Planetary Gears for Automatic Transmissions:
Ravigneaux, Simpson and Multi-Stage Powertrain Design
The automatic transmission planetary gear system is the most extensively engineered gear application in mass production — with hundreds of millions of units produced annually, designed to survive 200,000 km of vehicle life in a constantly varying torque and speed environment from engine idle to full throttle kick-down. This guide covers the Ravigneaux, Simpson, and compound multi-stage planetary gear arrangements used in passenger car, SUV, and commercial vehicle automatic transmissions — including material specification, surface treatment, and the micro-geometry requirements that control NVH in the modern 8–10 speed automatic gearbox.
200,000 km life · Hobbed DIN 6–7 · ATF oil
Ravigneaux · Simpson · 4–10 Speed · Hybrid
Planetary Gear Arrangements in Automatic Transmissions
The automatic transmission achieves its multiple forward speed ratios by selectively engaging and releasing clutch packs and brake bands that connect or hold specific members of the planetary gear set — sun gear, planet carrier, or ring gear. By connecting different members to the input shaft (from the torque converter), the output shaft (to the final drive), or ground (via a brake), the same planetary gear set produces different speed ratios without any gear change mechanism in the conventional sense. This is the elegance of the planetary gear automatic transmission: all gears are permanently in mesh, so speed changes require no synchromesh or jaw clutch engagement — only hydraulic or electronically-controlled clutch activation, producing smooth gearshifts.
Korea Ever-Power’s planetary gears for automatic transmissions are manufactured in SCM420H (JIS standard, equivalent to SAE 8620) and in 20CrMnTi (Chinese GB standard equivalent) — both case carburized to HRC 58–62, shot peened tooth root for improved bending fatigue under AT kick-down peak torques, and hobbed to DIN 6–7 quality class. Korea Ever-Power supplies AT planetary gears for transmission OEM programmes, replacement parts for transmission remanufacturers, and for commercial vehicle transmission suppliers where the quantities and specifications align with Korea Ever-Power’s production capability in M1.5–M4 module range. The very smallest AT planet gears (M1.0–M1.25 for passenger car sub-compact transmissions) are below Korea Ever-Power’s minimum practical module for case carburized production.
The NVH (noise, vibration, and harshness) requirement for automotive AT planetary gears is fundamentally different from industrial planetary gear NVH requirements — in industrial applications, a gear noise level below 75–80 dB(A) is the normal target; in a passenger vehicle AT, the planetary gear noise must be inaudible to the vehicle occupants at all speeds and in all gear ratios, against a background cabin noise level of 35–45 dB(A). This requires tooth quality class DIN 6–7 (which defines the maximum allowable transmission error at the speeds and loads of AT operation), specific tip and root relief geometries tuned to the specific tooth pair stiffness variation of the AT planetary gear geometry, and ATF (Automatic Transmission Fluid) oil formulation that maintains the EHL film thickness across all AT operating temperatures (-40°C cold start to +140°C continuous heavy towing).
COMMON AT PLANETARY GEAR ARRANGEMENTS
SIMPSON SET
Two simple planetary gear sets sharing a common sun gear. Used in early 3-speed automatics (Ford C4, GM Turbo-Hydramatic). Produces 3 forward + 1 reverse ratios. Simple design with minimum unique gear parts. Sun gear serves both planetary stages — single sun gear wears on both flanks from alternate driving and coasting torques. Still found in heavy commercial vehicle transmissions (Allison AT series derivatives).
RAVIGNEAUX SET
Compound planetary set with two sun gears, two sets of planet gears (long and short planets), and one shared ring gear. Produces 4–5 forward ratios from one compact assembly. Used in ZF 4HP and 5HP series, Aisin AW 55-series 5-speed transmissions. The long planet gears mesh with both sun gears at different positions along their length — the most complex individual gear geometry in any AT design.
MULTI-STAGE (6–10 SPEED)
Three or four independently-selectable simple or Ravigneaux planetary stages, each with dedicated clutch packs and brakes, combined to produce 6, 8, 9, or 10 forward ratios. ZF 8HP (BMW, Audi, Land Rover), GM 8L90 (Corvette, Tahoe), and Aisin AW TF-80 6-speed are common examples. The 8HP uses four simple planetary stages with five clutch elements — no single planetary set is the same size, requiring up to 8 distinct planet gear part numbers in one transmission.
Material, Heat Treatment and Quality Class
PASSENGER CAR AT GEARS
- ✓Material: SCM420H or SAE 8620 (guaranteed hardenability H-band for consistent carburized depth)
- ✓Module: M1.5–M2.5 (compact packaging, low NVH)
- ✓Case depth: 0.4–0.7 mm effective (small module — thin case to avoid distortion)
- ✓Quality: DIN 6–7 hobbed — profile relief applied by CNC hobbing programming
- ✓Life target: 200,000–300,000 km in ATF oil with lifetime oil fill
COMMERCIAL VEHICLE AT GEARS
- ✓Material: 18CrNiMo7-6 or SAE 9310 for higher core toughness under heavy duty cycle shock
- ✓Module: M2.5–M5 (higher torque per gear tooth area)
- ✓Case depth: 0.8–1.2 mm effective
- ✓Quality: DIN 6–7, shot peened, some ground for critical NVH positions
- ✓Life target: 500,000 km or B10 1 million km for fleet operators
Frequently Asked Questions
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Editor: Cxm
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