Pit Wheel
Part of Mill Construction
The pit wheel is the large gear mounted on the main water wheel shaft that transmits power to the vertical millstone spindle through the wallower gear.
Why This Matters
The water wheel turns slowly — typically 4 to 10 revolutions per minute. The millstones need to turn 80 to 120 RPM to grind effectively. Somewhere between these two speeds, a gear system must multiply the rotation rate. The pit wheel is the first and largest gear in this chain, and its design sets the stage for everything else in the power transmission system.
Getting the pit wheel right means the difference between a mill that transmits power smoothly and one that vibrates, wears rapidly, and breaks teeth under load. A pit wheel with wrong dimensions, poorly cut teeth, or bad alignment will destroy the wallower (the next gear in the chain) in a matter of weeks. Re-cutting a large pit wheel is weeks of skilled woodworking; building a replacement from scratch might take months. The investment in doing it correctly is enormous.
The pit wheel also sits in a damp, exposed environment — partially underground in the mill pit, subject to flooding during high water and condensation year-round. Material selection and preservative treatment are as important as the geometry.
Function and Position
In a traditional grain mill with a vertical water wheel (overshot, breastshot, or undershot), the main shaft is horizontal. The millstones are horizontal, driven by a vertical spindle. This requires converting horizontal rotation to vertical rotation — a right-angle drive.
The pit wheel accomplishes this. It is a large face gear (the teeth are on the rim face, not the outer edge) mounted on the horizontal main shaft. Its teeth mesh with the wallower — a lantern gear or spur gear mounted on the vertical millstone spindle. As the horizontal shaft turns, the pit wheel’s face drives the wallower, which turns the vertical spindle, which turns the upper millstone.
The pit wheel typically occupies the space between the water wheel and the main bearing on one side, and the mill building wall on the other. In mills where the main shaft enters the building through the wall, the pit wheel is inside the building, rotating in a pit (the “gear pit”) cut into the floor — hence the name.
Sizing the Pit Wheel
The gear ratio between the pit wheel and wallower determines how fast the millstone turns relative to the water wheel.
Required gear ratio = target stone RPM ÷ wheel RPM
Example: water wheel turns at 6 RPM, stones need to turn at 90 RPM. Required ratio = 90 ÷ 6 = 15:1. The pit wheel must have 15 times as many teeth as the wallower.
If the wallower has 8 staves (the wooden pins or iron bars in a lantern gear), the pit wheel needs 8 × 15 = 120 cogs.
The pitch circle diameter (the effective diameter at which the teeth mesh) must match between the two gears. For a wooden gear with a tooth pitch (the center-to-center distance between teeth) of 60mm:
Pitch circle circumference = number of teeth × tooth pitch Pit wheel: 120 × 60mm = 7,200mm circumference Diameter = 7,200 ÷ π = 2,292mm ≈ 2.3 meters
This is a large wheel — typical pit wheel diameters range from 1.5 to 3 meters. The larger the wheel, the slower its tooth speed (the speed at the tooth face), which reduces wear. Keeping tooth face speed below 1 meter per second greatly extends tooth life.
Construction
The pit wheel is built as a composite structure because no single timber is large enough for a 2-meter diameter wheel.
The arms: Six to eight arms radiate from the central hub. Each arm is a carefully shaped timber (oak or ash) with a rectangular cross-section, typically 150mm × 100mm. The inner ends are mortised into the hub; the outer ends are mortised into the rim segments.
The hub: A large central casting or laminated wooden block bored to fit the main shaft with a key and keyway to prevent slipping. Iron hubs are preferable — a wooden hub can split under the torque of a heavily loaded wheel. The hub must be bored precisely on-center; any eccentricity causes the teeth to run unevenly.
The rim: Built in segments (usually 6–8 segments forming a circle), each segment spanning between two arm tips. The segments are typically 200mm deep and 100mm thick, joined end-to-end with fish-plates or tenon-and-key joints. The rim must be true — any out-of-round causes the pitch circle to vary and teeth to skip or jam.
The cogs (teeth): Mortised into holes drilled through the face of the rim. Each cog is a shaped piece of dense hardwood (apple, pear, hornbeam, or boxwood) cut so the end grain faces outward (toward the mating gear). End-grain cogs wear more slowly and evenly than side-grain. The cogs are driven in from the back, slightly oversized, and the faces are pared to the correct profile after installation.
Cog profile
Traditional mill cogs use a cycloidal profile — the tooth face and flank are curves based on rolling circles. This ensures smooth entry and exit as teeth mesh and unmesh. Approximating this with a simple convex curve (planed to a slight crown) works adequately for slow-speed wooden gearing. Avoid flat-faced cogs — they jam at entry and exit.
Alignment and Installation
Mounting the pit wheel correctly is critical:
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True the rim: Before installing the cogs, rotate the assembled wheel on its shaft and check the rim for runout (variation in radius). Measure at multiple points with a stick gauge or trammel. Pare high spots and shim low spots until the rim runs within 3mm of true.
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Set the mesh depth: The cogs should mesh with the wallower staves at the pitch point, not too deep (which causes jamming) and not too shallow (which causes the teeth to ride on their tips and wear rapidly). Mesh depth = approximately half the cog height.
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Check the backlash: There must be a small clearance between the trailing face of one tooth and the leading face of the next — “backlash.” Backlash of 3–5mm for wooden gearing prevents binding when the teeth expand with moisture. Too much backlash and the drive is rough; too little and teeth jam.
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Adjust bearing position: The main shaft bearing position sets the pit wheel-to-wallower distance. Build the bearing support so it can be adjusted slightly (typically using wedges under the bearing block) to dial in the mesh depth after installation.
Maintenance
The pit wheel cogs wear continuously. In a working grain mill, expect to replace individual cogs every 2–4 months and do a full re-cog every 1–2 years.
Keep a stock of 20–30 pre-cut cog blanks at all times. When a cog shows visible wear (the crown is flat, the profile is eroded), replace it before it fails completely — a broken cog flying loose can damage multiple other cogs in a chain reaction.
The hub fit should be checked annually. A loose hub that rocks on the shaft wastes power and accelerates tooth wear. If the shaft key has worked loose, re-key immediately.
The pit environment is damp. Treat all wooden components with hot linseed oil annually to slow rot. Check for fungal growth (black or white patches on the wood) — affected cogs must be replaced immediately as fungus rapidly weakens wood fibers.