📐 Gear Reunion Bench: step-by-step build

Count five small-gear turns before both marked gears return to their own starts.

The small gear comes home once per turn, but its larger neighbour is still somewhere else. This original two-shaft 24/40 bench isolates simultaneous orientation return: five input turns and three opposite output turns. Inspired by Brick Technology’s multi-dial clock gearing, it is a counting experiment, not a clock, a creator replica or a timing test.

  • Level 2 of 5 (easy)
  • About 20 minutes
  • 9 steps, 25 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.
  • Axles: An axle is a cross-shaped rod — anything on it turns together.
  • Bushes & Spacers: Bushes stop things sliding off an axle.
  • Frames: A frame is a ready-made strong rectangle.
  • Pins: Grippy or Smooth: Pins join beams together — some hold tight, some let things swing.

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
  • 4 × Axle 3
  • 10 × Bush
  • 2 × Axle 6
  • 6 × Half bush
  • 1 × Medium gear (24 teeth)
  • 1 × Giant gear (40 teeth)

Steps

  1. Lay the 5×11 frame flat on a level table. The first and last holes on each long edge will carry retained feet. Count axle spacing by gaps between hole centres, not the number of holes.
  2. Fit a 3-module axle through this corner, with a full bush immediately underneath and another immediately above. Hold the axle while seating both bushes. Repeat at all four corners; the feet must sit level.
  3. Fit a 3-module axle through this corner, with a full bush immediately underneath and another immediately above. Hold the axle while seating both bushes. Repeat at all four corners; the feet must sit level.
  4. Fit a 3-module axle through this corner, with a full bush immediately underneath and another immediately above. Hold the axle while seating both bushes. Repeat at all four corners; the feet must sit level.
  5. Fit a 3-module axle through this corner, with a full bush immediately underneath and another immediately above. Hold the axle while seating both bushes. Repeat at all four corners; the feet must sit level.
  6. Fit the input 6-module axle through the third hole along one long edge. Hold it while adding a half bush immediately below the frame and another immediately above. These stop the whole shaft escaping in either direction while its round bearing permits rotation. Add a full bush onto the upper half bush. Fit both shafts and spacers before adding either gear.
  7. Fit the output 6-module axle through the seventh hole along one long edge. Hold it while adding a half bush immediately below the frame and another immediately above. These stop the whole shaft escaping in either direction while its round bearing permits rotation. Add a full bush onto the upper half bush. Fit both shafts and spacers before adding either gear.
  8. Slide the 24-tooth gear onto that spacer and a half bush immediately above the gear. This is this pair’s input; leave space for its output. Red top bushes identify inputs; blue identify outputs. Both gears sit at the same height. Their centres are four hole spacings (32 mm) apart. Rotate the whole output shaft to seat its teeth in the input gaps. The 4.5-degree modeled output phase is tooth seating, not an extra turn.
  9. Slide the 40-tooth gear onto that spacer and a half bush immediately above the gear. Turn the entire keyed shaft gently to seat its teeth in the neighbouring input’s gaps. Red top bushes identify inputs; blue identify outputs. Both gears sit at the same height. Their centres are four hole spacings (32 mm) apart. Rotate the whole output shaft to seat its teeth in the input gaps. The 4.5-degree modeled output phase is tooth seating, not an extra turn.

Play time

Hold the stationary frame on a level table. Place tiny removable dots on the top faces away from tooth flanks; do not add anything between the gears. Sketch each dot’s own starting direction on paper beside the bench. Turn the red 24-tooth input slowly, counting complete turns. After one input turn the 40-tooth output has made 0.6 opposite turns; after five input turns it has made three opposite turns and both dots return to their own starts. Repeat three times, then gently reverse. Do not brake an output, add a motor or force binding teeth. Stop for loose retainers or rubbing. We have not physically tested this model.

Credited demonstrations and component sources