Abstract: This invention relates to a mechanism employing fluid under pressure, comprising: a stator; a rotor; a device for assembling the rotor with respect to the stator, effecting relative rotational assembly of these two elements about a geometrical axis, and ensuring axial holding of said two elements in position, in the two opposite directions, parallel to said geometrical axis; a reaction cam fast with a first of said elements; cylinders made in the second element and disposed radially with respect to said geometrical axis; and pistons mounted in said cylinders and abutting on said cam. According to the invention, the assembly device is constituted by a single bearing which comprises: two rolling tracks made directly on an extension of the reaction cam itself, and on an extension of the element itself in which the cylinders are made, respectively; rolling members disposed between the tracks, capable of abutting on these two tracks so as to ensure axial holding of the rotor with respect to the stator.