The invention relates to a self-propelled apparatus 1 for replacing main such as a gas main 2, comprising a single double acting hydraulic piston and cylinder combination 3 connected with slidable devices 4, 6 operable by the device for step-by-step movement of the apparatus through the main on alternate...http://www.google.es/patents/US4674914?utm_source=gb-gplus-sharePatente US4674914 - Replacing mains
