A power transmission system for an aircraft turbomachine, comprising: - a speed reducer comprising a sun pinion which can rotate about an axis X, a ring gear which is mobile in rotation about the axis X, and planet pinions carried by a planet carrier, meshing with the sun pinion and the ring gear, said ring gear comprising two half-ring gears (40a, 40b) each carrying teeth for meshing with the planet pinions and comprising radially oriented external annular flanges (42a, 42b) which are tightened against each other by first attachment means, which pass through said flanges (42a, 42b), axes Y of said first attachment means being parallel to the axis X and distributed over a circumference of diameter D1, - an annular channel for recovering centrifuged oil, extending around said flanges (42a, 42b), this channel having in axial cross-section a general U shape surrounding the flanges (42a, 42b) and comprising an annular bottom wall of axis X connected to two annular lateral walls (66a, 66b) substantially perpendicular to the axis X, characterised in that: - the annular lateral walls (66a, 66b) of the channel comprise internal peripheral edges (72a, 72b) having a predetermined internal diameter D2 less than the diameter D1, - the bottom wall and the lateral walls (66a, 66b) of the channel are arranged, respectively, with respect to external peripheral edges (43a, 43b) of the flanges (42a, 42b) and with respect to the first attachment means with predetermined clearances J1 and J2, - said predetermined internal diameter D2 and said predetermined clearances J1, J2 of the channel being configured such that the channel covers the flanges (42a, 42b) and the first attachment means, - and in that the channel is formed by an axial assembly of at least two parts (74a, 74b), a first part of which comprises at least one portion of the bottom wall, and a second part of which comprises one of the lateral walls. - a speed reducer comprising a sun pinion which can rotate about an axis X, a ring gear which is mobile in rotation about the axis X, and planet pinions carried by a planet carrier, meshing with the sun pinion and the ring gear, said ring gear comprising two half-ring gears (40a, 40b) each carrying teeth for meshing with the planet pinions and comprising radially oriented external annular flanges (42a, 42b) which are tightened against each other by first attachment means, which pass through said flanges (42a, 42b), axes Y of said first attachment means being parallel to the axis X and distributed over a circumference of diameter D1, - an annular channel for recovering centrifuged oil, extending around said flanges (42a, 42b), this channel having in axial cross-section a general U shape surrounding the flanges (42a, 42b) and comprising an annular bottom wall of axis X connected to two annular lateral walls (66a, 66b) substantially perpendicular to the axis X, - the annular lateral walls (66a, 66b) of the channel comprise internal peripheral edges (72a, 72b) having a predetermined internal diameter D2 less than the diameter D1, - the bottom wall and the lateral walls (66a, 66b) of the channel are arranged, respectively, with respect to external peripheral edges (43a, 43b) of the flanges (42a, 42b) and with respect to the first attachment means with predetermined clearances J1 and J2, - said predetermined internal diameter D2 and said predetermined clearances J1, J2 of the channel being configured such that the channel covers the flanges (42a, 42b) and the first attachment means, - and in that the channel is formed by an axial assembly of at least two parts (74a, 74b), a first part of which comprises at least one portion of the bottom wall, and a second part of which comprises one of the lateral walls.