# Drag-link (double-crank) four-bar

> What is a drag-link mechanism?

- Category: Mechanisms › Linkages
- Kind: Hardware
- Basis: Xem Studio drawing; classic mechanism (Brown No. 231)
- Page: https://xemstudio.com/objects/drag-link-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=drag-link-mechanism · Plain text: https://xemstudio.com/objects/drag-link-mechanism/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** True size, with no printing tolerance added.

## Notes

- A four-bar whose shortest link is the fixed frame: both cranks turn fully round. Grashof: s + l ≤ p + q (20 + 50 ≤ 45 + 40).
- Input turns at constant speed, output at varying speed — the base of quick-return drives and some shaper and press mechanisms.
- Built from four-bar links of the lengths in the table.
- Parts (each has its own page): four-bar-links, pins-3mm. Brown's movement No. 231 shows it (1868).

## Sizes

| parameter | value |
|---|---|
| frame (shortest) s | 20 mm |
| input crank | 40 mm |
| coupler | 50 mm |
| output crank | 45 mm |

## Where the files are

- Henry T. Brown (1868, public domain): [Five Hundred and Seven Mechanical Movements — scan on archive.org](https://archive.org/details/fivehundredseven00browiala) — web — Publisher's page; dimensions are facts

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

Index: https://xemstudio.com/files/hardware/drag-link-mechanism/index.json

- drag-link-mechanism-assembly: [STEP](https://xemstudio.com/files/hardware/drag-link-mechanism/drag-link-mechanism-assembly.step) · [STL](https://xemstudio.com/files/hardware/drag-link-mechanism/drag-link-mechanism-assembly.stl) — Assembled mechanism, all parts placed (STEP keeps each part separate)

## Questions

**What is a drag-link mechanism?**

A four-bar whose shortest link is the fixed frame: both cranks turn fully round. Grashof: s + l ≤ p + q (20 + 50 ≤ 45 + 40).

## Fits with

- [Four-bar links: flat links with 20 to 100 mm hole centres](https://xemstudio.com/objects/four-bar-links/)
- [Crank and wrist pins Ø3 × 12, 16, 19, 20 and 23](https://xemstudio.com/objects/pins-3mm/)