A screw press squeezes crushed seed in a perforated chamber between a plate and a base, and the screw multiplies the pull on a handle many times over: a 1 m handle on a 10 mm pitch screw at 20-50% efficiency turns a 20 kg pull into about 24.7-61.6 kN on the seed. Size three things before building: the frame that holds that force, the screw that carries it, and the chamber that holds the seed and lets the oil out.
Part of Oil Pressing at Scale
The types of oil press a village can build, the mechanical advantage of a screw, and the frame, screw and chamber that every design has to size.
The Screw Makes The Force
A screw advances by one pitch for each turn. Pulled at a lever radius r, an ideal screw multiplies force by 2 x pi x r / pitch. For a 1 m handle (r = 1,000 mm) and a 10 mm pitch, the ideal multiplication is 2 x pi x 1,000 / 10 = 628. Real screws lose most of it to friction; the wiki’s screw page gives 20-50% efficiency (Screw Mechanisms), so the useful multiplication is 628 x 0.20 = 126 up to 628 x 0.50 = 314.
A 20 kg pull is 20 x 9.81 = 196 N. The force on the seed is then 196 x 126 = 24.7 kN up to 196 x 314 = 61.6 kN. Those are kilonewtons, not kilograms: dividing by 9.81 gives about 2.5 to 6.3 tonnes of force. The carrier’s requirement that the frame take 1,000+ lb (4.4 kN) is therefore a floor, not a design load; the construction article designs for the larger figure.
Four Press Types
| Type | Force comes from | Suits |
|---|---|---|
| Manual screw press | A person on a lever | A household or small village; the carrier gives 5-10 lb of prepared seed per pressing from a 6 in chamber |
| Powered screw, wedge or lever press | A water wheel or animal | Hundreds of pounds of seed (Powered Press) |
| Ram (hydraulic jack) press | A salvaged jack | Quick to build where a jack exists (Ram Press Alternative) |
| Lever press | A long beam and weights | Simple, lower force (Seed Pressing) |
The Three Things To Size
- Frame: it carries the whole force as tension in the uprights and bending in the crossbeam. A frame sized to the old 1,000 lb figure would fail.
- Screw and nut: the thread carries the force in shear. Steel is far stronger than wood; use salvaged steel rod if you have it.
- Chamber: a cylinder with small holes, for example 1/16 in holes in steel pipe, that holds the seed meal while the oil drains. The walls are loaded by the oil pressure.
What You Need
- A decision on the force you want, from the table above
- Strong frame material: hardwood such as oak or ash, or steel
- A screw and nut, or a salvaged jack for the ram press
- A collection tray that drains to a clean vessel
Method: Choosing A Design
Step 1: Estimate the seed you must press. From Community Oil Budget.
Step 2: Pick the press from the table. A manual press suits a household; for a village’s tonnes of seed a year, power the crusher at least.
Step 3: Size the frame for the force, not the old floor.
Step 4: Build, proof-test and then press.
Common Mistakes
- Mixing up kilograms and newtons. 20 kg of pull is 196 N, not 20 N and not 196 kN.
- Using the old 1,000 lb frame figure. It is a fraction of the real load.
- Skipping the proof test. A frame that fails at full load injures whoever stands beside it.
Stored force
A loaded press holds tens of kilonewtons. A failing frame, screw or chamber throws timber, metal and hot oil. Stand to the side of the press axis, never in line with the screw, release pressure slowly by backing the screw out, and keep hands and faces clear of the chamber and the swing of the handle.
What’s Next
- Learn next: Manual Screw Press Construction - the full sizing
- Combine with: Pressing and Extraction - how to use it