Electric linear motion actuator and electric disk brake system

An electric linear motion actuator comprising:

• a 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, and configured to be rotationally driven by an electric motor;
• a carrier supported on the rotary shaft so as to be rotatable about the rotary shaft;
• and
• planetary rollers mounted between a radially outer surface of the rotary shaft and a radially inner surface of the outer ring member, and rotatably supported by the carrier,
• wherein the outer ring member has a helical rib formed on the radially inner surface of the outer ring member and in engagement with 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 due to frictional contact between the rotary shaft and the respective planetary rollers, thereby moving the outer ring member in an axial direction,
• wherein the electric linear motion actuator further comprises a friction coupling arrangement configured to frictionally fixedly couple the carrier to the rotary shaft while an axial load applied to the outer ring member in a push-in direction is small, and to uncouple the carrier from the rotary shaft under the axial load when the axial load is large.