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Eccentric and strap: Ø30 eccentrics at 3, 5 and 8 mm offset with a split strap

How do I make an eccentric and strap drive with 3D printed parts? · Hardware · basis: Eccentric sheave: a disc on a shaft offset e from its centre gives a stroke of 2e; strap fits the Ø30 disc exactly

Long Earth Society build part. Quiet displacer and pump drive used on the LES rigs. The engines

Exact dimensions. Every file is drawn at true size with no printing tolerance. The holes and pins that meet another part are listed below under Mating features, with the fit each joint needs; open those by your printer's or process's clearance before making the part. Mating features

sizes
3 in table

Notes

  • A disc mounted off-centre on the shaft turns inside a strap; the strap's arm moves back and forth with a stroke of twice the offset.
  • Three discs: 3, 5 and 8 mm offset for 6, 10 and 16 mm stroke. All take a Ø5 shaft and have a Ø2.5 set-screw hole.
  • The strap is split so it goes over the disc without taking the shaft apart; its 40 mm arm ends in a Ø3 eye 51 mm from the strap's centre.
  • The load is spread round the whole strap, so it runs quietly; the usual way to drive a pump or a valve.

Sizes

offset estrokediscshaft
3 mm6 mmØ30 × 8Ø5
5 mm10 mmØ30 × 8Ø5
8 mm16 mmØ30 × 8Ø5

Mating features

Drawn at the exact size. Open press, slide, run and socket features by your process's clearance; leave designed gaps and clamp fits as drawn.

featureexactmates withfitfiles
eccentric boreØ5Ø5 shaftslideeccentric-d30-e*
set-screw holeØ2.5M3 tap drilldesignedeccentric-d30-e*
strap boreØ30Ø30 eccentric (turns in it)runeccentric-strap-*
strap split1.0closes on the eccentricclampeccentric-strap-*
strap eyeØ3Ø3 pin (turns in it)runeccentric-strap-*

Where the files are

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

WikipediaEccentric (mechanism)webCC BY-SA 4.0 (text)
Long Earth SocietyEngine and rig builds on extrusion framesweb, videoLES site

Free files

★ Free files 4 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json

eccentric-d30-e3eccentric Ø30, 3 mm offset (6 mm stroke), Ø5 shaft, Ø2.5 set-screw holeSTEP · STL
eccentric-d30-e5eccentric Ø30, 5 mm offset (10 mm stroke), Ø5 shaft, Ø2.5 set-screw holeSTEP · STL
eccentric-d30-e8eccentric Ø30, 8 mm offset (16 mm stroke), Ø5 shaft, Ø2.5 set-screw holeSTEP · STL
eccentric-strap-d30split strap for the Ø30 eccentrics, Ø30 bore, 40 mm arm with a Ø3 eyeSTEP · STL

Plain text: sheet.txt · JSON: /api/geometry?object=eccentric-and-strap

Eccentric and strap: Ø30 eccentrics at 3, 5 and 8 mm offset with a split strap, isometric render of a generated part
Render of eccentric-strap-d30 from the free STEP; not to scale.

How it fits

Goes in or on

Stirling shafts and rodsØ5 shaft through the disc

Connects to

Four-bar linksØ3 eye on the strap arm

Alternatives

Scotch yokepure sine motionCrank discs, connecting rods and flywheels for…crank and con-rod

You might also need

Shafts, D-flats, set-screw collars, E-clips (DIN 6799) and circlips (DIN 471), isometric render of a generated part
Shafts, D-flats, set-screw collars, E-clips (DIN 6799) and circlips (DIN 471)22 sizes★ 32 files
Swash (wobble) plate: Ø40 plates at 10, 15 and 20° with a ball-end follower, isometric render of a generated part
Swash (wobble) plate: Ø40 plates at 10, 15 and 20° with a ball-end follower3 sizes★ 4 files

Questions

What is the stroke of an eccentric?

Twice the offset: a 5 mm eccentric gives a 10 mm stroke.

Why use an eccentric instead of a crank?

It sits anywhere along a straight shaft with no crank throw, and the strap carries the load all the way round.