In a high-performance planetary gear set, each component experiences different stress cycles and thermal distortion risks, requiring differentiated metallurgy: (1) Sun gears and planet pinions undergo millions of high Hertzian contact and reverse-bending cycles, requiring case-carburized and tooth-ground alloy steels (18CrNiMo7-6, 20CrMnTi, or AISI 8620 at 58–62 HRC); (2) Thin-walled internal ring gears are prone to out-of-roundness during high-temperature quenching and are best made from 42CrMo (AISI 4140) or 38CrMoAlA with low-temperature plasma/gas nitriding (650–750 HV); and (3) Planet carriers require quenched-and-tempered 42CrMo forged steel (28–34 HRC) or QT600-3 ductile iron for high torsional stiffness.
Key Engineering & Manufacturing Takeaways
- ✓Planet pinions experience reverse tooth bending in every rotation (loaded in one direction by the sun gear and the opposite direction by the ring gear), requiring Derating Factor Y_M = 0.7 per ISO 6336-3 and tough carburized cores (32–40 HRC).
- ✓Carburizing a thin-walled internal ring gear at 920°C often causes oval distortion; low-temperature plasma nitriding at 500°C–540°C maintains DIN 6–7 pitch accuracy without expensive internal tooth grinding.
- ✓18CrNiMo7-6 (DIN 1.6587) is the gold-standard nickel-chromium-molybdenum steel for heavy-duty M4–M12 construction and wind planetary sets.
- ✓Effective Case Depth (ECD to 550 HV) should be specified at 0.15·m to 0.20·m, while avoiding over-carburizing thin tooth tips.
1. Why Planet Pinions Need Reverse-Bending Fatigue Protection
A critical metallurgical fact often overlooked in gear design is that a planet pinion is an idler gear. As a planet tooth rotates through the sun-gear mesh, its flank is loaded in one direction, putting the root fillet in tension. Half a revolution later, that exact same planet tooth enters the internal ring-gear mesh, bending the tooth in the OPPOSITE direction and subjecting the root fillet to alternating tensile-compressive stress!
According to ISO 6336-3 and AGMA standards, idler gears subjected to fully reversed bending stress must have their allowable bending stress derated by 30% (multiplier 0.70). Therefore, sun gears and planet pinions must be manufactured from clean, vacuum-degassed case-carburizing steels with fine austenitic grain size, shot-peened roots, and CNC profile-ground flanks.
2. Master Metallurgy & Heat Treatment Selection Matrix
Below is the proven material and heat-treatment pairing protocol used across ZhongYi Gear’s OEM production lines:
Table: Recommended Alloy Steels & Heat Treatment Routes by Planetary Component
| Planetary Component | Chinese GB Grade | International Equivalent (DIN / AISI) | Heat Treatment Process | Surface Hardness | Core Hardness & Effective Case Depth |
|---|---|---|---|---|---|
| Sun Gear & Planet Pinions (Medium/Servo Duty) | 20CrMnTi | 16MnCr5 / AISI 5120 / 8620 | Gas Carburizing + Quench + Temper + CNC Grind | 58 – 62 HRC | Core: 32–42 HRC; ECD: 0.5 – 1.2 mm |
| Sun Gear & Planet Pinions (Heavy Shock / Mining / Wind) | 18CrNiMo7-6 | DIN 1.6587 / 17CrNiMo6 / AISI 4320 | Deep Case Carburizing + Cryogenic/Temper + Grind | 58 – 62 HRC | Core: 35–45 HRC; ECD: 1.0 – 2.5 mm |
| Internal Ring Gear (Servo & Standard Reducers) | 42CrMo / 38CrMoAlA | 42CrMo4 (AISI 4140) / 41CrAlMo7 | Q&T Pre-hardening + Plasma/Gas Nitriding | 600 – 750 HV (≈ 55–62 HRC) | Core: 28–32 HRC; Nitrided layer: 0.3 – 0.6 mm |
| Internal Ring Gear (Extreme Heavy Impact) | 20CrMnTi / 18CrNiMo7-6 | 16MnCr5 / DIN 1.6587 | Press-Quench Carburizing + CNC Internal Grinding | 58 – 60 HRC | Core: 32–40 HRC; ECD: 1.0 – 2.0 mm |
| Planet Carrier (High Torque) | 42CrMo / 40Cr Forging | 42CrMo4 (AISI 4140) / 41Cr4 | Quenching & Tempering + Stress Relief | 28 – 34 HRC (270–320 HBW) | Through-hardened for pin bore stability |
3. Internal Ring Gears: Why Plasma Nitriding Beats Carburizing for 80% of Reducers
Unlike solid sun shafts or compact planet pinions, an internal ring gear is a large, relatively thin-walled ring (often featuring external mounting flanges or housing bolt holes). Furthermore, because internal teeth have a concave tooth profile, their Hertzian contact stress (σ_H) is naturally 25% to 40% LOWER than the convex sun-planet mesh for the same transmitted torque.
If you carburize and oil-quench a thin-walled ring gear at 900°C–920°C, phase transformation stresses cause the ring to warp into an oval or taper. Correcting that distortion requires an expensive CNC internal gear grinding operation. Instead, by pre-hardening 42CrMo (4140) or 38CrMoAlA to 30 HRC, finish-shaping or broaching the internal teeth to DIN 6, and then performing low-temperature ion/plasma nitriding at 510°C, the ring achieves 650–750 HV wear hardness with less than 0.01–0.02 mm dimensional change—eliminating post-heat-treat grinding entirely!
Sourcing Custom Planetary Gear Sets Built to Your Prints?
ZhongYi manufactures matched sun gears, CNC profile-ground planet pinions (with parabolic lead crowning and honed needle bearing bores), 5-axis machined planet carriers, and plasma-nitrided internal ring gears in our 2,800+ m² Zhejiang factory.
Engineering FAQ
Frequently Asked Technical Questions
Direct answers to common design, tolerance, and procurement questions on this topic.
When is carburizing and internal grinding mandatory for an internal ring gear?▼
When the planetary gearbox operates under severe impact loads and large modules (M5–M12 in mining excavators, tunnel boring machines, or wind turbine main drives) where subsurface shear stress occurs deeper than a 0.4 mm nitrided case can support, deep case carburizing (1.2–2.2 mm ECD) followed by CNC internal tooth profile grinding is required.
How do you prevent planet gear tooth tips from becoming brittle during carburizing?▼
Small-module gears (M1–M2.5) can become through-hardened at the narrow tooth tip if carburized too deeply. We strictly calculate the finished Effective Case Depth (after grinding stock removal of 0.08–0.15 mm per flank) at 0.15× to 0.18× module and verify the micro-Vickers hardness gradient curve on cross-sectioned test coupons.
Do you provide EN 10204 3.1 material and heat treatment certificates?▼
Yes. Every batch of custom planetary gear sets from ZhongYi includes a complete EN 10204 Type 3.1 Material Test Report (chemical composition by optical emission spectrometer), surface/core hardness reports, and effective case depth metallurgical charts.