Patent ID: 7353886

Claim:
A tractor assembly, comprising a tractor for moving within a borehole, said tractor configured to be powered by a pressurized operating fluid, said tractor comprising: an elongated body having a thrust-receiving portion longitudinally fixed with respect to said body, said body having an internal passage configured to receive the operating fluid; a gripper assembly longitudinally movably engaged with said body, said gripper assembly having an actuated position in which said gripper assembly limits relative movement between said gripper assembly and an inner surface of said borehole, and a retracted position in which said gripper assembly permits substantially free relative movement between said gripper assembly and said inner surface, said gripper assembly configured to be actuated by receiving operating fluid from said internal passage of said body; a valve system housed within said body, said valve system configured to receive operating fluid from said internal passage of said body and to selectively control the flow of operating fluid to at least one of said gripper assembly and said thrust-receiving portion; a pressure reduction valve; a first gripper fluid passage extending from said valve system to said pressure reduction valve; and a second gripper fluid passage extending from said pressure reduction valve to said gripper assembly; wherein said pressure reduction valve is configured to provide a flow path for operating fluid to flow from said first gripper fluid passage to said second gripper fluid passage when the pressure within said second gripper fluid passage is below a threshold, and wherein said pressure reduction valve is configured to prevent fluid from flowing from said first gripper fluid passage to said second gripper fluid passage when the pressure within said second gripper fluid passage is above said threshold.