⏱️ Unequal Return Timing Bench: step-by-step build

Turn one crank steadily and compare the unequal input angles of a rocker’s two strokes.

One complete input turn makes a rocker go out and return, but those strokes need not take half a turn each. This original horizontal four-bar bench uses a one-spacing crank, a five-spacing connecting link and a four-spacing rocker on shafts four spaces apart. Count input angle between output reversals before making a timing claim. Equal input speed is a model assumption, not something a hand crank guarantees.

  • Level 3 of 5 (medium)
  • About 30 minutes
  • 15 steps, 36 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.

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
  • 5 × Axle 3
  • 11 × Bush
  • 1 × Axle 5
  • 10 × Half bush
  • 1 × Axle 6
  • 2 × Medium gear (24 teeth)
  • 2 × Black pin (grippy)
  • 1 × Beam 9
  • 1 × Beam 7
  • 1 × Axle 4

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 input 5-module vertical axle through the fourth hole along the same long edge. Seat a half bush immediately below and above the frame, then a full bush above. Hold the shaft while adding its bottom stop. The shaft must turn without lifting or sliding sideways.
  7. Fit the output 6-module vertical axle through the eighth hole along the same long edge. Seat a half bush immediately below and above the frame, then a full bush above. Hold the shaft while adding its bottom stop. The shaft must turn without lifting or sliding sideways.
  8. Slide a yellow 24-tooth gear onto the input shaft spacer and seat a half bush directly above it. This gear is the crank plate, not a meshing drive. Its four round holes are one spacing from the shaft.
  9. Slide another 24-tooth gear onto the output shaft spacer. It is a keyed hub; the gears are four spaces apart and do not mesh.
  10. Put two black friction pins upward into opposite hub holes, one space either side of the output shaft. Fit a red 9-hole beam flat by its fourth and sixth holes, with the shaft in its fifth hole. Seat a half bush above. The end to your right is four spaces from the output shaft; the other end is an unloaded balance tail.
  11. Build the blue connecting link separately. Use the first and sixth holes of a 7-hole beam: five spaces between their centres, leaving one unused hole beyond the second joint. The first joint will sit over the input gear hole nearest the output shaft. The second will sit over the red beam’s right-hand end.
  12. Pass a 4-module axle down through the first blue link hole and add a half bush directly above it. Add a full bush below the blue link as a spacer. Hold the assembly; its lower stop comes after attachment.
  13. Pass a 3-module axle down through the sixth blue link hole and seat a half bush directly above it. Keep the seventh hole free.
  14. Point the input gear’s chosen round hole toward the output shaft. Keep the red rocker pointing right. Lower the link so the long first joint axle enters that gear hole and the second enters the red rocker’s right-hand end. The blue link sits above both main shaft tips. Its first spacer bridges the height difference above the gear. Support it temporarily while fitting the lower stops.
  15. Seat a half bush underneath the input gear around the long joint axle, and another underneath the red rocker around the short joint axle. Check every cap is seated: both whole main shafts and both complete joint axles need stops. Neither a loose shaft nor fingers holding the moving link are acceptable guides.

Play time

Keep the frame level and hold only the stationary frame. Gently turn the input gear through one complete turn by its rim, keeping fingers clear of the connecting link. The red output rocks rather than turning fully. Mark its two reversal positions on paper clear of the sweep, then count the input angle between reversals. Repeat in the same input direction. A shorter input-angle interval takes less time only if input speed is constant. The unloaded model has no motor, payload or calibrated speed controller and has not been physically tested. Stop for rubbing, loose pins or axial stops; do not force the output through a full turn.

Credited demonstrations and component sources

  • Four Bar Linkage Mechanism — Bricks Master Builders: Independent four-bar demonstration inspired the turn-versus-rock question. Our original horizontal timing bench has its own geometry, supports and instructions; it does not reproduce the motorized source model or claim measured source timing.
  • University of Almería — Four-bar linkage: Primary teaching reference for fixed pivots, crank/rocker names and the Grashof condition. Timing predictions are our ideal geometry calculations, not physical tests.