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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? · Hardware · basis: Xem Studio design following Brown No. 56 (1868)

Also called: back gear, eccentric bush, gear engagement lever, engranaje de retroceso con casquillo excéntrico, Rädervorgelege mit Exzenterbuchse

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

sizes
4 in table

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.

Gear thrown in and out of mesh by an eccentric bush size chart

parametervalue
gears40 / 20 T, module 1
eccentricity2.5 mm
engaged30 mm centres
out35 mm

Where the files are

Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.

Henry T. Brown (1868, public domain)Five Hundred and Seven Mechanical Movements — scan on archive.orgwebPublisher's page; dimensions are facts
507movements.comBrown's movement No. 56: figure, his text and an animationwebPublisher's page; dimensions are facts

Free files

★ Free files 1 parts. Xem Studio design: free to use, print and sell things from; not to be republished as files. Xem Studio Design License. index.json

eccentric-back-gear-mechanism-assemblyAssembled mechanism, all parts placed at one pose (STEP keeps each part separate)STEP · STL
Let your AI pull this. Add https://xemstudio.com/mcp to Claude, ChatGPT, Cursor or any MCP client (free, no account, one minute) and it can look up exact sizes, mating fits and free STEP/STL from every chat.
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Plain text: sheet.txt · JSON: /api/geometry?object=eccentric-back-gear-mechanism

Gear thrown in and out of mesh by an eccentric bush (lathe back gear), isometric render of a generated part
Render of eccentric-back-gear-mechanism-assembly from the free STEP; not to scale.

Fits with

Spur gears, module 1: 10 to 80 teeth (20° involute), isometric render of a generated part
Spur gears, module 1: 10 to 80 teeth (20° involute)26 sizes★ 26 files
Eccentric bush Ø16 × 12 with Ø6 bore offset 2.5 mm and a 50 mm lever, isometric render of a generated part
Eccentric bush Ø16 × 12 with Ø6 bore offset 2.5 mm and a 50 mm lever58.0×16.0×12.0 mm★ 1 file
Housing plate 70 × 40 × 6 with a Ø16 bush bore, isometric render of a generated part
Housing plate 70 × 40 × 6 with a Ø16 bush bore70.0×40.0×6.0 mm★ 1 file
Dowel pins, 2–6 mm (ISO 2338 / DIN 6325), isometric render of a generated part
Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)5 sizes★ 23 files

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.