# Rhombic drive kit (Philips beta Stirling): twin crank gears, links, yokes, frame

> How does a rhombic drive Stirling engine work and what parts does it need?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- 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
- Page: https://xemstudio.com/objects/rhombic-drive-kit/ · JSON: https://xemstudio.com/api/geometry?object=rhombic-drive-kit · Plain text: https://xemstudio.com/objects/rhombic-drive-kit/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- 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.

## Sizes

| 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 |

## Where the files are

- Wikipedia: [Rhombic drive](https://en.wikipedia.org/wiki/Rhombic_drive) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Stirling engine](https://en.wikipedia.org/wiki/Stirling_engine) — web — CC BY-SA 4.0 (text)
- Philips Research / Meijer: [The Philips Stirling engine (rhombic drive papers, 1958–1965)](https://en.wikipedia.org/wiki/Rhombic_drive#History) — web — reference (verify at link)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (10 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/rhombic-drive-kit/index.json

- crank-gear-30t-e8-left: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-30t-e8-left.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-30t-e8-left.stl) — crank gear 30 T module 1, Ø5 bore hub, Ø3 crank pin at 8.0 mm (stroke 16.0); left (identical; mesh them 180° apart)
- crank-gear-30t-e8-right: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-30t-e8-right.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-30t-e8-right.stl) — crank gear 30 T module 1, Ø5 bore hub, Ø3 crank pin at 8.0 mm (stroke 16.0); right (identical; mesh them 180° apart)
- link-40-1: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-1.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-1.stl) — rhombic link 40.0 mm centres, Ø3.2 pivots (4 needed)
- link-40-2: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-2.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-2.stl) — rhombic link 40.0 mm centres, Ø3.2 pivots (4 needed)
- link-40-3: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-3.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-3.stl) — rhombic link 40.0 mm centres, Ø3.2 pivots (4 needed)
- link-40-4: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-4.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/link-40-4.stl) — rhombic link 40.0 mm centres, Ø3.2 pivots (4 needed)
- yoke-upper-displacer: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/yoke-upper-displacer.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/yoke-upper-displacer.stl) — upper yoke (displacer rod Ø3) span 32 mm pivots
- yoke-lower-piston: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/yoke-lower-piston.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/yoke-lower-piston.stl) — lower yoke (piston on a Ø5 tube rod, displacer Ø3 rod passes through) span 32
- gear-frame-plate-c32: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/gear-frame-plate-c32.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/gear-frame-plate-c32.stl) — frame plate with Ø16.2 pockets for 625 bearings (Ø5 gear shafts) on 32 mm centres, a Ø16 rod clearance and 2 × M3 mounting holes at (±20, −15) for the rhombic frame head (print two)
- crank-pin-3x14: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-pin-3x14.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-pin-3x14.stl) — Ø3 × 14 crank pin (dowel) for the gears

## How it fits

### Goes in or on
- [Stirling linkage fittings: rod clevises, shaft adapter, 625 crank stands, Ø3 crank discs, rhombic frame heads](https://xemstudio.com/objects/stirling-linkage-fittings/): gear plates on the rhombic frame head

### Connects to
- [Tri-clamp power pistons and crank linkage: full-bore ring-less pistons, con-rods, crank webs and a clamp-on crank frame for 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-power-pistons/): -rod piston and its tube rod to the lower yoke
- [Tri-clamp displacers and engine heads: 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-displacers-and-heads/): displacer rod to the upper yoke
- [Tri-clamp free-piston kit: flexure head, stack ring, hub collars and piston plug for 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-free-piston-kit/): piston plug carries the tube rod

### Alternatives
- [Ross yoke kit (alpha Stirling with parallel cylinders)](https://xemstudio.com/objects/ross-yoke-kit/): alpha with parallel cylinders

### Also needed
- [Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)](https://xemstudio.com/objects/ball-bearings-metric/): 4 × 625 in the plates
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/): Ø3 link pins
- [Hobby brass tube (K&S), 1/16–21/32 in and 1–12 mm, telescoping](https://xemstudio.com/objects/brass-tube-ks/): 5 mm tube piston rod over the Ø3 rod
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/): flywheel

## Questions

**What is a rhombic drive?**

Two counter-rotating crank gears and four links forming a rhombus between two yokes; the top yoke drives the displacer, the bottom yoke the piston, both on one axis. Fully balanced, no side load, fixed phase angle.

**How do I set the phase angle on a rhombic drive?**

It is built into the geometry (gear spacing, throw and link length) and the 180° meshing of the gears; with c 32 / e 8 / L 40 it is about 90°. Change e/L to adjust.

**What stroke does this rhombic kit give?**

16 mm for both displacer and piston (2 × 8 mm throw); scale e and L together for other strokes, keeping L ≥ 4e and c ≈ 4e.

## Fits with

- [Spur gears, involute 20°: module 0.5, 1 and 2, 12–60 teeth](https://xemstudio.com/objects/spur-gears-involute/)
- [Tri-clamp displacers and engine heads: 1.5, 2 and 3 in spools](https://xemstudio.com/objects/tri-clamp-displacers-and-heads/)
- [Hobby brass tube (K&S), 1/16–21/32 in and 1–12 mm, telescoping](https://xemstudio.com/objects/brass-tube-ks/)
- [Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)](https://xemstudio.com/objects/ball-bearings-metric/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/)
- [Stirling engine configurations: which type needs which parts on this shelf](https://xemstudio.com/objects/stirling-configurations-guide/)