An overshot waterwheel turns only 5 to 15 RPM but delivers very large torque, so it drives a grain mill, saw or hammer directly and needs about 150:1 speed-up gearing to spin a 1,500 RPM generator. A flow of 100 L/s falling 2 meters at 70% efficiency gives 1,373 watts, which is a torque of 1,311 N·m on a wheel at 10 RPM but only 8.7 N·m at the generator.
Part of Micro-Hydro Turbine
An overshot wheel is a large wooden or steel wheel with buckets on its rim, filled from the top so that the weight of the water turns it.
Slow And Strong Is The Defining Trait
The carrier describes the overshot wheel as the traditional technology, effective but bulky: buckets around the rim fill at the top and gravity pulls them down. It states an efficiency of 60 to 85%, a rotation of 5 to 15 RPM, a minimum head of about 2 meters, and a use that favors mechanical drive over electricity. It can be built entirely from wood.
The Hydro Generator page sets the wheel diameter slightly below the available head: for 2 meters of head, a wheel 1.5 to 1.8 meters across, leaving room for the inlet above and the tailwater below. Its rim speed is small. At 1.8 m and 10 RPM, the rim moves π × 1.8 × 10 ÷ 60 = 0.94 m/s.
Worked Example: Power, Torque And Gearing For A 2-Meter Site
- Power at 70%: 9.81 × 100 L/s × 2 m × 0.70 = 1,373 W.
- Angular speed at 10 RPM: 10 × 2π ÷ 60 = 1.047 rad/s.
- Torque on the wheel shaft: 1,373 ÷ 1.047 = 1,311 N·m.
- Generator at 1,500 RPM: 1,500 × 2π ÷ 60 = 157.1 rad/s, so the torque it sees is 1,373 ÷ 157.1 = 8.7 N·m.
- Step-up needed: 1,500 ÷ 10 = 150, the same as the torque ratio 1,311 ÷ 8.7 = 150.
The power is the same on both shafts, apart from friction. What the gearing trades is torque for speed. The wheel shaft, hub and arms must carry 1,311 N·m, the torque of a 134 kg mass hanging on a 1-meter lever arm (1,311 ÷ 9.81 = 134), and every gear or belt stage must pass it.
Why The Carrier Prefers Mechanical Drive
The Hydro Generator page notes that a 100:1 ratio likely needs two belt stages to avoid slipping. A 150:1 step-up would need two stages of about 12:1 each (the square root of 150 is 12.2), which is very steep for a belt, or three stages of 5.3:1 each (the cube root of 150). Each stage has two bearings and a belt, and adds loss; belt stages above roughly 5:1 per stage are hard to keep from slipping, from my recall of belt practice.
A mill, saw or hammer wants low speed and high torque, which is what the wheel gives. Bolt the load to the wheel shaft and skip the gearing: see Overshot Wheel for the build and Power Transmission for line shafts. The carrier’s last line on this type is that it is best for mechanical drive rather than electricity.
A Flume And A Gate Feed And Stop The Wheel
A flume (a wooden trough) or a short penstock delivers water over the top of the wheel. A sluice gate at the head of the flume controls the flow, and the same gate is the stop. A flow below the design level still turns the wheel, since each bucket carries less water, so output falls in proportion; see Seasonal Variation.
Common Mistakes
- Choosing a wheel for a generator — the 150:1 gearing eats the saving over a turbine.
- A wheel as tall as the head — there is no room for the inlet flume above it or the tailwater below.
- No gate — a wheel that cannot be stopped from the bank is a hazard in a flood.
Safety
A wheel full of water will not stop when you let go
The wheel is heavy, loaded with water, and turns with the torque calculated above; a hand, rope or clothing caught between a bucket and the frame is pulled in with no chance to pull back. The stop is the sluice gate at the head of the flume, and the person who can reach it must be near the wheel whenever anyone is working close. Time how long the wheel coasts after the gate closes, write it on the gate post, and keep everyone clear until that time has passed. Do not stand on the wheel.
What’s Next
- Learn next: Generator Coupling (Belt vs Direct) for the step-up if you do drive a generator
- Combine with: Civil Works for the flume, gate and tailrace