You've probably never tried driving a manual trans- mission car using only third gear. If you did, you'd quickly find out that you had almost no acceleration when starting out, and at high speeds, the engine would be screaming along near the red-line. A car like this would wear out very quickly and would be nearly undriveable. So the transmission uses gears to make more effective use of the engine's torque, and to keep the engine operating at an appropriate speed. When towing or hauling heavy objects, your vehicle's transmission can get hot enough to burn up the transmission fluid. In order to protect the transmission from serious damage, drivers who tow should buy vehicles equipped with transmission coolers. The key difference between a manual and an automatic transmission is that the manual transmission locks and unlocks different sets of gears to the output shaft to achieve the various gear ratios, while in an automatic transmission, the same set of gears produces all of the different gear ratios. The planetary gearset is the device that makes this possible in an automatic transmission. Let's take a look at how the planetary gearset works When you take apart and look inside an automatic transmission, you find a huge assortment of parts in a fairly small space. Among other things, you see: An ingenious planetary gearset A set of bands to lock parts of a gearset A set of three wet-plate clutches to lock other parts of the gearset An incredibly odd hydraulic system that controls the clutches and bands A large gear pump to move transmission fluid around The center of attention is the planetary gearset. About the size of a cantaloupe, this one part creates all of the different gear ratios that the transmission can produce. Everything else in the transmission is there to help theplanetary gearset do its thing. This amazing piece of gearing has appeared on HowStuffWorks before. You may recognize it from the electric screwdriver article. An automatic transmission contains two complete planetary gearsets folded together into one component. Any planetary gearset has three main components: The sun gear The planet gears and the planet gears' carrier The ring gear Each of these three components can be the input, the output or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gearset.
The 6L50 transmission is a Hydra-Matic six-speed rear and all-wheel drive automatic transmission produced by GM. Left to right: the ring gear, planet carrier, and 2 sun gears.