Patent ID: 7121364

Claim:
A tractor for moving a component through a borehole, comprising: an elongate body; aft and forward gripper assemblies longitudinally movably engaged with said body, said aft and forward gripper assemblies each being hydraulically actuated and defining engagement surfaces configured to selectively engage an inner surface of the borehole; aft and forward propulsion assemblies configured to advance said body through the borehole relative to said aft and forward gripper assemblies, respectively; a gripper control valve having a first position in which said gripper control valve directs pressurized fluid to said aft gripper assembly and a second position in which said gripper control valve directs pressurized fluid to said forward gripper assembly; and aft and forward mechanically actuated valves positioned along said body and configured to detect advancement of said body relative to said aft or forward gripper assembly, respectively, each of said aft and forward mechanically actuated valves being actuated by mechanical contact between a portion of the mechanically actuated valve and a portion of one of the propulsion assemblies; wherein said aft and forward mechanically actuated valves are in fluid communication with said gripper control valve such that fluid pressure causes said gripper control valve to change positions after said body has completed an advancement stroke through the borehole relative to said aft or forward gripper assembly.