A centrifuge rotor is balanced when its center of mass lies on its axis of rotation, and the cost of failing is the force F = m × r × ω², which grows with the square of the speed. A 1 g imbalance at 75 mm radius pulls on the bearings with 3.3 N at 2,000 RPM, 13.2 N at 4,000 RPM and 52.6 N at 8,000 RPM, so balance statically and then at speed, and recheck after every speed increase.
Part of Centrifuge Building
Balancing a centrifuge means moving or adding mass until the rotor’s center of mass sits on its axis, so that spinning produces no rotating force that shakes the bearings and frame.
The Carrier Calls Balance The Most Critical Factor For Safety And Longevity
The reason is the square. An unbalance U (mass times its distance from the axis) produces a force F = U × ω² that rotates with the rotor and hits the bearings once per turn. Speed up by 4 times and the force goes up 16 times.
Worked Example: One Gram At 75 Mm
U = 1 g × 75 mm = 0.001 kg × 0.075 m = 7.5 × 10⁻⁵ kg·m.
| Speed | ω² | Force F = U × ω² |
|---|---|---|
| 2,000 RPM | 43,858 | 3.3 N |
| 4,000 RPM | 175,432 | 13.2 N |
| 8,000 RPM | 701,838 | 52.6 N |
Quadrupling the speed from 2,000 to 8,000 gave 52.6 ÷ 3.3 = 16 times the force. The 52.6 N is a force and weighs about 5.4 kg (52.6 ÷ 9.81). It reverses direction every half turn, a fatigue load on bearings and welds as well as a shake.
The carrier’s example is 10 g at 100 mm and 5,000 RPM: 0.010 × 0.1 × 523.6² = 274 N, about 28 kg-force.
Two Kinds Of Imbalance
- Static imbalance: the center of mass is off the axis. The rotor rolls to a heavy side when set on level supports. See Static Balancing.
- Dynamic imbalance: the center of mass is on the axis, but unequal masses at different points along the axis make the rotor rock like a wobbling shaft. It shows only when spinning. See Dynamic Balancing.
A short wide disc is mostly a static problem; a long shaft with two bowls is both. Whichever you have, do the static balance first (it is cheap and removes the larger part) and the dynamic second.
What You Need
- Level supports for static balance: two hardened bars, as in Static Balancing
- Trial weights and a way to fix them securely
- A fixed pointer or chalk to see the vibration
- A scale that reads in tenths of a gram for the small corrections
- A shield; see Containment Shield
Method: The Order
- Make the parts symmetric to begin with: turn the rotor from one piece, drill matching holes on matching circles, and use matched pairs of buckets or tubes.
- Static balance on knife edges until it stays still in any position.
- Dynamic balance at 10-20% of target speed; then step up.
- Isolate the machine, as in Vibration Isolation, to protect the floor and frame from what remains.
- Rebalance when anything changes: a new bowl, a replaced bearing, a new load pattern.
Common Mistakes
- Balancing empty and running loaded. In a tube centrifuge the tubes are part of the rotor; load them in matched pairs.
- Trusting static balance alone at speed.
- Balancing once and never checking. Wear, corrosion and sludge shift the mass.
- Continuing after vibration increases. It is the signal to stop.
Safety
A shaking centrifuge is telling you it is about to fail
Vibration that grows during a run, or that suddenly appears, means imbalance, a loose part or a failing bearing. Stop at once and let the rotor coast to rest; do not try to correct it while it spins. The mass you add to correct a balance is itself a part that can fly off at speed, so fix every weight by a clamp or screw and never by tape alone.
What’s Next
- Learn next: Static Balancing — the first step
- Combine with: Blade Balancing — the same technique on a wind rotor