Oldham Coupling Fidget — Offset-Shaft Constant-Velocity Demo Toy

⚠ This is USELESS — a pure desk fidget, just for fun. It holds nothing, organises nothing, improves nothing. Print it only because spinning two offset shafts at constant velocity is mildly hypnotic.

A working demo of the Oldham coupling. Spin the bottom (driver) knob and the top (driven) knob spins with it at exactly the same speed — even though their two shafts are parallel but laterally offset (they don't share an axis). The trick is the floating middle disc: it has a groove on each face, the two grooves at 90° to each other. A tongue on the driver hub slides in the lower groove, a tongue on the driven hub in the upper groove. As you spin, the disc's centre orbits a little circle while faithfully carrying the rotation across the offset — fun to watch through the open front of the yoke. This is the constant-velocity cousin of the U-joint (which bends through an angle instead of an offset).

Parts (5 printed, 4 unique designs):

• frame ×1 — the C-yoke; a post up from the base is the driver shaft

• hub ×2 — IDENTICAL part, print twice and mount one flipped (knob with a bore on one face and a drive tongue on the other)

• disc ×1 — the floating middle disc with perpendicular grooves

• pin ×1 — the driven shaft, a capped dowel dropped in last

Print:

• PLA is fine — nothing flexes during assembly

• All parts print flat and supportless; no brim needed

• 0.2 mm layers, 3+ walls

• Print the two hubs in different colours for the full effect

Exploded view — stack order, bottom → top: frame base post → driver hub → floating disc → driven hub → pin (dropped down through the arm):

Assembly — one part per step

Step 1 — driver hub. Drop a hub onto the base post (the up-post on the frame), bore-down, tongue-up. It seats on the base and spins freely.

Step 2 — floating disc. Lay the disc on the driver hub so the driver's tongue drops into the disc's lower groove. The disc can now only slide along that one direction relative to the driver.

Step 3 — driven hub. Set the second hub on top of the disc, tongue-down, so its tongue drops into the disc's upper groove — this groove runs 90° to the lower one, which is the whole trick. Line its top bore up under the hole in the arm.

Step 4 — drop the pin in. Push the pin down through the hole in the top arm; its journal enters the driven hub's bore and its collar press-fits the arm hole (firm — press with your thumb or tap the cap). The driven hub now spins freely on the journal and the pin stays captive. No flexing.

Now spin the bottom knob: the top knob tracks it 1:1 while the gold disc's centre traces a little circle of diameter e through the open front.

Parametric — the whole thing is one OpenSCAD file. Tune the shaft offset e (the orbit size), hub/disc dimensions, and all clearances.

Photos: printed in PLA, red + black knobs on a cream yoke.