Motion mechanisms: Scotch yoke, swash (wobble) plate, eccentric + strap, Geneva, four-bar links, bell crank, rack — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/motion-mechanisms/  (CC BY 4.0)
Basis: Textbook mechanism geometry (Scotch yoke, swash plate stroke = D·sin θ, eccentric stroke = 2e, Geneva with c = R / cos(π/n)); parts sized for Ø5 shafts, Ø3 pins and 623 bearings to match the rest of the shelf
Catalog version: 2026-10-20

Sizes: see table below
- Scotch yoke: pure sinusoidal motion, no con-rod; the crank pin (623 bearing on a Ø3 pin) rides in the yoke slot; stroke = 2 × pin offset. Runs the displacer on a beta Stirling without a wrist pin.
- Swash / wobble plate: a disc tilted θ on the shaft; a follower touching it at radius r reciprocates r·sin θ each side (Ø40 plate at 15° → 10.4 mm stroke at the rim). Several followers around one plate = multi-cylinder from one shaft; the follower here is an 8 mm square pushrod with a ball end.
- Eccentric + strap: a disc offset e on the shaft inside a split ring; stroke 2e, carries load on the whole ring, the quiet way to drive a pump or displacer. Geneva: intermittent motion, n stops per turn; drive-disc radius locks the wheel between indexes.
- Four-bar links (20–100 mm centres, Ø3.2) and the bell crank build crank-rockers, parallel motions and the Ross yoke; rack m1 meshes the module 1 gears for linear drives. All pivots are Ø3 dowel pins, Ø3.2 printed holes, or 623 bearings in Ø10.2 pockets.

mechanism  parts  input → output  key dimension
Scotch yoke  yoke + crank  rotary → sinusoidal linear  stroke 20 / 30 mm
swash plate  plate + follower  rotary → linear, multi-follower  Ø40 at 10 / 15 / 20°
eccentric  disc + split strap  rotary → linear, high load  e 3 / 5 / 8 mm (stroke 6 / 10 / 16)
Geneva  wheel + drive  continuous → intermittent  4 / 6 stations, R30
four-bar links  7 lengths  rockers, parallel motion, Ross yoke  20–100 mm centres
bell crank  30×30, 20×40  redirect / lever 90°  Ø3.2 pins
rack  m1 100, m2 150  rotary ↔ linear with the gears  module 1 / 2

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Wikipedia: Scotch yoke — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Scotch_yoke
- Wikipedia: Swashplate — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Swashplate
- Wikipedia: Geneva drive — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Geneva_drive
- Wikipedia: Four-bar linkage — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Four-bar_linkage
- Wikipedia: Eccentric (mechanism) — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Eccentric_(mechanism)
- Long Earth Society: Engine and rig builds on extrusion frames — web, video — LES site — https://longearthsociety.com/engineering/

Free files (CC BY 4.0, Xem Studio; STEP + STL, 27 parts): https://xemstudio.com/files/hardware/motion-mechanisms/index.json
API: https://xemstudio.com/api/geometry?object=motion-mechanisms