🌉 Speed-Cancelling Gear Bridge: step-by-step build

A 40–8–40 gear train: the middle turns five times, but the ends stay together.

Can a fast middle gear leave the final speed unchanged? Build a three-shaft bench with equal 40-tooth end gears and an 8-tooth idler. Count turns at every shaft to see a speed increase and reduction cancel. This is our compact educational model, inspired by long gear-train demonstrations, not a replica of a creator’s machine.

• Level 2 of 5 (easy)
• About 15 minutes
• 5 steps, 16 parts
• Powered by: Hand-crank
• Series: Starter Builds

What you will learn

• Gears: Gears are wheels with teeth that push each other round.
• Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.
• Idler Gears & Direction: A gear in the middle flips the direction back.
• Friction: Friction is the rubbing force that slows things down.
• Frames: A frame is a ready-made strong rectangle.
• Bushes & Spacers: Bushes stop things sliding off an axle.

Build these first

• ⚙️ Gear Friends: Three gears holding hands — spin one and they all dance. (level 1 of 5, about 8 minutes, 6 steps)

Parts you need

• 1 × Open frame 5×11
• 3 × Axle 4
• 6 × Bush
• 2 × Giant gear (40 teeth)
• 1 × Tiny gear (8 teeth)
• 2 × Half bush
• 1 × Axle 3

Steps

1. Lay the 5×11 frame flat. Find holes 2, 5 and 8 along one long edge; their centres are three modules apart.
2. At hole 2, fit a vertical 4-module axle, a full bush underneath, a 40-tooth gear above the frame and a red full bush on top. Hold the axle while fitting the lower bush. This red knob is the input.
3. At hole 5, fit another vertical 4-module axle with a full bush below, an 8-tooth gear above and a half bush on top. Rotate this entire shaft slightly so the tiny teeth enter the big gear’s gaps. Both gears must sit at the same height.
4. At hole 8, fit the final vertical 4-module axle, lower full bush, second 40-tooth gear and blue top bush. Seat its teeth in the tiny gear’s gaps without changing the axle spacing.
5. Add a fixed third foot on the opposite edge at hole 8: a vertical 3-module axle, full bush underneath and half bush on top. Keep the bench on a flat surface and check that every shaft is retained.

Play time

Use a removable mark on each top bush. Turn the red input once slowly: the middle makes five reverse turns and the blue output makes one forward turn. Repeat in reverse. A 1:1 speed ratio does not mean zero friction.