Centrifuge Building

Phase 4 — Village Scale

Separating substances by density using centrifugal force. Centrifuges are essential for dairy processing (cream separation), chemical purification, medical diagnostics (blood work), and industrial filtration.

Why This Matters

Gravity separates substances slowly — cream rises from milk overnight. A centrifuge accelerates separation by 100–10,000× gravity, completing in seconds what gravity takes hours. For dairy farms, this means fresh cream and butter within minutes of milking. For medical work, it means blood analysis capability. For chemistry, it means efficient separation and purification.

Centrifugal Principles

G-Force Calculation

Relative Centrifugal Force (RCF) in multiples of gravity:

RCF = 1.118 × 10⁻⁵ × r × N²

Where r = radius in cm, N = RPM.

RPM	Radius 5 cm	Radius 10 cm	Radius 15 cm
1,000	56 g	112 g	168 g
3,000	503 g	1,006 g	1,509 g
5,000	1,398 g	2,795 g	4,193 g
10,000	5,590 g	11,180 g	16,770 g

Practical targets:

• Cream separation: 5,000–8,000 g
• Blood serum separation: 1,000–3,000 g
• Chemical precipitation: 500–5,000 g
• Fine particle settling: 10,000+ g

Separation Theory

Heavier particles move outward, lighter particles move inward. Separation rate depends on:

• Density difference between particles and fluid
• Particle size (larger separates faster — proportional to diameter²)
• Viscosity of the fluid (lower viscosity = faster separation)
• G-force applied (more force = faster separation)

Hand-Cranked Centrifuge

Tube Centrifuge

The simplest useful design:

Materials:

• Steel or aluminum rotor arm, 200–300 mm total length
• Test tube holders welded to each end (swinging bucket type)
• Central shaft, 8–10 mm hardened steel
• Bearings: two ball bearings in a housing
• Hand crank with speed-multiplication gearing

Construction:

1. Machine the rotor arm from solid steel bar (for balance)
2. Drill pivot holes at each end for swinging bucket pins
3. Make bucket holders from bent steel strip
4. Mount on a central shaft with press-fit or keyway
5. Build a 10:1 gear ratio drive: small gear on crank → large gear on intermediate → small pulley on intermediate → large pulley on rotor shaft. Or use a stepped pulley system.
6. At 60 RPM crank speed × 10:1 ratio = 600 RPM rotor. With a 100 mm radius, that’s 40 g — enough for basic separation.

For higher g-forces, increase the gear ratio to 30:1 or 50:1 (achievable with two-stage belt drive).

String-Pull Centrifuge (Paperfuge)

Materials: A disc (cardboard, wood, or plastic), two holes near center, and a loop of string.

Thread string through both holes. Pull both ends outward, the disc spins. Release, let the string rewind, pull again. Achieves 20,000–100,000 RPM (!) with a lightweight disc.

Medical application

The paperfuge was developed for malaria diagnosis in field conditions. A drop of blood in a capillary tube, spun for 1.5 minutes, separates blood components for visual diagnosis. Build these for your medical kit — they cost nothing and work brilliantly.

Cream Separator

The most commercially valuable centrifuge for a village community.

Disc-Stack Design

A continuous-flow cream separator uses a stack of conical discs to maximize separation surface:

1. Bowl: A machined steel cylinder, 150–200 mm diameter, spinning at 6,000–10,000 RPM
2. Disc stack: 20–40 conical stainless steel discs with 0.5–1 mm spacing between them
3. Inlet: Milk enters through the center top
4. Separation: Cream (lighter) moves inward along disc surfaces; skim milk (heavier) moves outward
5. Outlets: Two separate outlets — cream from the center, skim milk from the periphery

Bowl Construction

Machine from a single piece of steel or assemble from turned components:

1. Outer shell: 3–4 mm wall thickness, seamless or welded longitudinally
2. Bottom plate: machined flat, press-fit or bolted
3. Disc stack: stamp conical discs from 0.5 mm stainless steel sheet, with spacer dimples
4. Top nut: secures the disc stack and provides the inlet/outlet channels

Precision is non-negotiable

A cream separator bowl spinning at 8,000 RPM with a 1-gram imbalance generates 35 N of radial force — enough to destroy bearings in minutes. Machine to ±0.05 mm, balance to ±0.5 grams.

Performance

A well-built 150 mm diameter separator processes 50–100 liters of milk per hour, extracting 95–98% of cream content. This handles the daily output of a 10–20 cow dairy herd.

Chemical/Industrial Centrifuge

Basket Centrifuge

For separating solids from liquids (sugar crystals from syrup, precipitates from solution):

1. Perforated basket (holes 0.5–2 mm depending on particle size)
2. Lined with cloth if needed for fine particles
3. Load with slurry, spin at 1,000–3,000 RPM
4. Liquid passes through perforations; solids build up on basket wall
5. Stop, scrape out solids

Construction: Weld a perforated cylinder from stainless steel sheet. Mount on a vertical shaft in a catch basin. Drive with belt from water wheel or motor.

Sedimenting Centrifuge

Solid bowl (no perforations) for liquid-liquid separation:

• Separate immiscible liquids of different densities
• Clarify liquids by spinning out suspended solids
• Concentrate slurries

Balancing

An unbalanced centrifuge is a bomb

At 5,000 RPM, a 10-gram imbalance on a 100 mm radius generates 280 N of force — oscillating. This tears bearings apart, cracks welds, and can launch fragments. Balance your rotor before every speed increase.

Static Balancing

1. Mount the rotor on knife edges (two parallel, level steel bars)
2. The heavy side rolls to the bottom
3. Add a small weight (tape, putty, or a setscrew) to the light side
4. Repeat until the rotor sits still in any position

Dynamic Balancing

Static balance isn’t enough for high-speed rotors. Dynamic imbalance shows up as vibration only when spinning:

1. Run the centrifuge at low speed (10–20% of target)
2. Feel for vibration
3. Add trial weight to the rotor at a marked position
4. Run again — if vibration decreases, you’re on the right side
5. Adjust weight amount and angular position until vibration is minimized
6. Increase speed incrementally, recheck

Vibration Isolation

Even a well-balanced centrifuge vibrates slightly. Mount on:

• Rubber pads (best)
• Thick leather pads
• Spring isolators
• Heavy base plate (mass absorbs vibration)

Safety

Centrifuge failure modes

1. Rotor burst: The rotor flies apart at overspeed. Fragments travel at bullet velocity. Always operate behind a containment shield.
2. Bearing seizure: Causes sudden deceleration and extreme vibration. Maintain bearings religiously.
3. Imbalance: Progressive vibration that worsens until something breaks. Stop immediately if vibration increases during a run.

Containment Shield

Surround the rotor with a steel cylinder (3–6 mm wall thickness) capable of containing fragments. Minimum: the shield should be 2× the rotor wall thickness and extend above and below the rotor.

Safe Maximum RPM

The stress on a spinning ring: σ = ρ × r² × ω²

For mild steel (yield 250 MPa, density 7,800 kg/m³), maximum safe surface speed is roughly 200 m/s. For a 100 mm radius rotor: max ~19,000 RPM. Apply a safety factor of 3× — never exceed 6,300 RPM with this rotor.

What’s Next

With centrifuge capability:

• Process dairy products efficiently (cream, butter, cheese production)
• Separate chemical precipitates for pharmaceutical and industrial chemistry
• Perform medical blood analysis (hematocrit, infection screening)
• Clarify vegetable oils, fruit juices, and other food products
• Concentrate valuable minerals from slurries