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Tube Centrifuge

4 min read

Parent
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Hand-Cranked Centrifuge
Simple designs achievable with basic tools
Current
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Tube Centrifuge
Swinging-bucket design for laboratory use

A swinging-bucket tube centrifuge holds sample tubes in pivoting buckets at the ends of a rotor arm so they swing horizontal under load. The carrier’s 10:1 drive at a 60 RPM crank gives a 100 mm radius only about 40 g, which is enough for crude settling but not the 1,000-3,000 g serum separation needs; that takes about 2,990 to 5,180 RPM, which at a 60 RPM crank is a 50:1 to 86:1 drive.

Part of Centrifuge Building

A tube centrifuge is a balanced arm with pivoting tube holders at each end, spun fast enough that the contents of each tube separate by density along the tube’s length.

The Gearing Decides Whether It Works At All

The carrier article’s build uses a 10:1 drive, so a 60 RPM crank turns the rotor at 600 RPM, and with a 100 mm radius that is 1.118 × 10⁻⁵ × 10 × 600² = 40 g. That is a real separation but a slow one. Blood-serum separation is quoted at 1,000-3,000 g in the carrier. Invert the formula in Centrifugal Force Calculations:

  • 1,000 g at r = 10 cm: N = √(1,000 ÷ (1.118 × 10⁻⁵ × 10)) = √(8.944 × 10⁶) = 2,990 RPM. Drive ratio: 2,990 ÷ 60 = 50:1.
  • 3,000 g: N = √(3,000 ÷ 1.118 × 10⁻⁴) = √(2.683 × 10⁷) = 5,180 RPM. Drive ratio: 5,180 ÷ 60 = 86:1.

So the carrier’s mention of 30:1 or 50:1 for higher g is the correct order for the lower end, and one 10:1 stage is not. A drive of that size takes several stages; see Speed Multiplication.

Worked Example: What The Tubes And Pins Feel

  • Imbalance. If the two tubes at 3,000 RPM differ by 1 g, F = m × ω² × r = 0.001 × 314.2² × 0.1 = 9.9 N, about the weight of a 1 kg mass, swinging around with the rotor.
  • Pin load. A bucket plus filled tube weighing 40 g (an assumption) at 1,000 g presses on its pivot pins with 0.040 × 9.81 × 1,000 = 392 N, about 40 kg-force. Two 4 mm steel pins in double shear have a combined area of 2 × 2 × π × 0.002² = 5.0 × 10⁻⁵ m², so the shear stress is 392 ÷ 5.0 × 10⁻⁵ = 7.8 MPa, small against the 250 MPa yield used in Speed Limits and Material Stress. The weak point is more often the bucket, the tube or the holes in the arm than the pin.
  • Rim speed. At 5,180 RPM and 0.1 m, v = 0.1 × 542 = 54 m/s, safely under the limit derived in the speed-limits article.

What You Need

  • Steel bar for the rotor arm, 200-300 mm total length (carrier), machined from solid for balance
  • Pivot pins and bucket holders from bent steel strip
  • A central shaft of 8-10 mm hardened steel and two ball bearings in a housing
  • A drive; see Hand-Cranked Centrifuge
  • Tubes (glass or plastic); glass is made as in Laboratory Ware

Method: Build And Run

Step 1: Machine the arm from one piece so its two halves are equal. Drill the pivot holes the same distance from the axis at each end.

Step 2: Fit buckets that swing freely and come to rest horizontal at speed.

Step 3: Mount on the shaft with a press fit or keyway and check it spins true by hand.

Step 4: Balance statically and dynamically; see Static Balancing.

Step 5: Load tubes in opposite pairs, matched by weight to better than the tolerance you can balance to, topping up the lighter with water.

Step 6: Run up slowly, hold for the time needed, let the rotor stop fully, then unload gently so the separated layers do not remix.

Common Mistakes

  • One tube only. An unpaired tube is a pure imbalance.
  • Tubes that are not capped. They spray the contents inside the chamber.
  • Starting at full speed. Run up in stages and watch for vibration.
  • Stopping the rotor by hand. Wait for it, or use a brake.

Safety

Blood spills and breaking glass

Samples of blood can carry disease; wear gloves, cap the tubes, and treat any breakage as a contamination to disinfect (see Sterilization Methods). A glass tube that cracks at 1,000 g throws fragments. Spin behind a shield; the rotor and shield rules are in Containment Shield.

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