Library › Hardware › Motion ·

Motion mechanisms: Scotch yoke, swash (wobble) plate, eccentric + strap, Geneva, four-bar links, bell crank, rack

Name: Motion mechanisms: Scotch yoke, swash (wobble) plate, eccentric + strap, Geneva, four-bar links, bell crank, rack dimension sheet
Creator: Xem Studio
License: https://creativecommons.org/licenses/by/4.0/

How do I convert rotary motion to linear motion with 3D printed parts? · Hardware · basis: Textbook mechanism geometry (Scotch yoke, swash plate stroke = D·sin θ, eccentric stroke = 2e, Geneva with c = R / cos(π/n)); parts sized for Ø5 shafts, Ø3 pins and 623 bearings to match the rest of the shelf

Long Earth Society build part. Displacer drives (yoke, swash plate, eccentric) and the linkages used on the LES rigs. The engines · all LES parts

sizes: 7 in table

Notes

Scotch yoke: pure sinusoidal motion, no con-rod; the crank pin (623 bearing on a Ø3 pin) rides in the yoke slot; stroke = 2 × pin offset. Runs the displacer on a beta Stirling without a wrist pin.
Swash / wobble plate: a disc tilted θ on the shaft; a follower touching it at radius r reciprocates r·sin θ each side (Ø40 plate at 15° → 10.4 mm stroke at the rim). Several followers around one plate = multi-cylinder from one shaft; the follower here is an 8 mm square pushrod with a ball end.
Eccentric + strap: a disc offset e on the shaft inside a split ring; stroke 2e, carries load on the whole ring, the quiet way to drive a pump or displacer. Geneva: intermittent motion, n stops per turn; drive-disc radius locks the wheel between indexes.
Four-bar links (20–100 mm centres, Ø3.2) and the bell crank build crank-rockers, parallel motions and the Ross yoke; rack m1 meshes the module 1 gears for linear drives. All pivots are Ø3 dowel pins, Ø3.2 printed holes, or 623 bearings in Ø10.2 pockets.

Sizes

mechanism	parts	input → output	key dimension
Scotch yoke	yoke + crank	rotary → sinusoidal linear	stroke 20 / 30 mm
swash plate	plate + follower	rotary → linear, multi-follower	Ø40 at 10 / 15 / 20°
eccentric	disc + split strap	rotary → linear, high load	e 3 / 5 / 8 mm (stroke 6 / 10 / 16)
Geneva	wheel + drive	continuous → intermittent	4 / 6 stations, R30
four-bar links	7 lengths	rockers, parallel motion, Ross yoke	20–100 mm centres
bell crank	30×30, 20×40	redirect / lever 90°	Ø3.2 pins
rack	m1 100, m2 150	rotary ↔ linear with the gears	module 1 / 2

Where the files are

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

Wikipedia	Scotch yoke	web	CC BY-SA 4.0 (text)
Wikipedia	Swashplate	web	CC BY-SA 4.0 (text)
Wikipedia	Geneva drive	web	CC BY-SA 4.0 (text)
Wikipedia	Four-bar linkage	web	CC BY-SA 4.0 (text)
Wikipedia	Eccentric (mechanism)	web	CC BY-SA 4.0 (text)
Long Earth Society	Engine and rig builds on extrusion frames	web, video	LES site

Free files

★ Free files 27 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json

scotch-yoke-20-yoke	Scotch yoke slider, 20 mm stroke, 10.4 mm slot for a 623 bearing on the crank pin, 8 mm rod with M3 end	STEP · STL
scotch-yoke-20-crank	Scotch yoke crank disc, pin at 10 mm (20 stroke), Ø5 bore	STEP · STL
scotch-yoke-30-yoke	Scotch yoke slider, 30 mm stroke	STEP · STL
scotch-yoke-30-crank	Scotch yoke crank disc, pin at 15 mm	STEP · STL
swash-plate-40-10deg	swash (wobble) plate Ø40, 10° tilt → 6.9 mm stroke at the rim, Ø5 shaft, M3 set screw	STEP · STL
swash-plate-40-15deg	swash (wobble) plate Ø40, 15° tilt → 10.4 mm stroke at the rim, Ø5 shaft, M3 set screw	STEP · STL
swash-plate-40-20deg	swash (wobble) plate Ø40, 20° tilt → 13.7 mm stroke at the rim, Ø5 shaft, M3 set screw	STEP · STL
swash-follower-8x30	swash follower: 8 mm square pushrod with a Ø8 ball end, M3 tapped tail	STEP · STL
eccentric-d30-e3	eccentric Ø30, 3 mm offset (6 mm stroke), Ø5 shaft, set screw	STEP · STL
eccentric-d30-e5	eccentric Ø30, 5 mm offset (10 mm stroke), Ø5 shaft, set screw	STEP · STL
eccentric-d30-e8	eccentric Ø30, 8 mm offset (16 mm stroke), Ø5 shaft, set screw	STEP · STL
eccentric-strap-d30	eccentric strap for the Ø30 eccentrics, split ring, 40 mm rod with M3 eye	STEP · STL
geneva-4-wheel-r30	Geneva wheel, 4 stations, R30, 5.6 mm slots, Ø5 bore	STEP · STL
geneva-4-drive-r30	Geneva drive disc + arm + Ø5 pin for the 4-station wheel (centre distance 42.4 mm)	STEP · STL
geneva-6-wheel-r30	Geneva wheel, 6 stations, R30, 5.6 mm slots, Ø5 bore	STEP · STL
geneva-6-drive-r30	Geneva drive disc + arm + Ø5 pin for the 6-station wheel (centre distance 34.6 mm)	STEP · STL
link-20	four-bar / rocker link, 20 mm centres, Ø3.2 holes, 10 wide × 5	STEP · STL
link-30	four-bar / rocker link, 30 mm centres, Ø3.2 holes, 10 wide × 5	STEP · STL
link-40	four-bar / rocker link, 40 mm centres, Ø3.2 holes, 10 wide × 5	STEP · STL
link-50	four-bar / rocker link, 50 mm centres, Ø3.2 holes, 10 wide × 5	STEP · STL
link-60	four-bar / rocker link, 60 mm centres, Ø3.2 holes, 10 wide × 5	STEP · STL
link-80	four-bar / rocker link, 80 mm centres, Ø3.2 holes, 10 wide × 5	STEP · STL
link-100	four-bar / rocker link, 100 mm centres, Ø3.2 holes, 10 wide × 5	STEP · STL
bell-crank-30x30-90	bell crank 30 × 30 mm arms at 90°, Ø10.2 pocket for a 623 bearing pivot, Ø3.2 arm holes	STEP · STL
bell-crank-20x40-90	bell crank 20 × 40 (2:1), 90°	STEP · STL
rack-m1-100	rack, module 1, 100 mm, 8 high × 8 wide, three M3 mounting holes; meshes with the module 1 gears	STEP · STL
rack-m2-150	rack, module 2, 150 mm	STEP · STL

Plain text: sheet.txt · JSON: /api/geometry?object=motion-mechanisms

Motion mechanisms: Scotch yoke, swash (wobble) plate, eccentric + strap, Geneva, four-bar links, bell crank, rack, isometric render of a generated part — Render of swash-plate-40-15deg from the free STEP; not to scale.

Fits with

Spur gears, involute 20°: module 0.5, 1 and 2, 12–60 teeth, isometric render of a generated part

Spur gears, involute 20°: module 0.5, 1 and 2, 12–60 teeth21 sizes★ 21 files

Crank discs, connecting rods and flywheels for model engines, isometric render of a generated part

Crank discs, connecting rods and flywheels for model engines3 sizes★ 15 files

Ratchet wheels and pawl (anti-reverse / one-way drive), isometric render of a generated part

Ratchet wheels and pawl (anti-reverse / one-way drive)4 sizes★ 4 files

Ball bearings, 683 to 6210 and flanged F6xx (ISO 15), isometric render of a generated part

Ball bearings, 683 to 6210 and flanged F6xx (ISO 15)44 sizes★ 44 files

Dowel pins, 2–6 mm (ISO 2338 / DIN 6325), isometric render of a generated part

Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)5 sizes★ 23 files

Flexible shaft couplers, 5–10 mm bores, isometric render of a generated part

Flexible shaft couplers, 5–10 mm bores6 sizes★ 6 files

T-slot attachments: slide-in and drop-in T-nuts, corner brackets, gussets, end caps, slot covers, feet, hole plates, isometric render of a generated part

T-slot attachments: slide-in and drop-in T-nuts, corner brackets, gussets, end caps, slot covers, feet, hole plates7 sizes★ 46 files

Questions

What is a Scotch yoke and when is it better than a con-rod?

A pin in a slot: the slider moves as a pure sine of crank angle with no connecting rod, so it is shorter and has no angularity; the pin bearing takes side load, so put a 623 on it. Good for displacers and pumps.

How much stroke does a swash plate give?

Stroke = 2 × r × sin θ for a follower at radius r on a plate tilted θ: Ø40 plate, follower at r = 18, 15° → 9.3 mm. Several followers around the plate run from one shaft.

What is the stroke of an eccentric?

Twice the offset: a 5 mm eccentric gives a 10 mm stroke. The split strap goes over the disc and the rod takes the motion.

What centre distance does a Geneva mechanism need?

c = R / cos(π/n): for a 6-station wheel of R30, 34.6 mm; the drive pin sits at R·tan(π/n) = 17.3 mm from the drive centre.