A rolling element cage for positioning a plurality of rolling elements between two rail elements of a telescopic rail or a linear guide which are mounted so as to be movable relative to each other is provided. The rolling element cage is composed or can be composed of two or more module elements which include a base module and one or more extension modules. Each module element has at least two receiving legs, each having one or more openings for receiving rolling elements, and at least one connecting section connecting the two receiving legs, the base module and the one or more extension modules being fixedly or releasably connected or connectable to one another in series in the longitudinal direction of the rolling element cage via fasteners.

One aspect of the present disclosure provides a strain wave gear device including an internal gear, an external gear and a wave generator. The wave generator has a cam and a fourth bearing. The cam has a non-circular outer peripheral surface, and the fourth bearing is positioned between the inner peripheral surface of the external gear and the outer peripheral surface of the cam. The fourth bearing has an outer ring, an inner ring, a plurality of balls, and a retainer. The strain wave gear device includes a restricting portion provided such that the restricting portion can avoid touching the retainer. The restricting portion can restrict the fourth bearing from moving in the direction extending along the axis of rotation.

An arc-shaped segment (1) for assembling a segmented bearing cage. An arc length L and a radian angle of the arc-shaped segment (1) satisfy the following relationship: 360L/D20360L/D+30 with D being a diameter of the segmented bearing cage. The arc length L refers to an arc length corresponding to the radian angle of the arc-shaped segment (1). A segmented bearing cage comprising a plurality of arc-shaped segments (1). A bearing comprising the segmented bearing cage. After being assembled in the bearing, a plurality of arc-shaped segments abut on or connect with each other, to form a complete cage.