📏 Rack Travel Bench: step-by-step build

Turn a small gear and measure the straight travel of a captured rack.

A round gear can move a straight row of teeth. Build a low tabletop counting bench and slide the red rack carrier inside a retained channel. Compare shaft angle with travel, then reverse and observe clearance. This original model takes the rack-motion idea from redshoebox’s demonstration, but uses its own layout and instructions; it is not a copy of the demonstrated mechanism.

  • Level 3 of 5 (medium)
  • About 25 minutes
  • 13 steps, 36 parts
  • Powered by: Hand-crank
  • Series: Starter Builds

What you will learn

  • Rack & Pinion Steering: A gear rolling on a straight rack slides it sideways.
  • Gears: Gears are wheels with teeth that push each other round.
  • Axles: An axle is a cross-shaped rod — anything on it turns together.
  • Friction: Friction is the rubbing force that slows things down.
  • Frames: A frame is a ready-made strong rectangle.

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
  • 2 × Axle 4
  • 7 × Bush
  • 1 × Tiny gear (8 teeth)
  • 2 × Beam 5
  • 4 × Axle 5
  • 10 × Half bush
  • 4 × Black pin (grippy)
  • 2 × Beam 7
  • 1 × Steering rack
  • 2 × Beam 3

Steps

  1. Lay the 5×11 frame flat on a smooth tabletop. Use the middle hole on a short edge for the pinion axle.
  2. Hold a 4-module axle in that hole. Fit a full bush beneath the frame, an 8-tooth gear above it and a red full bush on top. Keep the shaft free to turn.
  3. Fit a 5-hole lower crosspiece beneath the left side of the frame, extending two holes outward. Retain its guide axle underneath, and pin its next inner hole to the frame so it cannot swing.
  4. Fit the same retained and pinned lower crosspiece beneath the right side. Both crosspieces support the rack channel.
  5. Lay a 7-hole outer rail two modules outside the frame edge. Fit its left guide axle, lower half-bush foot and upper full-bush spacer.
  6. Fit the right outer guide axle with its lower foot and upper spacer. Two fixed axle lines now prevent the outer rail swinging.
  7. Add full-bush spacers to the two inner guide axles. All four spacers will support the top retainers.
  8. Support the far end of the long frame: fit a 4-module axle through its middle rear hole. Stack a full bush and half bush underneath as a foot, then retain it above with another half bush.
  9. Make the carrier: lay a red 7-hole beam flat, with its holes facing up. Its long edge will follow the outside of the frame.
  10. Fit two friction pins through the beam ends and press the rack onto them, teeth toward the frame. The pins retain the rack without hardware projecting below the carrier.
  11. Bring the carrier in from the left, outside the frame, until its middle is beside the pinion. Turn the whole shaft slightly so teeth fit gaps. The carrier sits on the lower crosspieces between the frame edge and outer rail; the next steps close the top. Limit travel to one module each side of this centre position.
  12. Close the guide with two 3-hole crosspieces across the channel. The carrier must still slide; the crosspieces keep it from lifting off.
  13. Retain each top crosspiece with two half bushes. Check the carrier cannot lift, yaw or move sideways, then slide it gently through the short stroke.

Play time

Hold the frame steady and slide the carrier in its captured channel. Mark the centre position with removable tape. Move the rack one module each way and observe the pinion. Record three slow forward and reverse trials; stop before the pinion reaches the toothless end tabs. Do not force a full turn or use this bench to lift a load.

Real-world inspiration

Mechanical Principles demonstrated with LEGO 03 — redshoebox — https://www.youtube.com/watch?v=CVL-9Cce38E