a slider assembly configured to move on a slide rail along an axial direction without limitation in distance is provided to solve the problems of the middle retaining members of a conventional circulation maintaining device , e . g ., insufficient connection rigidity and inconvenience in manufacture and assembly . the positioning portion of a middle retaining member and the positioning portion of the protrusion of a slider are connected in a direction perpendicular to both the axial direction and a transverse direction in which the protrusion projects , thereby connecting the middle retaining member and the protrusion securely and evenly along their entire lengths . the middle retaining member and the protrusion also feature ease of manufacture and assembly .