Centrifuge: Parts, Types, RCF vs. RPM, and How to Balance It Safely

Two things go wrong with centrifuges more than anything else, and neither is obvious from the dial. The first is invisible: a student follows a protocol that says "spin at 10,000 x g," sets the dial to 10,000 RPM, and gets a different result from the lab down the hall using a different machine, because RPM and x g are not the same number.

The second is impossible to miss: a centrifuge loaded with one tube on one side and nothing opposite begins to shake, then bang, then walk across the bench, and if it is not stopped it can wreck the rotor and injure someone. Both problems come from the same source, the enormous force a spinning rotor generates, and understanding that force is what this article is really about.

A centrifuge is an equipment that applies the principle of centrifugation to separate particles from the mixture. The particles in the mixture separates based on their density, i.e., the denser particle settles down as pellets in the container, and the lighter particle remains suspended as supernatant in the mixture.

Centrifugation separates particles based on their size, shape, density, and viscosity by centrifugal force. Centrifugal force is the force generated by the rotation of motors. The following factors influence the centrifugation:

• The density of the samples: The denser particles settle down, and particles with less density float or remain suspended in the solution.
• Viscosity and temperature: A more viscous (thicker) medium slows particles down, so separation takes longer or a higher force. Temperature matters the other way: the spinning rotor generates heat by friction, which can damage heat-sensitive samples such as proteins and nucleic acids, which is exactly why refrigerated centrifuges exist. Heat is a problem to control, not an aid to separation.
• The distance of particle displacement: The greater the distance of particle displacement, the greater the separation rate.
• The speed of rotation: The higher the speed, the faster the particles separate.

The force pushing particles outward and down is called the relative centrifugal force (RCF), also written as x g because it is expressed as multiples of gravity. Particles move outward and settle when this force exceeds the frictional resistance of the surrounding liquid. How RCF relates to the speed shown on the dial (RPM) is the single most important thing to understand about a centrifuge, and it is covered next.

Parts of a Centrifuge

The centrifuge machine has the following parts:

A motor

It is in the center that is very powerful and creates a spin.

A rotor

The rotor holds the tubes and is spun by the motor. Its radius matters, because the force on the sample depends on how far the tube sits from the center (see RCF vs. RPM below). Two rotor designs are common:

• Fixed-angle rotor: holds tubes at a set angle (often around 45°). Particles pellet against the side and bottom of the tube. Good for fast, hard pelleting of small volumes.
• Swinging-bucket rotor: the buckets hang vertically at rest and swing out to horizontal as the rotor spins, so the tube lines up with the force. This gives a flat pellet and clean layer separation, and it is used for density-gradient work and separating blood into layers.

Containers

These containers hold tubes with the materials/sample and it rests on the rotor.

Control

The types of control vary based on the centrifuge selected. Some are preprogrammed, and others are entirely customized with digital display. Regardless of the type of control, the centrifuge will run the motor based on the provided settings.

RCF vs. RPM: What the Dial Shows Versus What Acts on Your Sample

This is the concept students most often get wrong, and it matters for every protocol you follow.

RPM (revolutions per minute) is how fast the rotor turns. It is what most centrifuge dials display.

RCF (relative centrifugal force), written as x g, is the actual force experienced by the sample, expressed as multiples of gravity. This is what actually drives separation, and it is what protocols specify.

They are not the same, and the link between them is the rotor radius. A tube farther from the center travels a longer circle at the same RPM, so it experiences more force. That means the same RPM produces different RCF on different centrifuges with different rotor sizes. This is why a protocol that says "spin at 10,000 x g" cannot simply be set as "10,000 RPM", doing so gives a different force, and a different result, on every machine.

The conversion is:

RCF = 1.118 × 10⁻⁵ × r × (RPM)²

where r is the rotor radius in centimeters (center to tube bottom).

Worked example. A protocol calls for 3000 x g. Your rotor radius is 10 cm. Rearranging, RPM = square root of (RCF ÷ (1.118 × 10⁻⁵ × r)) = square root of (3000 ÷ (1.118 × 10⁻⁵ × 10)) ≈ 5180 RPM. On a machine with a 15 cm radius, the same 3000 x g needs only about 4230 RPM. Same force, different RPM, because the radius differs.

The rule: always follow the RCF (x g) in a protocol, not the RPM, and convert for your own rotor. Reporting a spin as "x g" makes it reproducible in any lab; reporting it as RPM does not.

Operation of Centrifuge

The operation of the centrifuge is easy. Steps of operating centrifuge:

1. Place the test tube with the sample into the container/portals.
2. Balance the samples, but insert water-filled tubes for balance when there is an odd number of samples.
3. Close the lid and select the required time and speed.
4. Start the centrifuge and wait for the cycle to complete.
5. Wait until the rotor has fully stopped on its own before opening the lid, then remove the tubes. The separated sample is now ready for analysis.

Why is balancing necessary?

At high speed, the rotor spins thousands of times a minute, and any unevenness in how weight is distributed around it is multiplied into a large, destructive wobble. A single unbalanced tube makes one side of the rotor heavier, so the spinning mass pulls harder on that side each revolution. The machine vibrates, then shakes violently, and can crack the rotor, break tubes, or move across the bench. In an ultracentrifuge the forces are large enough to cause serious damage or injury. Balancing keeps the center of mass on the axis of rotation, so there is nothing for the spin to amplify.

How to balance a centrifuge?

The ways to balance the centrifuge are as follows:

• Ensure that the tubes are evenly filled with liquid with a similar density.
• Match the mass of opposing tubes closely, balancing by weight, not just by eye. Balance by mass, not volume: two tubes of the same volume but different-density contents are not balanced.
• Place the tube just opposite one another to keep the gravity in the center.
• Use water to balance when the samples are in odd numbers.

Types of Centrifuge

There are different types of centrifuges based on optical detection, size, refrigeration capability, and rotor speed. The details of each type should be studied while choosing the perfect centrifuge.

Figure: Types of centrifuge

Benchtop centrifuges

Benchtop centrifuges are popular for their characteristic feature of requiring small space on the benchtop. Other features of benchtop centrifuges are the speed (RCF) of this centrifuge; it ranges from a few hundred to 50,000 x g and has interchangeable rotors; fixed angle, continuous flow, and swinging bucket rotors are available. Tubes of benchtop centrifuges range from less than 1 ml to a few liters.

Microcentrifuges

As the name suggests, the microcentrifuges accommodate tubes containing small/micro volumes of samples like 2 ml, 1.2 ml, 0.5 ml, and PCR (polymerase chain reaction) tubes. So, these apply most frequently in the microbiological laboratory for separating nucleic acid and proteins. It typically spins at speeds up to 16,000x g, but in the case of specialized forms, the rate can reach up to 30,000 x g. Some models also have interchangeable rotors and tube adaptors.

Vacuum centrifuges

Vacuum centrifuge uses centrifugal force, vacuum, temperature, and gas to remove liquid or gas from the sample to concentrate or desiccate the samples. Purifying nucleic acids, proteins, peptides, and other components used in research laboratories are some its uses.

Refrigerated centrifuges

A refrigerated centrifuge holds a set low temperature (commonly around 4°C, adjustable over roughly -20°C to +40°C depending on the model) while spinning at high speed. This protects heat-sensitive samples such as DNA, RNA, proteins, and antibodies from the heat that friction would otherwise generate. See the refrigerator article for why 4°C protects these samples.

These are also used for samples that need to be stored at a consistent temperature. These centrifuges must function at maximum speeds while maintaining a steady temperature.

Ultracentrifuges

Ultracentrifuges reach very high forces, typically up to about 100,000 x g and in some models 1,000,000 x g, used to separate viruses, subcellular organelles, and macromolecules.

Analytical centrifuges

Analytical centrifuges have different features that allow samples to detect as they spin in real-time. A light-based optical system like the light adsorption system, Rayleigh system, and the alternative Schlieren system helps observe the separated materials and monitor the speed.

Handling of Centrifuge

For proper functioning of the centrifuge, it requires constant care and maintenance. There are some measures to consider before, during, and after using the device to increase its lifespan. They are:

Lubricate the machine

The rings in the centrifuge are crucial as they prevent leaking from the sample. Hence, lubricating of rings after cleaning, rotor installation, and repairing with Hettich grease is necessary.

Regular repairing

The centrifuges should be checked regularly for any damage, unfamiliar noises, grinding and unnecessary vibration. The damages and dysfunctionality should be repaired regularly.

Regular cleaning

The rotors, chamber of rotors, the interior of the centrifuge, and electrical components are some of the parts that need constant cleaning using alcohol-based disinfectant and a soft cloth.

Laboratory Personnel

It should be ensured that the personnel using the centrifuge know how to operate it well. The tubes are balanced, and adjusting speed and compartment mass are things the operator should know before using the machine.

How to Remember

RPM is what it does, x g is what the sample feels. The dial shows revolutions; the sample feels force. Because force depends on how far the tube sits from the center, the same RPM means different force on different rotors. Always follow the x g in a protocol, and convert to RPM for your own machine.

Balance by mass, opposite each other, always. Every tube needs a partner of equal mass directly across the rotor. Odd number of samples? Add a water blank of matching mass. An unbalanced centrifuge is the one piece of routine lab equipment that can genuinely hurt you.

Let it stop by itself. Never open the lid or slow the rotor by hand while it is spinning. Wait for it to coast to a full stop.

Key Exam Facts in One Table

Where Students Get Confused

RCF versus RPM. The commonest error. RPM is the rotation speed on the dial; RCF (x g) is the force on the sample and depends on the rotor radius. A protocol's "x g" must be converted to the RPM for your specific rotor, not set as the same number.

Balancing by volume instead of mass. Two tubes with the same volume but different-density contents are not balanced. Balance by mass, and place equal-mass tubes directly opposite each other.

What happens to heat. Students sometimes think warmth helps separation. The heat from spinning is an unwanted side effect that can damage samples, which is why refrigerated centrifuges exist for DNA, RNA, and proteins.

Fixed-angle versus swinging-bucket. A fixed-angle rotor pellets fast and hard against the tube side; a swinging-bucket rotor lets tubes swing horizontal for a flat pellet and clean layer separation. The choice depends on whether you want a quick pellet or clean layers.

Opening too early. A spinning rotor holds enormous energy. Never open the lid or try to slow it by hand; wait for a full stop.

References

• Ohlendieck, K., & Harding, S. E. (2018). Centrifugation and Ultracentrifugation. In Wilson and Walker's Principles and Techniques of Biochemistry and Molecular Biology (8th ed.). Cambridge University Press.
• Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
• Cheesbrough, M. (2006). District Laboratory Practice in Tropical Countries, Part 2 (2nd ed.). Cambridge University Press.