Turning unit
A ball screw nut has a pair of through holes and a first passage that allows the through holes to communicate with each other and that opens radially outward. In each of the through holes, a ball screw mechanism has a circulation member having a second passage in which a ball can move between a ball raceway and the first passage. The ball screw mechanism has a circulation path for communicating the two connection points set on the ball raceway by the first passage and each of the second passages. A driven pulley has a thin portion that defines a radial clearance between an inner periphery of the driven pulley and an outer periphery of the ball screw nut. The ball screw mechanism has a pressing lid that presses down each of the circulation members from the outer periphery of the ball screw nut and that covers the first passage.
1. A turning unit comprising:
a turning shaft housed in a housing so that the turning shaft is reciprocable;
a belt mechanism having a driving pulley coupled to a motor, a driven pulley on an outer periphery of the turning shaft so that the driven pulley is rotatable, and a belt that is wound around between the driving pulley and the driven pulley; and
a ball screw mechanism that has a ball screw nut coupled to the driven pulley so that the ball screw nut is integrally rotatable, and a plurality of balls in a ball raceway that has a helical shape and is configured such that a screw groove on the outer periphery of the turning shaft faces a screw groove on an inner periphery of the ball screw nut, and that converts a rotation of the ball screw nut into reciprocating movement of the turning shaft, wherein
the ball screw nut has a pair of through holes extending through the inner periphery and an outer periphery of the ball screw nut, and a first passage that allows the through holes to communicate with each other and that opens radially outward,
the ball screw mechanism has a circulation member having a second passage through which the ball can move between the ball raceway and the first passage in each of the through holes,
the ball screw mechanism has a circulation path for communicating two connection points set on the ball raceway by the first passage and the second passages,
an inner periphery of the driven pulley has a thick portion and a thin portion,
a radial clearance bounded by the driven pulley at the thin portion and the outer periphery of the ball screw nut, and
the ball screw mechanism has a pressing lid that presses down each of the circulation members from the outer periphery of the ball screw nut and that covers the first passage.
2. The turning unit according to claim 1 , wherein the radial clearance extends over an entire circumference of the ball screw nut.
3. The turning unit according to claim 2 , wherein the pressing lid is a pipe that has a tubular shape and that is fitted to the outer periphery of the ball screw nut.
4. The turning unit according to claim 3 , wherein:
the ball screw nut has a recessed portion on an outer peripheral surface; and
the pipe is clinched at a position facing the recessed portion toward the recessed portion and is fixed to the ball screw nut.
5. The turning unit according to claim 1 , wherein the pressing lid is a curved plate that has a plate shape and that is curved so as to follow the outer periphery of the ball screw nut.
6. The turning unit according to claim 1 , wherein:
the driven pulley has on an inner periphery, a protruding portion protruding radially inward of the thin portion; and
the pressing lid is the protruding portion.
7. The turning unit according to claim 1 , wherein the outer periphery of the ball screw nut is a radially outermost portion of the ball screw nut where the ball screw nut and the driven pulley axially overlap.