Torque enters through the sun gear, which drives the planets. The planets simultaneously mesh with two ring gears: one is grounded to the housing (reaction ring), the other is coupled to the output shaft. The key is the small tooth-count difference between the fixed and the output ring gear โ for each orbit of the planets the output ring advances relative to the fixed one by only that difference, producing a very large speed reduction (a differential/Wolfrom principle: superposing a 'minus' and a 'plus' sub-train sharing one carrier). Planets may be single-rim or compound/stepped (two toothed rims on a common shaft). Because the carrier transmits no external torque, carrier-less designs are also possible, with the motor embedded inside the hollow sun gear.
Robot actuators need high reduction and high torque density in a small, coaxial package. Harmonic (strain-wave) drives meet these needs but can be costly, exhibit hysteresis and torque ripple, and are usually non-backdrivable. The 3K/Wolfrom type delivers very high single-stage reduction with a simpler, fully rigid gear set and intrinsic backdrivability.
Central gear driving the planets; typically the input, often hollow to embed the motor.
Gears orbiting the sun, meshing simultaneously with both ring gears; single-rim or compound/stepped.
Official
Ring gear grounded to the housing, providing the reaction reference for the reduction.
Ring gear with a slightly different tooth count than the reaction ring; coupled to the output shaft.
Holds the planet axes but is not a working member; enables carrier-less designs.
At very high ratios the recirculating (virtual/latent) power grows, sharply reducing efficiency (down to ~50-60% without optimization).
Selecting the tooth counts of both rings and the planets is subject to strict coaxiality and assembly conditions.
Planetary gear trains typically show 4-6 arcmin of lost motion vs <1 arcmin for harmonic drives โ lower positioning precision and stiffness.
Karaivanov and Arnaudov present design simplifications (fewer gears), e.g. replacing a 3-stage 15-gear train with a 6-gear Wolfrom.
'Compact Gearboxes for Modern Robotics' review positions the 3K/Wolfrom type as an alternative to harmonic and cycloidal drives.
Crispel et al. present a novel Wolfrom-based gearbox for robotic actuators, addressing harmonic-drive cost and hysteresis.
The VUB team (R2poweR) demonstrates a high-ratio, backdrivable Wolfrom reducer for safe physical human-robot interaction.
Primary determinant of the ratio โ the smaller the difference, the higher the reduction.
Affects load sharing and torque density.
Determines the ratio range and manufacturing complexity.
Key to efficiency; mitigates recirculating-power losses.
