# Scotch yoke: slotted yoke and crank disc, 20 and 30 mm stroke

> How do I make a Scotch yoke with 3D printed parts?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Textbook Scotch yoke: slider displacement = r·sin θ, stroke = 2r; slot sized to a 623 bearing (OD 10) on a Ø3 crank pin, Ø5 shaft
- Page: https://xemstudio.com/objects/scotch-yoke/ · JSON: https://xemstudio.com/api/geometry?object=scotch-yoke · Plain text: https://xemstudio.com/objects/scotch-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

- A crank pin rides in a slot across the yoke, so the yoke moves as a pure sine of crank angle with no connecting rod and no rod angularity.
- Stroke is twice the pin offset: the 20 mm crank has its pin hole at 10 mm, the 30 mm crank at 15 mm.
- The slot is 10 mm wide for a 623 bearing (OD 10) on a Ø3 pin through the crank disc; the rod end has a Ø3 hole for the next link.
- Shorter than a crank and con-rod for the same stroke; used to drive displacers and pumps.

## 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 slot width | 10 | 623 bearing OD 10 (rolls in it) | run | scotch-yoke-* |
| crank bore | Ø5 | Ø5 shaft | press | scotch-yoke-* |
| crank pin hole | Ø3 | Ø3 pin | press | scotch-yoke-* |
| rod-end hole | Ø3 | Ø3 pin (turns in it) | run | scotch-yoke-* |

## Sizes

| stroke | parts | slot | crank pin offset |
|---|---|---|---|
| 20 mm | yoke + crank disc | 10 × 30 mm | 10 mm |
| 30 mm | yoke + crank disc | 10 × 40 mm | 15 mm |

## Where the files are

- Wikipedia: [Scotch yoke](https://en.wikipedia.org/wiki/Scotch_yoke) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine and rig builds on extrusion frames](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (4 parts, CC BY 4.0, Xem Studio)

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

- scotch-yoke-20-yoke: [STEP](https://xemstudio.com/files/hardware/scotch-yoke/scotch-yoke-20-yoke.step) · [STL](https://xemstudio.com/files/hardware/scotch-yoke/scotch-yoke-20-yoke.stl) — Scotch yoke slider, 20 mm stroke, 10 mm slot for a 623 bearing on the crank pin, 8 mm rod with a Ø3 end hole
- scotch-yoke-20-crank: [STEP](https://xemstudio.com/files/hardware/scotch-yoke/scotch-yoke-20-crank.step) · [STL](https://xemstudio.com/files/hardware/scotch-yoke/scotch-yoke-20-crank.stl) — Scotch yoke crank disc, Ø3 pin hole at 10 mm (20 stroke), Ø5 bore
- scotch-yoke-30-yoke: [STEP](https://xemstudio.com/files/hardware/scotch-yoke/scotch-yoke-30-yoke.step) · [STL](https://xemstudio.com/files/hardware/scotch-yoke/scotch-yoke-30-yoke.stl) — Scotch yoke slider, 30 mm stroke, 10 mm slot
- scotch-yoke-30-crank: [STEP](https://xemstudio.com/files/hardware/scotch-yoke/scotch-yoke-30-crank.step) · [STL](https://xemstudio.com/files/hardware/scotch-yoke/scotch-yoke-30-crank.stl) — Scotch yoke crank disc, Ø3 pin hole at 15 mm (30 stroke), Ø5 bore

## How it fits

### Goes in or on
- [Stirling shafts and rods: Ø3 rods, Ø5 / Ø8 D-flat crankshafts, Ø5 tube rods, Ø3 pins, thrust spacers](https://xemstudio.com/objects/stirling-shafts-and-rods/): crank disc on a Ø5 shaft

### Connects to
- [Four-bar links: flat links with 20 to 100 mm hole centres](https://xemstudio.com/objects/four-bar-links/): Ø3 hole in the rod end

### Alternatives
- [Swash (wobble) plate: Ø40 plates at 10, 15 and 20° with a ball-end follower](https://xemstudio.com/objects/swash-plate/): multi-cylinder from one shaft
- [Eccentric and strap: Ø30 eccentrics at 3, 5 and 8 mm offset with a split strap](https://xemstudio.com/objects/eccentric-and-strap/): rotary to linear, no slot
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/): crank and con-rod

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

## Questions

**What is a Scotch yoke and when is it better than a con-rod?**

A pin in a slot: the slider moves as a pure sine of crank angle with no connecting rod, so it is shorter and has no rod angularity. The pin takes the side load, so it carries a 623 bearing in the slot.

**How do I get the stroke I want?**

Stroke = 2 × the crank pin offset. Pick the 20 or 30 mm crank disc, or move the pin hole to half the stroke you need.

## Fits with

- [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/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Stirling shafts and rods: Ø3 rods, Ø5 / Ø8 D-flat crankshafts, Ø5 tube rods, Ø3 pins, thrust spacers](https://xemstudio.com/objects/stirling-shafts-and-rods/)
- [Swash (wobble) plate: Ø40 plates at 10, 15 and 20° with a ball-end follower](https://xemstudio.com/objects/swash-plate/)
- [Eccentric and strap: Ø30 eccentrics at 3, 5 and 8 mm offset with a split strap](https://xemstudio.com/objects/eccentric-and-strap/)