Planetary Gears 101 – UnderhoodService

Planetary Gears 101

The most confusing systems on any car is the automatic transmission. The main stumbling block for most technicians is not the electronic control systems, but the humble planetary gear. The planetary gear sets in an automatic transmission provide forward underdrive and overdrive gear ratios, plus reverse. The machinery handbook describes planetary gearing as a compact means of driving inline shafts with speed reduction (underdrive) and induction (overdrive). Advantages of planetary gears over parallel axis gears include high power density, large reduction in a small volume, pure torsional reactions and coaxial shafting. Disadvantages include high bearing loads, inaccessibility and design complexity. The planetary gearbox arrangement is an engineering design that offers many advantages over traditional gearbox arrangements. One advantage is its unique combination of both compactness and outstanding power transmission efficiencies.

FIGURE 1: A planetary gear set is made up of a sun gear, planetary gears and a ring gear.  The ring is used as the final drive to the vehicles front or rear drive wheels.  The gears have three modes of operation: Driver, Fixed and Follower. 
 

 
FIGURE 2: Overdrive uses the sun gear in a fixed position, planetary gear in a driver position and the ring gear as the follower.  
 

 
FIGURE 3: Underdrive uses the sun gear in a driver position, the planetary gear set as the follower and the ring gear is in the fixed position.  To drive the ring gear in the proper direction, the planetary gear set uses a second gear to turn the ring gear. 

 
FIGURE 4: Reverse uses the sun gear in a drive position, the planetary gear set in a fixed position with the ring gear in a follower position. One of two types of clutches or bands are used to hold a particular member of the planetary gearset motionless, while allowing another member to rotate, thereby transmitting torque and producing gear reductions or overdrive ratios. These clutches are actuated by the valve body and their sequence controlled by the transmission’s internal programming. Principally, a type of device known as a sprag or roller clutch is used for routine upshifts/downshifts.
 

You May Also Like

Automotive Hall of Fame Announces Class of 2024 Inductees

Six honorees whose efforts helped shape the automotive and mobility marketplace will be recognized.

The Automotive Hall of Fame announced its 2024 Induction & Awards Ceremony will take place in Detroit on September 19, 2024, to recognize a selection of global mobility pioneers. Widely considered the single greatest honor an individual can receive in the automotive industry, induction into the Automotive Hall of Fame is reserved for noteworthy individuals whose efforts helped shape the automotive and mobility marketplace.

Deciphering Engine Misfire Codes

When the fuel/air mixture ignites, many factors must be met to pass a misfire monitor.

Duke Energy Launches EV Charger Rental Program in NC

The Charger Solution program offers a range of charger options and guides customers through evaluating charging station options.

Castrol Turns 125, Debuts New Market Strategy

As part of the new strategy, Castrol will explore opportunities in battery thermal management, digital and service solutions and data center immersion cooling.

Tensioner Tech Assist

The Lift and Pin is designed to simplify tensioner replacement and belt installation. This video is sponsored by Litens.

Other Posts

Detecting Misfire And Knock Conditions

Knock sensors are mounted on the block near the base of the cylinder head.

Ignition System Do’s and Don’ts

Why do ignition systems give technicians problems when diagnosing ignition-related misfires? The answer is that some technicians use tests that might give inconclusive results or do damage to the coil or drivers inside a module.

GM, Honda Begin Volume Production of Fuel Cell Systems

GM and Honda will produce fuel cells that will be two-thirds less expensive compared to Honda’s 2019 Clarity fuel cell.

EV Cooling Systems

The large batteries that EVs carry need to be maintained within a specific temperature range for optimal performance.