A planetary roller screw mechanism comprising: a screw comprising an outer circumference with an outer thread disposed thereon; a nut comprising an inner circumference with an inner thread disposed thereon, said nut being disposed around and positioned coaxially with the screw; and a plurality of rollers disposed around the outer circumference of the screw, between the screw and the nut; each of said rollers having an outer circumference with an outer thread disposed thereon, said roller thread engaging with said screw thread and said nut thread; and wherein each of said plurality of rollers has a V-shaped thread profile that has at least a portion that is convex extending between a trough and an adjacent peak of the roller thread profile; and wherein either one or both, of said screw and said nut has/have a V-shaped thread profile that has at least a portion (13', 23') that is concave extending between a peak (15', 25') and an adjacent trough (16', 26') of the screw or nut profile, and said convex portion of the roller thread profile has a convex ogive radius; said concave portion of the screw thread profile and/or the nut thread profile has a concave ogive radius and characterized in that said screw and nut concave ogive radii are twice the size of the roller convex ogive radius. a screw comprising an outer circumference with an outer thread disposed thereon; a nut comprising an inner circumference with an inner thread disposed thereon, said nut being disposed around and positioned coaxially with the screw; and a plurality of rollers disposed around the outer circumference of the screw, between the screw and the nut; each of said rollers having an outer circumference with an outer thread disposed thereon, said roller thread engaging with said screw thread and said nut thread; and wherein each of said plurality of rollers has a V-shaped thread profile that has at least a portion that is convex extending between a trough and an adjacent peak of the roller thread profile; and wherein either one or both, of said screw and said nut has/have a V-shaped thread profile that has at least a portion (13', 23') that is concave extending between a peak (15', 25') and an adjacent trough (16', 26') of the screw or nut profile, and said convex portion of the roller thread profile has a convex ogive radius; said concave portion of the screw thread profile and/or the nut thread profile has a concave ogive radius and characterized in that said screw and nut concave ogive radii are twice the size of the roller convex ogive radius. each of said rollers having an outer circumference with an outer thread disposed thereon, said roller thread engaging with said screw thread and said nut thread; and wherein each of said plurality of rollers has a V-shaped thread profile that has at least a portion that is convex extending between a trough and an adjacent peak of the roller thread profile; and wherein either one or both, of said screw and said nut has/have a V-shaped thread profile that has at least a portion (13', 23') that is concave extending between a peak (15', 25') and an adjacent trough (16', 26') of the screw or nut profile, and said convex portion of the roller thread profile has a convex ogive radius; said concave portion of the screw thread profile and/or the nut thread profile has a concave ogive radius and characterized in that said screw and nut concave ogive radii are twice the size of the roller convex ogive radius.