As the core component of mechanical transmission system, gear mainly faces two kinds of fatigue failure forms in long-term working state: bending fatigue fracture and contact fatigue pitting. In order to ensure its safe and reliable operation within the design life cycle, it is necessary to carry out systematic fatigue test and life prediction for gear.

This paper introduces the commonly used gear fatigue test methods in rail transit and industrial fields, and explains how to establish life prediction model through test data in combination with common standards, so as to predict the life of gears

It provides a reliable basis for structure optimization, material selection and heat treatment process.

Gear fatigue failure mode

Gear fatigue failure is mainly divided into the following two categories:

Reinforced fatigue (Tooth Root Bending Fatigue)

During the meshing process, the root area of the gear is subjected to periodic bending stress, which may form cracks and expand to fracture under long-term action.

Contact fatigue (Contact Fatigue /Pitting)

The tooth surface is subjected to cyclic contact stress during the process of meshing and contact, and cracks are produced on the surface and sub-surface, resulting in material spalling to form pitting (pitting) or large flakes (spalling).

Overview of test methods

The common test methods of gear fatigue test mainly include:

Reinforced fatigue test (Tooth Bending Fatigue Test)

ISO 6336-5《Calculation of load capacity of spur and helical gears – Part 5: Strength and quality of materials》

• AGMA 2001
• GB/T 14230
• Typical test equipment:
• Single gear testing machine: the load is applied to the root area of the tooth for repeated loading.
• Rotational bending testing machine: simulate the working stress state of gears.
• Data processing method:
• S-N curve (stress-life curve) fitting

• Contact fatigue test (Pitting Test)
• Standard basis:
• FZG gear testing machine (Fraunhofer Standard, Germany)
• ISO 14635 / ASTM D4998
• SH/T 0306-2017 (Evaluation of gear resistance to pitting by lubricating oil)
• Test content:
• Gear pair engagement test is carried out under specified speed, load and lubrication conditions to monitor the generation time and area of pitting.
• The fatigue life of tooth surface contact is determined and the influence of lubrication conditions on fatigue behavior is analyzed.
• 

Life prediction method

The data obtained from fatigue tests can be used to establish gear life models. Common methods are as follows:

Life prediction based on S-N curve

The S-N curve is fitted using fatigue life test data at different stress levels:

among:

N: Gear life (number of cycles)

σ: stress amplitude

A, b: fitting constants

Weibull Statistical law

Weibull distribution is used to analyze a set of failure life data to determine the reliable life, characteristic life and variability:

η：characteristics life

β：Shape parameter (determines the shape of the distribution curve)

F(t)：The probability of failure before time t

Miner Accumulative damage method (used in variable load conditions)

In actual working conditions, gears are often under multi-stage load. The Miner rule can be used to accumulate the predicted fatigue life:

Suggestions for improving gear life

According to the test results, the manufacturing enterprise can optimize the design from the following aspects:

The significance of gear fatigue test

Systematic fatigue test can not only evaluate the fatigue resistance of gear structure, but also serve as an important feedback tool for material, heat treatment process and processing quality control. In rail transit, aerospace and other fields, accurate prediction of fatigue life is the key basis for ensuring reliable operation and formulating maintenance strategies.