Planetary gearbox, NEMA 17, 27:1
Bolt-on torque multiplication for a stepper. Cheap ones have 1–3° of backlash, which is fine for a camera axis and fatal for a precision arm.
The role it plays
Transmission trades speed for torque between the motor and the load. Every option sits on three axes: ratio, backlash and stiffness. Low ratio with a good encoder is how quasi-direct-drive works; high ratio buys torque but hides the load behind compliance and friction. Pick the ratio from the torque budget first, then the technology that hits it.
Typical use: Camera axes, low-precision joints, test rigs.
What the numbers actually mean
The headline figure is 27:1 (ratio), with ~1 – 3° (typical cheap unit) as the next most load-bearing number (backlash). At $25 in the transmission tier it sits cheap enough to prototype with and replace without regret, which is the correct way to learn a new subsystem.
How it compares
Same subsystem, closest alternatives. The primary spec column is usually the one that decides it.
| Part | Price | Primary spec | Second spec |
|---|---|---|---|
| Planetary gearbox, NEMA 17, 27:1Planetary reducer | $25 | 27:1 | ~1 – 3° (typical cheap unit) |
| Harmonic Drive CSF-14-100Strain-wave reducer | $1.2k | 100:1 | < 1 arcmin |
| GT2 belt drive, 20T → 60TBelt reduction | $15 | 3:1 (20T driver, 60T driven) | GT2, 2 mm |
| T8 lead screw, 8 mm, 2 mm leadLinear conversion | $12 | 2 mm / rev | up to 500 mm typical |
| Printed cycloidal reducer3D-printed reducer | $5 | 20:1 – 40:1 (by design) | 0.5 – 2° depending on print quality |
Where it shows up
It is a component-level choice rather than a defining part of a named platform — pick it for the subsystem it serves, not to match a reference build.