How Wooden Gears Mesh and Why Tooth Spacing Matters

Gears look like the most forgiving part of a mechanism. They are the least. Two gears either mesh for their whole circumference or they do not work at all, and there is no adjustment in between.

Here is what actually has to be true, and how to find out which of the conditions is failing when a wooden mechanism grinds.

Teeth are a ratio, not a pattern

A gear is defined by how many teeth it has and how far apart those teeth sit. Two gears only mesh if the spacing is the same on both.

That spacing has a name — the pitch — and the practical consequence is simple: a gear with many small teeth drives a gear with few large ones only if the tooth size matches. A large gear and a small gear from the same mechanism always do, because they were cut from the same sheet to the same pitch.

This is why the mismatch you see in a struggling wooden model is almost never a badly cut gear. Within one kit, the gears are cut to match by construction. What goes wrong is that one of them is no longer sitting where it was designed to sit.

The three things that break a good mesh

Centre distance. Two gears need to sit at a specific distance apart. Too far and the teeth skip; too close and they jam. In a cut kit that distance is set by the holes in the frame, so the failure is not the gear — it is a frame that is not square, or an axle hole that has been widened.

A tilted axle. If a shaft is not perpendicular to the gear, the gear runs at an angle, and its teeth engage deeply on one side of the rotation and barely at all on the other. This is the most common cause of a mechanism that is tight once per revolution rather than tight everywhere.

A burr on a tooth. One rough tooth, or one tooth that is slightly thicker than its neighbours, produces a single tight spot per revolution. It feels like a lump rather than a bind.

How to tell those three apart

Turn the mechanism slowly by hand and notice the pattern of resistance. This one observation separates all three causes.

Tight at the same point, once per revolution ⇒ a burr on one tooth, or one gear is eccentric on its axle. Look at the tooth that is engaged at the moment it binds.

Tight the whole way round ⇒ the centres are too close, which on a wooden kit usually means the frame is slightly out of square and one bearing has moved inward.

Tight-and-loose alternating twice per revolution ⇒ an axle that is not perpendicular, or a gear that is oval because it was cut from a bowed sheet.

Why backlash is not a defect

A pair of gears needs a small amount of play. Too little and the teeth bind; the movement needs a fraction of a millimetre of slack to run.

People who build a mechanism and find a small wobble often try to shim it out. That is usually the wrong move — the wobble is the backlash doing its job, and removing it turns a free-running pair into a tight one. The thing to fix is a real bind, not a normal amount of play.

Gears in a mechanism with a load on them

A gear train that turns freely by hand can still stall when it has to lift something. On a mechanism that drives a load, the check is to run it with the load attached before closing anything up, because a mesh that is marginal will work unloaded and fail loaded.

This is why the trial cycle matters more on gear mechanisms than on anything else. Ten clean revolutions with the load attached is the standard; nine is not.

If a gear or a frame part breaks

Gear teeth are small and they are the part most likely to snap if a mechanism is forced. A clean break along the grain can be repaired, and the process is covered here.

A bowed gear cannot be repaired — it will never run true. Check gears against a flat surface before assembly, and that starts with keeping the sheets flat while you are not working.

Where gears show up

Gear trains are the quiet backbone of the kits that move: they turn a crank into a rhythm, or a wheel into a lift. You can see them in the current marble run collection, or browse every kit we carry.