# Bell cranks: 30 × 30 and 20 × 40 mm arms at 90°

> How do I make a bell crank with 3D printed parts?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Bell crank lever: output motion = input × (output arm / input arm); 90° between arms; Ø10 pivot pocket for a 623 bearing
- Page: https://xemstudio.com/objects/bell-cranks/ · JSON: https://xemstudio.com/api/geometry?object=bell-cranks · Plain text: https://xemstudio.com/objects/bell-cranks/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

- Turns a push or pull through 90°. The 30 × 30 crank keeps the same travel; the 20 × 40 doubles it (or halves it, driven the other way).
- Pivot is a Ø10 pocket for a 623 bearing (OD 10); arm ends have Ø3 holes for pins and links.
- Used in the beta Stirling bell-crank drive to phase the displacer and piston.

## 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 |
|---|---|---|---|---|
| pivot pocket | Ø10 | 623 bearing OD 10 | press | all |
| arm holes | Ø3 | Ø3 pins (turn in them) | run | all |

## Sizes

| arms | angle | ratio | pivot |
|---|---|---|---|
| 30 × 30 mm | 90° | 1:1 | Ø10 (623) |
| 20 × 40 mm | 90° | 1:2 | Ø10 (623) |

## Where the files are

- Wikipedia: [Bellcrank](https://en.wikipedia.org/wiki/Bellcrank) — 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 (2 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/bell-cranks/index.json

- bell-crank-30x30-90: [STEP](https://xemstudio.com/files/hardware/bell-cranks/bell-crank-30x30-90.step) · [STL](https://xemstudio.com/files/hardware/bell-cranks/bell-crank-30x30-90.stl) — bell crank, 30 × 30 mm arms at 90°, Ø10 pocket for a 623 bearing pivot, Ø3 arm holes
- bell-crank-20x40-90: [STEP](https://xemstudio.com/files/hardware/bell-cranks/bell-crank-20x40-90.step) · [STL](https://xemstudio.com/files/hardware/bell-cranks/bell-crank-20x40-90.stl) — bell crank, 20 × 40 mm arms (2:1) at 90°, Ø10 pivot pocket, Ø3 arm holes

## How it fits

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

### Also needed
- [Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)](https://xemstudio.com/objects/ball-bearings-metric/): 623 in the Ø10 pivot pocket
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/): Ø3 pivot and arm pins

## Questions

**What does a bell crank do?**

It changes the direction of a push or pull, usually by 90°, and scales the travel by the ratio of its arms.

## Fits with

- [Four-bar links: flat links with 20 to 100 mm hole centres](https://xemstudio.com/objects/four-bar-links/)
- [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 engine configurations: which type needs which parts on this shelf](https://xemstudio.com/objects/stirling-configurations-guide/)