{"id":1912,"date":"2026-07-21T06:54:43","date_gmt":"2026-07-21T06:54:43","guid":{"rendered":"https:\/\/gearrack.top\/?p=1912"},"modified":"2026-07-21T06:54:43","modified_gmt":"2026-07-21T06:54:43","slug":"planetary-gears-for-automatic-transmissions-2","status":"publish","type":"post","link":"https:\/\/gearrack.top\/fr\/planetary-gears-for-automatic-transmissions-2\/","title":{"rendered":"Planetary Gears for Automatic Transmissions"},"content":{"rendered":"
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GEAR APPLICATION GUIDE \u00b7 PLANETARY GEAR \u00b7 P04<\/span><\/p>\n

Planetary Gears for Automatic Transmissions:
\nRavigneaux, Simpson and Multi-Stage Powertrain Design<\/span><\/h1>\n

The automatic transmission planetary gear system is the most extensively engineered gear application in mass production \u2014 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 \u2014 including material specification, surface treatment, and the micro-geometry requirements that control NVH in the modern 8\u201310 speed automatic gearbox.<\/p>\n

SCM420H \u00b7 SAE 8620 \u00b7 Carburized HRC 58\u201362<\/span>
\n200,000 km life \u00b7 Hobbed DIN 6\u20137 \u00b7 ATF oil<\/span>
\nRavigneaux \u00b7 Simpson \u00b7 4\u201310 Speed \u00b7 Hybrid<\/span><\/div>\n<\/div>\n<\/div>\n
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DESIGN LIFE<\/p>\n

200,000 km<\/p>\n

Standard automotive automatic transmission design life. Premium segment vehicles: 300,000 km. Commercial vehicles and buses: 500,000 km or B10 life equivalent. Lifetime oil fill \u2014 no scheduled gear oil change in most modern AT designs<\/p>\n<\/div>\n

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GEAR STAGES<\/p>\n

4 \u2013 10 speeds<\/p>\n

Modern automatic transmission forward speed range. 4-speed (legacy): 2 planetary stages. 6-speed (most common): 3 planetary stages. 8\u201310 speed (premium): 3\u20134 compound planetary stages with multiple clutch packs<\/p>\n<\/div>\n

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TOOTH COUNT (PLANET)<\/p>\n

12 \u2013 25 teeth<\/p>\n

Typical automotive automatic transmission planet gear tooth count. Module M1.5\u2013M2.5 for most passenger car AT planetary gears \u2014 small module and high tooth count for noise reduction, maintained within strict gear noise (NVH) targets across all vehicle speed ranges<\/p>\n<\/div>\n

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MATERIAL<\/p>\n

SCM420H<\/p>\n

Japanese JIS grade SCM420H (closely equivalent to SAE 8620 \/ 20CrMo, guaranteed hardenability H band) \u2014 the dominant material for automotive AT planetary gears globally. Case carburized, HRC 58\u201362, hobbed DIN 6\u20137, shot peened tooth root<\/p>\n<\/div>\n<\/div>\n

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Planetary Gear Arrangements in Automatic Transmissions<\/h2>\n

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 \u2014 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 \u2014 only hydraulic or electronically-controlled clutch activation, producing smooth gearshifts.<\/p>\n

Korea Ever-Power’s planetary gears for automatic transmissions<\/a> are manufactured in SCM420H (JIS standard, equivalent to SAE 8620) and in 20CrMnTi (Chinese GB standard equivalent) \u2014 both case carburized to HRC 58\u201362, shot peened tooth root for improved bending fatigue under AT kick-down peak torques, and hobbed to DIN 6\u20137 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\u2019s production capability in M1.5\u2013M4 module range. The very smallest AT planet gears (M1.0\u2013M1.25 for passenger car sub-compact transmissions) are below Korea Ever-Power\u2019s minimum practical module for case carburized production.<\/p>\n

\"Planetary<\/p>\n

The NVH (noise, vibration, and harshness) requirement for automotive AT planetary gears is fundamentally different from industrial planetary gear NVH requirements \u2014 in industrial applications, a gear noise level below 75\u201380 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\u201345 dB(A). This requires tooth quality class DIN 6\u20137 (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\u00b0C cold start to +140\u00b0C continuous heavy towing).<\/p>\n

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COMMON AT PLANETARY GEAR ARRANGEMENTS<\/p>\n

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SIMPSON SET<\/p>\n

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 \u2014 single sun gear wears on both flanks from alternate driving and coasting torques. Still found in heavy commercial vehicle transmissions (Allison AT series derivatives).<\/p>\n<\/div>\n

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RAVIGNEAUX SET<\/p>\n

Compound planetary set with two sun gears, two sets of planet gears (long and short planets), and one shared ring gear. Produces 4\u20135 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 \u2014 the most complex individual gear geometry in any AT design.<\/p>\n<\/div>\n

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MULTI-STAGE (6\u201310 SPEED)<\/p>\n

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 \u2014 no single planetary set is the same size, requiring up to 8 distinct planet gear part numbers in one transmission.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n

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Material, Heat Treatment and Quality Class<\/h2>\n
\"automatic
Korea Ever-Power SCM420H case carburized automatic transmission planetary gear set \u2014 module M2.0, planet gear tooth count 18T, ring gear 72T, sun gear 36T, 3-planet carrier configuration for compact passenger car 6-speed AT stage. Case depth 0.5\u20130.8 mm, tooth surface HRC 59\u201362, core HB 290\u2013330. Shot peened tooth root for improved bending fatigue under kick-down peak torque. Hobbed to DIN 6\u20137 quality class; tooth tip and root relief profiles matched to AT stage tooth pair stiffness calculation for NVH performance. ATF-compatible surface finish Ra \u22640.6 \u03bcm. Planet bore H6 for precision needle bearing fit. Used in 6-speed AT OEM supply programmes for Korean and Chinese passenger car platforms.<\/figcaption><\/figure>\n
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PASSENGER CAR AT GEARS<\/h3>\n