Gear thrown in and out of mesh by an eccentric bush (lathe back gear) — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/eccentric-back-gear-mechanism/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Xem Studio design following Brown No. 56 (1868)
Catalog version: 2026-10-20

Sizes: see table below
- 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.

parameter  value
gears  40 / 20 T, module 1
eccentricity  2.5 mm
engaged  30 mm centres
out  35 mm

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Henry T. Brown (1868, public domain): Five Hundred and Seven Mechanical Movements — scan on archive.org — web — Publisher's page; dimensions are facts — https://archive.org/details/fivehundredseven00browiala
- 507movements.com: Brown's movement No. 56: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_056.html

Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/eccentric-back-gear-mechanism/index.json
API: https://xemstudio.com/api/geometry?object=eccentric-back-gear-mechanism