Gear failure

Last updated December 05, 2025

Sources of gear failure are varied, and are often based on the rotational speed and the load applied to the gear. It is possible that more than one of them occur at the same time. These sources are: wear, scuffing, pitting, micro-pitting, tooth flank fracture and tooth root fatigue fracture.^[1]^[2]^[3]

Wear

Wear is the damaging, gradual removal or deformation of material at solid surfaces.

Scuffing (or scoring)

Scuffing is related to metal-to-metal contact at high spots on the flank surfaces. Scuffing is a terminology mainly used in the automotive industry, while the term scoring is used in the aerospace industry.

Scuffing marks appear as streaks or scratches with sharpened bottoms and sides. They also frequently appear as bands of variable depth and width, oriented in the sliding direction. They can affect either isolated zones or the whole width of the face.^[4]

Pitting

Stress distribution in contacting surfaces due to rolling/sliding Rolling-sliding.png — Stress distribution in contacting surfaces due to rolling/sliding

Elastic stresses leads to crack nucleation near the surface, which can propagates into the gears. Pieces can break away, producing larger cavities. This is known as pitting, macropitting or micropitting. The initial stage of pitting is confined mostly to three areas along the profile of a gear tooth.^[5]

At low rotational speed, pitting is the predominant flank failure mode.^[2]

Tooth root and flank failure

Failure can occur by both fracture and fatigue.

Testing

Failure can happen by many different mechanisms, so gear testing procedures are designed to isolate the specific failure mechanism to study.^[2] Tooth root fatigue fracture can be studied through pulsator test. This test methodology consist in loading one or two teeth at the time using two anvils on which the load is applied. It provides different result with respect to the running gear test but they are still accepted.^[7]^[8]

There are several testing rigs with different dimensions and configurations to test gears of different shapes.^[6]

References

1 2 Niemann, Gustav; Winter, Hans; Höhn, Bernd-Robert; Stahl, Karsten (2019). Maschinenelemente 1: Konstruktion und Berechnung von Verbindungen, Lagern, Wellen (5. Aufl. 2019 ed.). Berlin, Heidelberg: Springer Berlin Heidelberg. ISBN 978-3-662-55482-1.
1 2 3 4 Totten, George E., ed. (2000-01-01). Mechanical Testing and Evaluation. Vol. 8. ASM International. doi:10.31399/asm.hb.v08.9781627081764. ISBN 978-1-62708-176-4.
↑ Totten, George E., ed. (2017-12-31). Friction, Lubrication, and Wear Technology. ASM International. doi:10.31399/asm.hb.v18.9781627081924. ISBN 978-1-62708-192-4.
↑ "ISO 10825-1:2022 Gears — Wear and damage to gear teeth Part 1: Nomenclature and characteristics". ISO standards.
1 2 Alban, Lester (1985). Systematic Analysis of Gear Failures.
1 2 Halgren, John A.; Wulpi, D. J. (1957). "Laboratory Fatigue Testing of Gears". SAE Transactions. 65: 452–470. ISSN 0096-736X. JSTOR 44564381.
↑ Bonaiti, Luca; Bayoumi, Ahmed Bayoumi Mahmoud; Concli, Franco; Rosa, Francesco; Gorla, Carlo (2021-05-03). "Gear Root Bending Strength: A Comparison Between Single Tooth Bending Fatigue Tests and Meshing Gears". Journal of Mechanical Design. 143 (103402) 103402. doi:10.1115/1.4050560. hdl: 11311/1192362 . ISSN 1050-0472.
↑ Bonaiti, Luca; Geitner, Michael; Tobie, Thomas; Gorla, Carlo; Stahl, Karsten (2023-01-25). "A Comparison between Two Statistical Methods for Gear Tooth Root Bending Strength Estimation Starting from Pulsator Data". Applied Sciences. 13 (3): 1546. doi: 10.3390/app13031546 . hdl: 11311/1233543 . ISSN 2076-3417.

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[:3-1] 1 2 Niemann, Gustav; Winter, Hans; Höhn, Bernd-Robert; Stahl, Karsten (2019). Maschinenelemente 1: Konstruktion und Berechnung von Verbindungen, Lagern, Wellen (5. Aufl. 2019 ed.). Berlin, Heidelberg: Springer Berlin Heidelberg. ISBN 978-3-662-55482-1.

[:2-2] 1 2 3 4 Totten, George E., ed. (2000-01-01). Mechanical Testing and Evaluation. Vol. 8. ASM International. doi:10.31399/asm.hb.v08.9781627081764. ISBN 978-1-62708-176-4.

[:5-3] Totten, George E., ed. (2017-12-31). Friction, Lubrication, and Wear Technology. ASM International. doi:10.31399/asm.hb.v18.9781627081924. ISBN 978-1-62708-192-4.

[:9-4] "ISO 10825-1:2022 Gears — Wear and damage to gear teeth Part 1: Nomenclature and characteristics". ISO standards.

[:7-5] 1 2 Alban, Lester (1985). Systematic Analysis of Gear Failures.

[:4-6] 1 2 Halgren, John A.; Wulpi, D. J. (1957). "Laboratory Fatigue Testing of Gears". SAE Transactions. 65: 452–470. ISSN 0096-736X. JSTOR 44564381.

[7] Bonaiti, Luca; Bayoumi, Ahmed Bayoumi Mahmoud; Concli, Franco; Rosa, Francesco; Gorla, Carlo (2021-05-03). "Gear Root Bending Strength: A Comparison Between Single Tooth Bending Fatigue Tests and Meshing Gears". Journal of Mechanical Design. 143 (103402) 103402. doi:10.1115/1.4050560. hdl: 11311/1192362 . ISSN 1050-0472.

[8] Bonaiti, Luca; Geitner, Michael; Tobie, Thomas; Gorla, Carlo; Stahl, Karsten (2023-01-25). "A Comparison between Two Statistical Methods for Gear Tooth Root Bending Strength Estimation Starting from Pulsator Data". Applied Sciences. 13 (3): 1546. doi: 10.3390/app13031546 . hdl: 11311/1233543 . ISSN 2076-3417.

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