Simpson planetary gearset
The Simpson planetary gearset is a compound planetary gear train consisting of two simple epicyclic gearsets that share a common sun gear, with the input connected to the rear ring gear and the output taken from the front planet carrier, which is mechanically linked to the rear ring gear.[1] This configuration enables the production of three forward speeds—typically with ratios of approximately 2.40:1 for first gear, 1.47:1 for second gear, and 1.00:1 for direct drive in third gear—along with one reverse gear, achieved through selective engagement of clutches and brakes to hold specific components stationary.[2] Invented by American engineer Howard W. Simpson and patented in 1950, the design was first implemented in the Chrysler TorqueFlite three-speed automatic transmission introduced in 1956 for 1957 model-year vehicles.[1] Its key advantages include mechanical simplicity, compact size due to the shared sun gear and often identical planet gears between the two sets, balanced internal forces that reduce wear and friction, and cost-effective manufacturing, making it a foundational element in numerous three-speed automatic transmissions from manufacturers like Ford (e.g., C4 and C6) and Chrysler throughout the mid-20th century.[3] In modern applications, variations of the Simpson gearset have been adapted for dual-motor electric vehicle powertrains, incorporating additional brakes and controls to optimize energy efficiency and provide up to six driving modes.[4]Overview
Description
The Simpson planetary gearset is a compound planetary gear train consisting of two planetary gearsets that share a common sun gear.[3][5] This design features a front gearset and a rear gearset, with the input connected to the rear ring gear, the front planet carrier mechanically linked to the rear ring gear to provide the output, and the rear planet carrier able to be held stationary.[1][3] Its primary function is to provide three forward speeds, typically with ratios of approximately 2.40:1 for first gear, 1.47:1 for second gear, and 1.00:1 for direct drive in third gear, one reverse, and neutral positions within automatic transmissions, enabling efficient power delivery across various vehicle speeds.[3][6][2] This configuration played a key role in 3-speed and 4-speed automatic transmissions, offering a compact and reliable means of achieving multiple gear ratios.[7][8] The gearset was invented by Howard W. Simpson and patented under US Patent 2518825A, granted on July 18, 1950.[1]Historical Significance
Following World War II, the U.S. automotive industry underwent a significant transformation, with growing consumer demand driving the proliferation of automatic transmissions to enhance driving convenience and accessibility. Pre-war and early post-war designs, such as General Motors' Hydramatic introduced in 1940, depended on intricate hydraulic systems featuring multiple bands, clutches, and planetary stages, which frequently resulted in maintenance challenges and reliability concerns. By the early 1950s, engineers and manufacturers prioritized simpler, more durable configurations to meet rising production volumes and improve performance in passenger vehicles.[9] Howard W. Simpson, born in 1892 and trained in engineering at the University of Michigan, contributed to this evolution through his extensive experience in transmission design. After working at Ford Motor Company's Fordson Tractor Plant from 1918 to 1938—where he developed early planetary gear systems—and briefly at Detroit Harvester Corporation, Simpson retired to pursue independent research. Recovering from health issues in the late 1940s, he innovated the compact three-speed planetary gearset comprising two simple planetary units with a shared sun gear, with the key patent (US 2518825A) filed in 1946 and granted in 1950.[3][10][1] Simpson's invention gained traction through strategic licensing, beginning with Ford Motor Company in 1953, though initial production lagged. In 1955, Chrysler Corporation acquired rights, integrating the gearset into its TorqueFlite automatic transmission, which entered mass production in mid-1956 for models like the Imperial and Chrysler 300B. This debut represented the gearset's commercial breakthrough, showcasing its potential for widespread automotive application.[3] The Simpson gearset's historical impact lay in its simplification of automatic transmission architecture, requiring only three friction elements to achieve all forward and reverse gears, thereby reducing the complexity of hydraulic controls that plagued earlier systems like the Hydramatic. Its public introduction via the TorqueFlite in 1956 highlighted this efficiency, accelerating industry adoption and paving the way for reliable three-speed automatics that dominated U.S. vehicles through the 1960s and beyond.[3][10]Technical Design
Components
The Simpson planetary gearset is a compound system built from two simple planetary gear units that share a common sun gear, enabling compact design and efficient power transmission in automatic transmissions. A basic planetary gear unit consists of a central sun gear, multiple planet gears (typically three or more) mounted on a carrier, and an outer ring gear with internal teeth. The planet gears mesh simultaneously with the sun gear and the ring gear, while the carrier holds the planets via pins, allowing them to orbit the sun gear and rotate on their axes; this arrangement provides fundamental torque multiplication or reduction based on which elements are fixed, driven, or output-connected.[11][12] The core components of the Simpson gearset include the shared sun gear, which meshes with the planet gears of both the front and rear units and is typically splined or lugged to a supporting shell for torque transfer. The front ring gear, an internal gear, meshes exclusively with the front planet gears and connects directly to the input shaft through a clutch hub. The rear ring gear similarly meshes with the rear planet gears and links to the output shaft via a drum or flange. The front planet carrier holds the front planet gears on pins supported by needle bearings, and it is splined to the output shaft for direct power delivery. The rear planet carrier performs an analogous function for the rear planet gears, with provisions for braking via a band. The planet gears themselves are spur or helical types, rotating freely on their pins to maintain constant mesh with the sun and ring gears.[11][12] Supporting elements include the input shaft, which drives the front ring gear through a forward clutch drum, and the output shaft, which receives power from the front planet carrier and rear ring gear. Brake bands encircle drums attached to the rear planet carrier and the sun gear shell to enable selective holding during operation. These components are typically constructed from alloy steels, such as case-hardened variants, to provide the necessary strength and wear resistance under high torque and speed conditions. Planet pins are precision-machined steel, with needle bearings ensuring low-friction rotation of the planets. In representative standard setups, the sun gear often features around 36 teeth, while the ring gears have approximately 72 teeth, though exact counts vary by application to optimize ratios and packaging.[11][12][13]Configuration
The Simpson planetary gearset is a compound arrangement comprising two simple epicyclic planetary gear units positioned in tandem, known as the front (or forward) gearset and the rear (or aft) gearset.[3] The front gearset receives input primarily through its ring gear, with its planet carrier serving as the output element, while the rear gearset utilizes input via the shared sun gear for certain modes and outputs through its ring gear in reverse configurations.[3] This layout enables the front gearset to provide torque multiplication or reduction, with the rear gearset configured to support directional reversal when its carrier is held stationary.[3] Key interconnections integrate the two gearsets into a cohesive assembly. The planet carrier of the front gearset is permanently linked to the ring gear of the rear gearset, allowing torque transfer between them, while both gearsets share a single central sun gear that aligns coaxially along the transmission axis.[3] The common sun gear is driven directly by the input shaft in higher-speed configurations, facilitating synchronized rotation across both units.[3] For stationary holding, brake drums or bands are incorporated on the rear planet carrier and the sun gear shell, enabling selective immobilization without complex nesting.[3] The output shaft is typically affixed to the rear ring gear (and thus the front carrier via their linkage), directing final drive to the vehicle's driveline.[3] Technical diagrams of the Simpson gearset often present a linear schematic that traces potential power paths axially, depicting the front gearset as a reduction stage with its ring gear at the input side and carrier extending to connect with the rear ring gear, while the shared sun gear spans both units centrally.[3] This visualization highlights the front gearset's role in initial torque amplification and the rear gearset's contribution to reverse motion through carrier braking, with clutch packs and bands positioned around the drums for selective engagement.[3] In contrast to compound designs like the Ravigneaux gearset, which integrates nested planet gears within a single carrier and multiple suns for compactness, the Simpson configuration employs two independent simple planetary units linked externally for greater manufacturing simplicity and load distribution across separate planet sets.[3]Operational Principles
Shifting Mechanisms
The shifting mechanisms of the Simpson planetary gearset consist primarily of two brake bands and two multi-disc clutches, which provide the necessary control for selecting gears in forward and reverse directions. The low/reverse brake band holds the rear planet carrier stationary during first gear and reverse operations, preventing rotation to establish the appropriate speed reduction or direction reversal. The intermediate brake band anchors the common sun gear to the transmission case during second gear, allowing the front ring gear to drive the system while the rear carrier serves as the overrun element. These bands are hydraulically actuated to ensure precise and timely engagement without interrupting power flow.[14] The two multi-disc clutches facilitate power input to key components of the gearset. The forward clutch connects the input shaft directly to the front ring gear, transmitting torque through the front planetary set in all forward gears. The direct/reverse clutch links the input shaft to the common sun gear, enabling direct drive in third gear and reverse rotation when combined with the low/reverse band. These clutches use multiple friction discs and plates, soaked in transmission fluid, to handle high torque loads with minimal slippage during engagement.[14] An overrunning clutch, typically implemented as a sprag, acts as a one-way holding device on the rear planet carrier, locking it against reverse rotation in first gear while permitting free overrun in higher gears for smoother automatic downshifts. This mechanical element eliminates the need for band release during certain transitions, reducing shift harshness. In implementations like the GM Turbo-Hydramatic 400, the sprag integrates with the overall system to support engine braking when required.[14] Control of these mechanisms is integrated through hydraulic actuators powered by the transmission's pump, with a valve body directing pressurized fluid based on vehicle speed, throttle position, and driver inputs. This setup, common in GM Turbo-Hydramatic applications, uses solenoid or governor-regulated valves to sequence engagements efficiently. The design's efficiency stems from requiring only four frictional elements—the two bands and two clutches—significantly reducing complexity and potential failure points compared to prior multi-planetary systems that demanded additional controls.[14]Gear Ratios and Power Flow
The Simpson planetary gearset, consisting of front and rear planetary units sharing a common sun gear, produces three forward speeds, reverse, and neutral by routing torque through specific elements while holding others stationary via brakes or clutches. The front planet carrier is mechanically linked to the rear ring gear, with input to the front ring gear and output from the front planet carrier. Power flow in each gear follows the principles of epicyclic motion, where the relative speeds of the ring gear, sun gear, and carrier determine the overall ratio. The basic ratio for a single planetary stage with the ring gear fixed, input to the sun gear, and output from the carrier is given by R = 1 + \frac{N_r}{N_s}, where N_r is the number of teeth on the ring gear and N_s is the number on the sun gear. Compound ratios in the Simpson configuration derive from the interactions between the front and rear units.[1] In neutral, all rotating elements—front and rear carriers, common sun gear, and both ring gears—are released from any holding mechanism, preventing torque transmission from the input to the output shaft and resulting in no power flow.[15] For first gear, the forward clutch connects the input to the front ring gear, and the rear carrier is held stationary by the low/reverse brake. Power flows from the front ring gear to the front planet gears, driving the common sun gear in the reverse direction at increased speed. The reverse-rotating sun gear then drives the rear planet gears, which with the fixed rear carrier, rotate the rear ring gear (and connected front carrier, the output) forward at reduced speed relative to the input, providing torque multiplication through the compound action. The overall ratio is the result of the two-stage interaction, typically approximately 2.5:1.[15][16] Second gear is achieved by connecting input torque to the front ring gear while braking the common sun gear. With the sun fixed, power flows from the front ring gear through the front planet gears, which walk around the stationary sun to drive the front carrier (output) forward at reduced speed; the rear set overruns freely via the sprag. The ratio is approximately 1.5:1, derived from the epicyclic action $1 + \frac{N_s}{N_{r,\text{front}}}.[15][16] Third gear operates at direct drive (1:1 ratio) by applying input torque simultaneously to the front ring gear and the common sun gear via both clutches, allowing the front and rear carriers to rotate freely together with all elements locked in unison. Power flows straight through without reduction or reversal, as the planetary units transmit torque synchronously from input to the front carrier output.[15] Reverse gear routes input torque to the common sun gear via the direct/reverse clutch while braking the rear carrier. Power flows from the sun gear through the rear planet gears to the rear ring gear (connected to the front carrier, output), producing backward rotation at reduced speed; the front ring gear freewheels. The ratio is approximately 2.0:1 in magnitude (negative for direction), resulting from the single-stage reversal in the rear unit with fixed carrier.[15][16] The following table illustrates representative gear ratios using typical tooth counts (sun gear: 36 teeth; front ring: 72 teeth; rear ring: 72 teeth), though actual values vary by application to optimize spread:| Gear | Approximate Ratio | Derivation Context |
|---|---|---|
| First | 2.5:1 | Compound double reduction via front drive to sun and rear reaction |
| Second | 1.5:1 | Single reduction with fixed sun |
| Third | 1:1 | Direct drive, no reduction |
| Reverse | -2.0:1 | Single-stage reversal with fixed rear carrier |