Electric linear motion actuator and electric disk brake system

An electric linear motion actuator comprising a cylindrical housing, an outer ring member mounted in the housing and having a center axis, a rotary shaft extending along the center axis of the outer ring member, a plurality of planetary rollers mounted between a radially outer surface of the rotary shaft and a radially inner surface of the outer ring member, a carrier supported on the rotary shaft so as to be rotatable about the rotary shaft and including as many roller shafts as the planetary rollers and rotatably supporting the respective planetary rollers, the roller shafts being movable in a radial direction of the rotary shaft, and elastic rings each having circumferentially separate ends and each fitted around the roller shafts so as to contact the respective roller shafts from radially outside, thereby biasing the respective planetary rollers against the radially outer surface of the rotary shaft, wherein the outer ring member has a helical rib formed on the radially inner surface of the outer ring member and in engagement with a helical groove or circumferential grooves formed on a radially outer surface of each of the planetary rollers, whereby when the rotary shaft rotates, the planetary rollers are configured to rotate about center axes of the respective planetary rollers while revolving around the rotary shaft, due to frictional contact between the rotary shaft and the respective planetary rollers, thereby moving the outer ring member and the carrier relative to each other in the direction of the center axis of the rotary shaft,

• wherein the electric linear motion actuator further comprises rotation stop arrangements provided between the respective elastic rings and the roller shafts and configured to prevent the respective elastic rings from rotating relative to the roller shafts in a direction of a circle circumscribed about the roller shafts.