What is a worm gear, and why does it lock?

A worm gear turns a gear super slowly — and never slips back. A worm gear is a screw. Each full turn only pushes the gear on by one tooth, so a 24-tooth gear needs 24 turns to go round once! And the gear cannot turn the worm backwards, so a crane stays up by itself.

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Try it yourself

Try to turn the big gear with your fingers. Now turn the worm crank. What is different?

Build it: 14 builds that teach worm gear

  • Worm-Gear Crane🏗️ Worm-Gear Crane: Crank the worm gear to lift the boom — then let go. It stays up by itself! (level 3 of 5, about 30 minutes, 15 steps)
  • 🏰 Castle Drawbridge: Wind the handle and the heavy bridge slowly rises — and stays up when you let go. (level 2 of 5, about 30 minutes, 15 steps)
  • 🌬️ Oscillating Fan: A desk fan that turns its head left and right all by itself! (level 3 of 5, about 40 minutes, 18 steps)
  • 🛢️ Nodding-Donkey Pump Jack: A slow crank rocks a long beam up and down — nod, nod, nod, all day long. (level 3 of 5, about 45 minutes, 30 steps)
  • 🛗 Scissor Lift: Turn the crank and the platform climbs straight up on criss-cross legs. (level 3 of 5, about 45 minutes, 29 steps)
  • 🎡 Ferris Wheel: A big wheel turns slowly while its gondolas always hang the right way up. (level 3 of 5, about 45 minutes, 28 steps)
  • 🦾 XL Power Crane: The strongest motor plus a worm gear: 24 times slower and super strong. (level 4 of 5, about 35 minutes, 27 steps)
  • 📦 Conveyor Belt: A factory belt that carries your parts from one end to the other — and drops them in a box. (level 3 of 5, about 50 minutes, 22 steps)
  • 🚜 Skid-Steer Loader: No steering at all: left motor, right motor — spin it round on the spot! (level 4 of 5, about 65 minutes, 25 steps)
  • 🧗 Worm-Drive Crawler: Super slow, super strong: worm gears drive both axles and it never rolls back. (level 4 of 5, about 50 minutes, 20 steps)
  • 🚞 Cog Railway Climber: A mountain train that grips a toothed rail — it climbs super steep ramps and never slides back. (level 4 of 5, about 55 minutes, 20 steps)
  • 🦾 Crank Excavator Arm: Crank the worm gears to swing the digger round and lift the boom — then dig! (level 4 of 5, about 50 minutes, 22 steps)
  • 🤖 Robot Arm with Servo Claw: Swing, lift and grab! Three motors and a remote turn you into a factory robot operator. (level 4 of 5, about 60 minutes, 30 steps)
  • 🚧 Twin-Motor Dozer: Wide caterpillar tracks, one motor per track, and a blade that cranks up with a worm gear. (level 5 of 5, about 100 minutes, 33 steps)

In the classroom

Common engineering questions

How can I try worm gear at home?

Try to turn the big gear with your fingers. Now turn the worm crank. What is different?

Which builds teach worm gear?

14 MightyCog builds teach it: Worm-Gear Crane, Castle Drawbridge, Oscillating Fan, Nodding-Donkey Pump Jack and Scissor Lift and more. Start with Worm-Gear Crane: 15 steps, about 30 minutes.

Ideas that go with it