# Rhombic drive crank gear, module 1.5, 44 T, 7 mm throw

> What are the dimensions of the crank gears in a rhombic drive?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Involute spur gear module 1.5, 44 teeth, 20° pressure angle (pitch Ø66); two of them on 66 mm centres; crank pin at e = 7, a quarter tooth pitch off a tooth centre; rhombic kinematics in tools/rhombic_geom.py
- Page: https://xemstudio.com/objects/rhombic-crank-gear/ · JSON: https://xemstudio.com/api/geometry?object=rhombic-crank-gear · Plain text: https://xemstudio.com/objects/rhombic-crank-gear/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 two counter-rotating gears of the rhombic drive are this one part used twice: module 1.5, 44 teeth, pitch Ø66, tip Ø69, 6 face, Ø12 × 5 hub with an M3 grub (Ø2.5 tapped), Ø5 bore (fit: slide).
- Crank-pin hole Ø3 (fit: press) at 7 mm from the axis and 2.045° (a quarter tooth pitch) from a tooth centre. With the two gears mirror images of each other (both pins at the same height), a tooth of one then sits in a gap of the other on the 66 mm centre line: the pin layout and the mesh agree.
- With the 30 mm links and yoke pivots at ±12.7 the displacer leads the piston by 90.0° and both strokes are 20.12 mm.

## 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 |
|---|---|---|---|---|
| gear bore | Ø5 | Ø5 gear shaft | slide | crank-gear-* |
| gear crank-pin hole | Ø3 | Ø3 crank pin | press | crank-gear-* |
| gear hub grub hole | Ø2.5 | M3 tap drill | designed | crank-gear-* |
| gear teeth | module 1.5, 44 T, 20°, on 66 mm centres | the other crank gear (no backlash drawn) | run | crank-gear-* |

## Sizes

| parameter | value |
|---|---|
| module | 1.5 |
| teeth | 44 |
| pitch Ø | 66.0 |
| tip Ø | 69.0 |
| face width | 6.0 |
| hub | Ø12 × 5 |
| bore | Ø5 |
| crank pin | Ø3 at 7, 2.045° off a tooth |
| gear centres | 66.0 |

## 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 (1 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/rhombic-crank-gear/index.json

- crank-gear-m1.5-44t-e7: [STEP](https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-m1.5-44t-e7.step) · [STL](https://xemstudio.com/files/hardware/rhombic-drive-kit/crank-gear-m1.5-44t-e7.stl) — crank gear module 1.5, 44 teeth (Ø66 pitch, Ø69 tip), 6 face, Ø12 × 5 hub with an M3 grub (Ø2.5 tapped), Ø5 bore, Ø3 crank-pin hole at 7 mm, 2.045° (a quarter tooth pitch) off a tooth centre; two per drive, meshed on 66 mm centres

## How it fits

### Goes in or on
- [Rhombic frame head for the 2 in tri-clamp beta: gear-shaft walls on a clamp-on cap](https://xemstudio.com/objects/rhombic-frame-head/): Ø5 gear shafts in the head's 625s, 66 mm centres

### Connects to
- [Rhombic drive crank pin Ø3 × 17](https://xemstudio.com/objects/rhombic-crank-pin/): Ø3 pin pressed in the gear at 7 mm
- [Rhombic drive link, 30 mm centres](https://xemstudio.com/objects/rhombic-link/): two links ride each crank pin

### Alternatives
- [Spur gears, involute 20°: module 0.5, 1 and 2, 12–60 teeth](https://xemstudio.com/objects/spur-gears-involute/): other involute gears

### Also needed
- [Crankshafts Ø3, Ø5 and Ø8 with full-length D-flat](https://xemstudio.com/objects/stirling-crankshafts/): Ø5 gear shafts

## Questions

**What gears does a rhombic drive use?**

Two identical spur gears turning opposite ways; here module 1.5, 44 T on 66 mm centres, each with a crank pin at 7 mm.

**How are the rhombic gears timed?**

Mesh them with the two crank pins level and mirror images of each other; the pin hole sits a quarter tooth pitch off a tooth so that position meshes.

**What stroke do these gears give?**

20.12 mm on both yokes with the 30 mm links (about 2√2 × the 7 mm throw at a 90° phase).

## Fits with

- [Rhombic drive link, 30 mm centres](https://xemstudio.com/objects/rhombic-link/)
- [Rhombic frame head for the 2 in tri-clamp beta: gear-shaft walls on a clamp-on cap](https://xemstudio.com/objects/rhombic-frame-head/)
- [Rhombic drive crank pin Ø3 × 17](https://xemstudio.com/objects/rhombic-crank-pin/)
- [Rhombic drive upper yoke, 25.4 mm pivot span](https://xemstudio.com/objects/rhombic-upper-yoke/)
- [Rhombic drive lower yoke, 25.4 mm pivot span](https://xemstudio.com/objects/rhombic-lower-yoke/)
- [Build list: Beta Stirling, rhombic drive, 2 in tri-clamp](https://xemstudio.com/objects/list-beta-rhombic-2in/)