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Triangular eccentric: curved triangle turning in a yoke (motion with dwells)

How does a triangular eccentric give a pause at each end of the stroke? · Hardware · basis: Xem Studio design following Brown No. 91 (1868)

Also called: triangular eccentric, reuleaux triangle cam, dwell cam, constant width cam, excéntrica triangular, Dreieckexzenter

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

sizes
4 in table

Notes

  • A curved triangle of constant width turns about one of its corners inside a frame whose faces are exactly that width apart, so it always touches both. The frame moves to and fro, but stands still while a corner-centred arc rolls on a face: an intermittent reciprocation from continuous rotation.
  • Here width 30 mm: stroke 30 mm, with a dwell at each end of the stroke; drawn at 25°.
  • Brown's No. 91 (triangular eccentric giving an intermittent reciprocating motion, used in France for slide valves).
  • Parts (each has its own page): triangular-eccentric-reuleaux-w30, triangular-eccentric-yoke-w30, dowel-pins-metric.
  • Brown's movement No. 91 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

Triangular eccentric: curved triangle turning in a yoke size chart

parametervalue
camReuleaux triangle, width 30
pivotone corner
stroke30 mm
yoke opening30 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. 91: 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

triangular-eccentric-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=triangular-eccentric-mechanism

Triangular eccentric: curved triangle turning in a yoke (motion with dwells), isometric render of a generated part
Render of triangular-eccentric-mechanism-assembly from the free STEP; not to scale.

Fits with

Triangular eccentric: curved (Reuleaux) triangle of width 30, pivoted at a corner, isometric render of a generated part
Triangular eccentric: curved (Reuleaux) triangle of width 30, pivoted at a corner30.0×37.0×18.0 mm★ 1 file
Yoke 42 × 72 with a 30 mm wide opening and two 8 × 8 rods, isometric render of a generated part
Yoke 42 × 72 with a 30 mm wide opening and two 8 × 8 rods122.0×72.0×8.0 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 a triangular eccentric give a pause at each end of the stroke?

A curved triangle of constant width turns about one of its corners inside a frame whose faces are exactly that width apart, so it always touches both. The frame moves to and fro, but stands still while a corner-centred arc rolls on a face: an intermittent reciprocation from continuous rotation.