A hand-cranked centrifuge turns a crank at about 60 RPM and multiplies it by gears or belts up to the rotor speed, which means a 50:1 step-up for a 3,000 RPM laboratory rotor and about 133:1 for an 8,000 RPM cream-separator bowl. One person can supply only tens of watts, so a hand machine suits tubes and small separators, and a large or continuous job needs a water wheel, treadle or animal to drive it.
Part of Centrifuge Building
A hand-cranked centrifuge is a rotor driven by a human-powered crank through a speed-multiplying drive, simple enough to build with ordinary workshop tools and run without electricity.
The Rotor Needs Far More Speed Than A Hand Can Give
A person turns a crank at about 60 RPM (the carrier article’s figure), and the rotor needs thousands. The gap is bridged by the drive, which is the subject of Speed Multiplication. Everything else in a hand machine follows from that and from how little power a hand has.
Worked Example: How Much A Hand Can Do
A person can sustain only some tens of watts at a hand crank over minutes; 50 W is used here as an assumption, a general-knowledge figure and not one from this wiki.
- Crank torque. At 60 RPM, ω = 6.28 rad/s, so torque = 50 ÷ 6.28 = 8.0 N·m. With a crank handle 0.15 m from the axis the hand pushes 8.0 ÷ 0.15 = 53 N, about the weight of 5.4 kg.
- Torque at the rotor. At 8,000 RPM (ω = 837.8 rad/s) 50 W gives 50 ÷ 837.8 = 0.060 N·m. The gearing has divided the crank torque by the ratio of 8,000 ÷ 60 = 133, which agrees: 8.0 ÷ 133 = 0.060. Real gears lose some power, so the torque at the rotor is a little less.
- Spin-up energy. A tube rotor with buckets and tubes weighing about 1 kg with its mass at about 0.08 m has moment of inertia I = m × r² = 1 × 0.08² = 0.0064 kg·m². At 3,000 RPM (ω = 314.2 rad/s) its energy is ½ × 0.0064 × 314.2² = 316 J. At 50 W that is about 6 seconds before losses.
- Machines that run continuously need more. A cream separator’s power is set by the milk flow; see Continuous Flow System, where 100 liters an hour comes out above what a hand can sustain.
Three Hand Designs
| Design | Rotor | Typical use | Article |
|---|---|---|---|
| Swinging-bucket tube rotor | 200-300 mm arm | Blood, samples | Tube Centrifuge |
| Pull-string disc | Disc on a loop of string | Field medical sample | String-Pull (Paperfuge) |
| Bowl and disc stack | 150-200 mm bowl | Cream from milk | Cream Separator |
What You Need
- Steel for the rotor and shaft, a lathe for round, concentric parts; see Lathe Fundamentals
- Two good bearings; see Bearings and Bushings
- A gear or belt train; see Gear Ratios and Belt Drives
- A rigid frame on a heavy base, a shield and a brake; see Containment Shield
Method: Design Order
Step 1: Fix the job and the g-force. From Centrifugal Force Calculations, choose the rotor radius and the speed.
Step 2: Check the rotor against its speed limit with Speed Limits and Material Stress before drawing anything else.
Step 3: Choose the step-up and number the stages; see Speed Multiplication.
Step 4: Build rigid, and balance before the first full-speed run; see Balancing and Vibration Control.
Step 5: Fit a brake and a stop you can reach, and write the coast-down time on the frame after you have timed it.
Common Mistakes
- Designing the rotor first and the drive afterward. The drive limits what the rotor can be.
- A light frame. Vibration moves it across the floor.
- No way to stop. A rotor at speed keeps turning; the crank keeps turning with it.
- Cranking faster than designed. A person can easily crank at 90 RPM instead of 60, which is 50% over the designed speed and 2.25 times the stress in the rotor.
Safety
The crank keeps turning after you let go
A step-up train works backwards too: the spinning rotor drives the crank handle. With a ratio of 133, a bowl coasting down from 8,000 RPM turns the handle at 60 RPM, hard enough to break a wrist if it catches your hand. Do not hold or grab the crank until the rotor has stopped, fit a freewheel or a brake, and keep your hands clear of gears and belts.
What’s Next
- Learn next: Speed Multiplication — getting the ratio
- Combine with: Safety Considerations — the rules of the whole family