Always in gear.

The gears stay meshed. The connections change.
Explore the quiet choreography inside an automatic transmission.

FIG. 02 / A conventional automaticCutaway view

Rotation slowed 120× · select a component belowDrive · 2nd gear

Driven memberHeld memberSectioned housing

THE PRINCIPLE

Change the connection.

The torque converter transfers engine power through fluid. Inside the gearbox, clutches connect rotating parts and brakes hold other parts still. Those choices make the same planetary gears reduce speed, turn together, or reverse the output.

DETAIL APlanetary set / from the output end

30-tooth sun · 3 × 21-tooth planets · 72-tooth ring
The front carrier connects the three planet pins.

DRIVE / SECONDConverter: fluid coupling

Hold the sun. Drive the ring.

The planets walk around the fixed sun and carry the output more slowly than the input.

MEMBER	CONNECTION	SPEED / RPM
	HELD	0
	INPUT	1381
	OUTPUT	975

Show the gear math & model limits

30 × sun rpm + 72 × ring rpm = 102 × carrier rpm.

These are ideal kinematic ratios, before losses. Reverse uses the ring as output; the forward examples use the carrier. Selectable couplings are simplified here. Production automatics usually combine multiple gearsets and have their own clutch maps and shift schedules.