To avoid load inhomogeneity when a radial force is acted on a screw axis and improve durability and silentness.
A top-type ball screw 20 is provided with a screw axis 28 having a male screw groove 28 on the outer circumferential surface, a ball nut 24 having a female groove 24a on the inner circumferential surface, a plurality of balls 72 housed rotatable and movable between the female groove 24a and the male screw groove 28, and four tops 70a-d fixed to the ball nut 24. The tops 70a-d guide the balls 72, etc. The screw groove 24a of the ball nut 24 is divided into four circulation passages 74a-d. If the nominal diameter of the balls used when the ball diameters of all the ball groups 72a-d are the same is d, the ball diameters of the balls 72a, 72d housed in the first and fourth circulation passages 74a, 74d are d1, and the ball diameters of the balls 72b, 72c housed in the second and third circulation passages 74b, 74c are d2, relationships d1=d and 1.005 d1≤d2≤1.1 d1 are provided.
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ISHIGAMI TOSHIYA
FUKUMURA KENICHI
