Drive the sun and watch the planets roll round inside the ring. Click a part to hold it, take the set apart, or look inside the drill. Drag to turn the model, scroll or pinch to zoom.
Try itPress play to drive the sun. Click the ring, the sun or the carrier to hold it.
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Scroll to take it apart
Four parts on one axis
Keep scrolling: the sun slides out, the planets come off their pins, and the ring lets go of the carrier.
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Step 1Pull out the sunThe sun gear and its splined input shaft slide out of the middle.
Step 2Slide off the planetsEach planet turns on a needle bearing round a steel pin.
Step 3Lift off the ringIts 60 teeth are on the inside. Behind it is the carrier and its output shaft.
Meet the parts
Three members and the bits that hold them
Tap any part of the exploded set or pick a name below. The sun, the carrier and the ring are the three members that can turn. The planets, pins and bearings link them. At the back are the band and the clutch an automatic gearbox adds.
Hover or tap the picture. Orange rings mark every part.
Parts diagram
From the motor to the bit
A cordless drill cut open, numbered along the path the power takes: the motor, three planetary stages and the speed switch, the clutch, then the chuck and the bit.
How it works, step by step
Twenty steps, from the drill to a car
Follow the same path as the video. Each step has a picture, a few lines, and a button that sets up the 3D model to show it.
A small motor, a big twist
Try it: hold, drive, take
Pick the parts, see the ratio
Choose which part is held and which one you drive. The third one is the output. Change the teeth and the planets too: the numbers come straight from the tooth counts.
Hold
Drive
Planets
3.5 : 1input to output
Same wayoutput turns
5.7output, rpm
× 3.5output torque
Each planet is pushed at two teeth, one on the sun and one on the ring, and passes both pushes to its pin.
Try it: share the load
Why three planets beat one
Every push from the sun reaches the carrier through the planets. With three planets, each tooth contact carries only a third of the force.
Spread evenly, the planets also push on the sun from three sides, so those pushes cancel and the shafts carry no sideways load. That is why the input and the output can sit on one straight axis.
Planets
0force at each tooth contact, N
0force on each pin, N
0sideways push on the sun, N
0torque out of the carrier, N·m
Try it: an automatic gearbox
One set, several gears
An automatic gearbox never slides a gear out of mesh. It squeezes clutches and brake bands instead, and each combination is a different gear.
Here one planetary set drives a car through its carrier. Two clutches choose what the engine turns, and two bands choose what is held. Pick a gear, or squeeze them yourself and see what happens.
A brake band round the ring, and a clutch pack behind the carrier.
Gear
Gear
Clutch A
Clutch B
Band 1
Band 2
Ratio
–
3.5 : 1
1.4 : 1
1 : 1
–gear ratio
800engine, rpm
0output shaft, rpm
0road speed, km/h
The screw is home: the last ring slips under the clutch balls and the drill clicks.
Try it: the drill’s clutch
Set the collar, drive the screw
Spring-loaded balls hold the drill’s last ring gear still. Each number on the collar squeezes the springs a little harder.
While the ring is held, the last stage drives the chuck. When the screw needs more torque than the springs allow, the ring pushes the balls back over its ramps, slips, and the drill clicks instead of stripping the screw.
Speed switch
0 mmscrew depth (50 mm screw)
0torque the screw needs, N·m
0clutch lets go at, N·m
Readydrill
More to know
Kinds, history and care
Six kinds of planetary gear
SimpleOne sun, one ring and a carrier of planets, like the set on this page. The most common kind.
Multi-stageSets in a row, each carrier driving the next sun. Drills, servo motors and wind turbines stack two or three.
Stepped planetsEach planet has two gear sizes on one shaft, meshing with two different suns or rings, for bigger ratios in one set.
Double planetsPlanets work in pairs between the sun and the ring. With the carrier held, the ring then turns the same way as the sun.
SimpsonTwo simple sets share one sun gear. With two clutches and two bands it makes a classic three-speed automatic with reverse.
RavigneauxTwo suns, short and long planets, and one ring in a single compact unit: four forward gears and reverse.
From the heavens to the gearbox
The Greek Antikythera mechanism mounts gears on a turning gear to copy the Moon’s uneven speed across the sky.
Ptolemy explains the planets’ wandering paths with epicycles: circles riding on circles.
James Watt patents the sun-and-planet gear, invented by his engineer William Murdoch, to turn a steam engine’s beam into rotation.
The Sturmey-Archer three-speed bicycle hub hides a planetary set inside the rear wheel.
The Ford Model T changes between two speeds with pedals that tighten bands round planetary drums.
General Motors’ Hydra-Matic, the first mass-produced fully automatic gearbox, shifts planetary sets with bands and clutches.
The Toyota Prius uses one planetary set to blend a petrol engine and two electric motors.
Eight, nine and ten-speed automatics use four planetary sets and five or six clutches and brakes.
Looking after the gears
Only move a drill’s speed switch when the motor has stopped. The sliding ring has to find its teeth.
Use speed 1 for big screws and holes. It has three and a half times the torque of speed 2.
Start with a low clutch number and turn it up until the screw head sits flush.
A grinding or slipping gearbox often means worn teeth or a cracked ring gear. Many drills use plastic rings.
Planetary gears carry big loads on small teeth, so keep them greased or in clean oil.
In a car with an automatic gearbox, stop completely before selecting reverse or park.
Things worth knowing
Planet size doesn’t matterThe ratio comes only from the sun and the ring. The planets just have to fit: ring teeth = sun teeth + 2 × planet teeth.
Why three fit evenlyEqual spacing works only if sun plus ring teeth divide by the number of planets: 24 + 60 = 84, and 84 ÷ 3 = 28. With five planets it wouldn’t.
Twice per strokeWatt’s sun-and-planet gear made the flywheel turn twice for every up-and-down stroke of the engine’s beam.
As wide as a golf ballThe drill’s three stages give a 43 : 1 reduction in a gearbox only about 45 mm across.
Wind powerWind turbine gearboxes use planetary stages to speed a slow rotor up to the generator, about a hundred times faster.
No belt, no conesThe Prius set works like a continuously variable gearbox, with the motors setting the ratio electrically.
Every Mechanism
