# 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?

- Category: Stirling engines › Drives & mechanisms
- Kind: 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
- Page: https://xemstudio.com/objects/eccentric-and-strap/ · JSON: https://xemstudio.com/api/geometry?object=eccentric-and-strap · Plain text: https://xemstudio.com/objects/eccentric-and-strap/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

**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.

## 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.

## Mating features (exact size; open press / slide / run / socket by your process clearance, leave designed and clamp as drawn)

| feature | exact | mates with | fit | files |
|---|---|---|---|---|
| eccentric bore | Ø5 | Ø5 shaft | slide | eccentric-d30-e* |
| set-screw hole | Ø2.5 | M3 tap drill | designed | eccentric-d30-e* |
| strap bore | Ø30 | Ø30 eccentric (turns in it) | run | eccentric-strap-* |
| strap split | 1.0 | closes on the eccentric | clamp | eccentric-strap-* |
| strap eye | Ø3 | Ø3 pin (turns in it) | run | eccentric-strap-* |

## Sizes

| offset e | stroke | disc | shaft |
|---|---|---|---|
| 3 mm | 6 mm | Ø30 × 8 | Ø5 |
| 5 mm | 10 mm | Ø30 × 8 | Ø5 |
| 8 mm | 16 mm | Ø30 × 8 | Ø5 |

## Where the files are

- Wikipedia: [Eccentric (mechanism)](https://en.wikipedia.org/wiki/Eccentric_(mechanism)) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine and rig builds on extrusion frames](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (4 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/eccentric-and-strap/index.json

- eccentric-d30-e3: [STEP](https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e3.step) · [STL](https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e3.stl) — eccentric Ø30, 3 mm offset (6 mm stroke), Ø5 shaft, Ø2.5 set-screw hole
- eccentric-d30-e5: [STEP](https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e5.step) · [STL](https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e5.stl) — eccentric Ø30, 5 mm offset (10 mm stroke), Ø5 shaft, Ø2.5 set-screw hole
- eccentric-d30-e8: [STEP](https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e8.step) · [STL](https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-d30-e8.stl) — eccentric Ø30, 8 mm offset (16 mm stroke), Ø5 shaft, Ø2.5 set-screw hole
- eccentric-strap-d30: [STEP](https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-strap-d30.step) · [STL](https://xemstudio.com/files/hardware/eccentric-and-strap/eccentric-strap-d30.stl) — split strap for the Ø30 eccentrics, Ø30 bore, 40 mm arm with a Ø3 eye

## How it fits

### Goes in or on
- [Stirling shafts and rods: Ø3 rods, Ø5 / Ø8 D-flat crankshafts, Ø5 tube rods, Ø3 pins, thrust spacers](https://xemstudio.com/objects/stirling-shafts-and-rods/): Ø5 shaft through the disc

### Connects to
- [Four-bar links: flat links with 20 to 100 mm hole centres](https://xemstudio.com/objects/four-bar-links/): Ø3 eye on the strap arm

### Alternatives
- [Scotch yoke: slotted yoke and crank disc, 20 and 30 mm stroke](https://xemstudio.com/objects/scotch-yoke/): pure sine motion
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/): crank and con-rod

## 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.

## Fits with

- [Stirling shafts and rods: Ø3 rods, Ø5 / Ø8 D-flat crankshafts, Ø5 tube rods, Ø3 pins, thrust spacers](https://xemstudio.com/objects/stirling-shafts-and-rods/)
- [Shafts, D-flats, set-screw collars, E-clips (DIN 6799) and circlips (DIN 471)](https://xemstudio.com/objects/shaft-collars-clips-and-shafts/)
- [Four-bar links: flat links with 20 to 100 mm hole centres](https://xemstudio.com/objects/four-bar-links/)
- [Scotch yoke: slotted yoke and crank disc, 20 and 30 mm stroke](https://xemstudio.com/objects/scotch-yoke/)
- [Swash (wobble) plate: Ø40 plates at 10, 15 and 20° with a ball-end follower](https://xemstudio.com/objects/swash-plate/)