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21 Capstone

Capstone: Gearbox

Design every gear, model the housing, assemble the full system, and prove it's ready to manufacture.

Advanced ~30 min

Learning Objectives

  • Specify a target output speed and torque, derive the gear ratio
  • Design every gear and shaft to fit the housing envelope
  • Assemble the full gearbox with the right joints and clearances
  • Generate the manufacturing pack: drawings, BOM, and STEP exports

Two-Stage Spur Gearbox - complete assembly with housing, shafts, and gear train (9:1 total ratio).

Planetary Gear Set - sun, planets, carrier, and ring gear in a compact coaxial arrangement.

1 Experience
2 Reflect
3 Theorize
4 Apply

Assembly Sequence

1
Design the Gears

Sketch involute profiles; keep module and pressure angle constant.

2
Design the Carrier

Transition-fit axle holes to hold shafts without binding.

3
Assemble

Ground ring gear, revolute joints for sun/carrier, pin planets to axles.

Gearbox DFM
  • Tooth clearance ≥ 0.15 mm (FDM) or ≥ 0.05 mm (CNC).
  • Bearing pocket walls ≥ 2 mm (printed) or ≥ 1.5 mm (machined).
  • Dowel-pin holes in housing halves for alignment.

Gear Parameters & Final Checks

Key Geometry (ratios in Module 11)

Module: m = d / z

All meshing gears must share the same module.

Center Distance: C = (d1 + d2) / 2

Use 20° pressure angle and ≥ 17 teeth to avoid undercut.

Pre-Export Checks
1
Interference Detection

Overlapping bodies mean parts won't fit.

2
Motion Study

Rotate input shaft and verify gear mesh ratios.

3
Bill of Materials

Confirm every part is listed, including fasteners.

Pause and Reflect
✓ Your reflections are saved automatically
Apply What You Learned

Complete the gearbox design by applying every concept from the course.

  • Verify all gear ratios match the required output speed and torque
  • Check that every part is fully constrained in the assembly
  • Run interference detection to find any collisions
  • Create a drawing package with dimensions, tolerances, and BOM
  • Evaluate manufacturability: can every part actually be built?
  • Prepare a presentation render showing the assembled gearbox
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Reference Assembly: T-Slot Frame

Example from AICAD AssemblyForge.

Parts & Assembly
  • 20x20 T-Slot Extrusion × 4 (160 × 20 × 20 mm)
  • L-Shaped Corner Bracket × 4 (40 × 20 × 4 mm)
  1. Lay out four extrusions in a 200×200 mm square with T-nuts.
  2. Place corner brackets at each joint over the T-nuts.
  3. Dowel pins for alignment, M5×8 screws torqued to 4 Nm.

Key Takeaways

  • Start from the output spec, work backward to the motor
  • Reuse standard gears and bearings — custom parts cost time you don't have
  • Always check shaft deflection separately from tooth strength
  • Ship the manufacturing pack, not just the model — a beautiful model the shop can't build is worthless
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