# Ross yoke a = h = 20 for two parallel syringes

> What shape and size is a Ross yoke that gives 90° between two pistons?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Linkage solved over a full turn (720 steps) in the generator: crank e 6, yoke a = h = 20, rocking link 40 from a pivot at (−40, 20), con-rods 60
- Page: https://xemstudio.com/objects/ross-yoke/ · JSON: https://xemstudio.com/api/geometry?object=ross-yoke · Plain text: https://xemstudio.com/objects/ross-yoke/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

**Exact dimensions.** Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part.

## Notes

- The triangular rocker of the Ross yoke alpha. Its half-width equals its height (a = h = 20), which puts the two top corners a quarter-turn apart: piston phase 90.0°, strokes 17.0 and 17.2 mm.
- Bottom vertex: a Ø3 hole (fit: run) that rides on the crank pin. Top corners at ±20: Ø3 holes (fit: press) for the con-rod pins. Between them, the top-middle Ø3 hole (press) for the rocking-link pin.
- Outline 52 × 32 × 6 with 3 mm corner radii.

## Mating features (exact size; open press / slide / run / socket by your process clearance, leave designed and clamp as drawn)

| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| yoke corner and top pins | Ø3 | Ø3 pins | press | ross-yoke-* |
| yoke crank-pin hole | Ø3 | crank pin (turns in it) | run | ross-yoke-* |

## Sizes

| parameter | value |
|---|---|
| a (half-width) | 20 |
| h (height) | 20 |
| outline | 52 × 32 × 6 |
| crank-pin hole | Ø3 run |
| con-rod and rocking pins | 3 × Ø3 press |
| phase | 90.0° |
| strokes | 17.0 / 17.2 |
| max con-rod angle | 0.91° |

## Where the files are

- Wikipedia: [Ross yoke](https://en.wikipedia.org/wiki/Ross_yoke) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (1 parts, CC BY 4.0, Xem Studio)

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

- ross-yoke-a20-h20: [STEP](https://xemstudio.com/files/hardware/ross-yoke-kit/ross-yoke-a20-h20.step) · [STL](https://xemstudio.com/files/hardware/ross-yoke-kit/ross-yoke-a20-h20.stl) — Ross yoke a = h = 20: Ø3 running hole on the crank pin at the bottom vertex, Ø3 press holes for the two con-rod pins at ±20 and the rocking pin between them

## How it fits

### Connects to
- [Ross yoke crank disc Ø24, 6 mm throw](https://xemstudio.com/objects/ross-crank-disc/): bottom vertex rides the disc's Ø3 crank pin
- [Con-rods with Ø3 pin ends, 45 and 60 mm, 8 × 5](https://xemstudio.com/objects/pin-end-con-rods/): conrod-60 on the ±20 corner pins
- [Ross yoke rocking link, 40 mm](https://xemstudio.com/objects/ross-rocking-link/): top-middle pin to the rocking link

### Also needed
- [Crankshafts Ø3, Ø5 and Ø8 with full-length D-flat](https://xemstudio.com/objects/stirling-crankshafts/): Ø3 × 12 and Ø3 × 20 pins, spacers
- [Metric shafts Ø3 to Ø10 with D-flats and clip grooves](https://xemstudio.com/objects/metric-shafts/): 3 mm collars on the free pin ends

## Questions

**What ratio gives 90° on a Ross yoke?**

Half-width equal to height, a = h. Here a = h = 20 with a 6 mm crank and 40 mm rocking link gives 90.0°, checked over a full turn.

**Why are Ross yoke con-rods almost vertical?**

The yoke's top corners move nearly straight up and down; the 60 mm rods stay within 0.91° of vertical, so the pistons see almost no side load.

## Fits with

- [Ross yoke alpha frame for two 10 mL syringes](https://xemstudio.com/objects/ross-yoke-frame/)
- [Ross yoke rocking link, 40 mm](https://xemstudio.com/objects/ross-rocking-link/)
- [Ross yoke crank disc Ø24, 6 mm throw](https://xemstudio.com/objects/ross-crank-disc/)
- [Con-rods with Ø3 pin ends, 45 and 60 mm, 8 × 5](https://xemstudio.com/objects/pin-end-con-rods/)
- [Build list: Ross yoke alpha, two parallel 10 mL syringes](https://xemstudio.com/objects/list-ross-yoke-alpha/)