Introduction

1. Mechanism of Gear Fatigue Failure

1.1 Basic Characteristics of Gear Fatigue Failure

• Cyclic load action: Caused by alternating contact stress and bending stress.
• Crack initiation and propagation: Typically undergoes three stages—crack initiation, stable propagation, and rapid fracture.
• Stress concentration effect: Fatigue is prone to occur at the tooth root transition curve and surface defects.

1.2 Main Fatigue Failure Modes

(1) Tooth Surface Contact Fatigue (Pitting)

• Microscopic mechanism: Cracks initiated by the maximum shear stress in the subsurface → propagate to the surface → material spalling.
• Manifestations:
  • Initial pitting: Isolated pits with diameter < 1mm.
  • Extended pitting: Connected pits forming spalling areas.
  • Macroscopic pitting: Large-area spalling with depth > 0.2mm.

(2) Tooth Root Bending Fatigue

• Fracture process: Cracks initiate on the tensile stress side of the tooth root → propagate to the compressive stress side → tooth breakage.
• Typical characteristics:
  • Shell-like fracture morphology.
  • Visible fatigue striations.
  • Coarse-grained final fracture zone.

(3) Surface Crushing (Plastic Deformation)

• Occurrence conditions: Extremely high contact stress exceeding the material yield limit.
• Common scenarios:
  • Soft tooth surface gears.
  • Impact load working conditions.
  • Improper heat treatment.

2. Gear Fatigue Strength Design Theories

2.1 Basic Theoretical Framework

• Stress-Life Method (S-N curve): Suitable for high-cycle fatigue (> 10⁴ cycles).
• Strain-Life Method (ε-N curve): Suitable for low-cycle fatigue (< 10⁴ cycles).
• Fracture Mechanics Method: Prediction based on crack growth rate da/dN.

2.2 Comparison of Key Design Standards