Summary of knowledge points on fatigue failure of gearbox gears

Release Time：

2025-07-17

Gear fatigue failure mainly includes tooth surface contact fatigue and tooth root bending fatigue. This article introduces the mechanism of fatigue failure, the specific failure forms of gear contact fatigue and bending fatigue, as well as the selection of gear materials and heat treatment, and gear strength calculation, so that everyone can have a comprehensive understanding of the failure forms of gears.

1、 Explanation of Fatigue Failure Mechanism
The failure under alternating stress is commonly referred to as fatigue failure.

Characteristics of fatigue failure

(1) The failure stress value of alternating stress is generally lower than the strength limit under static load, and sometimes even far below the yield limit of the material;
(2) Both brittle and plastic materials exhibit brittle fracture under alternating stress, with no obvious signs or plastic deformation before fracture;
(3) The propagation of cracks is intermittent, and the fracture surface can be clearly distinguished into two parts: smooth zone and rough zone.

The physical meaning of the life coefficient: It reflects the influence of stress cycle times on fatigue life, and is the degree of increase in finite life fatigue strength relative to infinite life fatigue strength, usually greater than 1.
From the S-N curve, it can be seen that the fatigue strength of the material is different from its static strength. The static strength can be expressed using only the strength limit; For the fatigue strength of materials, it is necessary to indicate that under the specified value of r, smax and the corresponding number of failure cycles N should also be specified. That is, only by using three physical quantities (r, N, smax) simultaneously can the fatigue strength of materials be described.

2、 The fatigue failure of gears is divided into contact fatigue and bending fatigue

Gear fatigue failure mainly includes tooth contact fatigue and tooth root bending fatigue. Tooth surface contact fatigue refers to the cyclic contact stress on the tooth surface of a gear during operation, which causes surface material fatigue peeling or crack formation, ultimately leading to tooth surface pitting corrosion. Tooth root bending fatigue refers to the occurrence of fatigue cracks at the root of gears under bending stress, leading to tooth breakage.

Failure form

Tooth surface pitting: This is one of the common forms of failure, usually occurring in closed gear transmission. Due to repeated contact stress, the surface material fatigue peels off, forming pitting pits.

Tooth fracture: Due to alternating bending stress exceeding the fatigue limit of the material, the tooth fractures. This type of failure usually occurs in situations of high-speed overload or overload impact.

Tooth surface bonding: Under high-speed and heavy load conditions, lubrication failure causes direct contact between two tooth surfaces, resulting in high-temperature adhesion and subsequent tearing, which belongs to instantaneous failure.