A modular pipe-crawling robot for in-pipe maintenance operations in aspects of the present disclosure may have one or more of the following features: (a) at least two locomotion modules, (b) each module has feet which can extend outward to grip a wall of a pipe while simultaneously reducing its length or disengaging its feet from the inner wall while increasing its length, (c) a gear mechanism built into mechanical linkage, wherein each module's feet are held perpendicular with respect to the inner wall of the pipe, and (d) a joint coupling the at least two modules.