As one of the core components of the transmission system, the fatigue performance of gear directly affects the service life of the whole equipment under the action of long-term alternating load. How to accurately calculate the fatigue life of gear and how to improve its fatigue resistance through material selection and tooth design are the key issues in the design of gear box.

Why calculate fatigue life?

The gear is subjected to periodic load (alternating stress) during operation. If the gear material is not resistant to fatigue, it is easy to appear fatigue cracks and even fracture in the process of use, resulting in equipment shutdown and heavy losses.

Fatigue failure is one of the most common failure modes of gears, which often occurs without obvious warning. Therefore, accurate evaluation of its fatigue life in the design stage is the premise to ensure the reliability of equipment.

Calculation method of gear fatigue life

Gear fatigue life is mainly evaluated from two directions:

1. Bend fatigue life

This is a fracture caused by alternating bending stress at the root of the tooth, and the calculation formula is as follows:

among:

Ft: Gear tangential load

b: breadth of tooth

m: modulus

YF: Tooth coefficient

The K series coefficient is affected by load distribution, dynamic load and other factors

The bending fatigue safety factor should be greater than the set value (generally 1.25~1.5) to ensure the reliability of gear long-term operation.

Contact fatigue life

Pitting corrosion on the surface of the material is caused by long-term alternating contact stress between the teeth. The calculation formula is as follows:

σH: contact stress

ZE: Elasticity influence coefficient (material dependent)

ZH: Gear geometric coefficient

The fatigue life of contact also needs to meet the set safety factor (generally 1.0~1.2).

Key factors affecting fatigue life

1. Material selection

The strength, hardness and toughness of different materials directly affect the fatigue performance of gears. For example, alloy steel after carburizing and quenching can significantly improve the hardness and fatigue limit of tooth surface.

Common high performance materials include:

18CrNiMo7-6 (European standard alloy carburized steel)

20CrMnTi (domestic commonly used carburized steel)

2. Gear shape optimization

The process of tooth root transition fillet design, tooth surface shaping (such as drum-shaped shaping) and tooth surface precision grinding can effectively relieve stress concentration and improve fatigue life.

Case study: fatigue life assessment of 18CrNiMo7-6 material

18CrNiMo7-6 is a low carbon alloy carburized steel with excellent hardenability and fatigue resistance. It is often used in high speed heavy load gear systems.Material performance parameters (after carburizing, quenching and tempering):

According to ISO 6336 standard for fatigue life assessment, gear parameters, load conditions and material data can be input to obtain the expected life of the gear (such as 10^7~10^8 cycles).

The fatigue life of gear not only depends on the performance of the material itself, but also depends on the rationality of the design, the precision of the process and the matching degree of the operating conditions.

A "good material" is the foundation, but only when combined:

Reasonable gear design

Precise heat treatment process

Strict manufacturing quality control

Only in this way can we truly realize the long life and high reliability of "good gears".