Planetary roller screw

A planetary roller screw comprising:

• a main screw including an outer periphery having a first external thread extending along an axis of the main screw, wherein the axis of the main screw defines an axial direction;
• a nut including a through-hole extending along the axial direction, wherein the nut includes an inner periphery surrounding the through-hole, wherein the inner periphery has an internal thread extending along the axial direction, wherein all or a portion of the first external thread of the main screw is located in the through-hole of the nut;
• and
• a planetary assembly including plural rollers each extending along another axis parallel to the axis of the main screw, each roller includes an outer periphery having a second external thread, wherein at least a portion of each roller is located in the through-hole of the nut, and wherein the second external thread of each roller meshes with the first external thread of the main screw and the internal thread of the nut,
• wherein a pitch of the first external thread of the main screw, a pitch of the internal thread of the nut, and a pitch of the second external thread of each roller are identical, wherein a helix direction of the first external thread of the main screw is opposite to a helix direction of the internal thread of the nut, and the helix direction of the internal thread of the nut is the same as a helix direction of the second external thread of each roller;
• wherein one of the main screw and the nut is a driving member, and the other two of the main screw, the nut and the planetary assembly that are not the driving member are defined as two followers, wherein the planetary roller screw satisfies a rule of configuration, wherein when the driving member rotates, the driving member displaces relative to the two followers in the axial direction, and no relative displacement occurs between the two followers in the axial direction, and wherein the rule of configuration requires that a ratio of pitch circle diameters between the two followers and a ratio of numbers of thread starts between the two followers to be the same, and the number of thread starts of the nut to be not equal to a sum of the number of thread starts of the main screw and twice the number of thread starts of each roller.