Abstract: This releasable screw and nut bearing mechanism comprises an internal central screw, a coaxial external nut constituting a socket made of several segments, the outer diameter of the screw being smaller than the inner diameter of the socket in order to provide a free gap therebetween in which a plurality of angularly spaced planet rolling members are disposed and adapted to mesh both with said screw and with said socket, this assembly being releasable by moving the socket segments radially outwards in order to free the screw from any meshing engagement with the rolling members, at any time, irrespective of the relative angular positions between the screw, the socket and the rolling members. Spring means may be provided for constantly urging the rolling members away from the screw, and cage means are provided for guiding the rolling members both axially and radially.