# Ross yoke kit (alpha Stirling with parallel cylinders)

> What is a Ross yoke and how do I make an alpha Stirling engine with parallel cylinders?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Andy Ross's yoke (1970s): a triangular rocker on a single crank gives two parallel pistons ~90° apart with near-straight-line motion; proportions a = 30, h = 20 for 16 mm strokes
- Page: https://xemstudio.com/objects/ross-yoke-kit/ · JSON: https://xemstudio.com/api/geometry?object=ross-yoke-kit · Plain text: https://xemstudio.com/objects/ross-yoke-kit/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- Alpha engines need two pistons 90° out of phase; the usual V-twin crank puts the cylinders at 90°. The Ross yoke lets both cylinders sit side by side and parallel: a triangular rocker pivots on a short rocking link, its bottom corner rides the crank pin, and its two top corners drive the pistons through short con-rods. The geometry makes each piston's motion almost straight (low side load) and roughly 90° apart.
- Proportions: top pivots at ±a, crank pivot at −h below, rocking pivot near the centroid; a/h ≈ 1.5, crank throw ≈ h/2.5. With a 30 / h 20 / e 8 the strokes are ~16 mm and phase ~90°. Cylinder spacing = 2a = 60 mm... here the syringe block uses 34 mm spacing with the a = 30 yoke angled; use a = 17 for a dead-parallel block, or the 2× syringe block as the compact version.
- Parts: yoke, rocking link (frame to yoke centroid, 623 bearing), two con-rods, crank disc, cylinder block for two 10 mL syringes (the simplest alpha: one hot, one cold, a tube between), flywheel from the crank kit.
- LES: the alpha demonstrator with two syringes; the hot syringe sits in a tri-clamp hot end via the power head.

## Sizes

| part | size | note |
|---|---|---|
| yoke | a 30, h 20 (or 40/25) | 623 at centroid |
| rocking link | 30 | frame pivot |
| con-rods | 45 × 2 | Ø3 pins |
| crank | e 8 (stroke 16) | Ø5 shaft |
| cylinder block | 2 × 10 mL syringes @ 34 | alpha |

## Where the files are

- Wikipedia: [Stirling engine](https://en.wikipedia.org/wiki/Stirling_engine) — web — CC BY-SA 4.0 (text)
- Andy Ross: [Making Stirling Engines (Ross yoke)](https://www.stirlingengine.com/) — book — reference (verify at link)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (6 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/ross-yoke-kit/index.json

- ross-yoke-a30-h20: [STEP](https://xemstudio.com/files/hardware/ross-yoke-kit/ross-yoke-a30-h20.step) · [STL](https://xemstudio.com/files/hardware/ross-yoke-kit/ross-yoke-a30-h20.stl) — Ross yoke rocker: piston pivots ±30 mm, crank pivot 20 mm below, 623 pocket at the rocking pivot
- ross-yoke-a40-h25: [STEP](https://xemstudio.com/files/hardware/ross-yoke-kit/ross-yoke-a40-h25.step) · [STL](https://xemstudio.com/files/hardware/ross-yoke-kit/ross-yoke-a40-h25.stl) — larger rocker ±40 / 25
- rocking-link-30: [STEP](https://xemstudio.com/files/hardware/ross-yoke-kit/rocking-link-30.step) · [STL](https://xemstudio.com/files/hardware/ross-yoke-kit/rocking-link-30.stl) — rocking link 30 mm (yoke centroid to frame pivot)
- conrod-45: [STEP](https://xemstudio.com/files/hardware/ross-yoke-kit/conrod-45.step) · [STL](https://xemstudio.com/files/hardware/ross-yoke-kit/conrod-45.stl) — piston con-rod 45 mm, Ø3.2 ends (2 needed)
- crank-disc-stroke16: [STEP](https://xemstudio.com/files/hardware/ross-yoke-kit/crank-disc-stroke16.step) · [STL](https://xemstudio.com/files/hardware/ross-yoke-kit/crank-disc-stroke16.stl) — crank disc, pin at 8 mm
- cylinder-block-2x-syringe-10ml: [STEP](https://xemstudio.com/files/hardware/ross-yoke-kit/cylinder-block-2x-syringe-10ml.step) · [STL](https://xemstudio.com/files/hardware/ross-yoke-kit/cylinder-block-2x-syringe-10ml.stl) — alpha cylinder block: two parallel 10 mL syringe barrel sockets (Ø16.05) 34 mm apart, pinch slot

## How it fits

### Goes in or on
- [Plastic syringes 1–60 mL as pistons and cylinders](https://xemstudio.com/objects/plastic-syringes/): 10 mL barrels in the cylinder block

### Connects to
- [Syringe pistons: ring-less printed pistons with a con-rod fork for 10, 20 and 30 mL syringe barrels](https://xemstudio.com/objects/syringe-pistons/): conrod-45 into the piston fork
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/): flywheel

### Alternatives
- [Alpha V-twin: 90° two-pin crank discs, V block, transfer tee](https://xemstudio.com/objects/alpha-twin-crank-90/): V-twin alpha
- [Rhombic drive kit (Philips beta Stirling): twin crank gears, links, yokes, frame](https://xemstudio.com/objects/rhombic-drive-kit/): balanced beta

### Also needed
- [Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)](https://xemstudio.com/objects/ball-bearings-metric/): 623 at the rocking pivot
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/): Ø3 pins

## Questions

**What is a Ross yoke?**

A triangular rocker on one crank that drives two parallel pistons about 90° apart with nearly straight-line motion: the way to build an alpha Stirling with side-by-side cylinders instead of a V.

**What phase angle does a Ross yoke give?**

About 90° with a/h ≈ 1.5; it varies a few degrees through the cycle, which is harmless.

**Can I make an alpha Stirling from two syringes?**

Yes: the cylinder block takes two 10 mL barrels; heat one, cool the other, connect them with a short tube and a regenerator plug, drive the plungers with the Ross yoke.

## Fits with

- [Motion mechanisms: Scotch yoke, swash (wobble) plate, eccentric + strap, Geneva, four-bar links, bell crank, rack](https://xemstudio.com/objects/motion-mechanisms/)
- [Plastic syringes 1–60 mL as pistons and cylinders](https://xemstudio.com/objects/plastic-syringes/)
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/)
- [Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)](https://xemstudio.com/objects/ball-bearings-metric/)
- [Stirling engine configurations: which type needs which parts on this shelf](https://xemstudio.com/objects/stirling-configurations-guide/)
- [Alpha V-twin: 90° two-pin crank discs, V block, transfer tee](https://xemstudio.com/objects/alpha-twin-crank-90/)