What is an axle, and why is it cross-shaped?

An axle is a cross-shaped rod — anything on it turns together. Axles have a cross (+) shape. Gears and wheels have cross holes, so they grab the axle and spin WITH it. Round beam holes let the axle spin freely inside them.

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Complete MightyCog booklet collection: 82 PDF booklets for $12.99. Instructions only; individual booklets remain free.

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Try it yourself

Put a gear on an axle and twist the axle. Does the gear turn too?

Build it: 11 builds that teach axles

  • Tiny Roller🚗 Tiny Roller: Your very first rolling car — only 13 pieces! (level 1 of 5, about 5 minutes, 6 steps)
  • 🔢 Odometer Cart: Five turns of the wheels make the flag go round just once — a cart that counts! (level 1 of 5, about 15 minutes, 9 steps)
  • ⚖️ Balance See-Saw: A see-saw that is also a scale. Can one gear beat two? (level 1 of 5, about 12 minutes, 10 steps)
  • 🌀 Coasting Wheel Bench: Spin a retained tabletop wheel gently, then measure how it slows down. (level 2 of 5, about 15 minutes, 8 steps)
  • 🔀 Dual Output Gear Bench: One central input drives two unequal outputs: count each branch independently. (level 2 of 5, about 20 minutes, 14 steps)
  • ⭕ Rim Distance Bench: Two unequal wheels share one shaft: equal turns, unequal rim travel. (level 2 of 5, about 15 minutes, 9 steps)
  • 📐 Crank Orbit Bench: Trace a crank pin’s circle and compare its one-axis travel at equal angle steps. (level 2 of 5, about 15 minutes, 10 steps)
  • 🐦 Woodpecker Automaton: Turn the handle and the woodpecker pecks: tok-tok-tok-tok! (level 2 of 5, about 20 minutes, 16 steps)
  • 📏 Rack Travel Bench: Turn a small gear and measure the straight travel of a captured rack. (level 3 of 5, about 25 minutes, 13 steps)
  • 🌬️ Bevel-Gear Windmill: Turn the handwheel at the bottom — the sails spin at the top! (level 3 of 5, about 25 minutes, 16 steps)
  • 🔬 Compound Counting Bench: Two five-to-one stages multiply: count 25 input turns for one output turn. (level 2 of 5, about 20 minutes, 7 steps)

In the classroom

Common engineering questions

How can I try axles at home?

Put a gear on an axle and twist the axle. Does the gear turn too?

Which builds teach axles?

11 MightyCog builds teach it: Tiny Roller, Odometer Cart, Balance See-Saw, Coasting Wheel Bench and Dual Output Gear Bench and more. Start with Tiny Roller: 6 steps, about 5 minutes.

Ideas that go with it