This is Part 2 of a 4-part series on automatic transmission fundamentals. Part 2 covers the fundamental mechanics of automatic transmission planetary gearsets, including their three core members, the five operational modes that produce different gear ratios, the Simpson geartrain architecture used in three-speed transmissions, and the Ravigneaux compound geartrain found in four-speed designs. Understanding how holding different members stationary changes output speed and torque reveals how automatic transmissions achieve multiple ratios without the complex shifting of manual gearboxes.
What Makes a Planetary Gearset Different From Ordinary Gears?
The planetary gearset operates on constant mesh. By holding one member stationary (reaction member) and applying power to a second (input), the third (output) rotates at different speed and torque.
Three Core Members:
The sun gear is the central axis. The planet carrier holds planet pinions in fixed relationship; the pinions rotate on their own shafts while circling the sun gear. The internal ring gear is the outermost member with internal teeth meshing with the planet gears.
Load Distribution:
Multiple planet pinions, usually 3 to 4, distribute torque across several teeth simultaneously, allowing high torque capacity in a compact diameter.
Which Member Gets Held, Driven, or Output? The Five Modes Explained
Mode: Torque Increase (Reduction)
Input: Sun gear
Held Stationary: Internal gear
Output: Planet carrier
Logic: Planet gears walk inside stationary internal gear
Mode: Small Torque Increase
Input: Internal gear
Held Stationary: Sun gear
Output: Planet carrier
Logic: Less reduction; internal gear larger than sun gear
Mode: Overdrive
Input: Planet carrier
Held Stationary: Internal gear
Output: Sun gear
Logic: Sun gear rotates faster than carrier
Mode: Reverse
Input: Sun gear
Held Stationary: Planet carrier
Output: Internal gear
Logic: Planet gears act as idlers; internal gear rotates opposite direction
Mode: Direct Drive (1:1)
Input: Any two members locked together
Held Stationary: N/A
Output: Entire assembly rotates as solid unit
Local Shop Note:
This reminds me of a fellow mechanic I knew who worked along Central Avenue in Yonkers, N.Y. He had a front-wheel-drive sedan come into the shop with a complaint that sounded familiar at first: during acceleration, especially on the 2–3 shift, engine RPM would rise briefly but vehicle speed would not increase at the same rate. The customer described it as the transmission slipping.
He started with fundamentals instead of assumptions.
Fluid level was correct. Fluid color looked normal. There was no excessive debris in the pan. He installed pressure gauges and checked line pressure through the operating ranges. Pressure readings remained within specification, which suggested the pump and pressure regulation system were functioning correctly.
Next came a road test while monitoring the commanded gear changes.
What caught his attention was this: the transmission control system was commanding the shift normally, hydraulic pressure remained stable, yet during the 2–3 shift engine speed increased momentarily instead of dropping while vehicle speed changed very little. That suggested the problem was probably not electronic control and pushed him toward looking for an internal mechanical problem inside the transmission.
Once the unit was removed and disassembled, the reasoning became clearer.
Because hydraulic pressure and commanded shift timing appeared normal, attention shifted to the components responsible for carrying torque after clutch application.
Inside the transmission, one of the planet pinion gears within the planetary carrier assembly showed excessive wear at the bushing and thrust surfaces. Under load, the pinion developed abnormal internal clearance and no longer maintained proper alignment with the mating members.
Remember what a planetary gearset does: one member is driven, one member provides reaction, and one becomes output. Multiple planet pinions normally share load across several gear teeth at the same time. When excessive clearance changes how load is distributed through the planetary members, torque is no longer carried through the gearset as efficiently.
That symptom could easily be mistaken for clutch slippage because both conditions can produce a temporary rise in engine RPM during a shift.
He replaced the worn planetary carrier assembly, inspected the sun and ring gear tooth contact surfaces, verified endplay, cleaned the valve body, flushed debris from the hydraulic circuits, and reassembled the transmission. After the repair, shift timing returned to normal and engine RPM once again matched vehicle acceleration.
My point to younger techs is simple: don’t diagnose an automatic transmission by symptoms alone. A brief rise in RPM during a shift (called a shift flare) does not automatically mean a burned clutch. First determine whether the problem comes from hydraulic control, clutch application, or mechanical torque transfer through the planetary gearset. If you understand which member is driving, which member is reacting, and where torque is flowing, the transmission starts telling you where the fault actually is.
How Does the Simpson Geartrain Deliver Three Speeds From Two Planetary Sets?
The Simpson geartrain uses two planetary gearsets sharing a common sun gear. The front carrier is typically splined to the output shaft, while the rear carrier serves as a reaction member.
Gear Ratios (Example):
First: 2.46:1
Input: Forward clutch locks front ring gear to input
Reaction: Low/reverse clutch or one-way clutch holds rear carrier stationary
Result: Power flows from the front ring gear to the front pinions to the common sun gear to the rear pinions against the held rear carrier.
Second: 1.46:1
Input: Forward clutch remains applied to front ring gear
Reaction: Intermediate band holds common sun gear stationary
Result: The front planetary unit acts as a simple reduction set. Power exits through the front carrier to the output shaft.
Third: 1.00:1
Input: Forward and high clutches applied
Reaction: N/A
Result: The sun gear is locked to the input shaft. The entire assembly rotates as a unit.
Reverse: 2.17:1
Input: Reverse and high clutch drives common sun gear
Reaction: Low/reverse clutch holds rear carrier stationary
Result: The sun gear drives the rear pinions against the stationary carrier, forcing the rear ring gear to rotate in the opposite direction.
Common Sun Gear:
Its status, whether driven, held, or free, is the critical factor determining the gear range.
One-Way Clutch in Low Range:
This provides a mechanical hold for the rear carrier in first gear (D1 range) but allows freewheeling during coasting to prevent engine braking, unless the manual low-and-reverse clutch is applied.
What Makes the Ravigneaux Compound Design Different for Four Speeds?
The Ravigneaux compound geartrain uses two sets of planet gears, short and long, sharing a single ring gear, providing multiple torque paths and reaction points.
Component Relationships:
The long planet gears engage the large sun gear and the ring gear. The short planet gears engage the small sun gear and the long planet gears. The forward sun gear is splined to the turbine shaft and acts as the primary input for forward motion. The reverse sun gear engages the long planet gears to provide directional reversal.
Power Paths:
In direct drive transition, the band is released and a clutch locks the sun gear and front ring gear together, causing the entire compound set to rotate as a unit for a 1:1 ratio. In the reverse power path, the front clutch is released and the rear planet carrier is held stationary by a band or clutch. Power enters through the reverse sun gear, which drives the long planet gears against the held carrier, reversing the output rotation at a ratio of approximately 2.1:1.
The key takeaway is that planetary gearsets produce different output ratios by selectively holding one of the three core members stationary while driving another. The Simpson geartrain uses two planetary sets sharing a common sun gear to achieve three forward speeds and reverse, while the Ravigneaux compound design uses long and short planet gears within a single ring gear to provide four forward speeds. The status of the sun gear, whether driven, held, or free, is the critical factor determining the gear range in both architectures. Continue to Part 3 which covers automatic transmission hydraulic holding & control.