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Counterweighted crank discs, crank webs and bolt-on counterweights
How do I balance the crank on a small Stirling engine? · 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
Long Earth Society build part. Balanced cranks for the 2 in gamma. The engines
- sizes
- 5 in table
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
Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.
| Wikipedia | Engine balance | web | CC BY-SA 4.0 (text) |
| Wikipedia | Crankshaft | web | CC BY-SA 4.0 (text) |
| Long Earth Society | Engine builds | web, video | LES site |
Free files
★ Free files 13 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json
| cw-crank-disc-stroke10 | 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 | STEP · STL |
| crank-web-stroke10 | crank web (bar crank) 10 mm stroke, Ø5 bore with set-screw hub, Ø3 pin; two make a full crankshaft throw | STEP · STL |
| cw-crank-disc-stroke15 | 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 | STEP · STL |
| crank-web-stroke15 | crank web (bar crank) 15 mm stroke, Ø5 bore with set-screw hub, Ø3 pin; two make a full crankshaft throw | STEP · STL |
| cw-crank-disc-stroke20 | 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 | STEP · STL |
| crank-web-stroke20 | crank web (bar crank) 20 mm stroke, Ø5 bore with set-screw hub, Ø3 pin; two make a full crankshaft throw | STEP · STL |
| cw-crank-disc-stroke25 | 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 | STEP · STL |
| crank-web-stroke25 | crank web (bar crank) 25 mm stroke, Ø5 bore with set-screw hub, Ø3 pin; two make a full crankshaft throw | STEP · STL |
| cw-crank-disc-stroke30 | 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 | STEP · STL |
| crank-web-stroke30 | crank web (bar crank) 30 mm stroke, Ø5 bore with set-screw hub, Ø3 pin; two make a full crankshaft throw | STEP · STL |
| bolt-on-counterweight-r15 | bolt-on counterweight arc, outer R30... (R30 OD, 120°), 3 × M3; add to any flywheel or disc opposite the pin | STEP · STL |
| bolt-on-counterweight-r20 | bolt-on counterweight arc, outer R40... (R40 OD, 120°), 3 × M3; add to any flywheel or disc opposite the pin | STEP · STL |
| bolt-on-counterweight-r25 | bolt-on counterweight arc, outer R50... (R50 OD, 120°), 3 × M3; add to any flywheel or disc opposite the pin | STEP · STL |
Plain text: sheet.txt · JSON: /api/geometry?object=counterweighted-cranks
How it fits
Goes in or on
Stirling linkage fittingsØ5 shaft in the 625 crank standsConnects to
Crank discs, connecting rods and flywheels for…con-rods on the Ø3 pinAlternatives
Crank discs, connecting rods and flywheels for…plain crank discsAlso needed
Dowel pins, 2–6 mmØ3 crank pins
Set screws, M3–M8M3 grubsIn these build lists
Gamma Stirling, 2 in tri-clampThe Long Earth Society engine: hot spool, shell displacer, graphite or syringe p…You might also need
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.