Design a hand screw press for a force of about 25-62 kN, not the 1,000 lb (4.45 kN) floor sometimes quoted, because a 1 m handle on a 10 mm pitch screw at 20-50% efficiency turns a 20 kg pull into that much. That force needs a frame of 150 x 200 mm oak or equivalent steel, a screw that locks by friction, and a perforated chamber wall of about 4.3-8.6 mm steel for a 6 in bore, and the press must be proof-tested before any seed goes in.
Part of Oil Pressing at Scale
Building a manual screw press from the force down: the load, the frame beam, the screw, the chamber wall, the capacity and the proof test.
Start From The Load The Handle Can Make
The carrier describes the manual press: a hardwood or steel frame, a large-diameter screw of 2-3 inches (50-76 mm), a press plate, a perforated pressing chamber and a base with a drain. It says the frame must withstand 1,000+ lb. Work out the real load first.
- Ideal multiplication for a 1 m handle on a 10 mm pitch: 2 x pi x 1,000 / 10 = 628.
- At 20% efficiency: 628 x 0.20 = 126; at 50%: 628 x 0.50 = 314 (efficiency range from Screw Mechanisms).
- Hand pull of 20 kg = 196 N: force on the seed = 196 x 126 = 24.7 kN up to 196 x 314 = 61.6 kN.
- The carrier’s floor is 1,000 lb = 4.45 kN, so the real load is 24.7 / 4.45 = 5.6 up to 61.6 / 4.45 = 13.8 times higher.
The seed-pressing page quotes a typical hand-cut screw with 1 cm pitch and a 40 cm handle at about 25:1 before friction (Seed Pressing). By the formula that combination is 2 x pi x 400 / 10 = 251:1, ten times higher, so the forces it implies are ten times too small.
The Frame: Tension In The Uprights, Bending In The Crossbeam
Each of two uprights carries half the load as tension: 61.6 / 2 = 30.8 kN at the top of the range. The crossbeam, carrying the nut at its center, bends. For a beam of span L = 0.5 m with a central load F, the bending moment is F x L / 4 = 61.6 x 0.5 / 4 = 7.7 kN-m. Take a working bending stress of 10 MPa for oak (a conservative design value, recalled, well below its breaking strength). The section modulus needed is 7,700 N-m / 10 x 10^6 Pa = 7.7 x 10^-4 m3, 770 cm3. A beam 150 mm wide and 200 mm deep has a section modulus of 150 x 200^2 / 6 = 1,000,000 mm3 = 1,000 cm3, which is enough, with 30% in hand. Joints matter as much as the beam: a mortise through the upright weakens it, so pin and strap the joint with iron, and keep the grain running along the load.
The Screw: Self-Locking And Strong Enough To Cut Straight
For a screw of mean diameter 76 mm and pitch 10 mm the lead angle is atan(10 / (pi x 76)) = atan(0.042) = 2.4 degrees. With oiled thread friction of 0.15 (recalled), the friction angle is atan(0.15) = 8.5 degrees. Because the lead angle is smaller than the friction angle, the screw does not run back when you let go of the handle: it holds the load. That is a safety feature and a nuisance: releasing needs a reverse turn against the stored force, so back it off slowly with the handle.
A steel rod or salvaged bolt of 50-76 mm is far stronger than a wooden screw, whose thread fails by the root shearing; use wood only if there is no steel, and cut a longer nut. Make the nut about as deep as the screw is wide, so the thread load is spread (a common proportion, recalled).
The Chamber: Hoop Stress From The Oil Pressure
The pressure on a 6 in (152.4 mm) bore of area pi x 0.0762^2 = 0.01824 m2 is 24,700 / 0.01824 = 1.35 MPa up to 61,600 / 0.01824 = 3.38 MPa. If the oil pressure acts fully on the wall (a safe assumption), the hoop stress in a thin cylinder is p x r / t. For a design stress of 60 MPa (a recalled safe working value for mild steel, about a quarter of its yield), the wall thickness needed is 3.38 x 10^6 x 0.0762 / 60 x 10^6 = 4.3 mm. Holes of 1/16 in (1.6 mm) in a pattern remove roughly half the section, so double it to 8.6 mm, or sleeve the chamber in a solid steel jacket with slots. A thin pipe, as the carrier suggests for light duty, is only right if the force is limited.
Capacity: Why One Manual Press Is Not A Village Press
The carrier gives 5-10 lb of prepared seed per pressing from a 6 in chamber, yielding 1-3 lb of oil, and 15-30 minutes of pressing per batch. For the carrier’s 1,100-2,200 lb of oil a year a village needs 1,100 / 3 = 367 up to 2,200 / 1 = 2,200 batches; at 15 minutes that is 92 hours of pressing and at 30 minutes it is 1,100 hours, before loading, heating and cleaning. A manual press is a household tool; a village needs the powered option (Powered Press).
What You Need
- Frame timber, 150 x 200 mm oak or ash for the crossbeam, similar for uprights; or steel sections
- A screw and nut, steel preferred
- Chamber: steel pipe or a stave cage in a steel jacket, with drain holes
- Base with a groove and spout, plate of hardwood or steel
- Iron straps and pins for the joints
Method: Build And Prove
Step 1: Set the uprights into a heavy base or footing so they cannot pull out.
Step 2: Fit the crossbeam with iron straps and pins at the joints; seat the nut in it.
Step 3: Fit the screw, plate and chamber and check alignment: the plate must stay square to the chamber.
Step 4: Proof-test with no seed. Close the plate on a solid block and load the handle with the design pull (20 kg at 1 m), standing to the side. Watch the beam, joints and nut for movement or cracking for several minutes, then back off.
Step 5: First pressing. Run a small batch warm and slow (Pressing and Extraction).
Worked Example: A Sound Design
Chamber 152 mm bore, handle 1 m, pitch 10 mm, steel screw 76 mm, crossbeam 150 x 200 mm oak, span 0.5 m, chamber wall 8.6 mm steel. Maximum force 61.6 kN, pressure 3.38 MPa, beam stress 7.7 kN-m / 1,000 cm3 = 7.7 MPa, below the 10 MPa working value.
Common Mistakes
- Using the 1,000 lb figure. The load is 5.6 to 13.8 times that.
- A thin pipe chamber. Hoop stress splits it; thicken or jacket it.
- Cutting a deep mortise in an upright. It weakens the member in tension.
- Skipping the proof test.
A press that holds 25-62 kN
Frame, screw and chamber failures throw timber, metal and hot oil. Stand beside the press, not in line with the screw, during the proof test and every pressing. Back the screw out slowly because it will not run back by itself, keep a clear space around the handle’s swing, and never lean over the chamber under load. Seed dust is combustible, so keep open flame away.
What’s Next
- Learn next: Animal) - scaling past a hand press
- Combine with: Ram Press Alternative - a jack in place of the screw