# Counterweighted crank discs, Ø5 bore, 10 to 30 mm stroke

> How is a counterweighted crank disc for a small engine shaped?

- Category: Stirling engines › Drives & mechanisms
- Kind: Hardware
- Basis: Crank disc ending just past the pin and thickened opposite it; pin at stroke / 2, Ø5 bore, M3 grub
- Page: https://xemstudio.com/objects/counterweighted-crank-discs/ · JSON: https://xemstudio.com/api/geometry?object=counterweighted-crank-discs · Plain text: https://xemstudio.com/objects/counterweighted-crank-discs/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 6 mm disc (Ø32, 32, 34, 39, 44 for 10–30 mm stroke) with the Ø3 pin hole at half the stroke. On the pin side the disc ends 6 mm past the pin centre, with a Ø10 boss round the pin.
- Opposite the pin a raised segment 4.8 thick (10.8 overall) carries the counterweight; the description sizes it at about 50 % of a con-rod and piston at the pin radius.
- Ø5 bore and a radial Ø2.5 hole (M3 tap drill) at mid-thickness for a grub on the shaft's flat.
- Balance condition: m_cw × r_cw = m_rotating × r_pin + 0.5 × m_reciprocating × r_pin. A crank that stops at any angle when spun bare is statically balanced; at 200–600 rpm that is enough.

## 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 |
|---|---|---|---|---|
| disc bore | Ø5 | Ø5 crankshaft (M3 grub) | slide | cw-crank-disc-* |
| disc pin hole | Ø3 | Ø3 crank pin | press | cw-crank-disc-* |
| disc grub hole | Ø2.5 | M3 tap drill | designed | cw-crank-disc-* |

## Sizes

| file | stroke | pin offset | disc Ø | thickness | envelope |
|---|---|---|---|---|---|
| cw-crank-disc-stroke10 | 10 | 5.0 | 32 | 6 / 10.8 | 27 × 32 |
| cw-crank-disc-stroke15 | 15 | 7.5 | 32 | 6 / 10.8 | 29.5 × 32 |
| cw-crank-disc-stroke20 | 20 | 10.0 | 34 | 6 / 10.8 | 33 × 34 |
| cw-crank-disc-stroke25 | 25 | 12.5 | 39 | 6 / 10.8 | 38 × 39 |
| cw-crank-disc-stroke30 | 30 | 15.0 | 44 | 6 / 10.8 | 43 × 44 |

## 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 (5 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/counterweighted-crank-discs/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
- 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
- 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
- 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
- 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

## How it fits

### Goes in or on
- [Crankshafts Ø3, Ø5 and Ø8 with full-length D-flat](https://xemstudio.com/objects/stirling-crankshafts/): Ø5 bore, M3 grub on the flat
- [Crank stands with a 625 pocket, 40 and 60 mm axis height](https://xemstudio.com/objects/crank-stands-625/): on the shaft beside the 625 stand

### Connects to
- [Connecting rods 30 to 80 mm centres with 623 pockets](https://xemstudio.com/objects/connecting-rods-623/): 623 on the Ø3 pin

### Alternatives
- [Crank discs for a Ø5 shaft, 10 to 30 mm stroke](https://xemstudio.com/objects/crank-discs-5mm-bore/): plain disc

## Questions

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

Enough that mass × radius opposite the pin equals the rotating mass plus about half the reciprocating mass at the pin radius.

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

The crank is out of balance: the piston and con-rod pull outward once a turn. A counterweight opposite the pin cancels it.

## Fits with

- [Crank discs for a Ø5 shaft, 10 to 30 mm stroke](https://xemstudio.com/objects/crank-discs-5mm-bore/)
- [Crank webs (bar cranks), Ø5 bore, 10 to 30 mm stroke](https://xemstudio.com/objects/crank-webs-5mm-bore/)
- [Bolt-on counterweight arcs R15, R20 and R25](https://xemstudio.com/objects/bolt-on-counterweights/)
- [Connecting rods 30 to 80 mm centres with 623 pockets](https://xemstudio.com/objects/connecting-rods-623/)
- [Crank stands with a 625 pocket, 40 and 60 mm axis height](https://xemstudio.com/objects/crank-stands-625/)