Brief description

The Archimedes’ screw is a machine historically used for transferring water from a low-lying body of water into irrigation ditches. It consists of a screw inside a hollow pipe.

Use / Function

• Irrigation: Lifting water from rivers to fields.
• Drainage: Removing water from mines or the bilges of ships.
• Handling Solids: Can move grain, sand, or sewage sludge without clogging.
• Reversibility: If water is poured into the top, it can turn the screw to generate power (screw turbine).

Operating principle

The device consists of a helical surface surrounding a central cylindrical shaft, inside a hollow pipe.

1. Scooping: As the bottom end turns in the water, a volume of water is trapped in the first “pocket” of the spiral.
2. Transport: As the shaft rotates, this pocket of water slides up the spiral. It doesn’t flow back down because the screw angle is chosen so that the pocket is always “cupped” by the lower blade.
3. Discharge: The water pours out from the top into the irrigation channel.

How to create it

1. Core: Start with a sturdy wooden log as the central axle.
2. Vanes: Construct the spiral vanes.
   • Method A (Carved): Carve the spiral from a single thick log (difficult).
   • Method B (Built-up): Attach thin flexible boards or shaped wooden pieces to the axle to form the helix. Use glue and nails.
3. Casing:
   • Option 1 (Fixed): The screw rotates inside a stationary trough or pipe. The fit must be tight to minimize backflow.
   • Option 2 (Rotating): The casing is attached to the screw and rotates with it. This is easier to seal (using pitch or tar) but heavier to turn.
4. Mounting: Install it at an angle (usually 30° to 45°).

Materials needed

• Structure: Wood is the traditional material.
• Sealing: Pitch, Tar, or Beeswax to make the casing watertight.
• Fasteners: Iron or Bronze nails/staples.
• Bearings: Simple Slide Bearings for the axle ends.

Variants and improvements

• Multiple Threads: Using 2 or 3 parallel spirals increases the volume of water lifted per turn.
• Screw Pump: Modern versions use metal and high-speed electric motors.
• Reverse Screw: Used as a hydroelectric turbine (Archimedes screw turbine) which is fish-friendly.

Limits and risks

• Lift Height: Limited by the length of the screw. Long screws act as heavy beams and can sag or break.
• Backflow: If the gap between the screw and the casing is too large, water leaks back down (loss of efficiency).
• Angle: Efficiency drops if the angle is too steep; water will just flow back down the spiral.