🔗 Parallel Motion Bench: step-by-step build

Move a connecting bar around an arc without turning the bar itself.

A bar can change position without changing its direction. This original four-bar bench uses equal three-spacing crank radii and equal four-spacing fixed and moving links. Rock either crank gently through the reviewed ±30° range and compare the moving bar with the fixed line between the shafts. Inspired by a parallel-motion demonstration, not a recreation of its motorized vehicle. The bar follows an arc, not a straight rail.

  • Level 3 of 5 (medium)
  • About 30 minutes
  • 16 steps, 38 parts
  • Powered by: Hand-crank
  • Series: Starter Builds

What you will learn

  • Linkages: Beams joined by pins change one motion into another.
  • Crank & Slider: A turning crank pushes a rod back and forth.
  • 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

  • 📐 Crank Orbit Bench: Trace a crank pin’s circle and compare its one-axis travel at equal angle steps. (level 2 of 5, about 15 minutes, 10 steps)

Parts you need

  • 1 × Open frame 5×11
  • 6 × Axle 3
  • 10 × Bush
  • 2 × Axle 6
  • 10 × Half bush
  • 2 × Medium gear (24 teeth)
  • 4 × Black pin (grippy)
  • 2 × Beam 7
  • 1 × Beam 5

Steps

  1. Lay the 5×11 frame flat. Its long edges have eleven holes; count spaces between hole centres. Keep the frame on a level table.
  2. Fit a 3-module axle at this corner. Hold it while adding a full bush directly underneath and another directly above. Repeat at all four corners so the feet sit level.
  3. Fit a 3-module axle at this corner. Hold it while adding a full bush directly underneath and another directly above. Repeat at all four corners so the feet sit level.
  4. Fit a 3-module axle at this corner. Hold it while adding a full bush directly underneath and another directly above. Repeat at all four corners so the feet sit level.
  5. Fit a 3-module axle at this corner. Hold it while adding a full bush directly underneath and another directly above. Repeat at all four corners so the feet sit level.
  6. Fit the first vertical 6-module axle through the fourth hole of the same long edge. Add a half bush directly below the frame, a half bush directly above it and a full bush above that. Hold the axle while fitting its lower stop. The shaft must turn without walking through its bearing.
  7. Fit the second vertical 6-module axle through the eighth hole of the same long edge. Add a half bush directly below the frame, a half bush directly above it and a full bush above that. Hold the axle while fitting its lower stop. The shaft must turn without walking through its bearing.
  8. Slide a 24-tooth gear onto the first shaft spacer. The two gears do not mesh: each is a keyed mounting hub. Their centres are four spaces apart.
  9. Put two black friction pins upward into opposite hub holes, one space either side of the axle. Fit the yellow 7-hole beam flat by its third and fifth holes, with the shaft through its middle hole. Add a half bush above. Both arms must initially point along the same direction. Two separated pins make each arm and hub one rigid crank.
  10. Slide a 24-tooth gear onto the second shaft spacer. The two gears do not mesh: each is a keyed mounting hub. Their centres are four spaces apart.
  11. Put two black friction pins upward into opposite hub holes, one space either side of the axle. Fit the red 7-hole beam flat by its third and fifth holes, with the shaft through its middle hole. Add a half bush above. Both arms must initially point along the same direction. Two separated pins make each arm and hub one rigid crank.
  12. Build the connecting bar separately: take a flat blue 5-hole beam. Its end holes are four spaces apart, equal to the fixed shaft spacing.
  13. Push a 3-module axle down through the first end round hole of the blue bar. Seat a half bush directly above the bar. It carries the axle while the bar is held for attachment.
  14. Push a 3-module axle down through the second end round hole of the blue bar. Seat a half bush directly above the bar. It carries the axle while the bar is held for attachment.
  15. Align the same-side outer ends of both cranks without forcing them. Lower the connecting bar assembly so its two joint axles pass through those end holes. Each joint is three spaces (24 mm) from its main shaft. The bar rests on the arms, closing a parallelogram. Support it temporarily until the lower stops are fitted.
  16. From underneath, seat a half bush against each crank around its joint axle. The top caps and these lower bushes retain both complete joint axles. Check free movement without pinching. Both main shafts also need their upper and lower stops, and each hub arm needs both mounting pins.

Play time

Hold only the stationary frame on a level table. Gently rock the yellow crank no more than 30° either side of its initial position. The red crank follows through the connected blue bar; no gears mesh. Compare the bar’s direction with the fixed shaft line, then track one end against paper placed clear of the mechanism. The bar stays parallel but its points follow circular arcs. There are no physical angle stops: do not force a full turn or a straight-line travel. Stop for a loose cap, rubbing or a hub arm that slips. This unloaded educational model has not been physically tested.

Credited demonstrations and component sources