What does an idler gear do?

A gear in the middle flips the direction back. Two gears spin opposite ways. Add a third gear in between — an idler — and the first and last gears now spin the SAME way. The idler does not change the speed, only the direction and the distance.

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Try it yourself

Count the gears in a row. With an odd number, do the ends turn the same way?

Build it: 7 builds that teach idler gears

  • ⚙️ Gear Friends: Three gears holding hands — spin one and they all dance. (level 1 of 5, about 8 minutes, 6 steps)
  • 🎠 Idler Parade Car: A parade of gears rides on top — each one spins the opposite way to its neighbour! (level 1 of 5, about 20 minutes, 13 steps)
  • 🌉 Speed-Cancelling Gear Bridge: A 40–8–40 gear train: the middle turns five times, but the ends stay together. (level 2 of 5, about 15 minutes, 5 steps)
  • 🥚 Egg Beater: Turn the handle and two whisks whizz round in opposite directions. (level 2 of 5, about 20 minutes, 12 steps)
  • ⚙️ Clutch-Gear 2-Speed Car: Push the gear lever and a driving ring grabs a different gear: 1st climbs, 2nd races! (level 3 of 5, about 45 minutes, 18 steps)
  • ↩️ Reverse-Gear Truck: The motor only spins one way — the gear lever decides forwards or backwards! (level 4 of 5, about 55 minutes, 18 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)

In the classroom

Common engineering questions

How can I try idler gears at home?

Count the gears in a row. With an odd number, do the ends turn the same way?

Which builds teach idler gears?

7 MightyCog builds teach it: Gear Friends, Idler Parade Car, Speed-Cancelling Gear Bridge, Egg Beater and Clutch-Gear 2-Speed Car and more. Start with Gear Friends: 6 steps, about 8 minutes.

Ideas that go with it