# Spur gear, module 1, 21 T, profile shift -0.46, Ø6 bore (meshes a 30 T at 25 mm)

> What is a negatively shifted 21 tooth gear?

- Category: Hardware › Mechanism parts
- Kind: Modern device
- Basis: Xem Studio design following Brown's figure (1868); true size, no printing allowance
- Page: https://xemstudio.com/objects/spur-gear-m1-21t-xm046-b6/ · JSON: https://xemstudio.com/api/geometry?object=spur-gear-m1-21t-xm046-b6 · Plain text: https://xemstudio.com/objects/spur-gear-m1-21t-xm046-b6/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); files Xem Studio Design License (free to use and sell from, not to redistribute as files: https://xemstudio.com/license/)
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

## Overall (mm)

| length | width | height |
|---|---|---|
| 22.0 | 22.06 | 8.0 |

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

## Notes

- Module 1, 20° involute, 21 teeth with a negative profile shift of -0.46 m: thinner teeth, tip at Ø22.08, so that it meshes with an unshifted 30-tooth gear at 25 mm instead of its natural 25.5 mm. Below the base circle (Ø19.73) the flank is radial; the mating idler's tips are shortened to stay clear of it. 8 mm face, Ø6 bore.
- Xem Studio design following Brown's figure; true size, no printing allowance (apply your own to the mating features).

## Sizes

| dimension | value |
|---|---|
| module | 1 |
| teeth | 21 |
| profile shift | -0.46 m |
| tip Ø | 22.08 mm |
| centre distance with 30 T | 25 mm |
| bore | Ø6 |

## 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
- 507movements.com: [Brown's movement No. 504: figure, his text and an animation](https://507movements.com/mm_504.html) — web — Publisher's page; dimensions are facts
- Xem Studio: [drawing](https://xemstudio.com/objects/fergusons-paradox-mechanism/) — web — Publisher's page; dimensions are facts

## Free STEP/STL (1 parts, Xem Studio Design License, Xem Studio)

Index: https://xemstudio.com/files/hardware/spur-gear-m1-21t-xm046-b6/index.json · license: https://xemstudio.com/files/hardware/spur-gear-m1-21t-xm046-b6/LICENSE.md

- spur-gear-m1-21t-xm046-b6: [STEP](https://xemstudio.com/files/hardware/spur-gear-m1-21t-xm046-b6/spur-gear-m1-21t-xm046-b6.step) · [STL](https://xemstudio.com/files/hardware/spur-gear-m1-21t-xm046-b6/spur-gear-m1-21t-xm046-b6.stl) — gear, z 0..8, a tooth centred on +X

## Questions

**What is a negatively shifted 21 tooth gear?**

Module 1, 20° involute, 21 teeth with a negative profile shift of -0.46 m: thinner teeth, tip at Ø22.08, so that it meshes with an unshifted 30-tooth gear at 25 mm instead of its natural 25.5 mm. Below the base circle (Ø19.73) the flank is radial; the mating idler's tips are shortened to stay clear of it. 8 mm face, Ø6 bore.

## Fits with

- [Ferguson's mechanical paradox (19 / 20 / 21 tooth wheels on one pin)](https://xemstudio.com/objects/fergusons-paradox-mechanism/)