Linear actuator

1. A linear actuator comprising:

an output shaft formed with a helical groove;

a retainer having a cylindrical shape which receives the output shaft therein, and provided with a plurality of concave portions spaced from each other at positions of an inner peripheral surface thereof along the helical groove;

a pair of sleeves having a cylindrical shape and mounted on a circumference of the retainer from a front side and a rear side in an axial direction;

a rotor magnet fixed in surrounding relation to the retainer; and

balls which are retained in the plurality of concave portions, respectively,

wherein when the retainer is rotated, the balls are moved along the helical groove of the output shaft in a state where the balls are retained in the concave portions,

wherein the pair of sleeves have edge portions opposing each other, and each of the edge portions is provided with a concavo-convex structure,

wherein the retainer is divided into segments along lines extending substantially parallel to a longitudinal axis thereof such that it includes a first retainer segment, a second retainer segment, and a third retainer segment, and

wherein the third retainer segment is provided with a protrusion inserted between and in contact with the concavo-convex structures of the sleeves so that the protrusion positions the concavo-convex structures of the pair of sleeves relative to each other and prevents rotation of the pair of sleeves.

2. The linear actuator according to claim 1 ,

wherein the concavo-convex structures provided on the edge portions of the pair of sleeves are fixed to the retainer with an adhesive in a state where a concave portion of one of the sleeves opposes a concave portion of the other of the sleeves to define a gap therebetween, and a convex portion of one of the sleeves opposes a convex portion of the other of the sleeves, and

wherein the adhesive is filled in the gap.

3. The linear actuator according to claim 1 ,

wherein the first retainer segment and the second retainer segment have a same shape.

4. The linear actuator according to claim 1 , wherein each concave portion is fully defined within one of the segments.

5. The linear actuator according to claim 1 , wherein the concavo-convex structures provided on the edge portions of the pair of sleeves each include multiple concave portions and multiple convex portions such that more than one said gap is defined between opposing concave portions of the sleeves, the protrusion has a size and shape substantially the same as that of each of the gaps, and the protrusion is fitted within one of the gaps.