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Linear-alternator stator that clamps round a 2 in tri-clamp tube: four coils outside the pressure wall

Name: Linear-alternator stator that clamps round a 2 in tri-clamp tube: four coils outside the pressure wall dimension sheet
Creator: Xem Studio
License: https://creativecommons.org/licenses/by/4.0/

How do I take electricity out of a pressurised free-piston engine without a wire through the wall? · Hardware · basis: Coil window and pitch from the alternator design in tools/gen_ta_power.py; gap field from a magnetic-circuit estimate

Used in Long Earth Society builds. Thermoacoustic engine spool (aluminum) · Designing a free-piston Stirling engine

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. Mating features

sizes: 6 in table

Notes

The coils sit outside the honed spool: a split bobbin clamps on the Ø50.8 tube with four 14 mm bays at the magnet rows' 32 mm pitch; the coils are wound in place, alternate hands, and joined in series; a split return-path shell (Ø66 / Ø74) clamps over them with three M4 × 16 and nuts each side. The shell has to carry the magnets' flux back; what it is made of is the builder's choice within that.
The spool wall (1.40 after honing) and a 0.5 bobbin wall sit in the magnetic gap: the wall must not carry magnetic flux, or it short-circuits the magnets. In the 25 Hz field the eddy-current loss in a 1.4 mm wall is a fraction of a watt.
Estimate: gap field 0.29 T (Br 1.3 T magnets, magnetic-circuit estimate with a 0.8 leakage factor), 369 turns of 0.45 mm wire per coil, Bl ≈ 79 T·m, coil resistance ≈ 31 Ω, inductance ≈ 132 mH; damping available Bl²/R ≈ 203 N·s/m. Measure Bl (EMF ÷ velocity) on the built engine; the model's power scales with it.
No electrical pass-through, no wire in the gas: the generator's pressure boundary is only tri-clamp joints.
Fits: bobbin bore clamps the tube (the 1 mm split is the take-up); shell bore clamps the bobbin flanges; ear holes are M4 clearance.

Sizes

part	value
bays	4 × 14 at 32
coil	Ø65.8 / Ø51.8
shell	Ø74 / Ø66 × 112
turns per coil	369
Bl	≈ 79 T·m
coil resistance	≈ 31 Ω (all four)

Mating features

Drawn at the exact size. Open press, slide, run and socket features by your process's clearance; leave designed gaps and clamp fits as drawn.

feature	exact	mates with	fit	files
bobbin bore	Ø50.8	2 in tube outside (honed spool)	clamp	stator-bobbin-*
bobbin flanges and dividers	Ø66.0	return-shell bore Ø66.0	clamp	stator-bobbin-*
split	1.0 total (0.5 per half)	the other half	clamp	all
coil bays	4 × 14 wide, Ø51.8–Ø66	coils wound in place (Ø51.8 / Ø65.8 winding)	designed	all
coil to shell	0.1 radial	return-shell bore	designed	stator-coil-*
ear holes	6 × Ø4.5	M4 clearance (ISO 273 medium)	designed	stator-iron-*

Where the files are

Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.

Xem Studio	tools/gen_ta_power.py: the network model and every dimension on this page	code	CC BY 4.0
Long Earth Society	Engine builds	web, video	LES site

Free files

★ Free files 3 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json

stator-bobbin-half-2in-112	bobbin half (y ≥ 0.5): wall Ø50.8 / Ø51.8, end flanges and dividers to Ø66, four 14 mm coil bays at 32 pitch, 112 long	STEP · STL
stator-coil-2in-14x7	coil winding volume Ø65.8 / Ø51.8 × 14: about 369 turns of 0.45 mm wire	STEP · STL
stator-iron-half-2in-112	return-path shell half (y ≥ 0.5) Ø74 / Ø66 × 112, ears x ±36..46 with Ø4.5 at x ±41.5, z 14, 56, 98	STEP · STL