quality materials, conduct regular non-destructive testing and avoid overloading/accidental impact for impact fracture.
Key Summary
Gear failure is a comprehensive result of material, design, process and operating conditions. Mastering the 28 failure modes in 6 categories is the basis for effective prevention and failure analysis. The core preventive principles include: rational material selection and surface treatment, optimized structural design to reduce stress concentration, strict control of manufacturing/heat treatment processes, proper lubrication and maintenance, and compliance with rated operating conditions. For different failure forms, targeted detection and prevention measures should be adopted to improve the reliability and service life of gear systems.