📐 Crank Orbit Bench: step-by-step build

Trace a crank pin’s circle and compare its one-axis travel at equal angle steps.

A crank pin follows a circle, not a straight line. This original measurement bench has a rigid two-pin crank arm on a retained vertical shaft. Turn it gently and compare the outer pin’s positions at equal angle steps. Inspired by redshoebox’s oscillating-mechanism demonstrations; it is not their machine or a piston assembly.

  • Level 2 of 5 (easy)
  • About 15 minutes
  • 10 steps, 25 parts
  • Powered by: Hand-crank
  • Series: Starter Builds

What you will learn

  • Crank & Slider: A turning crank pushes a rod back and forth.
  • Axles: An axle is a cross-shaped rod — anything on it turns together.
  • Frames: A frame is a ready-made strong rectangle.
  • Pins: Grippy or Smooth: Pins join beams together — some hold tight, some let things swing.
  • Bushes & Spacers: Bushes stop things sliding off an axle.

Build these first

  • Tiny Roller🚗 Tiny Roller: Your very first rolling car — only 13 pieces! (level 1 of 5, about 5 minutes, 6 steps)

Parts you need

  • 1 × Open frame 5×11
  • 4 × Axle 3
  • 8 × Bush
  • 5 × Black pin (grippy)
  • 2 × Beam 5
  • 1 × Axle 6
  • 3 × Half bush
  • 1 × Medium gear (24 teeth)

Steps

  1. Lay the 5×11 frame flat. Its four corner holes will hold the feet; the middle holes on its long sides will hold a bridge.
  2. Fit a 3-module axle through this corner. Hold it while fitting a full bush underneath as a foot and another full bush above as a retainer. Repeat at all four corners; the feet must rest level.
  3. Fit a 3-module axle through this corner. Hold it while fitting a full bush underneath as a foot and another full bush above as a retainer. Repeat at all four corners; the feet must rest level.
  4. Fit a 3-module axle through this corner. Hold it while fitting a full bush underneath as a foot and another full bush above as a retainer. Repeat at all four corners; the feet must rest level.
  5. Fit a 3-module axle through this corner. Hold it while fitting a full bush underneath as a foot and another full bush above as a retainer. Repeat at all four corners; the feet must rest level.
  6. Push two black friction pins upward through the middle holes of the long sides. Press the end holes of a flat 5-hole beam onto them, across the open centre. Two separated pins keep this bearing bridge rigid.
  7. Fit a vertical 6-module axle through the bridge’s centre round hole. Hold it while fitting a half bush directly underneath and another directly above. Leave the shaft free to turn, with minimal axial play.
  8. Slide a 24-tooth gear down onto the upper spacer. Its cross hole keys it to the shaft. It is a mounting hub here, not a meshing gear.
  9. Place two black pins upward in opposite gear holes, one hole spacing either side of the axle. Fit a flat 5-hole beam by its second and fourth holes. Two separated pins lock the crank arm to the hub. Add a half bush above the arm on the shaft.
  10. Fit a black friction pin pointing upward in the arm’s outer end hole. Its centre is two hole spacings from the shaft: a 16 mm crank radius. The opposite end stays empty.

Play time

Hold the stationary frame. Turn the top axle gently through a full revolution. Observe the outer pin from directly above; its radius is 16 mm. On separate paper, compare its position projected onto a fixed straight axis at equal angle steps. Keep paper clear of moving parts. The pin itself moves in a circle; there is no physical sliding follower. Stop if the bridge, arm or retainers are loose.

Real-world inspiration

Mechanical Principles demonstrated with LEGO 01 — redshoebox — https://www.youtube.com/watch?v=muU4fawAtZU