Gear Fatigue Strength: Design, Verification, And Engineering Applications

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