1. Gear Classification & Selection
| Type | Features | Typical Applications |
|---|---|---|
| Spur Gears | Simple structure, no axial thrust, economical | Low-speed drives, machine tool gearboxes |
| Helical Gears | Smooth meshing, high load capacity, axial thrust present | High-speed heavy-duty, auto transmissions |
| Bevel Gears | Power transmission between intersecting shafts, complex machining | Differentials, steering mechanisms |
| Worm Gears | High reduction ratio, self-locking, low efficiency | Lifting mechanisms, indexing devices |
| Planetary Gears | Compact, high power density, complex structure | Wind turbine gearboxes, robot joints |
2. Module Selection Principles
Module m = d/z (pitch diameter / teeth count) — the most fundamental gear parameter:
- Bending strength determines: Larger module = thicker tooth root = higher bending strength
- Minimum module principle: Choose the smallest module that meets strength requirements — lower cost, less vibration
- Standard module series (GB/T 1357): 0.5, 0.6, 0.8, 1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10…
- Preferred co-prime teeth: Avoid periodic wear; pinion typically 17-25 teeth (≥17 to avoid undercut)
3. Pressure Angle
- Standard 20°: Most common, balanced root strength and sliding ratio
- 14.5°: Older standard, smoother meshing but weaker root
- 25°: Heavy-duty gears, higher root strength but increased sliding ratio
4. Profile Shift
- Positive shift (x>0): Increases tooth thickness, improves bending strength, avoids undercut
- Negative shift (x<0): Reduces tooth thickness, adjusts center distance
- Equal shift: x1+x2=0, center distance unchanged
- Angular shift: x1+x2≠0, center distance changes
5. Practical Insights
Insight 1: The pinion is always the weak link
In any gear pair, the pinion has fewer teeth, more bending stress cycles, and higher failure risk.
Recommendations:
- Pinion should have slightly higher hardness (30-50 HB above the gear)
- Apply positive shift to the pinion, zero or negative shift to the gear
- Pinion: 20CrMnTi carburized; Gear: 40Cr quenched & tempered
Insight 2: Module selection trap
Customers often demand “larger module = stronger,” but over-sized modules bring:
- Increased tooth height → higher sliding ratio → faster wear
- Reduced contact ratio → lower meshing smoothness
- Higher material cost
Correct strategy: Under acceptable bending strength, prefer smaller module + appropriately wider face width.
Quick Reference — Material & Module Recommendations (Grade 7, medium load)
| Power Range | Pinion | Gear | Heat Treatment | Recommended Module |
|---|---|---|---|---|
| ≤5 kW | 45# Q&T | 45# Q&T | Tooth induction hardening | 2-3 |
| 5-30 kW | 40Cr Q&T | 45# Q&T | Pinion nitriding | 3-5 |
| 30-100 kW | 20CrMnTi | 40Cr Q&T | Carburizing | 4-8 |
| ≥100 kW | 20CrNi2Mo | 35CrMo | Carburizing | 6-12 |
Geyontech — Precision Gear Manufacturing & Transmission Systems