🔬 Compound Counting Bench: step-by-step build

Two five-to-one stages multiply: count 25 input turns for one output turn.

Build an open two-level gear bench to trace a compound reduction. The middle 40-tooth and 8-tooth gears share a cross axle and turn together, passing motion from the lower mesh to the upper mesh. This original educational bench takes inspiration from Brick Experiment Channel’s large reduction demonstration; it does not recreate that machine.

  • Level 2 of 5 (easy)
  • About 20 minutes
  • 7 steps, 18 parts
  • Powered by: Hand-crank
  • Series: Starter Builds

What you will learn

  • Gears: Gears are wheels with teeth that push each other round.
  • Compound Gears: Two gear stages in a row multiply their ratios.
  • Gear Ratio: Fast or Slow: A big gear turning a small gear makes it spin faster.
  • 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.
  • Axles: An axle is a cross-shaped rod — anything on it turns together.

Build these first

  • Gear Speed Lab🧪 Gear Speed Lab: Two cranks, two gear pairs: one makes things FAST, one makes things STRONG. (level 2 of 5, about 20 minutes, 13 steps)

Parts you need

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

Steps

  1. Lay the 5×11 frame flat. Locate holes 2, 5 and 8 on one long edge. The shafts will be three modules apart.
  2. At hole 2, fit a vertical 4-module axle, full bush below, 8-tooth gear above and red full bush on top. Hold the axle while fitting its lower stop; turn gently without squeezing the gear against the frame.
  3. At hole 5, fit a vertical 6-module axle with a full bush underneath and a 40-tooth gear at the same height as the input gear. Rotate the whole shaft slightly until teeth fit gaps. Keep the big gear resting on the frame until the next step adds the upper gear and retaining bush.
  4. Slide an 8-tooth gear onto the SAME middle axle directly above the big gear, then fit a blue full bush on top. The small gear centre is one module above the big gear centre. They cannot turn independently: both cross holes grip the axle.
  5. At hole 8, fit the second 40-tooth gear together with a vertical 6-module axle, full lower bush and full spacer bush immediately above the frame. Hold the big gear while threading the axle, before fitting the bushes. The output gear centre is level with the upper middle gear.
  6. Fit a yellow full bush above the output gear and check both meshes independently. The complete output shaft is rotated slightly so teeth fit gaps. Neither shaft should slide out of mesh.
  7. Fit a stationary foot at hole 8 on the opposite long edge: 3-module vertical axle, full lower bush and top half bush. Check retention on every shaft. Stop if teeth bind; never force the red input.

Play time

Mark the three top bushes with removable marks. Turn red slowly 25 times, tallying in groups of five: blue should make five reverse turns, yellow one forward turn. Repeat three trials and reverse direction. Compare actual counts with the ideal prediction; this is not a load-lifting model.

Real-world inspiration

Making a GOOGOL:1 Reduction with Lego Gears — Brick Experiment Channel — https://www.youtube.com/watch?v=QwXK4e4uqXY