Ultracentrifuge

Overview

An ultracentrifuge spins samples fast enough to sort molecules by density. Where a benchtop centrifuge pellets cells at a few thousand g, an ultracentrifuge reaches a million times gravity, enough to sediment viruses, ribosomes, proteins and the bands of a density gradient. It is a staple of molecular biology and virology, used to purify particles and to separate things that differ only slightly in mass.

Reaching those speeds safely is the whole engineering problem. A rotor turning at 150,000 rpm stores enormous energy, runs in a near-vacuum to avoid burning up on air friction, and must be held cold so the sample is not cooked by the little heat that remains. The machine is built around containing that energy and controlling that environment.

How it works

The High-Speed Drive turns the rotor through a slender flexible spindle. The upper end rides on a Magnetic Bearing that levitates the shaft on a controlled magnetic field, so at full speed almost nothing touches and wear is negligible; a touchdown bearing catches the shaft only if power is lost. A variable-frequency inverter ramps the motor smoothly through the critical speeds where the flexible quill lets the rotor self-center.

The sample sits in the Rotor & Buckets, a machined titanium rotor whose swinging buckets pivot horizontal under load so the density gradient forms along each tube. The rotor spins inside the Armored Vacuum Chamber, a steel-armored bowl that would contain the fragments if a rotor ever failed, sealed by an interlocked lid that cannot open while there is any rotation.

Air drag is removed by the Vacuum System, a backing pump roughs the chamber and a diffusion pump pulls it to high vacuum, and the heat that remains is carried off by the Refrigeration System, whose evaporator coil chills the chamber liner to hold the set temperature. The Sensor Suite watch speed, imbalance and rotor temperature, aborting instantly on a mis-loaded rotor, while the Control & Display runs the speed, time and temperature program behind the safety interlocks.