Tractor with improved valve system

Abstract

A hydraulically powered tractor includes an elongated body, two gripper assemblies, at least one pair of aft and forward propulsion cylinders and pistons, and a valve system. The valve system comprises an inlet control valve, a two-position propulsion control valve, a two-position gripper control valve, two cycle valves, and two pressure reduction valves. The inlet control valve spool includes a hydraulically controlled deactivation cam that locks the valve in a closed position, rendering the tractor non-operational. The propulsion control valve is piloted on both ends by fluid pressure in the gripper assemblies. The propulsion control valve controls the distribution of operating fluid to and from the propulsion cylinders, such that one cylinder performs a power stroke while the other cylinder performs a reset stroke. Each end of the gripper control valve is piloted by a source of high-pressure fluid selectively admitted by one of the cycle valves. The gripper control valve controls the distribution of operating fluid to and from the gripper assemblies. The cycle valves are spring-biased and piloted by fluid pressure in the propulsion cylinders, so that the gripper control valve shifts only after the cylinders complete their strokes. The pressure reduction valves limit the pressure within the gripper assemblies. These valves are spring-biased and piloted by the pressure of fluid flowing into the gripper assemblies. Some or all of the valves include centering grooves on the landings of the spools, which reduce leakage and produce more efficient operation. The propulsion control and gripper control valves include spring-assisted detents to prevent inadvertent shifting.

1 . 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.