SUN GEAR

• Material: 18CrNiMo7-6
• Heat treatment: Case carburized, HRC 60–62
• Quality class: DIN 4–5, ground
• Surface: Shot peened + ground
• Tooth form: Involute helical, helix 0°–10°
• Profile crowning: 8–15 μm
• Floating sun: most designs allow sun gear to float radially to self-centre among the planet loads

PLANET GEARS

• Material: 18CrNiMo7-6
• Heat treatment: Case carburized, HRC 60–62
• Quality class: DIN 4–5, ground
• Lead crowning: 10–25 μm (elastic deflection compensation)
• Bore: precision-bored for planet pin or needle bearing
• Count: 3 or 4 per stage; matched sets within 0.01 mm OD tolerance

RING GEAR

• Material: 34CrNiMo6 or 18CrNiMo7-6
• Heat treatment: Nitrided (case depth 0.35–0.55 mm, HV 600–750) — nitriding preferred over carburizing to minimise distortion on large OD ring
• Quality class: DIN 5–6 (internal tooth ground)
• Mounting: Tight interference fit or bolted flange to gearbox housing

Why is the ring gear typically nitrided rather than carburized in wind turbine planetary gearboxes?

Ring gear nitriding vs carburizing is a trade-off between case depth, distortion, and practicality for large-diameter thin-walled ring gears. Carburizing produces a deeper, harder case (1.0–1.8 mm, HRC 60–62) with higher contact fatigue resistance than nitriding (0.35–0.55 mm, HV 600–750 ≈ HRC 57–61). However, carburizing requires quenching from austenizing temperature (850–900°C) which causes large-diameter ring gears to distort significantly — a 600 mm OD ring gear can distort by 0.3–0.8 mm after carburize quench, requiring substantial finish-grinding to achieve the DIN 5–6 quality class. Nitriding is a low-temperature process (500–550°C gas nitriding) that causes minimal distortion — a 600 mm ring gear nitrided typically distorts less than 0.05 mm, making it much more practical to achieve the internal tooth quality class with minimal post-nitriding grinding. For wind turbine first-stage ring gears with OD above 400 mm: nitriding + hobbing to DIN 5 (with minimal stock removal after nitriding) is standard practice. For ring gears below 300 mm OD: carburizing with subsequent internal grinding is practical and gives higher contact fatigue resistance. Korea Ever-Power can supply either route depending on ring gear diameter and the client’s gearbox design contact stress requirement.

What is the correct procedure for replacing the first-stage planetary gear set in a wind turbine gearbox without removing the gearbox from the nacelle?

In-nacelle planetary gear set replacement (without gearbox removal) is possible on gearbox designs where the high-speed side cover can be removed and the planet carrier and HSS shaft assembly can be extracted through the nacelle service hatch. The procedure requires: (1) Rotor lock — mechanical or hydraulic rotor locking to prevent rotation during work. (2) HSS shaft disconnection — decouple generator coupling and HSS shaft from gearbox. (3) HSS and IMS shaft extraction — remove the IMS and HSS shaft assembly, giving access to the planetary stage. (4) Planet carrier extraction — on most designs, the carrier can now be pulled axially through the gearbox body. (5) Planet and sun gear exchange — replace planet bearings and planet gears on the extracted carrier (or replace the full carrier assembly with a pre-assembled spare). (6) Ring gear replacement (if required) — ring gear replacement in-nacelle requires partial gearbox housing disassembly and is typically deferred to a full gearbox exchange if the ring is also damaged. Korea Ever-Power can supply pre-assembled first-stage planet carrier assemblies (with new planet gears, bearings, and planet pins) as a direct exchange unit to support this procedure — contact Korea Ever-Power with the gearbox OEM model and turbine capacity for carrier assembly availability and lead time.

When replacing the first-stage planet gears in a wind turbine gearbox, should I also replace the ring gear and sun gear or only the damaged planet gears?

Planet gear replacement strategy depends on the failure mode and the hours since the last gear set replacement or installation: (1) Isolated planet gear failure (one planet, early life at below 50% of design life): if the ring gear and sun gear surfaces show no pitting, micro-pitting, or tooth profile wear (confirmed by borescope inspection), and if the failure was caused by a specific event (oil contamination, impact overload) rather than fatigue accumulation, it may be acceptable to replace only the failed planet and planet bearing. However, replace all three (or four) planet gears as a matched set — installing one new planet with two worn planets will cause unequal contact pattern and accelerate wear on the new planet. (2) Multiple planet gear failure or fatigue pitting reaching 10–20% of tooth face area: replace complete planetary gear set (sun + all planets + planet bearings). Inspect ring gear — if ring gear shows pitting exceeding 10% of any tooth face, replace ring gear at the same time to avoid new gear meshing against a worn ring profile. (3) At design life (175,000+ hours) or at any second failure in the same stage: replace full planetary gear set including ring gear as a proactive measure — the cost of leaving a partially worn ring gear with a new planet set and suffering an early re-failure in a turbine at 80+ m hub height far exceeds the cost of the ring gear itself. Korea Ever-Power supplies complete matched planetary gear sets (sun + planets + ring) as a standard replacement package for all common wind turbine gearbox first-stage configurations.

Spur Gears

M1–M20 · C45, 20CrMnTi, forged

Helical Gears

18CrNiMo7-6 · wind turbine IMS/HSS

Engrenages coniques

Right-angle drives · auxiliary equipment

Worm Gears

Self-locking · yaw brake · pitch drive

Ring Gears

Wind turbine · yaw & pitch ring · nitrided

Planetary Gears

Wind turbine · 3–4 planet · DIN 4–5

Plastic Gears

POM · Nylon · auxiliary · sensor drive