Abstract: A self-centering, self-aligning, anti-friction power transmission mechanism comprising a rotor cooperable with a screw to convert rotary movement to linear movement and vice versa. The rotor has one or more convolutions of rollers having their axes arranged in a helical path of the same pitch as the screw thread with their inner ends positioned to have non-scuffing, non-slipping rolling line contact with the screw thread, a result obtained by off-setting each roller axis downwardly along the helix from the line of contact by a distance such that the line of contact and the rotor axis lie in a common plane parallel to the roller axis. The screw thread is preferably of the Acme type with a sidewall taper initially slightly less than the taper of the roller ends in contact therewith whereby, in use under load, the rollers cause cold flow and work hardening of the thread sidewall thereby substantially increasing the length of the line of contact between the rollers and the screw thread.