1Five minutes to a first drive

1. Open the app. A single 15-tooth, module 2 spur gear is the driver (#1). It is always the root of the train.
2. Add New puts a second gear on it, meshing. Select it in the Gears list and set Number of teeth (N) — the ratio in the list updates as you type.
3. Turn the connection. Connection properties · Direction swings the child around its parent; the centre distance stays correct.
4. Press Space (or Start/Stop). The train turns at the drive speed. Drag the dot on tooth 0 to turn it by hand when stopped.
5. Check the numbers. Info (top right, or I) opens the data box: diameters, contact ratio, working pressure angle, tooth forces.
6. Export. Select a gear → DXF selected for cutting, STL for printing. SVG all gives a drawing of the whole train.

Everything is saved in the browser as you work. Save writes a .gg3 file; Copy share URL puts the whole design in a link.

2How a design is put together

The train is a tree. Every object hangs off one parent by a connection:

• Gear (mesh) — the two mesh; the centre distance follows from the teeth, module and profile shift, and the direction is yours to choose.
• Axle — the child sits on the same shaft: same centre, same speed, new layer.

A layer is one shaft line. Visibility · Same layer shows only the layer of the selection, which is how you work on a multi-stage box. The 3D view always shows one layer.

Ratios come out of the tree: the Gears list shows each object's ratio to drive 1 and its rpm, signed by direction (the info box spells out ↺ CCW / ↻ CW).

What can connect to what

A rack cannot be the driver. Type buttons that would break an existing connection are disabled and say why.

3The stage

100% means real size on screen at 96 dpi: hold a caliper to the screen and the pitch diameter is right. The bottom right corner always shows the current scale.

Overlays (Display section): gear guides (root, base, reference and tip circles), gear labels, centre marks, curve types (involute / trochoid / tip / root drawn in different colours), Bézier nodes for the selected object and its neighbours, and the line of action with live contact points. The same section holds Lost motion, which is a simulation setting rather than an overlay (§6.2).

4Designing gears

4.1Spur gear

Geometry follows the standard rack: addendum 1·m, dedendum 1.25·m, so root clearance is 0.25·m. The flank is a true involute; the fillet is the trochoid the generating rack's tip corner traces, so undercut appears exactly where a hobbed gear would have it — that is also why the app can warn about it rather than guess.

The info box reports reference, base, tip and root diameters, the diameter where the involute starts (Involute starts Ø — below this the flank is fillet, not involute), circular pitch, Béziers per tooth and the worst fit error of the curve fit (§9).

4.2Internal (ring) gear

Same parameters plus Rim thickness (material outside the roots); teeth run from 12 to 400. Rules the app enforces or warns about:

• The ring needs at least one more tooth than the largest gear meshing with it (hard limit), and 8 more to avoid interference in practice (warning).
• Pinion tips hit the bottom of the ring's tooth spaces means the pair as drawn would not assemble — raise the ring's teeth, lower the pinion's, or shift the profiles.
• Where a space closes up before the root (high pressure angle or shift), the bottom is cut the way a pinion-type cutter would cut it, instead of letting the flanks cross. The shape you get is what a shaper can actually produce.

Ring gears take screw holes in the rim (§7).

4.3Rack

Teeth, Module, Pressure angle, Body height (material below the roots), and Display:

• Fade — a window of rack fades out past the contacts (drawing style).
• Cut — the same window with hard ends.
• Length — a finite rack of Teeth teeth that travels; when it runs past the end it wraps by whole teeth so it never leaves the screen.

Contact distance places a rack child along its parent's rack, measured from the parent's contact point. The info box gives the rack's travel in mm/min at the current speed — the number you need for a feed calculation.

In the exports the rack is a piece of whole teeth. In Fade and Cut mode the piece follows the animation and jumps back one tooth each tooth of travel; export in Length mode when you want a definite bar.

4.4Planetary set

Sun gear (SN), Planet gears (PN), Ring gear (RN), Number of planets, Auto profile shift, Backlash, Ring rim, and the roles.

Two rules govern the set, both checked live:

• Teeth: RN = SN + 2 · PN, or the planets cannot mesh with both sun and ring.
• Assembly: (SN + RN) / n must be a whole number, or the planets cannot be spaced equally. The app then places them on the nearest valid tooth positions and warns.

Planetary setup gives each member a role: Input (driven by the parent), Fixed, Drive (its own drive — a differential), Output (children hang off it). The ratio follows: with the ring fixed, carrier / sun = SN / (SN + RN); the info box always shows the actual Output / input ratio for the roles you chose.

Auto profile shift applies the no-undercut shift to the sun and planets and balances the ring to keep both meshes right; the info box lists the three shifts used. Contact ratios for sun–planet and planet–ring are listed separately — check both.

The carrier is drawn as thin dashed arms; select the set to dimension them (radius of the planet axes and the angles between the arms).

4.5Chain wheel set

Pick an ISO 606 / ANSI chain (04C, 05B, 06B, 06C, 08A, 08B, 10A, 12A, 16A) and the two tooth counts. The tooth form is the simplified ISO 606 profile: roller seat radius ri = 0.505·d1 + 0.069·∛d1, seating angle (140 − 90/N)°, flank radius re = 0.008·d1·(N² + 180), tip between the standard minimum and maximum.

Wheel distance (D) is a target: the real distance in brackets is the nearest one that gives a whole, even number of links (so the chain closes with the same link types). The info box gives the link count, the chain length and both pitch diameters.

The chain also decides how the sprocket is built. A sprocket has to run between the chain's inner plates, so its toothed body is not a face width you choose: it is the ISO 606 tooth width for the chain you picked — 0.93·b1 up to 12.7 mm pitch and 0.95·b1 above, where b1 is the width between the plates. The app fixes it, states it in the 3D properties and in the info box, and gives you the two things that really are yours:

• Tooth chamfer — what lets the chain roll on and off instead of catching a square corner. It is a shallow cone turned onto the outer edge of the wheel: it starts where the roller seats end and the tooth flanks begin, and sinks towards the tips at the angle you give (Auto = 10°, a workshop chamfer), so the tips lose the most and the seats the rollers sit in keep their full width. It changes the solid only: the 2D outline, and the SVG and DXF exports, stay the flat profile you would cut.
• Hub — the collar around the bore, per sprocket: None, One side (flush with the teeth on the other face, as most machined sprockets are) or Both sides. Its width is the whole width of the wheel at the hub and can never be less than the body; its diameter is capped by what the chain plates clear where they wrap onto the wheel, p·cot(π/N) − 1 − h₂ (ISO 606) — the field shows that limit, and a hub asked to be fatter is trimmed to it with a ⚠. A hub too small to leave material around its own bore is left out and reported.

Holes and screw holes are set as usual (§7): the ones that fit inside the hub are bored through it, giving the grub screw or the shaft a proper length of grip, and any outside it go through the body alone. The hub is drawn as a thin circle in 2D, and comes out in the STL and OBJ solids; in SVG and DXF it lands in the optional guides layer, because a flat-cut part has no hub.

In the 3D view the chain itself is drawn: outer and inner links in turn, with their rollers and pins, wrapping both sprockets and running as the set turns — which is also the easiest way to see whether a hub is too fat for the plates to pass over. Every size of a link is that chain's own — plate depths, roller and pin diameters, and the width over the plates — in 2D as in 3D, so a roller always fits the seat the sprocket is cut with.

4.6Belt wheel set

Presets: GT2 2 mm and 3 mm, HTD 3M / 5M / 8M, T5, T10, and a flat belt. Choose the two tooth counts; Wheel distance is again a target, snapped to a whole number of belt teeth. The info box gives the belt tooth count, the pitch length and both pitch diameters — the numbers you order a belt with.

Sprockets and pulleys are always spur; there is no helical option for them.

5Profile shift and undercut

Profile shift moves the cutting rack in or out by x·m:

• Positive shift on a small pinion removes undercut, thickens the root and raises the tooth; it also pushes the pair apart (the working pressure angle rises).
• Negative shift brings a pair closer together than the standard centre distance.
• A pair keeps meshing at the centre distance the app computes from both shifts — it is never the naive m(N₁+N₂)/2 unless both shifts are zero.

No undercut uses x ≥ 1 − N·sin²α / 2. The warning Undercut: consider a profile shift appears only when the undercut actually eats into the involute (the involute start radius is more than 2 % above the base circle) — a hair of undercut on a large gear is not flagged.

Watch the contact ratio while shifting: below 1 the pair loses contact between teeth, and the app marks it in red in the info box and warns in the list.

6Backlash and lost motion

6.1What backlash does here

Backlash is a circumferential value in millimetres, entered per gear (and once per planetary set, where it is shared by both meshes). Each flank of the gear is thinned by half of it, so a pair with 0.2 mm on each gear has 0.4 mm of circumferential play.

What the app does with it:

• The teeth are really thinner — it is in the outlines, the DXF, the STL, everything.
• The driven member takes the gap up, so the flanks that carry the load touch and the trailing flanks show the gap. This holds for external pairs, internal meshes and both meshes of a planetary set.
• Direction matters. Reverse the drive (Rev) or drag a gear the other way by its marker and the contact moves to the other flank, as it does in a real drive.
• So does where you push. Turning the train by a wheel in the middle of it makes that wheel the source: it drives the gears after it in the usual way, but it also back-drives everything between it and the first gear, so those meshes take their play up on the opposite flank — again, as in a real drive when you turn an output shaft by hand.

6.2Lost motion — the dead band, simulated

Backlash on a drawing is a number per mesh. What you actually feel at a shaft is lost motion: the angle the input can turn, after a reversal, before the output moves at all. It is the sum of every mesh's play referred through the ratios, and it is what decides whether a positioning drive can repeat a move, whether a reversing load will hammer, and how much of a servo's resolution a gearbox throws away.

Lost motion (Display · Lost motion, on by default) simulates it properly rather than drawing it. Each mesh is a dead band: the driven wheel is pushed by one flank, and when the driver reverses it stands still until the other flank comes round to it. Reverse the drive (Rev) and watch a three-gear train: the first gear turns alone; the second waits out its gap, then starts; only then does the third begin to wait out its own. The delays add up down the train, one mesh at a time, exactly as the real one does.

Reading your design's lost motion off the screen. Freeze the animation, then take the output wheel by its first-tooth marker and turn it in the direction that unloads it. It will turn freely while the gaps close one after another, and the input gear will not move at all — the angle the output covers before the input twitches is the lost motion of your train, referred to the output shaft. Turn it back the other way and the whole train follows instantly, because those flanks are already touching. That asymmetry is real, and it is the same thing you feel with a shaft between your fingers.

Other things the simulation shows you:

• No contact means no contact. While a mesh sits inside its gap neither flank is touching, so its contact dots disappear and the info line counts it as 0 tooth pairs in contact. The line of action stays drawn — that is where the teeth will meet. Over a reversal you can watch contact leave one flank, vanish, and arrive on the other.
• Where you push matters. Turning the train by a wheel in the middle makes that wheel the source: the wheels after it follow through their own gaps, and the wheels between it and the drive are back-driven through theirs, one at a time, before the drive itself moves.
• It does not drift. However long the animation runs, and however many reversals, a mesh that has been driven one way sits exactly on its flank again — the state is pinned, not integrated.

Switch it off to compare, or to read the exact meshing position of a train under load: the take-up then closes the whole gap the instant the direction changes, as a drawing would show it.

What is not in it: torsional windup of shafts and teeth, friction, inertia (a free wheel does not coast through its gap — it waits to be pushed), bearing and keyway clearance, and chain or belt slack. The number you read off is the geometric lost motion of the gearing alone, which is the part you can do something about while designing the teeth.

One simplification: a planetary set takes its two meshes (sun–planet and planet–ring) up together rather than one after the other, so a set behaves as a single dead band of the size of both. The total is right; only the order within the set is not.

6.3Choosing a value

Circumferential, total per mesh:

Backlash is not the same as the root clearance (0.25·m, always present) and not the same as the centre-distance tolerance; if you widen the centre distance instead, use a positive shift rather than thinning the teeth.

7Holes, joints and screw holes

Per wheel (Holes & shaft joints):

Screw holes are a bolt circle: number of holes, bolt circle radius, hole diameter, and a rotation for the first hole. On a ring gear they sit in the rim.

The app will not produce a part that cannot exist:

• A hole that would break into the teeth, into the centre hole or into its neighbour is left out, and the reason appears as ⚠ in the list and in the info box.
• The same for a centre hole larger than the wheel allows.
• The limit is the tooth roots less 0.8 · module, with at least 0.8 mm of material between holes. If a hole is dropped, either shrink it, move the bolt circle, or give the wheel more material (a bigger rim on a ring, more teeth on a gear).

Bores are drawn at their nominal size — add your own fit allowance (H7/h6 as your process needs, or 0.1 – 0.3 mm for printed parts).

8Tooth form in 3D: helical, double helical, herringbone

Press 3D to enter the 3D settings view. It shows the layer of the selection as solids and holds the settings that only exist in 3D:

A chain wheel set has no face width to set — the chain fixes the toothed body, and the panel says how wide it is and why. Its 3D section holds the tooth chamfer and the two hubs instead (§4.5).

Module and pressure angle are normal values. A helix angle converts them into the transverse plane the profile is cut in:

m_t = m_n / cos β        tan α_t = tan α_n / cos β        x_t = x_n · cos β

So the 2D outline, every diameter and every centre distance grow by 1 / cos β. The panel shows a ⚠ line saying so, and the info box lists the transverse module and transverse pressure angle next to the normal ones you entered. At β = 20°, a 15-tooth m2 gear has D = 31.93 mm, not 30 mm.

What to check in the info box:

• Overlap ratio εβ = b · sin β / (π · m_n). Aim for ≥ 1 if you want the smooth running helical gears are chosen for; below 1 the face is too narrow for the helix angle.
• Axial force Fa = Ft · tan β per mesh — the load your bearings have to take. Double helical and herringbone report balanced instead, because the two halves cancel.
• Hands: for a planetary set the sun carries the set's hand and the planets and ring take the other.

The twist is about the mid-plane, so the section at half the face width is exactly the 2D profile and conjugate partners mesh in every slice. Holes and ring rims stay straight.

9Accuracy

9.1What the curves are

• Involute flanks are fitted with cubic Béziers to a tolerance you set — they are not polylines and not arc approximations.
• Fillets are the true trochoid of the generating rack's tip corner, including undercut.
• Tip and root arcs are exact circular arcs (as exact as a cubic arc can be, error < 0.02 % of the radius per 90° segment).
• Circles (guides, bores, bolt circles) are exact arcs.

Curve precision (Document section) is the largest distance a Bézier may sit from the exact curve. Default 2 µm; range 0.1 µm – 50 µm. It drives the stage, the SVG, the DXF, the meshes — everything.

The info box reports, per gear:

• Béziers per tooth and the total for the outline;
• Max fit error in µm — the worst actual deviation of the fitted curve from the exact involute for this gear. This is the number to quote when someone asks how accurate the profile is.

At the default setting a typical gear comes out with a handful of segments per flank and a fit error well under a micrometre — smaller than any cutting process will hold.

9.2Accuracy of each export

Exports carry marks, labels and guides according to the Display checkboxes, on their own layers/groups, so you can switch them off for a cutting file and on for a drawing.

9.3What is verified, and how

The geometry is covered by an automated test suite (88 tests, run with npm test), not judged by eye:

• Conjugate action: external pairs (with shifts and backlash), internal pairs, racks and planetary sets are sampled at many angles and the outlines checked for overlap — penetration stays at the sampling noise (a few micrometres), i.e. the flanks roll without digging in.
• Backlash take-up: with 0.4 mm of backlash the outlines pass within ~1 µm of the contact points the line of action predicts — the load side really touches, in both directions, and whichever wheel of the train the hand turns (including the meshes that then run backwards).
• Lost motion: the wait is exactly the width of the gap and not a hair more, it passes down a train mesh by mesh, no teeth dig in anywhere along the crossing, and the state neither widens nor drifts after hundreds of reversals — a long run one way puts every mesh back on its flank to the last digit.
• Ring gears: the outline never crosses itself over a wide sweep of pressure angles and shifts, and the pinion always clears the space bottoms.
• Solids: every tooth form (spur, helical, double helical, herringbone) on gears with keyways, on rings and on racks comes out as a closed 2-manifold with the expected volume; helical partners are checked slice by slice.
• Sprockets: every chain, a wide range of tooth counts and each kind of hub give a closed solid of the volume the body, the hub and the holes add up to; every vertex of a chamfered wheel is shown to lie on the cone the chamfer angle describes, with the roller seats at full width; a hole on the hub is bored through it and one outside it is not; and the chain links come out closed, alternating and one pitch apart along the path.
• Units and I/O: unit maths, .gg3 round trips, share links and Gear Generator 2 import.

9.4What the app does not compute

It is a geometry and kinematics tool. It does not do:

• strength, stress, Lewis/AGMA/ISO 6336 ratings, or material selection;
• efficiency, friction or losses — all torques are ideal (power in = power out);
• lubrication, temperature, noise, or dynamic effects;
• tolerance stack-ups, fits, or manufacturing allowances (kerf, shrinkage, over-extrusion);
• bevel, worm, hypoid or non-circular gears;
• chain wear, elongation or the sag of a long span — the chain is drawn at its nominal pitch, running on the pitch polygon, and the links are rigid.

Treat its numbers as exact geometry, and your own engineering judgement as the rest.

10Loads and forces

Set the input on drive 1: Torque (N·m) or Power (W); the line under it shows both. Load flows down the tree, split at branches by Load share (Auto divides equally).

The info box, per object, gives:

• Torque and Power at that object;
• Tooth forces with each neighbour: tangential Ft, radial Fr = Ft·tan α_w, axial Fa = Ft·tan β (helical), and the resultant normal force Fn — the load for a bearing calculation;
• Chain / belt tension and an approximate shaft load (twice the tension);
• Planetary: sun, carrier and ring torques and the force per planet mesh.

Working pressure angle α_w is used, not the nominal one, so shifted pairs give the right force directions.

Planetary sets with a second drive. The three member torques are in fixed ratios Ts : Tr : Tc = 1 : RN/SN : −(1 + RN/SN), so only one load can be set: drive 1's. A second drive's load box is read-only and shows what its member must supply, labelled Coupled to Drive 1. A member held at 0 rpm carries torque but no power — that is correct, not a bug.

11Warnings

Every warning appears as ⚠ next to the object in the Gears list, with the text in the info box.

12Recipes

A two-stage reducer. Driver 15 T → mesh a 45 T (3:1). Select the 45 T, Add New, set the new gear's connection to Axle — it is now the second shaft. Mesh another gear on that, 15 T → 60 T (4:1). The list shows 12:1 overall. Use Visibility · Same layer to work on one shaft at a time.

A planetary gearbox. Add a planetary set. Sun 12, planet 9, ring 30 (30 = 12 + 2×9 ✓, (12+30)/3 = 14 ✓ for 3 planets). Roles: sun Input, ring Fixed, carrier Output → ratio 1 : 3.5. Check both contact ratios in the info box, then set Auto profile shift if the sun is small.

Rack and pinion travel. Mesh a rack on a gear. The info box gives travel in mm/min at the drive speed; travel per revolution = π · m · N. Export in Length mode to get a definite bar.

A printed helical pair. Design the pair in 2D first (the numbers are normal module values). Press 3D, choose Helical, set β = 15–20° and a face width of at least π·m / sin β to keep εβ ≥ 1. Add 0.2 – 0.3 mm of backlash for FDM. Export both gears as STL — they are closed solids, no repair needed.

Laser-cut plate gears. Keep the tooth form spur. Add backlash equal to your kerf plus a little (0.15 – 0.25 mm typical). Switch off labels and guides in Display, then DXF selected per part, or DXF all for a nested sheet.

A sprocket you can fit to a shaft. Add a chain wheel set and pick your chain: the toothed body is now as wide as that chain allows, which on a small pitch is only a few millimetres — not enough to grip a shaft. Press 3D, set the primary wheel's Hub to One side, and give it a width that suits the shaft (a bore's worth of grip is a fair rule) and a diameter within the limit the field shows. Back in 2D, put the bore and a couple of screw holes on a bolt circle that stays inside the hub — the note under the screw holes says how far that reaches — so the screws are drilled through the full hub. STL selected exports the sprocket as one closed solid, chamfered teeth, hub and all. The flat SVG or DXF of the same wheel is the plain profile, as it must be: a laser has no way to cut a hub.

Measuring the lost motion of a train. Enter the backlash you can really hold on every gear, leave Lost motion on, and press Freeze. Note where the last gear's tooth-0 dot sits, then drag that dot in the direction that unloads the train: it turns freely, mesh after mesh, while the first gear stands still. The moment the first gear twitches, stop — the angle the output has covered is the lost motion of the whole train at the output shaft (§6.2). Halve a mesh's backlash, or move the big ratio nearer the output, and repeat to see what it buys you.

13Files, sharing and compatibility

• .gg3 — the whole document as JSON. Save / Open…. Older files are upgraded silently.
• Share URL — the entire design compressed into the link; nothing is stored on a server.
• Gear Generator 2 — Import URL… reads a GG2 share link; Open… reads a GG2 .gg file. GG2 chains map to the nearest preset.
• Autosave — your work is kept in the browser between sessions.
• Export passes — exports are gated by a time-limited pass; the panel shows the time left. The desktop build has no store.

14Reference card