# Four-bar links: flat links with 20 to 100 mm hole centres

> Where can I get 3D printable linkage bars for a four-bar mechanism?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Flat links 10 wide × 5 thick, Ø3 holes at exact centres from 20 to 100 mm
- Page: https://xemstudio.com/objects/four-bar-links/ · JSON: https://xemstudio.com/api/geometry?object=four-bar-links · Plain text: https://xemstudio.com/objects/four-bar-links/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

- Seven lengths: 20, 30, 40, 50, 60, 80 and 100 mm between hole centres.
- Ø3 holes at both ends for Ø3 pins, so links pin to each other, to the bell cranks and to the Scotch yoke rod.
- Builds crank-rockers, parallel motions, rocking beams and the Ross yoke linkage.

## 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 |
|---|---|---|---|---|
| end holes | Ø3 | Ø3 pins (turn in them) | run | all |

## Sizes

| centres | width × thickness | holes |
|---|---|---|
| 20 mm | 10 × 5 | Ø3 |
| 30 mm | 10 × 5 | Ø3 |
| 40 mm | 10 × 5 | Ø3 |
| 50 mm | 10 × 5 | Ø3 |
| 60 mm | 10 × 5 | Ø3 |
| 80 mm | 10 × 5 | Ø3 |
| 100 mm | 10 × 5 | Ø3 |

## Where the files are

- Wikipedia: [Four-bar linkage](https://en.wikipedia.org/wiki/Four-bar_linkage) — 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 (7 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/four-bar-links/index.json

- link-20: [STEP](https://xemstudio.com/files/hardware/four-bar-links/link-20.step) · [STL](https://xemstudio.com/files/hardware/four-bar-links/link-20.stl) — four-bar / rocker link, 20 mm centres, Ø3 holes, 10 wide × 5 thick
- link-30: [STEP](https://xemstudio.com/files/hardware/four-bar-links/link-30.step) · [STL](https://xemstudio.com/files/hardware/four-bar-links/link-30.stl) — four-bar / rocker link, 30 mm centres, Ø3 holes, 10 wide × 5 thick
- link-40: [STEP](https://xemstudio.com/files/hardware/four-bar-links/link-40.step) · [STL](https://xemstudio.com/files/hardware/four-bar-links/link-40.stl) — four-bar / rocker link, 40 mm centres, Ø3 holes, 10 wide × 5 thick
- link-50: [STEP](https://xemstudio.com/files/hardware/four-bar-links/link-50.step) · [STL](https://xemstudio.com/files/hardware/four-bar-links/link-50.stl) — four-bar / rocker link, 50 mm centres, Ø3 holes, 10 wide × 5 thick
- link-60: [STEP](https://xemstudio.com/files/hardware/four-bar-links/link-60.step) · [STL](https://xemstudio.com/files/hardware/four-bar-links/link-60.stl) — four-bar / rocker link, 60 mm centres, Ø3 holes, 10 wide × 5 thick
- link-80: [STEP](https://xemstudio.com/files/hardware/four-bar-links/link-80.step) · [STL](https://xemstudio.com/files/hardware/four-bar-links/link-80.stl) — four-bar / rocker link, 80 mm centres, Ø3 holes, 10 wide × 5 thick
- link-100: [STEP](https://xemstudio.com/files/hardware/four-bar-links/link-100.step) · [STL](https://xemstudio.com/files/hardware/four-bar-links/link-100.stl) — four-bar / rocker link, 100 mm centres, Ø3 holes, 10 wide × 5 thick

## How it fits

### Connects to
- [Bell cranks: 30 × 30 and 20 × 40 mm arms at 90°](https://xemstudio.com/objects/bell-cranks/): Ø3 arm holes
- [Scotch yoke: slotted yoke and crank disc, 20 and 30 mm stroke](https://xemstudio.com/objects/scotch-yoke/): Ø3 rod-end hole
- [Eccentric and strap: Ø30 eccentrics at 3, 5 and 8 mm offset with a split strap](https://xemstudio.com/objects/eccentric-and-strap/): Ø3 strap eye

### Alternatives
- [Ross yoke alpha for two parallel 10 mL syringes: frame, yoke, rocking link, crank and con-rods, solved for 90°](https://xemstudio.com/objects/ross-yoke-kit/): solved Ross linkage for an alpha

### Also needed
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/): Ø3 pins through the joints

## Questions

**Which lengths make a crank-rocker?**

Grashof: shortest + longest ≤ the sum of the other two, with the shortest as the crank. For example a 20 crank, 60 coupler, 50 rocker and 60 ground.

**What pins go through the holes?**

Ø3 pins, dowels or shafts; the holes are Ø3 exactly, so apply your fit to them.

## Fits with

- [Bell cranks: 30 × 30 and 20 × 40 mm arms at 90°](https://xemstudio.com/objects/bell-cranks/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Ross yoke alpha for two parallel 10 mL syringes: frame, yoke, rocking link, crank and con-rods, solved for 90°](https://xemstudio.com/objects/ross-yoke-kit/)
- [Scotch yoke: slotted yoke and crank disc, 20 and 30 mm stroke](https://xemstudio.com/objects/scotch-yoke/)
- [Clevis pins, R-clips, cotter pins, clevises, universal joint, rod ends (heim joints)](https://xemstudio.com/objects/pins-clevises-and-joints/)