A tractor for moving within a borehole comprises an elongated tractor body and two propulsion assemblies that are longitudinally movably engaged with the body. The tractor body has annular pistons configured to receive hydraulic thrust to propel the body longitudinally. Each propulsion assembly includes...http://www.google.de/patents/US6241031?utm_source=gb-gplus-sharePatent US6241031 - Electro-hydraulically controlled tractor
