📐 Opposite Output Gear Bench: step-by-step build

One input drives two equal-speed outputs in opposite directions through different paths.

One 40-tooth input drives a direct 8-tooth output and a second 8-tooth output through a 24-tooth idler. Both outputs make five turns per input turn, but one reverses and the other follows the input. This original fixed-path comparison is inspired by the reversal question in Dr. Engine’s multi-stage gear-train demonstration. It does not recreate its selectable gearbox, scissor lift or motor-driven assembly.

  • Level 2 of 5 (easy)
  • About 20 minutes
  • 13 steps, 37 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
  • 12 × Bush
  • 4 × Axle 6
  • 12 × Half bush
  • 1 × Giant gear (40 teeth)
  • 2 × Tiny gear (8 teeth)
  • 1 × Medium gear (24 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 sixth 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 all four shafts and these spacers before adding any gear.
  7. Fit the direct output 6-module axle through the ninth 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 all four shafts and these spacers before adding any gear.
  8. Fit the idler 6-module axle through the sixth hole along the opposite 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 all four shafts and these spacers before adding any gear.
  9. Fit the indirect output 6-module axle through the eighth hole along the opposite 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 all four shafts and these spacers before adding any gear.
  10. Slide the 40-tooth gear onto that spacer and a half bush immediately above the gear. This is the shared input; leave room for both branches. The red top bush marks the input; blue marks both outputs; yellow marks the idler. All four gears sit at the same height. Input to direct output is three hole spacings; input to idler is four; idler to indirect output is two. Do not add a gear between the two outputs.
  11. Slide the 8-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 40-tooth input gear’s gaps. The red top bush marks the input; blue marks both outputs; yellow marks the idler. All four gears sit at the same height. Input to direct output is three hole spacings; input to idler is four; idler to indirect output is two. Do not add a gear between the two outputs.
  12. Slide the 24-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 40-tooth input gear’s gaps. The red top bush marks the input; blue marks both outputs; yellow marks the idler. All four gears sit at the same height. Input to direct output is three hole spacings; input to idler is four; idler to indirect output is two. Do not add a gear between the two outputs.
  13. Slide the 8-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 24-tooth idler gear’s gaps. The red top bush marks the input; blue marks both outputs; yellow marks the idler. All four gears sit at the same height. Input to direct output is three hole spacings; input to idler is four; idler to indirect output is two. Do not add a gear between the two outputs.

Play time

Hold the stationary frame on a level table. Mark both outputs with removable dots and view all three shafts from the same side. Gently turn the red input once: predict five opposite turns at the direct output and five same-direction turns at the indirect output. Repeat slowly in reverse. Do not brake either output, add a motor or force binding teeth. Check seated retainers, gear height and spacing first. The two outputs are not independent inputs. We have not physically tested this model.

Credited demonstrations and component sources