Grades 9–12 · Machines
How does an automatic transmission change gears?
An automatic transmission changes the relationship between engine rotation and wheel rotation without a driver moving a gear lever for each shift. Many conventional designs use planetary gear sets. These contain a central sun gear, planet gears around it, an outer ring gear, and a carrier that supports the planet shafts.
In the diagramClutches and brakes select a planetary gear ratioSun input • carrier output • one ideal gear setSRingSunCarrierBrake holds ringAPPLIED: ring 0 rpmClutch joins sun + ringReleasedController → valvesSun: 2400 rpmCarrier: 720 rpm • reduced speedPitch circles • teeth and shift transients omitted
The gears stay meshed. A clutch joins two rotating members, while a brake holds a member against the stationary transmission housing. Changing these connections changes the allowed motion of the whole set. The planets can spin on their own shafts while their carrier travels around the sun.
In the ideal example, the sun has 30 teeth and the ring has 70. With the ring held still, the carrier turns at 30 divided by 100 of the sun speed. A sun turning at 2400 revolutions per minute therefore drives the carrier at 720 rpm. Joining the sun and ring instead makes the set rotate together: the carrier also turns at 2400 rpm.
A controller uses information such as vehicle speed and accelerator demand to select a ratio. Electrically operated valves direct hydraulic pressure to the appropriate clutches and brakes. Actual gearboxes combine several gear sets and coordinate their connections during a shift. This single-set picture explains ratio selection; it does not reproduce a complete transmission or the brief slipping that can occur while connections change.
About this explanation
Starting knowledge: Understand gear ratios, rotational speed and a clutch as a connection between rotating parts.
Sources
- MathWorks — Planetary Gear
- Toyota Motor Corporation — Toyota introduces microcomputer-controlled automatic transmission ECT