Rhombic drive kit (Philips beta Stirling): twin crank gears, links, yokes, frame — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/rhombic-drive-kit/  (CC BY 4.0)
Basis: Philips/Meijer rhombic drive: two counter-rotating crank gears, four equal links, two yokes; displacer rod through the piston; geometry here e = 8, link 40, gear centres 32 (module 1, 30 T), scaled for a 2 in tri-clamp beta
Catalog version: 2026-10-20

Sizes: see table below
- The rhombic drive is the classic beta mechanism (Philips 1950s): two identical gears mesh and turn opposite ways, each carrying a crank pin; four equal links join the pins to an upper yoke (displacer rod) and a lower yoke (piston rod, a tube the displacer rod runs through). Because the two cranks are mirror images every side load cancels: the piston and displacer move on the axis with no piston slap, perfectly balanced, and the phase angle is fixed by the geometry (about 90° for these proportions).
- Geometry rules: gear centre distance c, crank throw e, link length L. Stroke ≈ 2e for both yokes; the displacer leads the piston by an angle set by c/L and e/L (Meijer's charts); with c = 32, e = 8, L = 40 you get ~85–95° and strokes of 16 mm. Keep L ≥ 4e. Gears must be meshed with the pins 180° apart (one at top dead centre when the other is at bottom): mark a tooth.
- Parts: 2 crank gears (module 1 30 T, Ø5 shafts in 625 bearings in the frame plates), 4 links (40 mm, Ø3 pins), upper yoke (Ø3 displacer rod clamp), lower yoke (Ø5 tube piston rod, Ø3.6 clearance for the displacer rod), 2 frame plates, 4 Ø3 crank pins. Rod tube: 5 mm K&S brass (3.1 ID) is the piston rod with the Ø3 displacer rod inside.
- LES: the 2 in beta on the rhombic drive is the balanced, quiet one; matches the Ø46 × 90 tri-clamp displacer at 16 mm stroke, with the syringe or a lapped Ø20 power piston.

parameter  value_mm  note
gear centres c  32  30 T module 1 gears
crank throw e  8  stroke 16
link length L  40  ≥ 4e
yoke span  32  = c at mid-stroke
displacer rod  Ø3  through Ø5 tube
piston rod  Ø5 tube (3.1 ID)  K&S brass
bearings  4 × 625  frame plates

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Wikipedia: Rhombic drive — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Rhombic_drive
- Wikipedia: Stirling engine — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Stirling_engine
- Philips Research / Meijer: The Philips Stirling engine (rhombic drive papers, 1958–1965) — web — reference [verify license at link] — https://en.wikipedia.org/wiki/Rhombic_drive#History
- Long Earth Society: Engine builds — web, video — LES site — https://longearthsociety.com/engineering/

Free files (CC BY 4.0, Xem Studio; STEP + STL, 10 parts): https://xemstudio.com/files/hardware/rhombic-drive-kit/index.json
API: https://xemstudio.com/api/geometry?object=rhombic-drive-kit