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Folded Compound Gear Train (alternative)

Status: in progress

A live alternative, not built. The most print-forgiving option — worth prototyping if the planetary's tolerance sensitivity keeps biting. TODO: prototype + measure backlash.

Folded compound spur-gear train

Idea

A spur reduction folded back onto two axes (motor + column) so it doesn't sprawl sideways. Each stage is a single gear pair; compound idlers (a big gear and a small gear fixed together) ride on bearings and ping-pong the reduction between the two axes. Motor at the bottom, steering column out the top.

Why it could beat the planetary

Each stage is one gear pair, and a pair only has to get one thing right — centre distance — which you can tune after printing by sliding the bearing mounts. There is no simultaneous-mesh tolerance stack (the planetary breaks because three planets must mesh the sun and ring at once, unadjustable post-print). Same printed flaws, far less consequence.

Ratio & the odd-stage rule

Each mesh hops between the two axes, so the output lands back on the column only with an odd mesh count:

  • 3 stages × ~2.2:1 ≈ 11:1 on two shafts (the diagram).
  • A 2-stage × ~3.3:1 version is fewer meshes/gears/backlash, but an even mesh count lands back on the motor axis → it needs a third countershaft.

So the trade is shaft count vs mesh count, not a free win.

Costs

  • Two parallel axes → wider than the coaxial planetary.
  • Backlash stacks across the meshes (the column encoder handles position, but stiffness/feel takes the hit).
  • Lowest torque capacity of the live options — a single tooth-pair contact per mesh, no load sharing (see the torque ranking).

CAD

Diagram only

The schematic above is the concept. If prototyped: model in Fusion (two-axis layout, compound idlers on 608 bearings, slotted mounts for centre-distance tuning), add a public-link embed + STEP.