Geared CVT Dynamics

The Concept

Italian company RatioZero describes their system as the "world's first geared CVT," based on a 2016 patent by Edyson Pavilcu. The mechanism achieves continuously variable transmission through a planetary gear set in which an adjustable linkage offset between the carrier and the planet gear axes modifies the transmission ratio in real time — including a zero-ratio state, which acts as an integrated clutch.

The topology differs from conventional CVTs (belt-based, toroidal, or hydraulic) in that all torque is transmitted through hard gear contacts rather than friction interfaces, which in principle allows higher power density and better efficiency than belt-based alternatives.

Simulation

The dynamic model is composed of rigid bodies interconnected by gear contacts and revolute joints. Several of these joints are unidirectional — without them, the output gear would not rotate in a controlled direction. The simulation sweeps the linkage offset continuously from zero to approximately 2.5, covering the full ratio range.

For visualization clarity, Pavilcu's internal (ring) gear at the output was replaced by an external sun gear; the operating principle is unchanged. Red markers at the planet gear joints indicate blocked rotation events that enable eccentric torque transfer between planets and output.

The transmission ratio as a function of linkage offset is approximately u × R / Rs, where u is the ratio between the input ring gear and the first planet gear, R is the offset distance, and Rs is the output gear pitch radius. This simple expression captures the mean ratio well, but the variation of the ratio — the instantaneous oscillation around the mean — requires dynamic simulation rather than geometric analysis.

Key Trade-offs

Transmission ratio variation is a significant design challenge. RatioZero addresses this with elliptical gears in their latest design. An alternative approach — increasing the number of planet gears — reduces variation but constrains the orbital radius, limiting the maximum achievable transmission ratio. High power losses relative to conventional parallel-axis gearboxes represent a further design constraint, though the system may rank among the more efficient CVT solutions currently available.

The mechanism's relative simplicity, integrated zero-ratio clutch, and broad range make it technically interesting as an alternative to belt-based CVTs in applications where gear contact efficiency and compactness are priorities.