A centrifuge separates a mixture by spinning it, so that the denser material is thrown outward and the lighter material collects nearer the axis, in seconds instead of the hours gravity needs. The pull on the mixture grows with the square of the rotation speed and in direct proportion to the radius, so doubling the speed gives four times the force; a 10 cm radius at 5,000 RPM gives about 2,795 times gravity.
Part of Centrifuge Building
Centrifugal separation is the use of rotation to replace gravity as the force that sorts a mixture by density, so that separation that takes hours at rest happens in seconds on a spinning rotor.
Gravity Sorts By Density Slowly; Rotation Does It Fast
Cream rises from milk because fat is lighter than the watery milk around it. In a bowl of milk at rest, the only force sorting the two is gravity, and it takes a day. Put the same milk in a rotor and spin it, and each bit of milk is pulled outward with a force far greater than gravity. The denser skim milk moves outward faster than the lighter fat, so the cream collects closer to the axis.
What is happening physically is that the mixture is trying to travel in a straight line while the rotor keeps turning it, and the container wall pushes it back. Nothing mysterious is flung out; the heavier material resists the turning more, and arrives at the wall first. The practical rule is the only one needed to build and use one.
The Rule: Force Depends On Speed Squared And Radius
The relative centrifugal force, in multiples of gravity, is RCF = 1.118 × 10⁻⁵ × r × N², with r in centimeters and N in revolutions per minute. See Centrifugal Force Calculations for the derivation. For a 10 cm radius at 5,000 RPM: 1.118 × 10⁻⁵ × 10 × 5,000² = 2,795 times gravity. Three consequences follow:
- Speed matters most. Doubling the RPM quadruples the force. Radius matters less: doubling the radius only doubles it.
- The force differs along the radius. Material at the rim feels more force than material nearer the axis, so “the g-force” always means “at this radius.”
- Strength is the limit. The same force that separates the mixture pulls on the rotor, and a rotor spun too fast breaks; see Speed Limits and Material Stress.
Which Machine For Which Job
| Job | Machine | Article |
|---|---|---|
| Blood, test tubes, small samples | Swinging-bucket tube centrifuge | Tube Centrifuge |
| Fat from milk, continuous | Disc-stack cream separator | Cream Separator |
| Solids out of a slurry | Basket or solid-bowl machine | Industrial Centrifuge |
| Field medical sample, no tools | String-pull paperfuge | String-Pull (Paperfuge) |
The cream separator and the laboratory centrifuge are the same machine at different duties, and the same formula covers both. The dairy side of the question is in Cream Separation Techniques.
What You Need
- A rotor, a shaft, bearings and a drive that gets the shaft to speed; see Hand-Cranked Centrifuge
- A way to balance the rotor; see Balancing and Vibration Control
- A shield and a rule for maximum speed
Common Mistakes
- Quoting RPM without a radius. RPM alone says nothing about the force; a small rotor and a large one at the same RPM differ many times over.
- Assuming more speed is always better. The rotor’s stress grows as the square of speed too.
- Thinking a centrifuge is a gentle tool. The same energy that separates also stores energy in the rotor.
Safety
A centrifuge is a stored-energy machine
A rotor at speed holds energy that can destroy bearings and throw fragments if it fails, and it keeps spinning after the drive stops. Never operate one without balance, a shield and a speed limit; the full set of rules is in Safety Considerations.
What’s Next
- Learn next: Centrifugal Force Calculations — the formula and its derivation
- Combine with: Separation Theory — why denser material settles faster