# Gear thrown in and out of mesh by an eccentric bush (lathe back gear)

> How does an eccentric bush engage a lathe's back gear?

- Category: Mechanisms › Gears
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 56 (1868)
- Page: https://xemstudio.com/objects/eccentric-back-gear-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=eccentric-back-gear-mechanism · Plain text: https://xemstudio.com/objects/eccentric-back-gear-mechanism/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/)

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

## Notes

- The pinion's axle runs in a bush whose bore is off centre; the bush turns in a fixed housing. With its lever one way the axle sits at the exact centre distance and the gears mesh; half a turn of the lever moves the axle away by twice the eccentricity and the pinion comes clear.
- Here module 1, 40 and 20 teeth (catalog gears) at 30 mm; housing bore 32.5 mm from the spindle, bush eccentricity 2.5 mm: engaged at 30 mm, thrown out to 35 mm (2.75 mm clear of the 2.25 mm tooth depth). Drawn engaged.
- Brown's No. 56 (lever moving a lathe's back gear in and out of gear by an eccentric).
- Parts (each has its own page): spur-gears-module-1, eccentric-bush-16-bore6-e2-5-lever, back-gear-housing-plate-70x40, dowel-pins-metric.
- Brown's movement No. 56 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| gears | 40 / 20 T, module 1 |
| eccentricity | 2.5 mm |
| engaged | 30 mm centres |
| out | 35 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
- 507movements.com: [Brown's movement No. 56: figure, his text and an animation](https://507movements.com/mm_056.html) — web — Publisher's page; dimensions are facts

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

Index: https://xemstudio.com/files/hardware/eccentric-back-gear-mechanism/index.json · license: https://xemstudio.com/files/hardware/eccentric-back-gear-mechanism/LICENSE.md

- eccentric-back-gear-mechanism-assembly: [STEP](https://xemstudio.com/files/hardware/eccentric-back-gear-mechanism/eccentric-back-gear-mechanism-assembly.step) · [STL](https://xemstudio.com/files/hardware/eccentric-back-gear-mechanism/eccentric-back-gear-mechanism-assembly.stl) — Assembled mechanism, all parts placed at one pose (STEP keeps each part separate)

## Questions

**How does an eccentric bush engage a lathe's back gear?**

The pinion's axle runs in a bush whose bore is off centre; the bush turns in a fixed housing. With its lever one way the axle sits at the exact centre distance and the gears mesh; half a turn of the lever moves the axle away by twice the eccentricity and the pinion comes clear.

## Fits with

- [Spur gears, module 1: 10 to 80 teeth (20° involute)](https://xemstudio.com/objects/spur-gears-module-1/)
- [Eccentric bush Ø16 × 12 with Ø6 bore offset 2.5 mm and a 50 mm lever](https://xemstudio.com/objects/eccentric-bush-16-bore6-e2-5-lever/)
- [Housing plate 70 × 40 × 6 with a Ø16 bush bore](https://xemstudio.com/objects/back-gear-housing-plate-70x40/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)