A step-up drive multiplies a slow crank into a fast rotor by chaining gear or pulley pairs whose ratios multiply, so 60 RPM becomes 8,000 RPM with a total ratio of 133:1, which three stages of about 5:1 (5 × 5 × 5 = 125, giving 7,500 RPM) come close to. Each stage divides the torque by its ratio, loses a few percent to friction, and runs backwards when the rotor coasts, so the crank handle keeps turning after you let go.
Part of Centrifuge Building
Speed multiplication is the use of gear or pulley chains to turn a slow input into a fast output, trading torque for speed in the same proportion.
Speed Up Means Torque Down, Stage By Stage
A gear pair or a pulley pair with a driver of D_in and a driven of D_out multiplies speed by D_in ÷ D_out and divides torque by the same factor (ignoring loss). The carrier article’s centrifuge needs a large step-up, and one stage cannot do it: Compound Trains notes that single gear pairs top out around 6:1 or 8:1 before the size difference gets absurd. Chaining stages multiplies the ratios.
Worked Example: A 133:1 Train
Target: a 150 mm separator bowl at 8,000 RPM from a 60 RPM crank. The total ratio is 8,000 ÷ 60 = 133:1.
- Three equal stages: the cube root of 133 is 5.1, so three stages of 5:1 give 5 × 5 × 5 = 125:1, and 60 × 125 = 7,500 RPM. That is 1.118 × 10⁻⁵ × 7.5 × 7,500² = 4,717 g at 75 mm, a little below the 5,000-8,000 g quoted for cream separation. To reach 8,000 RPM, make one stage 5.33:1.
- A possible build: a first stage of gears, 60 teeth driving a 12-tooth pinion (5:1); a second gear stage of the same (5:1); a final stage by belt with a pulley ratio of 5:1, for example 250 mm on a 50 mm pulley. The belt is last because the fast end runs at thousands of RPM, where it is quieter than gears and slips if the rotor jams; see Belt Drives.
- Efficiency. If each stage is 95-98% efficient (a general-knowledge range, not a wiki figure), the train as a whole is 0.95³ to 0.98³, which is 0.857 to 0.941.
- Torque at the rotor. If the hand supplies 8.0 N·m at 60 RPM (see Hand-Cranked Centrifuge), the rotor sees 8.0 ÷ 133 = 0.060 N·m at best, and less after losses.
Which Drive For Which Stage
| Stage | Choice | Why |
|---|---|---|
| Slow, high torque | Gears | No slip under the hand’s load; see Gear Ratios |
| Fast, low torque | Belt and pulleys | Quiet, forgiving, slips on a jam; see Belt and Pulley Systems |
| Speed reduction only | Worm gear | A worm gear gives large ratios in one step as a reduction (see Torque Multiplication), but a worm that locks cannot be driven by its wheel, so it does not work as a step-up |
What You Need
- Gear stock and gear-cutting tools, or salvaged gears of matching pitch
- Pulleys of different diameters and flat or round belt
- Bearings at every shaft; see Bearings and Bushings
- Lubrication; see Lubrication
- A rigid frame that holds the shafts parallel
Method
Step 1: Fix the total ratio from the target RPM and crank RPM.
Step 2: Split it into stages no larger than about 6:1 each.
Step 3: Choose small gears no smaller than the tooth count you can cut cleanly (12 teeth in the example).
Step 4: Mount every shaft in two bearings set as far apart as the frame allows.
Step 5: Check by hand, turning slowly and counting turns of each shaft against the ratio.
Common Mistakes
- Counting the rotor’s speed from the gear ratio alone. Slip in the belt reduces it; measure the speed.
- A fast shaft on bushings in a flimsy frame. It whines and wobbles.
- A tight belt. It loads the bearings; the belt need only be tight enough not to slip, as in Belt Drives.
- Forgetting the backward drive. See below.
Safety
A step-up train turns the crank when the rotor coasts
With a ratio of 133, a bowl spinning at 8,000 RPM turns the handle at 60 RPM when you stop cranking, and the stored energy of the bowl keeps it going for minutes. A handle that hits a hand at that speed breaks bones. Fit a freewheel between the crank and the train so the crank stops while the rotor coasts, guard every gear and belt, and never reach into the train while the rotor turns.
What’s Next
- Learn next: Tube Centrifuge — a use for a 50:1 train
- Combine with: Cream Separator — a use for a 133:1 train