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Materials Helicopter Gears

Applications and Design

Materials for Helicopter Gears - National Research Council


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Some of the power train transmission gears in helicopter drive systems can become critical components as performance requirements are increased; accordingly, increasing attention must be paid to new alloys in order to obtain required performance, reliability, and survivability. The major limitation of the alloy presently used, SAE 9310 steel, is its tendency to score and scuff under high-temperature conditions. Candidate advanced alloys, with improved high-temperature properties, while increasing the resistance to scoring and scuffing, tend to have lower ductility and fracture toughness.

In this report, an attempt is made to identify design, materials, and process problems and requirements. In addition, it is recommended that the characterization of candidate steels be accelerated; preliminary investigation indicates that new alloys may provide improved capability against surface distress. Other short- and long-term recommendations also are presented.

Key Words

  • Carburizing
  • Fatigue
  • Gears
  • Helicopters
  • Scuffing
  • Spalling
  • Transmission




Chapter 1: Summary, Conculsions, and Recommendations 1
A. Summary 1
B. Conclusions 2
C. Recommendations 4

Chapter 2 : Introduction 7

Chapter 3: Gear Failures 13
A. Definition 13
B. Principal Failure Modes 13
1. Tooth Bending Fatigue 16
2. Pitting 16
3. Scoring/Scuffing 20
C. Gear Lubrication Oils 21 References 27

Chapter 4: Status of Technology 29
A. Gear Steel Compositions and Physical 29 Static Stress Characteristics
B. Hot Hardness 29
C. High Temperature Strength 29
D. High Temperature Score Resistance 33
E. Fracture Toughness and Damage Toler- 33 ance
F. Fatigue Crack Growth 39 References 47

Chapter 5: Processing 49
A. Introduction 49
B. Testing 54
1. Fatigue Tests 56
2. Bend Testing 61
3. High-Cycle Fatigue 61
4. Impact Fatigue and Impact Fracture 63 Stress
5. Nondestructive Evaluation 66

APPENDIX A: Pitting Fatigue Analysis 71
1. Test Procedures 71

APPENDIX B: Frictional Forces and Shear Stresses 75

APPENDIX C: Scoring Phenomena 79


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