🔀 Differential Hand Bench: step-by-step build

Turn a standard differential by hand and compare its two independently supported outputs.

Two outputs can share one input without sharing one speed. This original tabletop bench captures a standard assembled differential between retained half shafts, with a bearing tower on either side. A slow hand experiment reveals the average-speed rule. The animation represents that ideal rule with a simplified housing; it does not show the internal tooth engagement or predict friction.

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

What you will learn

  • The Differential: Lets the two driven wheels spin at different speeds in a turn.
  • 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

  • ⚙️ 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 × Blue long pin
  • 4 × Split pin block
  • 3 × Axle 3
  • 6 × Bush
  • 1 × Differential
  • 3 × Corner gear (12 teeth)
  • 2 × Axle 4
  • 6 × Half bush
  • 2 × Beam 5
  • 4 × Red axle-pin

Steps

  1. Lay the 5×11 frame flat. This is the bench's base.
  2. Push two blue long pins up into the 8th and 10th holes of the left long side. Press two split pin blocks onto them, one on top of the other.
  3. The same on the right long side: two blue long pins and two split pin blocks.
  4. Give the bench two more feet at the other end: push a 3-axle into each corner hole so it pokes out underneath, and slide a bush on below.
  5. Lock the two feet with a bush on top of each.
  6. Add a third stationary foot below the middle of the opposite short edge, with an axle and bushes above and below the frame.
  7. Assemble the bare 62821 housing with three loose 6589 single-bevel 12T gears (not 32270 double-bevel gears). Seat one gear on the housing’s transverse internal post; place the other two as opposite side gears, their teeth facing inward to mesh with it. Check the official page-20 assembly reference. This step’s internal view is symbolic; the three real loose gears are counted in the parts tray and booklet.
  8. Hold the two side gears seated and insert one 4-module half shaft from each side into its own gear’s cross hole. Leave a half-module gap between the inner tips: each shaft seats through its own side-gear cross hole, and the shafts must remain separate outputs. Turn one shaft gently with the housing held and check that the other turns oppositely.
  9. Fit two half bushes on each half shaft beside the housing. Their inner faces meet the housing necks; the outer faces will meet the tower bearings.
  10. Differential module: slide one upright tower over its half-shaft end, the mounting pins are inserted after seating the module.
  11. Lower the module from above, then insert the two tower pins in the stationary split blocks.
  12. Fit the two red axle-pins on the opposite side and slide the second tower over the shaft end. Both tower bearings support the output; neither may tilt.
  13. Retain each half shaft outside its bearing tower with a coloured half bush. Check that the outputs turn independently and that neither can slide out.

Play time

Keep the frame on the table. Turn the housing gently by hand while allowing both output ends to turn. Then hold one coloured output lightly and repeat: the other turns faster. Finally hold the housing and turn one output; the other reverses. Do not squeeze the gear teeth or force a blocked assembly. Count turns, not torque or efficiency.

Real-world inspiration

2in1Bricking demonstrates differential action with a custom construction-part differential. This bench instead uses a standard assembled differential and its own supported layout.

Credited demonstrations and component sources