Rotational anchoring of drill tool components

A rotational anchor mechanism is mounted on an operatively nonrotating housing that forms part of a drill string. An anchor linkage forming part of the anchor mechanism is radially extendable and contractible to move an anchor member, such as a roller, mounted on the anchor linkage radially towards and away from the housing, to engage a borehole wall for resisting rotation of the housing relative to drill pipe that is drivingly rotated within the housing. The anchor linkage includes operatively coupled mounting links mounted on the housing to pivot about respective mounting axes which are parallel to one another in a fixed spatial relationship. An actuating mechanism is coupled to the anchor linkage to exert an actuating force on the anchor linkage. An angular orientation of the actuating force relative to the housing varies in response to variation in radial expansion of the anchor linkage.