Gear Making

Why This Matters

Gears are the fundamental link between any power source and the work it performs. Without gears, a waterwheel can only turn things at its own slow speed, a hand crank cannot generate enough force, and no machine can change the direction of power. Every mill, generator, lathe, and vehicle depends on gears.

Understanding Gear Types

Each gear type solves a specific mechanical problem. You will use all of them as your technology advances.

Spur Gears

The simplest and most common type. Two flat wheels with teeth around their rims, mounted on parallel shafts. When one turns, its teeth push on the teeth of the other, transferring rotation.

When to use: Any time you need to change speed or torque between two parallel shafts. Examples: mill gearing, lathe drive, hand-crank mechanisms.

Bevel Gears

Cone-shaped gears that change the axis of rotation, typically by 90 degrees. The teeth are cut on the conical surface rather than the rim.

When to use: Converting horizontal waterwheel rotation to vertical millstone rotation. Converting a horizontal hand crank to a vertical drill press spindle.

Worm Gears

A threaded shaft (the worm) meshes with a gear wheel (the worm wheel). One full rotation of the worm advances the wheel by exactly one tooth.

When to use: When you need very high gear reduction in a small space, or when you need self-locking (the load cannot drive the worm backward). Examples: lifting mechanisms, tensioning devices.

Rack and Pinion

A circular gear (pinion) meshes with a flat toothed bar (rack), converting rotation to linear motion or vice versa.

When to use: Lathe carriage feed, gate openers, press mechanisms, sawmill log carriage advance.

Gear Type	Shaft Orientation	Typical Ratio Range	Difficulty to Make
Spur	Parallel	1:1 to 8:1	Moderate
Bevel	90-degree intersecting	1:1 to 4:1	Hard
Worm	90-degree non-intersecting	10:1 to 100:1	Hard
Rack & pinion	Rotary to linear	N/A	Moderate
Lantern pinion	Parallel	3:1 to 10:1	Easy

Gear Ratios and Torque

The Fundamental Trade-Off

Gears trade speed for torque (turning force), or vice versa. This relationship is absolute and unavoidable:

Gear Ratio = Teeth on driven gear / Teeth on driving gear

If the driven gear has 40 teeth and the driving gear has 10 teeth, the ratio is 4:1. The driven gear turns 4 times slower but with 4 times the torque.

Quick Rule

Count the teeth. If the big gear drives the small gear, speed goes up and torque goes down. If the small gear drives the big gear, speed goes down and torque goes up. The ratio is always the tooth count of the output divided by the input.

Compound Gear Trains

To get large ratios without enormous gears, use multiple stages. Mount two gears of different sizes on the same shaft — the small one driven by the previous stage, the large one driving the next stage.

Example: 16:1 reduction

• Stage 1: 10-tooth pinion drives 40-tooth gear (4:1)
• The 40-tooth gear is on the same shaft as a new 10-tooth pinion
• Stage 2: That 10-tooth pinion drives another 40-tooth gear (4:1)
• Total: 4 x 4 = 16:1

This is how mill gearing achieves the 4:1 to 6:1 ratio needed to speed up a slow waterwheel (8-12 RPM) to millstone speed (40-60 RPM).

Wooden Gear Construction

Start here. Wooden gears are easier to make, cheaper to replace, and perfectly adequate for moderate loads.

Lantern Pinion

The simplest gear to build. It looks like a small wooden cage or lantern.

Materials: Two hardwood discs (end plates), 6-12 hardwood dowels (staves)

Construction:

1. Cut two identical circular discs from seasoned hardwood (oak, maple, or ash), 15-25 cm diameter, 2 cm thick
2. Mark evenly spaced holes around the perimeter of both discs, 1 cm from the edge
3. Drill holes sized to fit your dowels snugly (typically 2-3 cm diameter dowels)
4. Insert dowels into one disc, apply hide glue, then press the second disc onto the other ends
5. The dowels act as the “teeth” — other gears mesh between them

Wood Grain Direction

Always orient the grain of the end plates perpendicular to the dowels. If the grain runs parallel, the disc will split along the grain when the dowels are loaded. Use quarter-sawn wood for maximum strength.

Compass-Arm Gear (Mortise Gear)

For larger gears (60 cm and above), solid discs are impractical. Use the compass-arm design instead.

Construction:

1. Make a hub — a thick hardwood cylinder (15 cm diameter, 15 cm long) with a square hole for the shaft
2. Cut 4-6 arms (spokes) from straight-grained hardwood, mortised into the hub
3. Build the rim in segments — curved sections of hardwood joined together around the arms
4. Insert pegs (cogs) into holes drilled in the rim face — these are the teeth
5. The cogs should be made from a different, harder wood than the rim (apple, hornbeam, or dogwood are traditional)

Why different wood for cogs? When teeth wear out (and they will), you knock out individual cogs and replace them. This is far cheaper and faster than replacing the entire gear. The cogs should be slightly softer than the metal gear they mesh with, so the wooden teeth wear preferentially — protecting the expensive metal gear.

Wood Selection Guide

Wood	Hardness	Best Use	Notes
Apple	Very hard	Cogs/teeth	Excellent wear resistance
Hornbeam	Very hard	Cogs/teeth	Traditional choice
Oak	Hard	Gear body, arms, rim	Strong and durable
Ash	Hard	Shafts, arms	Flexible, absorbs shock
Elm	Medium	Large gear rims	Resists splitting
Maple	Hard	Lantern pinion discs	Fine grain, machines well

Seasoning

All gear wood must be thoroughly seasoned — at least one year of air drying per 2.5 cm of thickness. Green wood shrinks as it dries, loosening joints and changing tooth spacing. If you must use partially seasoned wood, leave gear teeth slightly oversized and trim to fit after 6 months.

Metal Gear Cutting

Metal gears are necessary when loads are high, space is limited, or precision matters (clocks, instruments, generators).

The Dividing Plate

The key to cutting evenly spaced teeth is a dividing plate — a disc with precisely spaced holes around its circumference. Mount it on the gear blank’s shaft and use a pin to lock the shaft at each tooth position.

Making a dividing plate:

1. Cut a brass or iron disc, 20-30 cm diameter
2. Scribe circles at multiple radii (e.g., at 8 cm, 9 cm, 10 cm from center)
3. Along each circle, drill holes at specific divisions:
   • Inner circle: 24 holes (for gears with 6, 8, 12, or 24 teeth)
   • Middle circle: 30 holes (for 5, 6, 10, 15, or 30 teeth)
   • Outer circle: 36 holes (for 4, 6, 9, 12, 18, or 36 teeth)
4. Space holes using careful geometric construction with dividers (compass)

Cutting Teeth on a Lathe

1. Mount the gear blank on the lathe spindle with the dividing plate behind it
2. Set a cutting tool at the correct depth for the tooth gap
3. Lock the dividing plate pin in the first hole
4. Cut the first tooth gap by traversing the tool across the gear face
5. Retract the tool, advance the dividing plate by the correct number of holes, lock the pin
6. Repeat for each tooth

Tooth Profile — The Involute Curve

The ideal tooth shape follows an involute curve, which ensures smooth meshing and constant speed ratio. For practical purposes:

1. The tooth face (the part that contacts the mating gear above the pitch circle) should curve outward
2. The tooth flank (below the pitch circle) should curve inward
3. A rough approximation: make the tooth face an arc of a circle whose radius equals the pitch circle radius of the mating gear

For low-speed applications (under 100 RPM), straight-sided teeth with slightly rounded tips work acceptably. Save involute precision for clocks and generators.

Bearings

Every rotating shaft needs bearings. Poor bearings waste power through friction and wear out shafts.

Plain Bearings (Journal Bearings)

The simplest type — a hole in which the shaft turns.

Materials ranked by performance:

1. Bronze (copper-tin alloy) — the best practical bearing material. Cast it into a sleeve that fits around the shaft
2. Lignum vitae (or other dense, oily hardwood) — naturally self-lubricating, excellent for water-powered mills
3. Hardwood with tallow — functional but requires frequent re-greasing
4. Iron on iron — high friction, wears quickly, avoid if possible

Sizing: The bearing length should be 1-2 times the shaft diameter. The clearance between shaft and bearing should be about 0.1-0.2 mm (a piece of paper should just slide through).

Roller Bearings

Place cylindrical rollers between the shaft and the housing. This converts sliding friction to rolling friction, reducing power loss by 80-90%.

Construction:

1. Turn a set of identical steel or bronze rollers (6-12 pieces, 1-2 cm diameter)
2. Make an inner race (fits on the shaft) and outer race (fits in the housing) with shallow grooves to contain the rollers
3. Space the rollers evenly using a cage — a thin ring with notches cut to hold each roller in position

Thrust Bearings

Handle axial loads (force along the shaft’s length). Critical for vertical shafts like millstone spindles.

The simplest thrust bearing is a hardened steel button on the end of the shaft, resting in a hardened steel cup. Keep it lubricated with tallow. For higher loads, use a ring of bronze or steel balls between two flat races.

Shafts and Keyways

Shaft Materials

Material	Max Diameter	Best For	Limitations
Hardwood (oak, ash)	30 cm	Mill main shafts	Cannot transmit high torque at small diameter
Wrought iron	10 cm	General machinery	Adequate strength
Steel	10 cm	High-stress applications	Best, but hardest to produce

Keyways

A key is a small metal bar that locks a gear to its shaft, preventing the gear from spinning freely. Cut a rectangular groove (keyway) along the shaft and a matching groove inside the gear bore. Insert a tight-fitting key into both grooves.

Key sizing rule: Key width = shaft diameter / 4. Key depth = shaft diameter / 8 in both the shaft and the gear.

Clutches and Engagement

A clutch lets you connect and disconnect a gear from its shaft without stopping the power source.

Jaw Clutch

Two facing discs with interlocking teeth (jaws). Slide one disc along the shaft to engage or disengage. Simple but must be engaged at low speed or when stopped — engaging at high speed will break the jaws.

Friction Clutch

A disc pressed against another by spring or lever pressure. Can be engaged gradually at any speed. Line the friction surfaces with leather or woven material for grip.

The Simplest Clutch

For a mill or shop, the easiest clutch is a sliding gear on a splined shaft. Cut shallow grooves along the shaft so the gear can slide back and forth but cannot rotate independently. Push the gear into mesh with its partner to engage; pull it out to disengage. A forked lever makes this easy to operate.

Lubrication

Without lubrication, wooden gears last months. With proper lubrication, they last years.

Lubricant	Source	Best For	Reapply
Tallow (rendered fat)	Animal fat	Plain bearings, slow gears	Weekly
Lard oil	Pig fat, minimally rendered	Metal bearings	Weekly
Vegetable oil	Seeds (flax, rapeseed)	Light-duty bearings	Weekly
Beeswax + oil mix	Bees, seeds	Wooden gear teeth	Monthly
Graphite (if available)	Natural deposits	High-speed metal bearings	Monthly

Never Use on Food-Contact Surfaces

If your gear train drives a grain mill, use only food-safe lubricants on gears near the millstones. Tallow or vegetable oil only — never mineral oils, tar, or graphite near flour.

Troubleshooting Common Failures

Symptom	Likely Cause	Fix
Grinding noise	Teeth not meshing properly	Check alignment, adjust shaft positions
Vibration	Uneven tooth spacing	Recut affected teeth or replace gear
Rapid tooth wear	Insufficient lubrication	Increase lubricant, check for grit contamination
Shaft wobble	Worn bearing	Replace bearing, check shaft for scoring
Gear slipping on shaft	Worn or missing key	Replace key, check keyway for wear
Teeth breaking	Overloaded or shock loading	Add a clutch, reduce load, use larger gear
Squealing	Metal-on-metal without lubricant	Lubricate immediately

What’s Next

Gears connect to nearly every mechanical system you will build:

• Mill Construction — Apply your gear knowledge to build grain mills and sawmills
• Generators and Motors — Precise gearing is essential for matching generator speed to power source RPM
• Simple Machines — Review fundamental mechanical advantage principles that underpin all gear design

Gear Making — At a Glance

Start with: Wooden lantern pinions — simplest to build, easy to repair Gear ratio formula: Output teeth / Input teeth = ratio Compound trains: Multiply individual ratios for total reduction Best bearing material: Bronze sleeve for durability; lignum vitae for water mills Tooth count rule: Minimum 8 teeth on any gear to avoid jamming Keyway sizing: Width = shaft diameter / 4 Lubrication: Tallow for slow gears, oil for fast; reapply weekly Wood for cogs: Apple or hornbeam — harder than the gear body so only cogs wear Critical rule: Meshing gears must have identical tooth pitch (spacing) Failure prevention: Alignment first, lubrication second, load limits third