Linear motion mechanism and SCARA robot

A linear motion mechanism comprising:

• a shaft extending along a predetermined axis;
• a structural member provided with a cavity through which the shaft extends;
• a spline nut and a ball screw nut that are detachably attached to the structural member from both sides sandwiching the structural member via attachment flanges provided thereon and that support the shaft so as to be rotatable about the predetermined axis and so as to be movable in a direction of the predetermined axis, respectively, with respect to the structural member;
• a power transmission member fixed to, inside the cavity, at least one of the spline nut and the ball screw nut, the power transmission member having an outer diameter larger than an outer diameter of each of the attachment flanges provided on at least one of the spline nut and the ball screw nut to which the power transmission member is fixed;
• and
• an adapter detachably attached to the structural member and having a larger outer diameter than the power transmission member,
• wherein the structural member includes an opening configured to be opened by removing the adapter from the structural member, and
• the at least one of the spline nut and the ball screw nut is attached to the structural member via the adapter so that the power transmission member is taken out of the cavity through the opening when the adapter is removed from the structural member.