# Linear alternator module tuning parts: add-on mass discs, the soft 0.5 mm leaf and its rim shim

> How do I retune the clamp-in linear alternator module to another engine's frequency?

- Category: Electronics › Generators & alternators
- Kind: Hardware
- Basis: Natural frequency f = √(k/m) / 2π of the mover on its leaves and bounce gas spring (k_b = γ p A² / V_b, adiabatic); masses from the CAD at 7.9 g/cm³ (assumed); leaf stiffness from the generator's frame model (E 200 GPa assumed)
- Page: https://xemstudio.com/objects/linear-alternator-module-tuning-2in/ · JSON: https://xemstudio.com/api/geometry?object=linear-alternator-module-tuning-2in · Plain text: https://xemstudio.com/objects/linear-alternator-module-tuning-2in/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**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

- What moves the frequency, largest first: the charge behind the piston, the bounce volume, the leaves, the mass. The discs are the fine trim: four of them lower the frequency by about 5 %, because the mover already carries 865 g of magnets and return path.
- Coarse tuning without new parts: a stock 2 in spool between face B and the cap (one more gasket and clamp) softens the bounce gas spring. With a 3 in spool: 295 cm³, 17.5 / 28.3 Hz (air 1 bar / helium 10 bar, standard leaves); with a 6 in spool: 430 cm³, 17.1 / 25.1 Hz.
- Mass down, frequency down: each disc adds 24.1 g. Discs stack on the crown inside the cup (Ø26 bore on the hub boss) and are held by three M3 screws into the crown's tapped holes: 1 disc M3 × 8, 2 discs M3 × 12, 3 discs M3 × 16, 4 discs M3 × 20 (always 4 mm of thread in the crown; the heads stay inside the cup).
- Spring down: the soft leaf is the 0.8 leaf's outline at 0.5 thick, 1135 N/m against 4426 N/m axially (stiffness goes with thickness cubed). Fit both leaves soft, each between two 0.6 half gaskets (0.6 + 0.5 + 0.6 = 1.7, the joint). One leaf per joint is the rule: two leaves in one joint would rub on each other, and a joint holds only 1.7. In the open-frame module each soft leaf sits on a 0.3 rim shim in the 0.8 seat.
- Spring up: charge. With face B closed the bounce gas is the stiffest spring in the module: 3.0 kN/m at 1 bar air, 29.2 kN/m at 10 bar air, 34.8 kN/m at 10 bar helium (157 cm³ behind the piston). A stock 2 in spool between face B and the cap adds volume and softens it (k_b falls in proportion).
- The table is the module alone. On an engine the working gas on face A adds its own stiffness (and the engine's power): the module's frequency on the engine is the engine's, and it is measured, not set.
- Open-frame module: no bounce space, so the leaves and the mass are the whole module spring: 15.8 Hz with the 0.8 leaves and no disc, 15.0 Hz with four discs, 8.0 Hz on the soft leaves (moving mass 899 g, its tube rod is longer). The engine's own gas on face A adds to it; the datasheet lists that for engine volumes from 50 to 400 cm³.

## 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 |
|---|---|---|---|---|
| bore | Ø26 | piston cup hub boss | slide | la-mass-disc* |
| OD | Ø41 | piston cup skirt bore Ø42 (0.5 radial gap, no contact) | designed | la-mass-disc* |
| holes | 3 × Ø3.4 on r 17 | M3 clearance (ISO 273 medium) | designed | la-mass-disc* |
| rim | Ø64 × 0.5, clamped from Ø47.5 out | two 0.6 half gaskets in the joint; centred on the Ø64 flange rims by the clamp | clamp | leaf-2in-piston-t0.5* |
| hub bore | Ø18 | tube rod, hub clamped by two collars | slide | leaf-2in-piston-t0.5* |
| arms | three spiral arms inside Ø47.5 | free (no contact) | designed | leaf-2in-piston-t0.5* |
| rim | Ø64 × 0.5 on a 0.3 shim (open frame) | open frame's Ø64 × 0.8 leaf seat under a clamp ring | clamp | leaf-2in-piston-t0.5* |
| OD | Ø64 | open frame's Ø64 leaf seat (under a 0.5 leaf's rim) | clamp | la-rim-shim* |
| faces | 0.3 thick, Ø47.5–Ø64 | seat floor and the 0.5 leaf's rim | clamp | la-rim-shim* |

## Sizes

| discs | mass_g | std_open_Hz | std_air1_Hz | std_air10_Hz | std_He10_Hz | soft_open_Hz | soft_air1_Hz | soft_He10_Hz |
|---|---|---|---|---|---|---|---|---|
| 0 | 865 | 16.1 | 18.6 | 33.4 | 35.8 | 8.2 | 12.4 | 33.0 |
| 1 | 889 | 15.9 | 18.3 | 32.9 | 35.3 | 8.1 | 12.2 | 32.6 |
| 2 | 913 | 15.7 | 18.1 | 32.5 | 34.8 | 7.9 | 12.1 | 32.1 |
| 3 | 937 | 15.5 | 17.9 | 32.1 | 34.4 | 7.8 | 11.9 | 31.7 |
| 4 | 961 | 15.3 | 17.6 | 31.7 | 33.9 | 7.7 | 11.8 | 31.3 |

## Where the files are

- Xem Studio: [tools/gen_la_module.py: the module's geometry, masses, bounce volume, natural frequencies and every number on these pages](https://xemstudio.com/) — code — CC BY 4.0
- Xem Studio: [tools/gen_fpse_gen.py: the free-piston generator whose stator, magnets, leaves and collars this module reuses; its leaf frame model and magnetic-circuit estimate](https://xemstudio.com/) — code — CC BY 4.0
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (3 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/linear-alternator-module-tuning-2in/index.json

- la-mass-disc-41x26x4: [STEP](https://xemstudio.com/files/hardware/linear-alternator-module-tuning-2in/la-mass-disc-41x26x4.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-module-tuning-2in/la-mass-disc-41x26x4.stl) — add-on mass disc Ø41 / Ø26 × 4, 3 × Ø3.4 on r 17: up to 4 stack on the cup's crown inside its skirt, 24.1 g each at 7.9 g/cm³
- leaf-2in-piston-t0.5: [STEP](https://xemstudio.com/files/hardware/linear-alternator-module-tuning-2in/leaf-2in-piston-t0.5.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-module-tuning-2in/leaf-2in-piston-t0.5.stl) — soft leaf: the 0.8 piston leaf's outline (Ø64 rim, Ø18 hub bore, 3 arms 4 wide sweeping 330°) at 0.5 thick; clamped between two 0.6 half gaskets (closed module) or on a 0.3 rim shim in the open frame's 0.8 seat
- la-rim-shim-64x0.3: [STEP](https://xemstudio.com/files/hardware/linear-alternator-module-tuning-2in/la-rim-shim-64x0.3.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-module-tuning-2in/la-rim-shim-64x0.3.stl) — rim shim Ø64 / Ø47.5 × 0.3: under a 0.5 soft leaf's rim in the open frame's Ø64 × 0.8 leaf seat, so the clamp ring clamps it as it clamps a 0.8 leaf

## Questions

**Which way does each change go?**

Discs lower the frequency; soft leaves lower it (most at 1 bar, little at 10 bar where the bounce gas dominates); more charge or less bounce volume raise it.

**Is the soft leaf strong enough?**

Its bending stress is 114 MPa at ±6 mm against 177 MPa for the 0.8 leaf: lower, because it is thinner.

## Fits with

- [Linear alternator module mover: Ø48 piston cup, 40-magnet carrier and tube rod, sprung on two leaves](https://xemstudio.com/objects/linear-alternator-module-mover-2in/)
- [Linear alternator module open frame, 2 in: frame base, leaf seats, clamp rings, stator sleeve and top plate](https://xemstudio.com/objects/linear-alternator-module-open-frame-2in/)
- [Tri-clamp flexure leaf: spiral planar spring clamped in a 2 in joint like a gasket (displacer and piston)](https://xemstudio.com/objects/tri-clamp-flexure-leaf/)
- [Tri-clamp half gasket for a clamped leaf or rim: 2 in, web 0.6 or 0.45](https://xemstudio.com/objects/tri-clamp-leaf-half-gasket/)
- [Linear alternator module, 2 in tri-clamp: datasheet, closed and open frame (mass, stiffness, frequency, Bl, R, L, stroke, pressure) and test procedure](https://xemstudio.com/objects/linear-alternator-module-2in-datasheet/)
- [Build list: Linear alternator module, 2 in, closed (sealed bounce space, chargeable)](https://xemstudio.com/objects/list-linear-alternator-module-2in-closed/)
- [Build list: Linear alternator module, 2 in, open frame (exposed flexures)](https://xemstudio.com/objects/list-linear-alternator-module-2in-open/)