The invention relates to a rotary mechanism comprising a first component, such as a hub base of a wind turbine hub, and a second component, such as a blade base of a wind turbine blade, which are configured for coaxial rotation relative to one another around a first axis (206). The first component (202) and the second component (204) are coupled via a mechanical linkage that is configured for constraining a relative movement of the first component and the second component to the coaxial rotation around the first axis. The mechanical linkage comprises: a first rod (304); a second rod (306); a swiveling member (308); and a swiveling shaft. The swiveling shaft has a swiveling axis (312). The swiveling member has a top part (314) and a bottom part (316). The top part of the swiveling member has a first end (318) and a second end (320), different from the first end. The bottom part (316) is mounted to the swiveling shaft for swiveling around the swiveling axis. The swiveling axis (312) and the first axis (206) span a specific plane that is stationary with respect to a specific one of the first component and the second component. The first rod (304) is coupled between the first end (318) of the top part of the swiveling member and a first portion of the specific one of the first component and the second component. The second rod (306) is coupled between the second end (320) of the top part of the swiveling member and a second portion of the specific one of the first component and the second component, different from the first portion. The mechanical linkage is a modified embodiment of the Roberts' linkage, so as to allow relative rotation between the first and second components while keeping a spacing between first and second components constant.