Wood Selection

Part of Gear Making

Choosing the right wood species and grain orientation for gear teeth that are strong, wear-resistant, and dimensionally stable.

Why This Matters

Wood was the dominant gear material for over two thousand years of mechanical civilization. The great mills of Europe — grain mills, saw mills, fulling mills, paper mills — ran on wooden gears transmitting substantial power with acceptable reliability. The choice of wood species and how the wood is oriented within the tooth made the difference between gears that lasted decades and gears that failed in weeks.

Understanding wood selection for gears matters beyond historical curiosity. In a post-collapse world, metal gears will be scavenged and precious. Wood will be available, renewable, and workable with basic tools. Properly selected and cut wooden gears can run a water mill for grain grinding, a bellows pump, a timber saw, or similar low-to-moderate speed machinery for years before requiring replacement teeth — and when they do wear, the replacement takes hours, not weeks.

The key insight from historical millwrights: not all woods are equal, grain orientation is as important as species, and the wood must be properly seasoned before cutting. Freshly cut wood will shrink, warp, and split after being shaped into gear teeth, destroying the geometry that makes them work.

Species Selection

End-grain hardwoods are the traditional choice for gear cogs (individual teeth). The following properties matter: hardness (resistance to wear), toughness (resistance to splitting under impact load), dimensional stability when dry, and the ability to hold a sharp-cut profile without splintering.

Apple wood is the traditional European millwright’s first choice for cogs — extremely hard, dense, and dimensionally stable once dry. It has an interlocked grain that resists splitting under the impact loads of gear meshing. Apple grows throughout temperate regions; any old orchard provides raw material.

Hornbeam (Carpinus betulus) was widely used in medieval European mills. Exceptionally hard and wear-resistant — one of the densest European hardwoods. The interlocked grain resists splitting. Available wherever the tree grows; identify by its distinctive muscular-looking trunk.

Osage orange (Maclura pomifera) is the American equivalent — extremely hard, naturally decay-resistant, and holds a sharp cut profile well. Native to central North America; widely planted as hedgerows in the 19th century.

Lignum vitae is the hardest, densest wood in the world, and self-lubricating due to natural resin content. Used for bearings, propeller shaft bushings, and gear teeth in marine applications. Found in tropical regions; rarely available in temperate zones but worth knowing.

Acceptable alternatives: Hickory (tough, shock-resistant, good for heavily loaded cogs), white oak (widely available, good hardness), beech (very uniform grain, good for turned gear components).

Avoid: Soft woods (pine, spruce, fir) crush and wear rapidly. Ash and elm are tough but prone to checking (cracking) when drying. Walnut and cherry are intermediate — acceptable for light-duty gears but not heavily loaded mill work.

Grain Orientation

This is the most important factor that most amateur gear makers get wrong. There are three possible orientations for a wood cog:

End grain (growth rings run perpendicular to the tooth face): The absolute best for gear cogs. End grain surfaces are harder than side grain, wear more slowly, and compress under load without splitting along grain lines. The tooth takes load against the end of the fibers, which are strongest in compression. Traditional millwrights always used end-grain cogs.

To achieve end grain in a cog: cut the cog blank from a log cross-section, like cutting a disc off a log. The tooth runs from the outer surface toward the center, with end grain on the working faces.

Radial grain (grain runs outward from center to rim, growth rings perpendicular to the gear face): Also good — load is applied along the grain (compression along fibers), which is strong. The gear blank looks like a disc with grain running outward. Less common in traditional practice but mechanically sound.

Tangential grain (growth rings run parallel to the gear face, grain tangent to the pitch circle): The weakest orientation. Load is applied across the grain direction, tending to split the cog along the growth rings. Avoid for any heavily loaded application.

Practical guidance: For individual cogs (inserted into a larger wooden wheel rim), cut each cog from short lengths of branch where the grain runs straight along the cog’s length — this gives approximately end grain on the tooth faces. For gear blanks cut from planks, orient the plank so growth rings run through the blank face-to-face (end grain on the tooth surfaces).

Seasoning Requirements

Green (freshly cut) wood shrinks dramatically as it dries — 4-10% tangentially, 2-5% radially, less than 0.3% along the grain. This shrinkage, and the differential stresses it creates, will crack any precision-cut gear tooth.

Minimum drying time: 1 year per inch of thickness for air drying in a covered, ventilated space. A 2-inch thick gear blank needs 2 years minimum before cutting.

Accelerated drying: Small blanks (under 3 inches) can be kiln-dried in a low-temperature kiln (100-120°F / 38-49°C) for 2-4 weeks. Higher temperatures cause surface checking. Or dry in a warm, well-ventilated building with weights on the blank to prevent warping.

Conditioning: After initial drying, allow the wood to equilibrate to the environment where it will be used. Wood in a wet mill environment will absorb moisture; it should be equilibrated at that humidity level before final dimensioning and tooth cutting. Otherwise it will swell after installation and jam the gears.

Testing for readiness: Weigh a sample piece weekly; when weight stabilizes, the wood has reached equilibrium moisture content. Or use a moisture meter — target 12-16% for typical covered outdoor environments, 8-12% for dry indoor settings.

Surface Preparation and Sealing

Once shaped, apply a penetrating oil finish to all surfaces to stabilize moisture content and reduce future movement:

Linseed oil (raw or boiled): Traditional and effective. Multiple coats applied over several days. Penetrates into the wood and polymerizes, reducing moisture absorption.

Tallow or beeswax: Rub into the wood while warm. Provides good moisture exclusion and acts as a mild lubricant on the tooth surfaces — directly useful for gears.

Avoid film-forming finishes (varnish, paint) on tooth surfaces — they crack under load and peel, introducing debris into the gear mesh. Penetrating finishes only.

Tooth lubrication: Working wooden gear teeth should be lubricated with animal fat, tallow, or a heavy grease. This both lubricates the meshing surfaces and provides additional moisture exclusion. Apply when assembling and check weekly during operation.

Storage: Store finished gear blanks and cogs in a dry, covered location away from direct sunlight. Sunlight causes rapid surface drying and checking (cracking). Mark the grain direction clearly on stored blanks so the correct orientation is maintained during assembly.