OVERSHOT
GRISTMILL

Water turns the wheel. The wheel turns the stones. t = 0.0 s

Wheel 0.0 rpm
Stones 0 rpm
Flour 0 kg/h

Runner speed
pattern

Dashed band: 110 to 130 rpm,
where millers kept the stones

Drag the dial to turn the model

0°

01Water supplysluice gate

Gate opening a

075150 mm

Sets how much water flows. More flow fills the buckets fuller.

Head over the gate h

0.20.61.0 m

Depth of water behind the gate. It sets how fast the jet leaves, and so where it lands.

Flow Q

Jet speed v

Lands at

v = C_v√(2gh)

Q = C_dba√(2gh)

P_in = ρgQH

02Overshot wheel4.8 m, 36 buckets

Speed n_w

Rim speed

Torque τ_w

Shaft power

Efficiency η

Water held

Fig. 1 Torque vs wheel speed. Dashed: bucket model. Solid: load of the stones. Dots: fluid simulation, last 20 s. The mill settles where they cross.

03Gear traini = 18

Shaft	Teeth	Speed	Torque
Wheel shaftpit wheel, after bearings	96
speed × 3, torque ÷ 3, less 7% in the bevel pair
Uprightwallower, great spur	32, 120
speed × 6, torque ÷ 6, less 5% in the spur pair
Stone spindlestone nut, per pair	20

Gears trade torque for speed. The wheel turns slowly with great force; the stones need speed. About is lost to friction on the way.

04Millstones1.2 m French burr

Each pair in gear adds its load to the wheel. Lift a stone nut out of gear to stop that pair.

Grain feed ṁ

0125250 kg/h

More grain means more flour, and more load on the wheel.

Stone gap δ

0.10.81.5 mm

Closer stones grind finer, but each kilo takes more energy and the meal runs hotter.

Runner n_s

Throughput

Grinding

Energy per kg

Meal d₅₀

Meal temp

Product Sacks of 50 kg

05Energy balancekW, to scale

Fig. 2 Power from the falling water to the grinding, with the losses at the wheel and in the gears.

OVERSHOTGRISTMILL

Runner speedpattern

OVERSHOT
GRISTMILL

Runner speed
pattern