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.

What causes automatic transmission planetary gear whine, and is it always a gear quality problem?

AT planetary gear whine — a tonal noise at a specific vehicle speed that tracks precisely with the transmission gear engaged — is caused by the mesh frequency of the active planetary stage exciting a resonance in the transmission housing, powertrain, or vehicle body. It is not always a gear quality defect; the root cause is often in one of four areas: (1) Gear quality exceeding NVH limit: if tooth profile or spacing error is above the design specification for the active stage, transmission error at the mesh frequency excites the resonance more strongly. Verify by measuring the noise level against the gear specification; if TE is within spec, the problem is not the gear itself. (2) ATF viscosity: low ATF level or degraded ATF viscosity reduces the EHL film thickness at the planet mesh, causing increased dynamic mesh force at the gear mesh frequency — the noise increases and the gear tooth surface may develop micro-pitting. Solution: ATF level check and replacement with fresh ATF. (3) Planet pin wear: worn planet pin bore in the carrier increases planet gear radial play, causing periodic load variation as each planet passes through the top-of-roll position. Solution: replace carrier or re-bush planet pin bores to H6 tolerance. (4) Resonance body: the frequency matches a structural resonance of a bracket, shield, or body panel that amplifies the gear noise — detectable by damping the suspected panel and checking if the noise changes. Korea Ever-Power can supply replacement AT planetary gear sets for commercial vehicle transmissions (Allison, ZF Ecomat, Voith DIWA series) at M2.5–M4.5 — contact with the transmission model and affected speed range for assessment.

Can Korea Ever-Power supply planetary gear sets for ZF 6HP or 8HP automatic transmission repair?

Korea Ever-Power can supply planetary gear sets for ZF 6HP and 8HP automatic transmissions in two scenarios: (1) Commercial vehicle and bus AT repair (ZF Ecomat 4HP/5HP bus transmission, ZF 6HP28/6HP32 commercial vehicle variants): these use module M2.5–M4 planet gears with case carburized 18CrNiMo7-6 material — within Korea Ever-Power’s production capability for replacement supply to transmission remanufacturers. (2) Passenger car ZF 8HP (M1.5–M2.0 planet gears): the smallest planet gears in the ZF 8HP passenger car transmission are below Korea Ever-Power’s minimum practical module for carburized gear production — Korea Ever-Power would refer these to specialist automotive AT gear suppliers. For the larger commercial vehicle planetary gear sizes in the ZF 6HP/8HP range, contact Korea Ever-Power with the specific stage number, planet gear tooth count and module (measured from the failed gear), and the vehicle application for a feasibility assessment. Korea Ever-Power’s standard response to an AT planetary gear inquiry is to first confirm the module and whether it falls within the production range before providing a delivery or pricing commitment.

How do hybrid vehicle automatic transmissions differ in planetary gear requirements from conventional AT?

Hybrid vehicle planetary gear systems (exemplified by Toyota’s Power Split Device in the Prius hybrid system) differ from conventional AT planetary gears in three key ways: (1) Continuous ratio variation: the Toyota THS-II uses the planetary gear set as a continuously variable power split mechanism — the engine drives the ring gear while Motor Generator 1 (MG1) controls the sun gear speed to vary the effective transmission ratio. There is no stepped gear change; the planet gear speed continuously adjusts. This means the planetary gear set operates across a wide speed range continuously, including very low speed near the zero-output condition where conventional AT planet gears are only momentarily loaded during gearshifts. (2) Regenerative braking torque reversal: the planetary gear experiences full torque reversal on each transition between propulsion and regenerative braking — the drive flank becomes the coast flank and vice versa — at frequencies determined by the braking cycle. This places higher demands on the tooth root bending fatigue in both bending directions than a conventional AT where coast torques are much lower than drive torques. (3) Thermal management: the hybrid planetary gear set generates heat at all operating conditions including low-speed urban driving where conventional AT gears run cool — the gear oil must maintain adequate viscosity for EHL film formation at all temperatures including hot stop-and-go traffic. Korea Ever-Power can supply hybrid power split planetary gear sets for commercial vehicle hybrid retrofit programmes in the M2.5–M5 range — contact with the vehicle specification and required ratio range.

Spur Gears

Automotive · SCM420H · carburized

Helical Gears

Automotive · NVH optimised · M1.5–M5

ベベルギア

Hypoid · differential · final drive

Worm Gears

Power seat · steering column · actuator

Ring Gears

Internal · AT planetary · nitrided

Planetary Gears

AT · Simpson · Ravigneaux · 4–10 speed

Plastic Gears

POM · PEEK · sensor · HVAC actuator