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Vibration Isolation

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Balancing and Vibration Control
The most critical factor in centrifuge safety and longevity
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Vibration Isolation
Mounting systems to absorb residual vibration

Mount a centrifuge on soft pads or springs so that its natural frequency on the mounts is far below the rotor’s running frequency, and most of the residual vibration never reaches the floor. Mounts that compress 5 mm under the machine’s weight have a natural frequency near 7 Hz, which for an 8,000 RPM rotor at 133 Hz passes under 1% of the shake, but the machine must run up through that resonance, so keep the run-up quick and limit the travel with stops.

Part of Centrifuge Building

Vibration isolation is the mounting of a machine on flexible supports tuned so that the shaking force the rotor leaves after balancing is absorbed by the supports instead of the floor and frame.

Mount Softly Enough That The Resonance Is Far Below The Running Speed

The carrier lists the options: rubber pads (best), thick leather pads, spring isolators and a heavy base plate whose mass absorbs vibration. A machine on a flexible mount behaves like a mass on a spring and has a natural frequency of its own. The isolation works when the shake frequency is well above that.

Worked Example: A Machine On Rubber Mounts

For a mass on a spring that compresses by δ under its own weight, the natural frequency is f_n = (1 ÷ 2π) × √(g ÷ δ). The compression is the easy thing to measure.

  • δ = 5 mm: f_n = (1 ÷ 6.283) × √(9.81 ÷ 0.005) = 0.1592 × 44.3 = 7.05 Hz, which is 423 RPM.
  • Rotor at 8,000 RPM: 8,000 ÷ 60 = 133.3 Hz, about 133 Hz. Frequency ratio r = 133.3 ÷ 7.05 = 18.9.
  • Transmissibility (undamped): T = 1 ÷ (r² − 1) = 1 ÷ (357 − 1) = 0.0028, so 0.28% of the force reaches the floor. That is consistent with the rubber-mount figure for an engine in Vibration Isolation, which finds a 5 mm compression resonating near 7 Hz and cutting 100 Hz vibration by 99%. (That article is about vehicle engines; the mathematics is the same.)
  • The cost. Every run-up and run-down passes through 7 Hz, 423 RPM. At resonance the motion is amplified by about 1 ÷ (2ζ), where ζ is the damping; for ζ = 0.05, a value from general knowledge for rubber, that is a factor of 10. For a machine of 30 kg (an assumption) and a residual unbalance of 7.5 × 10⁻⁵ kg·m (1 g at 75 mm), the motion at resonance is about (7.5 × 10⁻⁵ ÷ 30) × 10 = 25 µm, harmless. For 10 g, 0.25 mm. The danger is not the residual imbalance, but a badly unbalanced rotor or a slow, hand-driven run-up that lingers near resonance.
  • Mount stiffness. For 30 kg at 5 mm: k = m × g ÷ δ = 30 × 9.81 ÷ 0.005 = 58,900 N/m in total, split among the mounts.

A Heavier Machine Moves Less

The motion at a given unbalance falls in proportion to the mass of the machine. A heavy base plate under the same mounts lowers the amplitude and also lowers the natural frequency, a double gain. This is the reason to bolt the centrifuge to a heavy plate of concrete or steel and put the plate on the mounts.

What You Need

  • Rubber pads or springs rated for the weight, with the compression you want under load
  • A heavy base plate to carry the machine, and stops that limit its sideways and upward travel
  • A tether or restraint that stops the machine leaving its mounts if the rotor fails
  • A ruler to measure the compression under load

Method

Step 1: Weigh the machine and base and choose the compression: 5 mm gives 7 Hz, and more compression gives a lower frequency, with a limit set by stability.

Step 2: Spread the load evenly over at least three or four mounts, so none carries much more than its share.

Step 3: Measure the compression with the machine in place and adjust the number or hardness of the pads.

Step 4: Fit stops that allow a few millimeters of travel, not more.

Step 5: Run up promptly through the low-frequency resonance; do not hold the speed near 423 RPM, and keep a record of it.

Common Mistakes

  • Mounts that are too stiff. The resonance sits close to the running speed and the vibration is amplified, not reduced.
  • Rubber that is old, hot or oiled. It hardens or swells; check yearly.
  • No tether. A soft mount lets the machine travel when something fails.
  • A rigid pipe or shield bolted to the floor. It bridges the isolation.

Safety

A soft mount lets a failing machine move

Isolation reduces the shake but gives the machine freedom to move: an out-of-balance rotor can walk the machine off its mounts, and a rotor burst can throw it. Use stops and a tether, and fix the containment shield to the floor, not to the isolated machine. See Containment Shield.

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