# Counterweighted crank discs, crank webs and bolt-on counterweights

> How do I balance the crank on a small Stirling engine?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Single-cylinder balancing practice: counterweight = all rotating mass + ~50% of the reciprocating mass placed opposite the pin at the pin radius; discs cut away on the pin side and thickened opposite; bolt-on arcs for trimming on the engine
- Page: https://xemstudio.com/objects/counterweighted-cranks/ · JSON: https://xemstudio.com/api/geometry?object=counterweighted-cranks · Plain text: https://xemstudio.com/objects/counterweighted-cranks/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- Why: the piston, con-rod and crank pin all pull outward once per turn; without a counterweight the engine walks across the table and the bearings hammer. Put mass opposite the pin: m_cw × r_cw = m_rotating × r_pin + 0.5 × m_reciprocating × r_pin. Over-balance toward 60–70% if the engine shakes mostly along the cylinder axis.
- The discs here remove material beyond the pin and add a 150° thickened sector opposite it (about 50% balance for a printed con-rod + syringe plunger); the bolt-on arcs trim the rest: drill the flywheel rim at the three M3 positions and move the arc until it runs smooth. Crank webs are the bar-crank alternative: two webs + a Ø3 pin make a throw in the middle of a shaft so the con-rod can sit between two bearings.
- Check: spin the bare crank on its bearings; it should stop anywhere (static balance). Then run and feel the stands. Running speed of a hobby gamma is 200–600 rpm, so static balance is enough; dynamic balance only matters past ~1500 rpm.
- LES: counterweighted discs on the 2 in gamma; bolt-on arcs on the Ø100 flywheel for the final trim.

## Sizes

| stroke_mm | pin_offset_mm | disc_dia_mm | parts |
|---|---|---|---|
| 10 | 5.0 | 32 | disc + web |
| 15 | 7.5 | 32 | disc + web |
| 20 | 10.0 | 34 | disc + web |
| 25 | 12.5 | 39 | disc + web |
| 30 | 15.0 | 44 | disc + web |

## Where the files are

- Wikipedia: [Engine balance](https://en.wikipedia.org/wiki/Engine_balance) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Crankshaft](https://en.wikipedia.org/wiki/Crankshaft) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (13 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/counterweighted-cranks/index.json

- cw-crank-disc-stroke10: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/cw-crank-disc-stroke10.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/cw-crank-disc-stroke10.stl) — counterweighted crank disc, 10 mm stroke (pin at 5.0), Ø5 bore, M3 grub, mass opposite the pin ≈ 50% of the con-rod+piston at the pin radius
- crank-web-stroke10: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/crank-web-stroke10.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/crank-web-stroke10.stl) — crank web (bar crank) 10 mm stroke, Ø5 bore with set-screw hub, Ø3 pin; two make a full crankshaft throw
- cw-crank-disc-stroke15: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/cw-crank-disc-stroke15.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/cw-crank-disc-stroke15.stl) — counterweighted crank disc, 15 mm stroke (pin at 7.5), Ø5 bore, M3 grub, mass opposite the pin ≈ 50% of the con-rod+piston at the pin radius
- crank-web-stroke15: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/crank-web-stroke15.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/crank-web-stroke15.stl) — crank web (bar crank) 15 mm stroke, Ø5 bore with set-screw hub, Ø3 pin; two make a full crankshaft throw
- cw-crank-disc-stroke20: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/cw-crank-disc-stroke20.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/cw-crank-disc-stroke20.stl) — counterweighted crank disc, 20 mm stroke (pin at 10.0), Ø5 bore, M3 grub, mass opposite the pin ≈ 50% of the con-rod+piston at the pin radius
- crank-web-stroke20: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/crank-web-stroke20.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/crank-web-stroke20.stl) — crank web (bar crank) 20 mm stroke, Ø5 bore with set-screw hub, Ø3 pin; two make a full crankshaft throw
- cw-crank-disc-stroke25: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/cw-crank-disc-stroke25.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/cw-crank-disc-stroke25.stl) — counterweighted crank disc, 25 mm stroke (pin at 12.5), Ø5 bore, M3 grub, mass opposite the pin ≈ 50% of the con-rod+piston at the pin radius
- crank-web-stroke25: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/crank-web-stroke25.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/crank-web-stroke25.stl) — crank web (bar crank) 25 mm stroke, Ø5 bore with set-screw hub, Ø3 pin; two make a full crankshaft throw
- cw-crank-disc-stroke30: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/cw-crank-disc-stroke30.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/cw-crank-disc-stroke30.stl) — counterweighted crank disc, 30 mm stroke (pin at 15.0), Ø5 bore, M3 grub, mass opposite the pin ≈ 50% of the con-rod+piston at the pin radius
- crank-web-stroke30: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/crank-web-stroke30.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/crank-web-stroke30.stl) — crank web (bar crank) 30 mm stroke, Ø5 bore with set-screw hub, Ø3 pin; two make a full crankshaft throw
- bolt-on-counterweight-r15: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r15.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r15.stl) — bolt-on counterweight arc, outer R30... (R30 OD, 120°), 3 × M3; add to any flywheel or disc opposite the pin
- bolt-on-counterweight-r20: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r20.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r20.stl) — bolt-on counterweight arc, outer R40... (R40 OD, 120°), 3 × M3; add to any flywheel or disc opposite the pin
- bolt-on-counterweight-r25: [STEP](https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r25.step) · [STL](https://xemstudio.com/files/hardware/counterweighted-cranks/bolt-on-counterweight-r25.stl) — bolt-on counterweight arc, outer R50... (R50 OD, 120°), 3 × M3; add to any flywheel or disc opposite the pin

## How it fits

### Goes in or on
- [Stirling linkage fittings: rod clevises, shaft adapter, 625 crank stands, Ø3 crank discs, rhombic frame heads](https://xemstudio.com/objects/stirling-linkage-fittings/): Ø5 shaft in the 625 crank stands

### Connects to
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/): con-rods on the Ø3 pin

### Alternatives
- [Nut-weighted flywheels: Ø100–200 with hex pockets for M8, M10, M12, 3/8 and 1/2 in nuts](https://xemstudio.com/objects/nut-weighted-flywheels/): counterweight with nuts in one half of the flywheel
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/): plain crank discs

### Also needed
- [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 D-flat crankshaft
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/): Ø3 crank pins
- [Set screws (grub screws), M3–M8 (ISO 4026)](https://xemstudio.com/objects/set-screws-metric/): M3 grubs

## Questions

**How much counterweight does a single-cylinder engine need?**

Mass opposite the pin such that m·r equals the rotating mass plus about half the reciprocating mass at the pin radius; trim on the engine with the bolt-on arcs until the stands stop buzzing.

**Why does my Stirling engine walk across the table?**

Unbalanced crank: the piston and con-rod mass pull outward every turn. Fit a counterweighted disc or add an arc to the flywheel opposite the crank pin.

**Disc crank or crank web?**

A disc is simplest (one part, pin on the face, con-rod outboard). Webs let the con-rod sit between two bearings on a straight shaft: stiffer, better for two-cylinder or beam engines.

## Fits with

- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/)
- [Shafts, D-flats, set-screw collars, E-clips (DIN 6799) and circlips (DIN 471)](https://xemstudio.com/objects/shaft-collars-clips-and-shafts/)
- [Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)](https://xemstudio.com/objects/ball-bearings-metric/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)
- [Set screws (grub screws), M3–M8 (ISO 4026)](https://xemstudio.com/objects/set-screws-metric/)