Patent Publication Number: US-2010108387-A1

Title: Tractor with improved valve system

Description:
CLAIM FOR PRIORITY 
     This application is a continuation of and claims priority to U.S. application Ser. No. 12/046,283, filed Mar. 11, 2008, now U.S. Pat. No. 7,607,495, which is a continuation of U.S. application Ser. No. 11/717,467, filed Mar. 12, 2007, now U.S. Pat. No. 7,353,886, which is a continuation of U.S. application Ser. No. 11/418,546, filed May 3, 2006, now U.S. Pat. No. 7,188,681, which is a continuation of U.S. patent application Ser. No. 10/759,664, filed Jan. 19, 2004, now U.S. Pat. No. 7,080,700, which is a continuation of U.S. application Ser. No. 10/004,965, filed Dec. 3, 2001, now U.S. Pat. No. 6,679,341, which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 60/250,847, filed Dec. 1, 2000. 
    
    
     INCORPORATION BY REFERENCE 
     This application incorporates by reference the entire disclosures of (1) U.S. Pat. No. 6,347,674 to Bloom et al.; (2) U.S. Pat. No. 6,241,031 to Beaufort et al.; (3) U.S. Pat. No. 6,003,606 to Moore et al.; (4) U.S. Pat. No. 6,464,003 to Bloom et al.; (5) U.S. Provisional Patent Application Ser. No. 60/250,847, filed Dec. 1, 2000; and (6) U.S. Pat. No. 6,715,559 to Bloom et al. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to tractors for moving equipment within passages. 
     2. Description of the Related Art 
     The art of moving equipment through vertical, inclined, and horizontal passages plays an important role in many industries, such as the petroleum, mining, and communications industries. In the petroleum industry, for example, it is often required to move drilling, intervention, well completion, and other forms of equipment within boreholes drilled into the earth. 
     One method for moving equipment within a borehole is to use rotary drilling equipment. In traditional rotary drilling, vertical and inclined boreholes are commonly drilled by the attachment of a rotary drill bit and/or other equipment (collectively, the “Bottom Hole Assembly” or BHA) to the end of a rigid drill string. The drill string is typically constructed of a series of connected links of drill pipe that extends between ground surface equipment and the BHA. A passage is drilled as the drill string and drill bit are together lowered into the earth. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string to the drill bit. The drilling fluid is used to cool and lubricate the bit, and only recently for drilling to remove debris and rock chips from the borehole, which are created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole. As the drill string is lowered or raised within the borehole, it is necessary to continually add or remove links of drill pipe at the surface, at significant time and cost. 
     Another method of moving equipment within a borehole involves the use of a downhole tool, such as a tractor, capable of gripping onto the borehole and thrusting both itself and other equipment through it. Such tools can be attached to rigid drill strings, but can also be used in conjunction with coiled tubing equipment. Coiled tubing equipment includes a non-rigid, compliant tube, referred to herein as “coiled tubing,” through which operating fluid is delivered to the tool. The operating fluid provides hydraulic power to propel the tool and the equipment and, in drilling applications, to lubricate the drill bit. The operating fluid also can provide the power for gripping the borehole. In comparison to rotary equipment, the use of coiled tubing equipment in conjunction with a tractor should be generally less expensive, easier to use, less time consuming to employ, and should provide more control of speed and downhole loads. Also, a tractor, which thrusts itself within the passage and pushes and pulls adjoining equipment and coiled tubing, should move more easily through inclined or horizontal boreholes. In addition, due to its greater compliance and flexibility, the coiled tubing permits the tractor to perform much sharper turns in the passage than rotary equipment. 
     A tractor can be utilized for drilling boreholes as well as many other applications, such as well completion and production work for producing oil from an oil well, pipeline installation and maintenance, laying and movement of communication lines, well logging activities, washing and acidizing of sands and solids, retrieval of tools and debris, and the like. 
     One type of tractor comprises an elongated body securable to the lower end of a drill string. The body can comprise one or more connected shafts in addition to a control assembly housing or valve system. This tractor includes at least one anchor or gripper assembly adapted to grip the inner surface of the passage. When the gripper assembly is actuated, hydraulic power from operating fluid supplied to the tractor via the drill string can be used to force the body axially through the passage. The gripper assembly is longitudinally movably engaged with the tractor body, so that the body and drill string can move axially through the passage while the gripper assembly grips the passage surface. A gripper assembly can transmit axial and even torsional loads from the tractor body to the borehole wall. Several highly effective designs for a fluid-actuated gripper assembly are disclosed in U.S. Pat. No. 6,464,003, which is incorporated by reference herein. In one design, the gripper assembly includes a plurality of flexible toes that bend radially outward to grip onto the passage surface by the interaction of ramps and rollers. 
     Some tractors have two or more sets of gripper assemblies, which permits the tractor to move continuously within the passage. Forward longitudinal motion (unless otherwise indicated, the terms “longitudinal” and “axial” are herein used interchangeably and refer to the longitudinal axis of the tractor body) is achieved by powering the tractor body forward with respect to an actuated first gripper assembly (a “power stroke” with respect to the first gripper assembly), and simultaneously moving a retracted second gripper assembly forward with respect to the tractor body (a “reset stroke” of the second gripper assembly). At the completion of the power stroke with respect to the first gripper assembly, the second gripper assembly is actuated and the first gripper assembly is retracted. Then, the tractor body is powered forward while the second gripper assembly is actuated (a power stroke with respect to the second gripper assembly), and the retracted first gripper assembly executes a reset stroke. At the completion of these respective strokes, the first gripper assembly is actuated and the second gripper assembly is retracted. The cycle is then repeated. Thus, each gripper assembly operates in a cycle of actuation, power stroke, retraction, and reset stroke, resulting in longitudinal motion of the tractor. A number of highly effective tractor designs utilizing this configuration are disclosed in U.S. Pat. No. 6,003,606 to Moore et al., which discloses several embodiments of a tractor known as the “Puller-Thruster Downhole Tool;” U.S. Pat. No. 6,241,031 to Beaufort et al., which discloses an “Electro-Hydraulically Controlled Tractor;” and U.S. Pat. No. 6,347,674 to Bloom et al., which discloses an “Electrically Sequenced Tractor” (“EST”). 
     The power required for actuating the gripper assemblies, longitudinally thrusting the tractor body during power strokes, and longitudinally resetting the gripper assemblies during reset strokes may be provided by pressurized operating fluid delivered to the tractor via the drill string—either a rotary drill string or coiled tubing. For example, the aforementioned Puller-Thruster Downhole Assembly includes inflatable engagement bladders and uses hydraulic power from the operating fluid to inflate and radially expand the bladders so that they grip the passage surface. Hydraulic power is also used to move forward cylindrical pistons residing within sets of propulsion cylinders slidably engaged with the tractor body. Each set of cylinders is secured with respect to a bladder, so that the cylinders and bladder move together longitudinally. Each piston is longitudinally fixed with respect to the tractor body. When a bladder is inflated to grip onto the passage wall, operating fluid is directed to the proximal side of the pistons in the set of cylinders secured to the inflated bladder, to power the pistons forward with respect to the borehole. The forward hydraulic thrust on the pistons results in forward thrust on the entire tractor body. Further, hydraulic power is also used to reset each set of cylinders when their associated bladder is deflated, by directing drilling fluid to the distal side of the pistons within the cylinders. 
     A tractor can include a valve system for, among other functions, controlling and sequencing the distribution of operating fluid to the tractor&#39;s gripper assemblies, thrust chambers, and reset chambers. Some tractors, including several embodiments of the Puller-Thruster Downhole Tool, are all-hydraulic. In other words, they utilize pressure-responsive valves and no electrically controlled valves. One type of pressure-responsive valve shuttles between its various positions based upon the pressure of the operating fluid in various locations of the tractor. In one configuration, a spool valve is exposed on both ends to different fluid chambers or passages. The valve position depends on the relative pressures of the fluid chambers. Fluid having a higher pressure in a first chamber exerts a greater pressure force on the valve than fluid having a lower pressure in a second chamber, forcing the valve to one extreme position. The valve moves to another extreme position when the pressure in the second chamber is greater than the pressure in the first chamber. Another type of pressure-responsive valve is a spring-biased spool valve having at least one end exposed to fluid. The fluid pressure force is directed opposite to the spring force, so that the valve is opened or closed only when the fluid pressure exceeds a threshold value. 
     Other tractors utilize valves controlled by electrical signals sent from a control system at the ground surface or even on the tractor itself. For example, the aforementioned EST includes both electrically controlled valves and pressure-responsive valves. The electrically controlled valves are controlled by electrical control signals sent from a controller housed within the tractor body. The EST is preferred over all-hydraulic tractors for drilling operations, because electrical control of the valves permits very precise control over important drilling parameters, such as speed, position, and thrust. In contrast, all-hydraulic tractors, including several embodiments of the Puller-Thruster Downhole Tool, are preferred for so-called “intervention” operations. As used herein, “intervention” refers to re-entry into a previously drilled well for the purpose of improving well production, to thereby improve fuel production rates. As wells age, the rate at which fuel can be extracted therefrom diminishes for several reasons. This necessitates the “intervention” of many different types of tools. Hydraulic tractors, as opposed to electrically controlled tractors, are preferred for intervention operations because intervention, as opposed to drilling, does not require precise control of speed or position. The absence of electrically controlled valves makes hydraulic tractors generally less expensive to deploy and operate. 
     Tractors in combination with coiled tubing equipment are particularly useful for intervention operations because, in many cases, the wells were originally drilled with rotary drilling equipment capable of drilling very deep holes. It is more expensive to bring back the rotary equipment than it is to bring in a coiled tubing unit. However, the coiled tubing unit may not be capable of reaching extended distances within the borehole without the aid of a tractor. 
     In one known design, exemplified by FIG. 3 of U.S. Pat. No. 6,003,606 (which discloses the Puller-Thruster Downhole Tool), a tractor includes a spool valve whose spool has two main positions. In one main position, the valve directs pressurized fluid to a first gripper and to propulsion chambers of a first set of propulsion cylinders. In this position of the spool, the pressure is permitted to decrease in a second gripper and in reset chambers of a second set of propulsion cylinders. In the other main position, the valve does the opposite—it directs pressurized fluid to the second gripper and propulsion chambers of the second set of cylinders, and permits pressure to decrease in the first gripper and in propulsion chambers of the first set of cylinders. The spool of the valve is piloted by fluid pressure on both ends of the spool. A pair of cycle valves selectively administers high pressure to the ends of the spool. Each cycle valve is in turn piloted by the pressure in the fluid passages to the cylinders and grippers. 
     The Puller-Thruster all-hydraulic tractor design has proven to be a major advance in the art of tractors for moving equipment within boreholes. However, it operates most effectively within a limited zone of parameters, including the pressure, weight, and density of the operating fluid, the geometry of the tractor components, and the total weight of the equipment that the tractor must pull and/or push. Thus, it is desirable to provide an improved design for a tractor, which will work within a much larger zone of such parameters. 
     Another prior design consists of a wellbore tractor having wheels that roll along the surface of the well casing. This design is problematic because the wheels do not have the ability to provide significant gripping force to move heavier downhole equipment. Also, the wheels can lose traction in certain conditions, such as in regions including sand. 
     A typical process of extracting hydrocarbons from the earth involves drilling an underground borehole and then inserting a generally tubular casing in the borehole. In order to access oil reserves from a given underground region through which the well passes, the casing must be opened within that region. In one method, perforation guns are brought to the desired location within the well and then utilized to cut openings through the casing wall and/or the earth formation. Oil is then extracted through the openings in the casing up through the well to the surface for collection. Perforation guns can also be used to penetrate the formation in an “open hole” to access desired oil reserves. An open hole is a borehole without a casing. Perforation guns can be ignited by different means, such as by pressurized operating fluid or electricity provided through electrical lines (“e-lines”). However, the practice of igniting the perforation guns with e-lines poses the risk of a spark leading to explosion and potential loss of life. Thus, it is desirable to fully hydraulic tractors, without e-lines, for operations that involve the use of perforation guns. 
     Perforation guns are commonly used in conjunction with rotary drilling equipment, due to the large weight of the guns. Long strips of perforation guns can weigh up to 20000 pounds or more. The rotary drilling equipment, consisting of the rigid drill string formed from connected links of drill pipe, has been used because of its ability to absorb the weight in tension. However, the use of rotary equipment is very expensive and time-consuming, due in part to the necessity of assembling and disassembling the portions of drill pipe. 
     In the prior art, shafts designed for downhole tools used in drilling and intervention applications have been formed from more flexible materials, such as copper beryllium (CuBe). This is because in drilling it is not uncommon to experience sharp turns, and the tool is preferably capable of turning at sharp angles. Also, shafts have been formed with relatively large internal passages for the flow of operating fluid to the valves and other equipment of the BHA. This is because in drilling the operating fluid is typically drilling mud, which often contains larger solids and necessitates a larger flow passage. The drilling mud is preferred because it provides better lubrication to the drill bit and more effectively carries the drill cuttings up through the annulus back to the ground surface. 
     The shaft of a downhole tool typically must include multiple internal passages (e.g., for fluid to the gripper assemblies, propulsion chambers, and the other downhole equipment) that extend along the shaft length. In the past, such passages have been formed by gun-drilling, which is well known. Unfortunately, it is typically not possible to gun-drill the entire length of the shaft (in most applications, the length of a shaft for a downhole tool can be anywhere in the range of 50 to 168 inches). The distance that a passage can be gun-drilled is limited by (1) the inherent length limitations of known gun-drilling tools, and (2) the limitations imposed by the geometry and material characteristics of the shaft. In the past, it has been necessary to limit the length of gun-drilled passages in shafts of downhole tools to a relatively great degree. This is because the larger internal passage required for drilling mud leaves less room for other fluid passages. This shortage of available “real estate” in the shaft requires higher precision gun-drilling and increases the risk of inadvertent damage to other passages caused by the gun-drilling process. These problems are exacerbated by the fact that the more flexible materials used for the shaft (e.g., CuBe) are softer, more difficult to drill through, and more prone to damage. 
     The limitations on the length that passages can be gun-drilled have necessitated forming the shafts from a plurality of shaft portions of reduced length. The fluid passages are gun-drilled in each shaft portion, and then the shaft portions are attached to each other. Due in large part to the use of CuBe, shaft portions have been attached together by electron beam welding. Electron beam welding is favored because it maintains the structural integrity of the material and of the fluid passages contained therein. Unfortunately, electron beam welding is a very expensive process. Most conventional welding processes have not been used because they do not facilitate the welding together of thick objects (i.e., the weld does not fuse completely through the objects). In shaft manufacturing for downhole tools, it is necessary to soundly fuse together all of the mating surfaces in order to maintain zero leakage between the various internal fluid passages and to provide structural integrity. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to overcome the aforementioned limitations of the prior art by providing a hydraulically powered and substantially or completely hydraulically controlled tractor to be used preferably with coiled tubing equipment. This invention represents a major advancement in the art of tractors, and particular in the art of well intervention tools. Compared to the prior art, the preferred embodiments of the tractor of the invention operate very effectively within a much larger zone of parameters, such as the pressure, weight, and density of the operating fluid, the geometry of the tractor components, and the total weight of the equipment that the tractor must pull and/or push. 
     As explained below, the tractor preferably includes a two-position propulsion control valve that directs fluid to and from the tractor&#39;s propulsion cylinders. In order for the propulsion control valve spool to shift, two cycle valves are provided for sensing the completion of the strokes of the propulsion cylinders. The cycle valves shift in order to begin a sequence of events that results in a fluid pressure force causing the propulsion control valve spool to shift, so that the propulsion cylinders can switch between their power and reset strokes. However, rather than administering high pressure fluid directly to the propulsion control valve spool, the cycle valves shift to send a pressure force to an additional two-position valve. The additional valve controls the flow of pressurized fluid to control the position of the propulsion control valve spool. Thus, the additional valve isolates the propulsion control valve from direct interaction with the cycle valves. Advantageously, the shift action of the additional valve creates a longer time lag between the shift action of either cycle valve and the shift action of the propulsion control valve spool. Due to the time lag, the propulsion cylinders are more likely to complete their strokes before the propulsion control valve shifts. In addition, better shifting can be effected by spring-assisted detents on the propulsion control valve spool. In the illustrated embodiments of the invention, the additional valve comprises a gripper control valve that controls the distribution of fluid to and from the gripper assemblies. 
     The preferred embodiments include an inlet control valve having a feature that allows the valve to be hydraulically restrained in a closed position, so that the tractor is assured of being non-operational and in a non-gripping state. This permits the operation of downhole equipment adjoined to the tractor or other portions of the bottom hole assembly, such as perforation guns, substantially without the risk of inadvertent movement of the tractor. It also assures that the gripper assemblies are retracted from the borehole surface during the operation of other downhole equipment, thus reducing the risk of damage to the gripper assemblies. 
     In addition, the invention provides a new method of manufacturing the shafts that form the body of the tractor, which is much less expensive than prior art shaft manufacturing methods. According to this method, shaft portions are silver brazed together to form the shafts. Silver brazing is less expensive than prior art welding methods, such as electron beam welding. Also, the preferred material characteristics and internal fluid passage configuration permits longer gun-drilled holes. Advantageously, fewer shaft portions are necessary. 
     In one aspect, the present invention provides a tractor assembly comprising a tractor for moving within a borehole. The tractor comprises an elongated body, first and second gripper assemblies, first and second elongated propulsion cylinders, and a valve system. The body has first and second pistons longitudinally fixed with respect to the body. Each piston has aft and forward surfaces configured to receive longitudinal thrust forces from fluid from a pressurized source. The body has a flow passage. 
     Each gripper assembly is longitudinally movably engaged with the body. Each gripper assembly has an actuated position in which the gripper assembly limits relative movement between the gripper assembly and an inner surface of the borehole, and a retracted position in which the gripper assembly permits substantially free relative movement between the gripper assembly and said inner surface. Each gripper assembly is configured to be actuated by fluid. 
     The first propulsion cylinder is longitudinally slidably engaged with respect to the body and has an elongated internal propulsion chamber enclosing the first piston. The first piston is slidable within and fluidly divides the internal propulsion chamber of the first cylinder into an aft chamber and a forward chamber. Similarly, the second propulsion cylinder is longitudinally slidably engaged with respect to the body and has an elongated internal propulsion chamber enclosing the second piston. The second piston is slidable within and fluidly divides the internal propulsion chamber of the second cylinder into an aft chamber and a forward chamber. 
     The valve system comprises a propulsion control valve and a gripper control valve. The propulsion control valve has a first position in which it provides a flow path for the flow of fluid to the aft chamber of the first cylinder. The propulsion control valve also has a second position in which it provides a flow path for the flow of fluid to the aft chamber of the second cylinder. The gripper control valve has a first position in which it provides a flow path for the flow of fluid to the first gripper assembly. The gripper control valve also has a second position in which it provides a flow path for fluid to the second gripper assembly. When the gripper control valve is in its first position and the propulsion control valve is in its first position, the gripper control valve must move from its first position to its second position before the propulsion control valve can move from its first position to its second position. 
     In another aspect, the present invention provides a method of moving the tractor assembly (described immediately above) within a borehole. The method comprises providing pressurized fluid from a source, directing the pressurized fluid toward the gripper control valve, directing the pressurized fluid toward the propulsion valve, and, when the gripper control valve and propulsion control valves are in their first positions, preventing the propulsion control valve from moving from its first position to its second position until the gripper control valve moves from its first position to its second position. 
     In another aspect, the invention provides a tractor assembly, comprising a tractor for moving within a borehole. The tractor comprises an elongated body, first and second gripper assemblies, first and second elongated propulsion cylinders, and a valve system. The elongated body has first and second pistons longitudinally fixed with respect to the body. Each of the pistons has aft and forward surfaces configured to receive longitudinal thrust forces from fluid from a pressurized source. The body also has a flow passage. Each of the first and second gripper assemblies is longitudinally movably engaged with the body, and has actuated and retracted positions as described above. The first and second propulsion cylinders are configured as described above. 
     The valve system comprises a propulsion valve and a control valve. The propulsion valve has a first position in which it provides a flow path for the flow of fluid to the aft chamber of the first cylinder, and a second position in which it provides a flow path for the flow of fluid to the aft chamber of the second cylinder. The control valve has a first position in which it provides a flow path for the flow of fluid to urge the propulsion valve toward the first position of the propulsion valve. The control valve has a second position in which it provides a flow path for the flow of fluid to urge the propulsion valve toward the second position of the propulsion valve. When the control valve and the propulsion valve are in their first positions, the control valve must move from its first position to its second position before the propulsion valve can move from its first position to its second position. 
     In another aspect, the invention provides a method of moving the tractor assembly (described immediately above) within a borehole. The method comprises providing pressurized fluid from a source, directing the pressurized fluid toward the gripper control valve, directing the pressurized fluid toward the propulsion valve, and, when the control valve and the propulsion valve are in their first positions, preventing the propulsion valve from moving from its first position to its second position before the control valve moves from its first position to its second position. 
     In another aspect, the invention provides a tractor assembly, comprising a tractor for moving within a borehole. The tractor is configured to be powered by operating fluid received from a conduit extending from the tractor through the borehole to a source of the operating fluid. The tractor comprises an elongated body, a gripper assembly, a valve system housed within the body, a pressure reduction valve, and first and second gripper fluid passages. The elongated body has a thrust-receiving portion longitudinally fixed with respect to the body. The body also has an internal passage configured to receive the operating fluid from the conduit. The gripper assembly is longitudinally movably engaged with the body and has actuated and retracted positions as described above. The valve system is configured to receive operating fluid from the internal passage of the body and to selectively control the flow of operating fluid to at least one of the gripper assembly and the thrust-receiving portion. The first gripper fluid passage extends from the valve system to the pressure reduction valve, while the second gripper fluid passage extends from the pressure reduction valve to the gripper assembly. The pressure reduction valve is configured to provide a flow path for operating fluid to flow from the first gripper fluid passage to the second gripper fluid passage when the pressure within the first gripper fluid passage is below a threshold. The pressure reduction valve is also configured to prevent fluid from flowing from the first gripper fluid passage to the second gripper fluid passage when the pressure within the first gripper fluid passage is above the threshold. 
     In another aspect, the invention provides a method of moving a tractor assembly within a borehole. The tractor assembly includes a tractor having an elongated body, a gripper assembly longitudinally movably engaged with the body, a valve system housed within the body, and first and second gripper fluid passages. The body has a thrust-receiving portion longitudinally fixed with respect to the body. The body also has an internal passage configured to receive the operating fluid from the conduit. The gripper assembly has actuated and retracted positions as described above, and is configured to be actuated by receiving operating fluid from the internal passage of the body. The valve system is configured to receive operating fluid from the internal passage of the body and to selectively control the flow of operating fluid to at least one of the gripper assembly and the thrust-receiving portion. The first gripper fluid passage extends from the valve system, and the second gripper fluid passage extends to the gripper assembly. According to the method of this aspect of the invention, pressurized fluid is provided from a source. The pressurized fluid is permitted to flow from the first gripper fluid passage to the second gripper fluid passage when the pressure within the first gripper fluid passage is below a threshold. Fluid is prevented from flowing from the first gripper fluid passage to the second gripper fluid passage when the pressure within the first gripper fluid passage is above the threshold. 
     In another aspect, the invention provides a tractor assembly, comprising a tractor for moving within a borehole. The tractor is configured to be powered by pressurized operating fluid received from a conduit extending from the tractor through the borehole to a source of the operating fluid. The tractor comprises an elongated body, a gripper assembly longitudinally movably engaged with the body, and a valve system housed within the body. The body has a thrust-receiving portion longitudinally fixed with respect to the body, and an internal passage configured to receive the operating fluid from the conduit. The gripper assembly has actuated and retracted positions as described above. 
     The valve system is configured to receive fluid from the internal passage of the body and to selectively control the flow of operating fluid to at least one of the gripper assembly and the thrust-receiving portion. The valve system includes an entry control valve controlling the flow of operating fluid from the internal passage of the body into the valve system. The entry control valve comprises a valve passage and a body movably received therein. The valve passage has at least two secondary passages and is configured to conduct the operating fluid between the secondary passages. The entry control valve has first and third position ranges in which it provides a flow path for operating fluid within the valve system to flow through the entry control valve to the exterior of the tractor, and in which the valve body prevents the flow of operating fluid from the internal passage of the tractor body into the valve system. The entry control valve also has a second position range in which it provides a flow path for operating fluid from the internal passage of the tractor body to flow into the valve system, and in which the valve body prevents the flow of operating fluid within the valve system to the exterior of the tractor. The entry control valve is in its first position range when the fluid pressure in the internal passage of the tractor body is below a lower shut-off threshold. The entry control valve is in the second position range when the fluid pressure in the internal passage is above the lower shut-off threshold and below an upper shut-off threshold. The entry control valve is in the third position range when the fluid pressure in the internal passage is above the upper shut-off threshold. 
     In another aspect, the invention provides a method of moving a tractor assembly within a borehole, the tractor assembly including a tractor having an elongated body and gripper assembly configured as in the previously described aspect of the invention. The tractor also comprises a valve system housed within the body, the valve system including an entry control valve. According to the method, fluid is received from the internal passage of the body, and the flow of operating fluid from the internal passage of the body into the valve system is controlled with the entry control valve. The flow of operating fluid from the internal passage of the body into the valve system is prevented with the entry control valve when the fluid pressure in the internal passage of the body is below a lower shut-off threshold and when the fluid pressure in the internal passage is above an upper shut-off threshold. The flow of operating fluid from the internal passage of the body into the valve system is permitted when the fluid pressure in the internal passage is above the lower shut-off threshold and below the upper shut-off threshold. 
     In another aspect, the present invention provides a tractor assembly, comprising a tractor for moving within a borehole. The tractor is configured to be powered by pressurized operating fluid received from a conduit extending from the tractor through the borehole to a source of the operating fluid. The tractor comprises an elongated body, a gripper assembly longitudinally movably engaged with the body, and a valve system. The elongated body has a thrust-receiving portion longitudinally fixed with respect to the body. The body also has an internal passage configured to receive the operating fluid from the conduit. The gripper assembly has actuated and retracted positions as described above. 
     The valve system of the tractor is configured to receive fluid from the internal passage of the body and to selectively control the flow of operating fluid to at least one of the gripper assembly and the thrust-receiving portion. The valve system includes an entry control valve controlling the flow of operating fluid from the internal passage of the body into the valve system. The entry control valve comprises a housing defining a valve passage, a body movably received within the passage, and at least one spring. The housing has at least two side passages, the valve passage being configured to conduct the operating fluid between the side passages. The valve body has a first surface configured to be exposed to operating fluid from the internal passage of the tractor body, the first surface being configured to receive a longitudinal pressure force in a first direction. The valve body has first and third position ranges in which the body provides a flow path for operating fluid within the valve system to flow through the entry control valve to the exterior of the tractor, and in which the valve body prevents the flow of operating fluid from the internal passage of the body into the valve system. The valve body has a second position range between the first and third position ranges in which the valve body provides a flow path for operating fluid from the internal passage of the tractor body to flow into the valve system, and in which the valve body prevents the flow of operating fluid within the valve system to the exterior of the tractor. 
     The at least one spring biases the valve body in a direction opposite to that of the pressure force received by the first surface of the valve body, such that the magnitude of the fluid pressure in the internal passage determines the deflection of the at least one spring and thus the position of the valve body. The at least one spring is configured so that the valve body occupies a position within the first position range when the fluid pressure in the internal passage of the tractor body is below a lower shut-off threshold, so that the valve body occupies a position within the second position range when the fluid pressure in the internal passage is above the lower shut-off threshold and below an upper shut-off threshold, and so that the valve body occupies a position within the third position range when the fluid pressure in the internal passage is above the upper shut-off threshold. 
     In another aspect, the invention provides a tractor assembly, comprising a tractor for moving within a borehole while connected to an injector by a drill string. The tractor comprises an elongated body, first and second gripper assemblies, elongated first and second propulsion cylinders, and a valve system. The body has first and second pistons longitudinally fixed with respect to the body. Each of the pistons has aft and forward surfaces configured to receive longitudinal thrust forces from fluid from a pressurized source. The body also has a flow passage. The first gripper assembly is longitudinally movably engaged with the body and has actuated and retracted positions as described above. Similarly, the second gripper assembly is longitudinally movably engaged with the body and has actuated and retracted positions as described above. The first propulsion cylinder is longitudinally slidably engaged with respect to the body. The first cylinder has an elongated internal propulsion chamber enclosing the first piston. The first piston is slidable within and fluidly divides the internal propulsion chamber of the first cylinder into an aft chamber and a forward chamber. Similarly, the second propulsion cylinder is longitudinally slidably engaged with respect to the body. The second cylinder has an elongated internal propulsion chamber enclosing the second piston. The second piston is slidable within and fluidly divides the internal propulsion chamber of the second cylinder into an aft chamber and a forward chamber. 
     The valve system of the tractor comprises a propulsion control valve and a gripper control valve. The propulsion control valve has a first position in which it provides a flow path for the flow of fluid to the aft chamber of the first cylinder, and a second position in which it provides a flow path for the flow of fluid to the aft chamber of the second cylinder. The gripper control valve has a first position in which it provides a flow path for the flow of fluid to the first gripper assembly, and a second position in which it provides a flow path for fluid to the second gripper assembly. The speed of movement of the tractor is controlled by the pressure and flow rate of the operating fluid and the tension exerted on the tractor by the drill string. 
     In another aspect, the invention provides a tractor assembly, comprising a tractor for moving within a borehole. The tractor comprises an elongated body, a first gripper assembly longitudinally movably engaged with the body, an elongated first propulsion cylinder longitudinally slidably engaged with respect to the body, and a valve system. The body has first and second pistons longitudinally fixed with respect to the body. Each of the pistons has aft and forward surfaces configured to receive longitudinal thrust forces from fluid from a pressurized source. The body also has a flow passage. The first gripper assembly has actuated and retracted positions as described above. The first propulsion cylinder has an elongated internal propulsion chamber enclosing the first piston. The first piston is slidable within and fluidly divides the internal propulsion chamber of the first cylinder into an aft chamber and a forward chamber. 
     The valve system comprises a propulsion valve and a control valve. The propulsion valve has a first position in which it provides a flow path for the flow of fluid to the aft chamber of the first cylinder, and a second position in which it does not provide a flow path for the flow of fluid to the aft chamber of the first cylinder. The control valve has a first position in which it provides a flow path for the flow of fluid to urge the propulsion valve toward the first position, and a second position in which it provides a flow path for the flow of fluid to urge the propulsion valve toward the second position. When the control valve and the propulsion valve are in their first positions, the control valve must move from its first position to its second position before the propulsion valve can move from its first position to its second position. 
     For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above and as further described below. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. 
     All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the major components of one embodiment of a tractor of the present invention, utilized in conjunction with a coiled tubing system; 
         FIG. 2  is a front perspective view of a preferred embodiment of the tractor of the present invention; 
         FIG. 3  is a schematic diagram illustrating a preferred configuration of the tractor and the valve system of the present invention; 
         FIG. 4  is a front perspective view of the control assembly of the tractor of  FIG. 2 , shown partially disassembled; 
         FIG. 5  is a longitudinal sectional view of the control assembly of  FIG. 4 , illustrating the inlet control valve of the tractor; 
         FIG. 6  is an exploded view of the inlet control valve shown in  FIG. 5 ; 
         FIG. 7  is an exploded view of the deactivation cam shown in  FIG. 6 ; 
         FIG. 8  is a longitudinal sectional view of the deactivation cam of  FIG. 7 ; 
         FIG. 9  is a longitudinal sectional view of the control assembly of  FIG. 4 , illustrating the propulsion control valve of the tractor; 
         FIG. 10  is an exploded view of the propulsion control valve shown in  FIG. 9 ; 
         FIG. 11  is a perspective view of a portion of the propulsion control valve spool; 
         FIG. 12  is a longitudinal sectional view of the aft cycle valve shown in  FIG. 4 ; 
         FIG. 13  is a longitudinal sectional view of the aft pressure reduction valve of the control assembly shown in  FIG. 4 ; 
         FIG. 14  is a perspective view of a forward shaft assembly a tractor according to one embodiment of the invention, with the gripper assembly not shown for clarity; 
         FIG. 15  is a perspective view of a male braze joint of a shaft portion of the shaft of  FIG. 14 ; 
         FIG. 16  is a longitudinal sectional view of a braze joint of the shaft of  FIG. 14 , as well as a connection of a preferred embodiment of a piston to the shaft; 
         FIG. 17  is a schematic diagram illustrating a valve system according to an alternative embodiment of a tractor of the invention, which includes a hydraulically controlled reverser valve that toggles in response to a pressure spike to permit the tractor to power out of a borehole; 
         FIG. 18  is a schematic diagram illustrating a valve system according to another alternative embodiment of a tractor of the invention, which includes an electrically controlled reverser valve; 
         FIG. 19  is a schematic diagram illustrating a valve system according to yet another alternative embodiment of a tractor of the invention, which includes a pair of inlet control valves, one hydraulically controlled and the other electrically controlled to provide electric starting or stopping of the tractor; 
         FIG. 20  is a schematic diagram illustrating a valve system according to yet another alternative embodiment of a tractor of the invention, which includes both the pair of inlet control valves of the valve system of  FIG. 19  and the electrically controlled reverser valve of the valve system of  FIG. 18 ; 
         FIG. 21  is a perspective view of a preferred embodiment of a gripper assembly having flexible toes with rollers; 
         FIG. 22  is a longitudinal sectional view of the toe supports, slider element, and a single toe of the gripper assembly of  FIG. 21 , shown at a moment when there is substantially no external load applied to the toe; 
         FIG. 23  is an exploded view of the aft end of the toe shown in  FIG. 22 ; 
         FIG. 24  is an exploded view of one of the rollers of the toe shown in  FIG. 22 ; 
         FIG. 25  is an exploded view of the forward end of the toe shown in  FIG. 22 ; 
         FIG. 26  is a longitudinal sectional view of the toe supports, slider element, and a single toe of the gripper assembly of  FIG. 21 , shown at a moment when an external load is applied to the toe; 
         FIG. 27  is an exploded view of the aft end of the toe shown in  FIG. 26 ; 
         FIG. 28  is an exploded view of one of the rollers of the toe shown in  FIG. 26 ; 
         FIG. 29  is an exploded view of the forward end of the toe shown in  FIG. 26 ; 
         FIG. 30  is a partial cut-away side view of the toe supports, slider element, and a single toe of the gripper assembly of  FIG. 21 , shown at a moment when the toe is relaxed; 
         FIG. 31  is an exploded view of one of the spacer tabs of the toe shown in  FIG. 30 ; 
         FIG. 32  is an exploded view of one of the rollers of the toe shown in  FIG. 30 ; 
         FIG. 33  is a side view of the slider element and a portion of one of the toes of the gripper assembly of  FIG. 21 , shown at a moment when the toe is radially deflected or energized; and 
         FIG. 34  is an exploded view of one of the alignment tabs of the toe shown in  FIG. 33 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a hydraulic tractor  100  for moving equipment within a passage, configured in accordance with a preferred embodiment of the present invention. In the embodiments shown in the accompanying figures, the tractor of the present invention may be used in conjunction with a coiled tubing drilling system  20  and adjoining downhole equipment  32 . The system  20  may include a power supply  22 , tubing reel  24 , tubing guide  26 , tubing injector  28 , and coiled tubing  30 , all of which are well known in the art. The tractor  100  is configured to move within a borehole having an inner surface  42 . An annulus  40  is defined by the space between the tractor  100  and the inner surface  42  of the borehole. 
     The downhole equipment  32  may include various types of equipment that the tractor  100  is designed to move within the passage. For example, the equipment  32  may comprise a perforation gun assembly, an acidizing assembly, a sandwashing assembly, a bore plug setting assembly, an E-line, a logging assembly, a bore casing assembly, a measurement while drilling (MWD) assembly, or a fishing tool. Also, the equipment  32  may comprise a combination of these items. If the tractor  100  is used for drilling, the equipment  32  will preferably include an MWD system  34 , downhole motor  36 , and drill bit  38 , all of which are also known in the art. Of course, the downhole equipment  32  may include many other types of equipment for non-drilling applications, such as intervention and completion applications. While the equipment  32  is illustrated on the forward end of the tractor, it will be understood that such downhole equipment can be connected both aftward and forward of the tractor. 
     It will be appreciated that a hydraulic tractor of a preferred embodiment of the present invention may be used to move a wide variety of tools and equipment within a borehole or other passage. For example, the tractor can be utilized for applications such as well completion and production work for producing oil from an oil well, pipeline installation and maintenance, laying and movement of communication lines, well logging activities, washing and acidizing of sands and solids, retrieval of tools and debris, and the like. Also, while preferred for intervention operations, the tractor can be used for drilling applications, including petroleum drilling and mineral deposit drilling. The tractor can be used in conjunction with different types of drilling equipment, including rotary drilling equipment and coiled tubing equipment. 
     For example, one of ordinary skill in the art will understand that oil and gas well completion typically requires that the reservoir be logged using a variety of sensors. These sensors may operate using resistivity, radioactivity, acoustics, and the like. Other logging activities include measurement of formation dip and borehole geometry, formation sampling, and production logging. These completion activities can be accomplished in inclined and horizontal boreholes using a preferred embodiment of the hydraulic tractor of the invention. For instance, the tractor can deliver these various types of logging sensors to regions of interest. The tractor can either place the sensors in the desired location, or it can idle in a stationary position to allow the measurements to be taken at the desired locations. The tractor can also be used to retrieve the sensors from the well. 
     Examples of production work that can be performed with a preferred embodiment of the hydraulic tractor of the invention include sands and solids washing and acidizing. It is known that wells sometimes become clogged with sand, hydrocarbon debris, and other solids that prevent the free flow of oil through the borehole  42 . To remove this debris, specially designed washing tools known in the industry are delivered to the region, and fluid is injected to wash the region. The fluid and debris then return to the surface. Such tools include acid washing tools. These washing tools can be delivered to the region of interest for performance of washing activity and then returned to the ground surface by a preferred embodiment of the tractor of the invention. 
     In another example, a preferred embodiment of the tractor of the invention can be used to retrieve objects, such as damaged equipment and debris, from the borehole. For example, equipment may become separated from the drill string, or objects may fall into the borehole. These objects must be retrieved, or the borehole must be abandoned and plugged. Because abandonment and plugging of a borehole is very expensive, retrieval of the object is usually attempted. A variety of retrieval tools known to the industry are available to capture these lost objects. The tractor can be used to transport retrieving tools to the appropriate location, retrieve the object, and return the retrieved object to the surface. 
     In yet another example, a preferred embodiment of the tractor of the invention can also be used for coiled tubing completions. As known in the art, continuous-completion drill string deployment is becoming increasingly important in areas where it is undesirable to damage sensitive formations in order to run production tubing. These operations require the installation and retrieval of fully assembled completion drill string in boreholes with surface pressure. The tractor of the invention can be used in conjunction with the deployment of conventional velocity string and simple primary production tubing installations. The tractor can also be used with the deployment of artificial lift devices such as gas lift and downhole flow control devices. 
     In a further example, a preferred embodiment of the tractor of the invention can be used to service plugged pipelines or other similar passages. Frequently, pipelines are difficult to service due to physical constraints such as location in deep water or proximity to metropolitan areas. Various types of cleaning devices are currently available for cleaning pipelines. These various types of cleaning tools can be attached to the tractor so that the cleaning tools can be moved within the pipeline. 
     In still another example, a preferred embodiment of the tractor of the invention can be used to move communication lines or equipment within a passage. Frequently, it is desirable to run or move various types of cables or communication lines through various types of conduits. The tractor can move these cables to the desired location within a passage. 
     Overview of Tractor Components 
       FIG. 2  shows a preferred embodiment  100  of a tractor of the present invention, shown with the aft end on the right and the forward end on the left. The tractor  100  comprises a central control assembly  102 , an uphole or aft gripper assembly  104 , a downhole or forward gripper assembly  106 , an aft propulsion cylinder  108 , a forward propulsion cylinder  114 , tool joint assemblies  116  and  129 , shafts  118  and  124 , and flex joints or adapters  120  and  128 . The tool joint assembly  116  connects a drill string, such as coiled tubing, to the shaft  118 . The aft gripper assembly  104 , aft propulsion cylinder  108 , and flex joint  120  are assembled together end-to-end and are all axially slidably engaged with the shaft  118 . Similarly, the forward gripper assembly  106 , forward propulsion cylinders  114 , and flex joint  128  are assembled together end-to-end and are axially slidably engaged with the shaft  124 . The tool joint assembly  129  couples the tractor  100  to downhole equipment  32  ( FIG. 1 ). The shafts  118  and  124  and control assembly  102  are axially fixed with respect to one another and are sometimes referred to herein as the body of the tractor. The body of the tractor is thus axially fixed with respect to the drill string and the downhole tools. 
     The tractor  100  can be made to have the capability of pulling and/or pushing downhole equipment  32  of various weights. In one embodiment, the tractor  100  is capable of pulling and/or pushing a total weight of 100 lbs, in addition to the weight of the tractor itself. In three other embodiments, the tractor is capable of pulling and/or pushing a total weight of 500, 3000, and 15,000 lbs. 
     In order to prevent damage to a surrounding formation or casing wall, the tractor can be designed to limit the radial gripping load that it exerts on a surface surrounding the tractor. In one embodiment, the tractor exerts no more than 25 psi on a surface surrounding the tractor. This embodiment is particularly useful in softer formations, such as gumbo. In three other embodiments, the tractor exerts no more than 100, 3000, and 50,000 psi on a surface surrounding the tractor. At radial gripping loads of 50,000 psi or less, the tractor can be used safely in steel tube casing. 
     The tractor components shown in  FIG. 2  are assembled in a manner similar to the components of the aforementioned EST, disclosed and illustrated in U.S. Pat. No. 6,347,674. Two notable differences between the tractor  100  shown in  FIG. 2  and the EST are (1) the tractor  100  of the present invention utilizes gripper assemblies of a different type, and (2) the control assembly  102  of the tractor  100  is different than the control assembly of the EST. In the preferred embodiment, the gripper assemblies  104  and  106  of the tractor  100  are preferably of a design similar to a gripper assembly disclosed and illustrated in U.S. Pat. No. 6,464,003, with a number of improvements described below. The control assembly  102  houses a valve system that controls the distribution of operating fluid to and from the gripper assemblies and propulsion cylinders. The control assembly  102  is described below. 
     The control assembly  102  includes internal fluid passages for flow between the valves and flow to the gripper assemblies, propulsion cylinders, and downhole equipment. In a preferred embodiment, some of the fluid passage sizes are similar to or larger than the fluid passages of the control assembly of the EST. As in the EST design, the fluid passages are sized and located to fit within the available space constraints of the tractor. The sizes of the various components (e.g., the shafts, propulsion cylinders, pistons, control housing, valves, etc.) are generally similar to the sizes of analogous components of the EST. Using principles of design and space management made apparent by U.S. Pat. No. 6,347,674 (which discloses the EST) in combination with the specification and figures of the present application, one of ordinary skill in the art will understand how to build a tractor according to the present invention. 
     The tractor  100  can be any desirable length, but for typical oilfield applications the length is approximately 25 to 30 feet. The maximum diameter of the tractor will typically vary with the size of the hole, thrust requirements, and the restrictions that the tractor must pass through. The gripper assemblies can be designed to operate within boreholes of various sizes, but typically can expand to a diameter of 3.75 to 7.0 inches. 
     The flex adapters  120  and  128  are hollow structural members that provide a region of reduced flexural rigidity in the tractor. This region of increased flexibility facilitates the negotiation of sharp turns. The adapters are preferably formed of a relatively low modulus material such as Copper Beryllium (CuBe) and Titanium. Occasionally, there are applications that require the use of non-magnetic materials for the tractor. Otherwise, depending on the required turning capability of the tractor and resultant stresses, it is possible that various stainless steels may be used in many areas of the tractor. 
     In the preferred embodiment, the tool joint assembly  116  couples the shaft  118  to a coiled tubing drill string, preferably via a threaded connection. However, downhole tools can also be placed aftward of the tractor, connected to the tool joint assembly  116 . The tool joint assembly  129  will normally be coupled to downhole tools. The interface threads of the tool joint assemblies are preferably API threads or proprietary threads (such as Hydril casing threads). The tool joint assemblies can be prepared with conventional equipment (tongs) to a specified torque (e.g., 1000-3000 ft-lbs). The tool joint assemblies can be formed from a variety of materials, including CuBe, steel, and other metals. 
     The shafts  118  and  124  can be formed from any suitable material. In one embodiment, the shafts are formed from a flexible material, such as CuBe, in order to permit the tractor  100  to negotiate sharper turns. In other embodiments CuBe is not used, as it is relatively expensive. Other acceptable materials include Titanium and steel (when low flexibility is sufficient). In a preferred configuration, each shaft includes a central internal bore (forming a portion of the passage  44  discussed below and shown in  FIG. 3 ) for the flow of pressurized operating fluid to the downhole equipment and to the valve system of the tractor. This bore extends the entire length of each shaft. Each shaft also includes numerous other passages for the flow of fluid to the gripper assemblies and propulsion cylinders. These fluid passages range in length and are equal to or less than the overall length of the tractor. Multiple fluid passages can be drilled in the shaft for the same function, such as to feed a single propulsion chamber. Preferably, the bore and the other internal fluid passages are arranged so as to minimize stress and provide sufficient space and strength for other design features, such as the pistons within the cylinders. Each shaft is preferably provided with threads on one end for connection to the tool joint assemblies  116  and  129 , and with a flange on the other end to allow bolting to the control assembly  102 . 
     In one embodiment, the tractor  100  is specifically designed for intervention applications. While intervention tractors can be made any size, they are typically operated within 5-inch or 7-inch casing. The inside diameter of a 5-inch casing can range from 4.5 to 4.8 inches. The inside diameter of a 7-inch casing can range from 5.8 to 6.4 inches. The primary structural components of the tractor  100  are the shafts  118  and  124 . In a preferred embodiment, the shafts have an outside diameter of 1.75 inches and an inside bore diameter of 0.8 inches. The remaining fluid passages of the shafts are preferably smaller. The pistons can have varying outside diameters. 
     For intervention applications, the tractor  100  saves time and money. Prior art intervention tools that utilize rotary drill strings are as much as 150% more expensive than the illustrated tractor  100  using coiled tubing equipment. In addition, the tractor  100  is more time-conservative, as the longer rig-up time associated with rotary equipment is avoided. The use of coiled tubing is particularly advantageous when operating perforation guns. 
       FIG. 3  schematically illustrates a preferred configuration of the major components of the tractor  100 . The tractor  100  includes an internal passage  44  extending from the aft end of the aft shaft  118  through the control assembly  102  to the forward end of the forward shaft  124 . In use, pressurized operating fluid is pumped through the drill string into the internal passage  44 . The operating fluid can be used for various applications to be undertaken by the downhole equipment, such as for powering perforation guns utilized for cutting holes in a casing wall of an oil well. The valve system  133  is configured to receive a portion of the operating fluid flowing through the internal passage  44 . 
       FIG. 3  also schematically illustrates a preferred configuration of the valve system  133  of the tractor  100 . The valve system  133  is housed within the control assembly  102  shown in  FIG. 2 . The valve system  133  selectively controls the flow of operating fluid to and from the gripper assemblies  104  and  106  and to and from the propulsion cylinders  108  and  114 . The operation of the valve system  133  is described in detail below. 
     In the aft shaft assembly, the aft propulsion cylinder  108  is longitudinally slidably engaged with the aft shaft  118  and forms an internal annular chamber surrounding the shaft. An annular piston  180  resides within the annular chamber formed by the cylinder  108 , and is at least longitudinally fixed to the shaft  118 . The piston  180  fluidly divides the internal annular chamber formed by the cylinder  108  into an aft chamber  154  and a forward chamber  156 . Preferably, the chambers  154  and  156  are fluidly sealed to substantially prevent fluid flow between the chambers or leakage to the annulus  40 . The piston  180  is longitudinally slidable within the cylinder  108 . 
     In the forward shaft assembly, the forward propulsion cylinder  114  is configured similarly to the aft propulsion cylinder  108 . The cylinder  114  is longitudinally slidably engaged with the forward shaft  124 . An annular piston  186  is at least longitudinally fixed to the shaft  124 , and is enclosed within the cylinder  114 . The piston  186  fluidly divides the internal annular chamber formed by the cylinder  114  into a rear chamber  166  and a front chamber  168 . The piston  186  is longitudinally slidable within the cylinder  114 . 
     Thus, the chambers  154 ,  156 ,  166 , and  168  have varying volumes, depending upon the positions of the pistons  180  and  186  within the cylinders. It will be understood that the cylinders and pistons can have any of a variety of different shapes and sizes (including non-circular cross-sections), preferably keeping in mind the goals of providing an elongated thrust chamber for a suitable power stroke, as well as concerns of simplicity, prevention of leakage, ease of manufacturing, and compatibility with existing downhole tools. 
     Although one aft propulsion cylinder  108  and one forward propulsion cylinder  114  (along with a corresponding aft piston and forward piston) are shown in the illustrated embodiment, any number of aft cylinders and forward cylinders may be provided. The hydraulic thrust provided by the tractor increases as the number of propulsion cylinders increases. In other words, the hydraulic force provided by the cylinders is additive. Thus, the number of cylinders is selected according to the desired thrust. It will be understood that the number of cylinders may be limited by the capability of the gripper assemblies to transfer radial loads to the borehole wall. In other words, the thrust produced by the cylinders should not be so high as to cause the gripper assemblies to slip in their actuated positions. In a preferred embodiment, the cylinder outside diameter is 3.75 inches. In this embodiment, the gripper assemblies are designed to transmit a radial gripping force of approximately 6,500 pounds, and each piston is designed to produce a stall force of 8,835 pounds at 1500 psi. Thus, in this embodiment, only one aft and one forward cylinder are preferred. The load transmission capability of the gripper assemblies varies by design of the gripper assembly. 
     The tractor  100  is hydraulically powered by an operating fluid pumped down the drill string, such as brine, sea water, drilling mud, or hydraulic fluid. In a preferred embodiment, the same fluid that may operate downhole equipment  32  ( FIG. 1 ) powers the tractor. This avoids the need to provide additional fluid channels in the tool for the fluid powering the tractor. Preferably, liquid brine or sea water is used in an open system. Alternatively, fluid may be used in a closed system, if desired. Referring to  FIG. 1 , in operation, operating fluid flows from the drill string  30  through the tractor  100  and down to the downhole equipment  32 . Referring again to  FIG. 3 , a diffuser or filter  132  in the control assembly  102  diverts a portion of the operating fluid into the valve system  133  to power the tractor. Preferably, the diffuser  132  filters out larger fluid particles that can damage internal components of the valve system, such as the valve spools. 
     Preferred Configuration of Valve System 
     With reference to  FIG. 3 , a preferred embodiment of the valve system  133  includes an inlet or entry control valve  136 , a propulsion control valve  146 , a gripper control valve  148 , an aft cycle valve  150 , and a forward cycle valve  152 . In addition, pressure reduction valves  244  and  246  are preferably provided to limit the fluid pressure in the gripper assemblies, as described in further detail below. The operation of each of these valves is discussed below. 
     Fluid diverted to the valve system  133  through the diffuser  132  enters an inlet galley  134  upstream of the inlet control valve  136 . As used herein, the terms “galley,” “chamber,” and “passage” refer to regions of the tractor that are configured to contain operating fluid, and are not limited to any particular shape. Some of these regions are illustrated as flow paths or lines in  FIG. 3 . 
     The inlet control valve  136  is preferably a spool valve, a preferred embodiment of which is illustrated in  FIGS. 4-8 . The valve  136  serves as a gateway for fluid to flow into a main galley  144  of the valve system  133 . The spool of the valve  136  has first, second, and third position ranges, the second range being interposed between the first and third ranges. In the first and third position ranges, the spool provides a flow path (represented by arrow  174  for the first position range and arrow  176  for the third position range) for fluid within the main galley  144  to flow through the valve  136  to the annulus  40  on the exterior of the tractor. Also, in the first and third position ranges, the spool prevents the flow of fluid from the inlet galley  134  through the valve  136  into the main galley  144 . Thus, in the first and third position ranges of the inlet control valve spool, fluid exits the valve system  133  to render the tractor non-operational. In the second position range, the spool provides a flow path (represented by arrow  172 ) for fluid in the inlet galley  134  to flow into the main galley  144 . In the second position range, the spool also prevents the flow of fluid from the main galley  144  through the valve  136  to the annulus  40 . Thus, in the second position range of the inlet control valve spool, fluid enters the valve system  133  such that the tractor is operational. In  FIG. 3 , the spool of valve  136  is shown in its second position range. When shifted vertically downward in  FIG. 3 , the spool occupies its first position range. When shifted vertically upward in  FIG. 3 , the spool occupies its third position range. 
     The spool of the inlet control valve  136  has a first end or surface  139  biased by one or more springs  140  and a second end or surface  138  exposed to fluid in the inlet galley  134 . In the illustrated embodiment, the spring  140  is also in fluid communication with the annulus  40 , as indicated by the broken lines  142 . The spring  140  imparts a spring force on the first end surface  139  that tends to push the spool toward its first position range. In the illustrated embodiment, fluid from the annulus  40  also imparts a pressure force onto the first end surface  139 . The fluid in the galley  134  imparts a pressure force on the second surface  138  that tends to push the spool toward its third position range. Thus, the spring force and fluid pressure force on the first end surface  139  act against the fluid pressure force on the second surface  138 . The differential fluid pressure in the inlet galley  134  required to move the spool from the first position range to the lower endpoint of the second position range (i.e., the position at which the valve opens a flow path between the galleys  134  and  144 ) depends upon the effective spring constant of the spring  140  and is defined as the lower shut-off threshold. Likewise, the differential fluid pressure required to move the spool from the second position range to the lower endpoint of the third position range (i.e., the position at which the valve closes the flow path between the galleys  134  and  144 ) also depends upon the effective spring constant of the spring  140  and is defined as the upper shut-off threshold. Unless otherwise indicated, as used herein, “differential pressure” or “pressure” at a particular location within the tractor refers to the difference between the pressure at that location and the pressure in the annulus  40 . Advantageously, the inlet control valve  136  thus permits the fluid pressure within the valve system  133  to be limited to within a specific range. In a preferred embodiment, the lower shut-off threshold is 800 psid and the upper shut-off threshold is 2100 psid. 
     It will be understood that the spring  140  can bear against any suitable surface of the spool or any component having a fixed relationship with the spool. It will also be understood that the spring  140  can be configured to operate primarily in tension or primarily in compression, keeping in mind the goal of biasing the spool toward its first position. 
     In the preferred embodiment, discussed in greater detail below, the inlet control valve  136  includes a locking feature to lock the valve spool in its third position range and to thus prevent fluid from entering the valve system  133 . The locking feature is schematically represented in  FIG. 3  by a latch  137 . The purpose and preferred configuration of the locking feature is discussed below. 
     The main galley  144  fluidly communicates with and provides incoming pressurized operating fluid to the propulsion control valve  146 , the gripper control valve  148 , the aft cycle valve  150 , and the forward cycle valve  152 . The propulsion control valve  146  is preferably a two-position spool valve. The spool of the valve  146  has a first position, shown in  FIG. 3 , in which the valve  146  provides a flow path (represented by arrow  192 ) for the flow of fluid from the main galley  144  into a chamber or passage  196 . The chamber  196  leads from the valve  146  to the aft chamber  154  of the aft cylinder  108 , and also to the forward chamber  168  of the forward cylinder  114 . When the spool of the valve  146  is in its first position, the valve  146  also provides a flow path (represented by arrow  194 ) for the flow of fluid within a chamber or passage  198  to the annulus  40 . The chamber  198  leads from the valve  146  to the forward chamber  156  of the aft cylinder  108 , and also to the aft chamber  166  of the forward cylinder  114 . 
     The spool of the propulsion control valve  146  also has a second position, shifted to the left in  FIG. 3 . When the spool of the valve  146  is in its second position, the valve  146  provides a flow path (represented by arrow  200 ) for the flow of fluid from the main galley  144  to the chamber  198 . When the spool of the valve  146  is in its second position, the valve  146  also provides a flow path (represented by arrow  202 ) for the flow of fluid from the chamber  196  to the annulus  40 . 
     With continued reference to  FIG. 3 , the spool of the propulsion control valve  146  has a first end surface  188  and a second end surface  190 . The first end surface  188  is exposed to fluid within a chamber  204  that leads to the aft gripper assembly  104  (or, if present, to an aft pressure reduction valve  244 ). The second end surface  190  is exposed to fluid within a chamber  206  that leads to the forward gripper assembly  106  (or, if present, to a forward pressure reduction valve  246 ). The first and second end surfaces  188  and  190  are configured to receive respective fluid pressure forces that act against each other. The first end surface  188  receives a pressure force from the fluid in the chamber  204  that tends to move the spool of the valve  146  toward its first position, as shown in  FIG. 3 . The second end surface  190  receives a pressure force from the fluid in the chamber  206  that tends to move the spool toward its second position, which would be shifted to the left in  FIG. 3 . Preferably, the valve  146  includes detents (mechanical catches or restraints) for retaining the spool in its first and second positions until the pressure difference between the chambers  204  and  206  reaches a shifting threshold. In a preferred embodiment, the detents include resilient elements, such as springs, that interact with tapered surfaces of the spool landings, as described in further detail below and illustrated in  FIG. 10 . Alternatively, the detents may be conventional mechanical detents. 
     Like the propulsion control valve  146 , the gripper control valve  148  is preferably a two-position spool valve. The spool of the valve  148  has a first position, shown in  FIG. 3 , in which the valve  148  provides a flow path (represented by arrow  208 ) for the flow of fluid from the main galley  144  into the chamber  204 . When the spool of the valve  148  is in its first position, the valve  148  also provides a flow path (represented by arrow  210 ) for the flow of fluid within the chamber  206  to the annulus  40 . The spool of the gripper control valve  148  also has a second position, not shown in  FIG. 3 . The second position is that which the spool would be in if it is shifted to the left in  FIG. 3 . When the spool of the valve  148  is in its second position, the valve  148  provides a flow path (represented by arrow  212 ) for the flow of fluid from the main galley  144  to the chamber  206 . When the spool of the valve  148  is in its second position, the valve  148  also provides a flow path (represented by arrow  214 ) for the flow of fluid from the chamber  204  to the annulus  40 . 
     The spool of the gripper control valve  148  has a first end surface  216  and a second end surface  218 . The first end surface  216  is exposed to fluid within a chamber or passage  220  that leads to the aft cycle valve  150 . The second end surface  218  is exposed to fluid within a chamber or passage  222  that leads to the forward cycle valve  152 . The first and second end surfaces  216  and  218  are configured to receive respective fluid pressure forces that act against each other. The first end surface  216  receives a pressure force from the fluid in the chamber  220  that tends to move the spool of the valve  148  toward its first position, as shown in  FIG. 3 . The second end surface  218  receives a pressure force from the fluid in the chamber  222  that tends to move the spool toward its second position, which would be shifted to the left in  FIG. 3 . Preferably, the valve  148  includes detents for retaining the spool in its first and second positions until the pressure difference between the chambers  220  and  222  reaches a shifting threshold. In a preferred embodiment, the detents include resilient elements, such as springs, that interact with tapered surfaces of the spool landings. Alternatively, the detents may be conventional mechanical detents. 
     The aft cycle valve  150  is preferably a two-position spring-biased spool valve. The spool of the cycle valve  150  has a first position, shown in  FIG. 3 , in which the valve  150  provides a flow path (represented by arrow  224 ) for the flow of fluid from the chamber  220  to the annulus  40 . The spool also has a second position, not shown in  FIG. 3 . The second position is that which the spool would be in if it is shifted vertically downward in  FIG. 3 . When the spool of the cycle valve  150  is in its second position, the valve  150  provides a flow path (represented by arrow  226 ) for the flow of fluid from the main galley  144  to the chamber  220 . 
     The spool of the cycle valve  150  has an end surface  228  exposed to fluid in the chamber  198 . The fluid in the chamber  198  imparts a pressure force onto the end surface  228 , which tends to move the spool toward its second position. An opposite end surface  230  of the spool is biased by one or more springs  232 . In the illustrated embodiment, the end surface  230  is also in fluid communication with fluid in the annulus  40 . The spring  232  imparts a spring force onto the spool, which tends to move the spool to its first position. Thus, the fluid pressure force on the end surface  228  and the spring force on the end surface  230  act against each other. When the differential fluid pressure in the chamber  198  is below a threshold, the fluid pressure force is less than the spring force and the spool occupies its first position. When the differential fluid pressure in the chamber  198  exceeds the threshold, the fluid pressure force exceeds the spring force and the spool moves to its second position. Any desired threshold can be achieved by careful selection of the spring  232 . It will be understood that the spring  232  can bear against any suitable surface of the spool or any component having a fixed relationship with the spool. It will also be understood that the spring  232  can be configured to operate primarily in tension or primarily in compression, keeping in mind the goal of biasing the spool toward its first position. 
     The forward cycle valve  152  is preferably configured similarly to the aft cycle valve  150 . The valve  152  is preferably a two-position spring-biased spool valve. The spool of the cycle valve  152  has a first position, shown in  FIG. 3 , in which the valve  152  provides a flow path (represented by arrow  234 ) for the flow of fluid from the chamber  222  to the annulus  40 . The spool also has a second position, not shown in  FIG. 3 . The second position is that which the spool would be in if it is shifted vertically downward in  FIG. 3 . When the spool of the cycle valve  152  is in its second position, the valve  152  provides a flow path (represented by arrow  236 ) for the flow of fluid from the main galley  144  to the chamber  222 . 
     The spool of the cycle valve  152  has an end surface  238  exposed to fluid in the chamber  196 . The fluid in the chamber  196  imparts a pressure force onto the end surface  238 , which tends to move the spool toward its second position. An opposite end surface  240  of the spool is biased by one or more springs  242 . In the illustrated embodiment, the end surface  240  is also in fluid communication with fluid in the annulus  40 . The spring  242  imparts a spring force onto the end surface  240 , which tends to move the spool to its first position. Thus, the fluid pressure force on the end surface  238  and the spring force on the end surface  240  act against each other. When the differential fluid pressure in the chamber  196  is below a threshold, the fluid pressure force is less than the spring force and the spool occupies its first position. When the differential fluid pressure in the chamber  196  exceeds the threshold, the fluid pressure force exceeds the spring force and the spool moves to its second position. Any desired threshold can be achieved by careful selection of the spring  242 . It will be understood that the spring  242  can bear against any suitable surface of the spool or any component having a fixed relationship with the spool. It will also be understood that the spring  242  can be configured to operate primarily in tension or primarily in compression, keeping in mind the goal of biasing the spool toward its first position. 
     The gripper control valve  148  acts as a pilot for the propulsion control valve  146 , which would stall without this pilot. The pilot action of valve  148  improves the operation of valve  146  since the operation of valve  146  controls the pressure signal to the cycle valves  150  and  152 . Without the gripper control valve  148  to isolate the valve  146  from the cycle valves  150  and  152 , the valve  146  would stall or oscillate. For example, consider a configuration in which the valve  146  controls fluid flow to the passages  196 ,  198 ,  204 , and  206  (which is not the case in the illustrated embodiment), and in which the valve  148  is eliminated. In a worst-case scenario, the system would operate as follows. When the piston  180  reaches the end of its stroke, rising pressure in the passage  196  would “open” the valve  152  (i.e., would cause the valve  152  to shift to its second position, downward in  FIG. 3 ). This would cause a pressure rise in the passage  222 , causing the spool of valve  146  to shift toward the left position (in  FIG. 3 ). As the flow path  192  begins to close, the pressure in passage  196  would decrease, causing the cycle valve  152  to close. The high pressure force on the end surface  190  of the spool of the valve  146  would be lost. Without a pressure force on the surface  190 , the spool of the valve  146  would not be able to finish the shift and would either stall in a partially shifted position or return to the first position (i.e., to the right in  FIG. 3 ). If the spool of the valve  146  returns to its first position, the pressure signal would be restored to the cycle valve  152 , which would again shift to provide a pressure signal to the spool of the valve  146 . The spool would again start to shift. This cycle would continue without the spool of the valve  146  ever completing a full shift. In the illustrated embodiment of the valve system  133 , the gripper control valve  148  ensures that the spool of the propulsion control valve  146  completes each of its shifts. A complete sequence of operation is described below. 
     As shown in  FIG. 3 , the valve system  133  preferably includes two pressure reduction valves  244  and  246 . The pressure reduction valves limit the pressure of the fluid in the gripper assemblies, and thus provide a means for preventing possible failure of the gripper assembly components. 
     The aft pressure reduction valve  244  preferably comprises a spool valve. In a first position of the spool, shown in  FIG. 3 , the valve  244  provides a flow path (represented by arrow  250 ) for the flow of fluid within the chamber  204  to a chamber or passage  248  that leads to the aft gripper assembly  104 . The valve spool is designed to be in its first position when the gripper assembly  104  is being purposefully actuated or retracted according to the operational cycle of the valve system  133 . A second position of the spool is that in which the spool is shifted partially to the left in  FIG. 3 . In the second position of the spool, the valve  244  blocks communication between the chambers  204  and  248 . The valve spool is designed to be in its second position when the gripper assembly  104  is actuated during the normal operational cycle of the valve system  133 . The second position of the spool prevents fluid from exiting the gripper assembly  104 . 
     A third position of the spool of the pressure reduction valve  244  is that in which the spool is shifted further to the left. In the third position, the valve  244  provides a flow path (represented by arrow  252 ) for the flow of fluid within the chamber  248  to the annulus  40 . In the preferred embodiment, the valve spool is designed to shift to the third position when the toes  612  (see  FIG. 21 ) of the preferred gripper assembly experience external forces, such as sliding friction between the toes and the borehole surface. These external forces can cause over-pressurization of the fluid in the gripper assembly  104 . The third position of the spool of the valve  244  allows the excess pressure to bleed to the annulus  40 . The spool has a surface  254  exposed to fluid within the chamber  248 , and an opposing surface  256  biased by one or more springs  258 . Fluid within the chamber  248  imparts a fluid pressure force onto the surface  254 , which tends to move the spool toward its third position. The spring  258  exerts a spring force that counteracts the fluid pressure force and tends to move the spool toward its first position. When the pressure in the chamber  248  exceeds a threshold determined by the spring  258 , the spool shifts to its third position. Thus, the valve  244  imposes an upper limit on the pressure in the passage  248  and thereby prevents over-pressurization of the aft gripper assembly  104  by bleeding excess pressure to the annulus  40 . 
     It will be understood that the spring  258  can bear against any suitable surface of the spool or any component having a fixed relationship with the spool. It will also be understood that the spring  258  can be configured to operate primarily in tension or primarily in compression, keeping in mind the goal of biasing the spool toward its first position. 
     The forward pressure reduction valve  246  is preferably configured similarly to the aft pressure reduction valve  244 . The forward pressure reduction valve  246  preferably comprises a spool valve. In a first position of the spool, shown in  FIG. 3 , the valve  246  provides a flow path (represented by arrow  262 ) for the flow of fluid within the chamber  206  to a chamber or passage  260  that leads to the forward gripper assembly  106 . The valve spool is designed to be in its first position when the gripper assembly  106  is being purposefully actuated or retracted according to the operational cycle of the valve system  133 . A second position of the spool is that in which the spool is shifted partially to the left in  FIG. 3 . In the second position of the spool, the valve  246  blocks communication between the chambers  206  and  260 . The valve spool is designed to be in its second position when the gripper assembly  106  is actuated during the normal operational cycle of the valve system  133 . The second position of the spool prevents fluid from exiting the gripper assembly  106 . 
     A third position of the spool of the pressure reduction valve  246  is that in which the spool is shifted further to the left. In the third position, the valve  246  provides a flow path (represented by arrow  264 ) for the flow of fluid within the chamber  260  to the annulus  40 . In the preferred embodiment, the valve spool is designed to shift to the third position when the toes  612  (see  FIG. 21 ) of the preferred gripper assembly experience external forces, such as sliding friction between the toes and the borehole surface. These external forces can cause over-pressurization of the fluid in the gripper assembly  106 . The third position of the spool of the valve  246  allows the excess pressure to bleed to the annulus  40 . The spool has a surface  266  exposed to fluid within the chamber  206 , and an opposing surface  268  biased by one or more springs  270 . Fluid within the chamber  260  imparts a fluid pressure force onto the surface  266 , which tends to move the spool toward its third position. The spring  270  exerts a spring force that counteracts the fluid pressure force and tends to move the spool toward its first position. When the pressure in the chamber  260  exceeds a threshold determined by the spring  270 , the spool shifts to its third position. Thus, the valve  246  imposes an upper limit on the pressure in the passage  260  and thereby prevents over-pressurization of the forward gripper assembly  106  by bleeding excess pressure to the annulus  40 . 
     It will be understood that the spring  270  can bear against any suitable surface of the spool or any component having a fixed relationship with the spool. It will also be understood that the spring  270  can be configured to operate primarily in tension or primarily in compression, keeping in mind the goal of biasing the spool toward its first position. 
     It will also be understood that some of the illustrated valves of the valve system  133  can be combined to provide a more condensed configuration of the valve system. The valves can be formed from various different materials, but are preferably made of a hard erosion-resistant material such as Tungsten Carbide, Ferrotic (a proprietary metal formulation), or possibly a ceramic blend. 
     Valve System Operation 
     With reference to  FIG. 3 , when the inlet control valve  136  is open, i.e., in its second position range, pressurized operating fluid flows from the inlet galley  134  to the main galley  144  of the valve system  133 . With the valves in the positions shown in  FIG. 3 , the pressurized operating fluid in the main galley  144  flows through the gripper control valve  148 , the chamber  204 , the aft pressure reduction valve  244 , the chamber  248  (which extends through the aft shaft  118 ), and into the aft gripper assembly  104 . Thus, the aft gripper assembly  104  becomes actuated and grips onto the borehole surface  42 . At the same time, fluid within the forward gripper assembly  106  flows through the chamber  260  (which extends through the forward shaft  124 ), the forward pressure reduction valve, the chamber  206 , the gripper control valve, and into the annulus  40 . Thus, the forward gripper assembly  106  becomes retracted from the borehole surface  42 . 
     With the aft gripper assembly  104  actuated and the forward gripper assembly  106  retracted, pressurized fluid within the main galley  144  flows through the propulsion control valve  146 , the chamber  196  (which extends through both shafts), and into the aft chamber  154  of the aft cylinders  108 , as well as into the forward chamber  168  of the forward cylinder  114 . Simultaneously, fluid within the forward chamber  156  of the aft cylinder  108 , as well as fluid within the aft chambers  166  of the forward cylinder  114 , flows through the chamber  198  (which extends through both shafts) and the propulsion control valve  146  into the annulus  40 . This causes the aft piston  180 , and thus the entire tractor body, to be thrust forward (to the right in  FIG. 3 ) with respect to the actuated aft gripper assembly  104 . In other words, the aft cylinder  108  performs a power stroke. Simultaneously, the forward cylinder  114  is thrust forward with respect to the piston  186  and the tractor body. In other words, the forward cylinder  114  performs a reset stroke. 
     During the above strokes of the cylinders, note that the fluid within the chamber  204  is pressurized and the fluid within the chamber  206  is depressurized. Thus, the fluid pressure force acting on the first end surface  188  of the spool of the propulsion control valve  146  is significantly larger than the fluid pressure force acting on the second end surface  190  of the spool. As a result, the spool of the valve  146  is maintained in its first position (the position shown in  FIG. 3 ). 
     Also, during the above strokes of the cylinders, the cycle valves  150  and  152  remain in their first positions (the positions shown in  FIG. 3 ). Since there is flow into the valve system  133  filling the cylinders, there is a pressure drop from the full system pressure available in the central passage  44 . This decrease in pressure maintains the cycle valves in their first positions. Thus, the chambers  220  and  222  remain in fluid communication with the annulus  40 . In this state, the fluid pressure forces on the end surfaces  216  and  218  of the spool of the gripper control valve  148  are approximately equal (the pressure within the annulus  40  may vary depending upon position). Hence, the gripper control valve  148  will remain in the position shown in  FIG. 3 , particularly since the detents (described below) require a threshold force to shift the valve spool. 
     When the cylinders complete their respective strokes, the fluid pressure in the chamber  196  will begin to rise. In contrast to when the cylinders are still stroking, the incoming flow of fluid into the system is halted. As a result, the pressure in the tractor valve system  133  will rise to the full pressure available in the center passage  44 . When the pressure in the chamber  196  exceeds a threshold associated with the spring(s)  242  of the forward cycle valve  152 , the spool of the valve  152  will shift to its second position (downward in  FIG. 3 ), permitting pressurized fluid from the main galley  144  to enter the chamber  222 . At this point, the spool of the aft cycle valve  150  is still in its first position, due to the low pressure in chamber  198 . Due to the pressure imbalance on the end surfaces  216  and  218 , the spool of the gripper control valve  148  overcomes the retaining forces of the detents and shifts to its second position (to the left in  FIG. 3 ). As a result, pressurized fluid within the galley  144  flows through the gripper control valve  148 , the chamber  206 , the forward pressure reduction valve  246 , the chamber  260 , into the forward gripper assembly  106 . This causes the forward gripper assembly to actuate and grip onto the borehole surface  42 . Simultaneously, fluid within the aft gripper assembly  104  flows through the chamber  248 , the aft pressure reduction valve  244 , the chamber  204 , the gripper control valve  148 , into the annulus  40 . This causes the aft gripper assembly to retract from the borehole surface  42 . Thus, when the gripper control valve  148  switches positions, both gripper assemblies switch between their actuated and retracted positions. 
     After the gripper control valve  148  switches its position, the fluid within the chamber  204  becomes depressurized and the fluid within the chamber  206  becomes pressurized. The resulting pressure imbalance on the end surfaces  188  and  190  causes the spool of the propulsion control valve  146  to overcome the retaining forces of its detents and shift to its second position (to the left in  FIG. 3 ). This happens when the flow of fluid into the valve system  133  stops, which occurs when the gripper assembly has come into contact with the borehole wall. When the flow stops, there is no longer a pressure drop (due to flow), and the pressure will rise to full system pressure. As a result of the shifting of the spool of the valve  146 , pressurized fluid within the main galley  144  flows through the propulsion control valve  146 , the chamber  198 , and into the forward chamber  156  of the aft cylinder  108  and the aft chamber  166  of the forward cylinder  114 . Simultaneously, fluid within the aft chamber  154  of the aft cylinder  108 , as well as fluid within the forward chamber  168  of the forward cylinder  114 , flows through the chamber  196  and the propulsion control valve  146  into the annulus  40 . This causes the forward piston  186 , and thus the entire tractor body, to be thrust forward (to the right in  FIG. 3 ) with respect to the actuated forward gripper assembly  106 . In other words, the forward cylinder  114  performs a power stroke. Simultaneously, the aft cylinder  108  is thrust forward with respect to the piston  180  and the tractor body. In other words, the aft cylinder  108  performs a reset stroke. The depressurization of the chamber  196  causes the spool of the forward cycle valve  152  to shift back to its first position (the position shown in  FIG. 3 ). 
     During the above strokes of the cylinders, the fluid within the chamber  206  is pressurized and the fluid within the chamber  204  is depressurized. Thus, the fluid pressure force acting on the second end surface  190  of the spool of the propulsion control valve  146  is significantly larger than the fluid pressure force acting on the first end surface  188  of the spool. As a result, the spool of the valve  146  is maintained in its second position (shifted to the left in  FIG. 3 ). 
     Also, during the above strokes of the cylinders, with the cycle valves  150  and  152  in their first positions (the positions shown in  FIG. 3 ), the chambers  220  and  222  are in fluid communication with the annulus  40 . In this state, the fluid pressure forces on the end surfaces  216  and  218  of the spool of the gripper control valve  148  are again equal. Hence, the gripper control valve  148  will remain in its position, particularly since the detents (described below) require a threshold force to shift the valve spool. 
     When the cylinders complete their respective strokes, the fluid pressure in the chamber  198  will begin to rise. When the pressure in the chamber  198  exceeds a threshold associated with the spring(s)  232  of the aft cycle valve  150 , the spool of the valve  150  will shift to its second position (downward in  FIG. 3 ), permitting pressurized fluid from the main galley  144  to enter the chamber  220 . At this point, the spool of the forward cycle valve  152  is still in its first position, due to the low pressure in chamber  196 . Due to the pressure imbalance on the end surfaces  216  and  218 , the spool of the gripper control valve  148  overcomes the retaining forces of the detents and shifts back to its first position (the position shown in  FIG. 3 ). As a result, pressurized fluid flows from the galley  144  through the gripper control valve  148 , the chamber  204 , the aft pressure reduction valve  244 , the chamber  248 , into the aft gripper assembly  104 . This causes the aft gripper assembly to actuate. Simultaneously, fluid within the forward gripper assembly  106  flows through the chamber  260 , the forward pressure reduction valve  246 , the chamber  206 , the gripper control valve  148 , into the annulus  40 . This causes the forward gripper assembly  106  to retract. 
     After the gripper control valve  148  switches its position, the fluid within the chamber  204  again becomes pressurized and the fluid within the chamber  206  again becomes depressurized. The resulting pressure imbalance on the end surfaces  188  and  190  causes the spool of the propulsion control valve  146  to overcome the retaining forces of its detents and shift back to its first position (the position shown in  FIG. 3 ). With the valve  146  back in its first position, pressurized fluid again flows into the aft chamber  154  of the aft cylinder  108 , and into the forward chamber  168  of the forward cylinder  114 . Simultaneously, fluid within the forward chamber  156  of the aft cylinder  108 , as well as fluid within the aft chamber  166  of the forward cylinder  114 , flows into the annulus  40 . This causes the aft cylinder  108  to perform a new power stroke. Simultaneously, the forward cylinder  110  performs a new reset stroke. The depressurization of the chamber  198  causes the spool of the aft cycle valve  150  to shift back to its first position (the position shown in  FIG. 3 ). 
     At this point, all of the valves have returned back to their original positions (the positions shown in  FIG. 3 ). Thus, the above describes a complete cycle of operation of the valve system during forward motion. Note that during forward (or backward) motion, the gripper assemblies shuttle between two extreme positions: First, the gripper assemblies move as far apart as possible toward opposite ends of the tractor. Second, the gripper assemblies move as close together as possible (with the propulsion cylinders and control assembly between them). During most of the operation of the tractor, one gripper assembly is in a power stroke while the other is in a reset stroke. When they switch directions they also switch gripper action. Hence, the tractor continually moves in one longitudinal direction. 
     A significant advantage of the preferred configuration of the valve system  133  is that the cylinders are assured of completing their respective strokes before the gripper assemblies are switched between their actuated and retracted positions. This result is achieved by (1) the provision of separate valves for controlling the flow of fluid to the gripper assemblies and to the propulsion cylinders (in the illustrated embodiment, these are the propulsion control valve  146  and the gripper control valve  148 ), and (2) piloting the gripper control valve by cycle valves that are themselves piloted by the pressure in the cylinders. This ensures that the cycle valves will open only when the pressure in the cylinders increases significantly, which in turn will occur only when the cylinders complete their strokes or when the tractor is stalled by an overload. 
     In a preferred embodiment, the valve system  133  requires an incoming flow of operating fluid of about 16 gallons per minute. Typically, large positive displacement pumps are utilized at the ground surface to pump fluid down the coiled tubing and through the internal passage  44  of the tractor. Such pumps usually supply a flow rate of about 80 to 120 gpm. Thus, since the valve system only requires a relatively small portion of the flow, the operation of the tractor has little effect on the pressure in the passage  44 . This makes the system more stable. Preferably, an orifice is provided downstream of the tractor. The orifice is designed to provide the desired back pressure (which the tractor utilizes to push/pull a specified load) at a predetermined flow rate within the passage  44 . 
     The speed of the tractor is determined by the pressure and flow rate of fluid pumped through the coiled tubing, as well as the loads experienced by the tractor. The pressure and flow rate of the fluid in the coiled tubing, which are substantially controlled by the actions of surface equipment operators, together determine the amount of hydraulic energy available in the tractor. The loads experienced by the tractor include the weight of equipment (such as the equipment  32  shown in  FIG. 1 ) pushed and pulled by the tractor, tension in the coiled tubing from the surface, frictional drag forces between the coiled tubing and the borehole, etc. The surface operators also control the injector and coiled tubing reel and thus the feed rate of the coiled tubing into the borehole. 
     Because the valve system  133  is all-hydraulic, its maximum speed is greater than an electrically controlled tractor. The valve system does not include electrical conductors and other electrical elements, which allows for larger internal fluid passages, greater flow rates, and improved power density. The faster maximum speed of the tractor results in lower operational costs, especially for intervention applications. In a preferred embodiment of the invention, the tractor is capable of moving at speeds greater than or equal to 1350 feet per hour. 
     Control Assembly 
     According to the preferred embodiment, the tractor  100  includes a control assembly  102  which houses the valve system  133  described above. One embodiment of the control assembly  102  is shown partially disassembled in  FIG. 4 . The illustrated control assembly includes a control housing  280 , an aft transition housing  282 , and a forward transition housing  284 . 
     The control housing  280  houses the inlet control valve  136 , the propulsion control valve  146 , the gripper control valve  148  (not visible, as it is located on the backside of the view of  FIG. 4 ), and the cycle valves  150  and  152 . Each valve includes an elongated valve housing defining a spool passage, and a spool. The valves are positioned within recesses in the outer surface of the control housing  280 . 
     For example, the inlet control valve  136  includes a housing  290  having a spool passage  292  sized to receive a spool. The valve housing  290  also has an external vent  294  configured to vent operating fluid into the annulus  40  between the tractor and the borehole surface. The housing  290  is positioned within a recess  296  in the outer surface of the control housing  280 . In contrast to the housings of the other valves, the inlet control valve housing  290  includes two pin receiving side portions  298  configured to receive pins or slot engagement portions  300 , for purposes described below. The ends of the housing  290  are slightly inclined from the radial direction, such that the housing has a trapezoidal axial cross-section. Two valve housing clamp elements  304  are secured into the recess  296  at each end of the valve housing  290  by bolts  306 . The clamp elements have surfaces  308  that mate closely with the inclined surfaces  302  of the valve housing  290 , thus securing the valve housing rigidly onto the control housing  280 . The aft clamp element has a vent  305 , and the forward clamp element has a vent  307 . The inner configuration of the valve housing  290  and the spool of the inlet control valve  136  are described below. 
     The propulsion control valve  146 , gripper control valve  148 , and cycle valves  150  and  152  are configured somewhat similarly to the inlet control valve  136 . Specifically, the valve housings of the valves  146 ,  148 ,  150 , and  152  are include similarly configured spool passages and vents and are secured to the control housing  280  in similar fashion. In the illustrated embodiment, the housings of the valves  146 ,  148 ,  150 , and  152  include two vents as opposed to one. Also, each of the clamp elements for the valves  146 ,  148 ,  150 , and  152  receives a single bolt as opposed to two bolts. 
     The control housing  280  includes numerous internal fluid passages for the controlled flow of operating fluid to the downhole equipment  32  ( FIG. 1 ), between the valves, to the gripper assemblies, and to the propulsion cylinders. The fluid passages are configured to effect the hydraulic circuit shown in  FIG. 3 . Some of the fluid passages extend to openings  312  in the end surfaces  310  of the control housing  280 , where they connect to openings of corresponding fluid passages in the end surfaces  316  of the transition housings  282  and  284 . Some of these fluid passages extend through the shafts  118  and  124  ( FIG. 2 ) to the gripper assemblies, the propulsion cylinders, or to downhole equipment connected to the tractor. As in the EST, within the housing  280  the internal passage  44  is shifted to one side (i.e., it is not in the center of the housing), to maximize available space for the various valves and internal fluid passages. Also, if liquid brine is used as the operating fluid, the passage  44  is not required to be as large as in the EST design, further maximizing the available space. 
     The control housing  280  is bolted to the transition housings  282  and  284  by a plurality of studs  318  and nuts  319 . The studs extend though holes  322  in the end surfaces  310  of the housing  280  into holes  324  in the end surfaces  314  of the transition housings. Recesses  320  are provided in the outer surfaces of the housing  280 , which facilitate access to the studs  318 . In the illustrated embodiment, five studs  318  are provided in the end surfaces of the housing  280  and the transition housings. 
     The aft transition housing  282  houses the diffuser  132  and the aft pressure reduction valve  244 . The aft end  326  of the housing  282  receives the internal passage  44  from the aft shaft  118  at the center axis of the tractor. Within the housing  282 , the passage  44  transitions toward one side of the housing. Thus, the housing  282  moves the passage  44  to one side to maximize space for the valves and various fluid passages within the control housing  280 . The diffuser  132  is positioned on the forward end  314  of the housing  282 . As in the EST, the diffuser  132  is generally cylindrical and has a plurality of side holes  328  for directing the flow from the passage  44  into the inlet galley  134  of the inlet control valve  136 . In one embodiment, the side holes  328  are angled so that the fluid passing forward through the diffuser must turn somewhat aftward to enter the inlet galley  134 . This prevents larger particles within the operating fluid from entering the valve system  133 , as it is more difficult for the larger particles to overcome forward momentum and flow through the side holes  328 . Those of ordinary skill in the art will understand that any of a variety of different types of filters can be used instead of the illustrated diffuser  132 . 
     The aft pressure reduction valve  244  includes a valve housing  330 . The valve housing  330  is configured similarly to the housings of the valves within the control housing  280 . Specifically, the valve housing  330  includes a similarly configured spool passage  332  and vents  334 . In the illustrated embodiment, the valve housing  330  includes two vents  334 . Also, the valve housing  330  is secured into a recess  338  of the aft transition housing  282  by the use of clamp elements  336 , in similar fashion as the aforementioned valve housings are secured to the control housing  280 . The recess  338  includes several openings  344 . The openings  344  comprise ends of fluid passages that conduct fluid to and from corresponding side passages in the valve housing  330  of the valve  244  (such as the side passages  477  and  479  shown in  FIG. 13 ), as described in further detail below. It will be understood that the corresponding recesses for all of the valve housings of the housings  280  and  284  (such as the recess  296  of the inlet control valve  136 ) have openings of fluid passages that communicate flow through the valves. 
     The forward transition housing  284  is configured generally similarly to the aft transition housing  282 . One difference is that the aft housing  282  is configured to accommodate the diffuser  132  and has a fluid passage for the inlet galley  134 , whereas the forward housing  284  does not require these features. Also, the forward housing  284  transitions the internal passage  44  back to the center axis of the tractor. 
       FIG. 5  shows a longitudinal cross-section of the assembled control assembly  102  of  FIG. 4 , with the aft end on the right and the forward end on the left. This particular section shows the configuration of the inlet control valve  136 . Also shown in  FIG. 5  are several internal fluid passages, which comprise some of the flow lines, chambers, passages, and galleys schematically illustrated in  FIG. 3 . One of skill in the art will understand that the internal fluid passages can have any of a large variety of configurations. 
     Inlet Control Valve 
       FIG. 6  is an exploded view of the inlet control valve  136  shown in  FIG. 5 , which includes the valve housing  290 , an elongated spool  346 , and a set of springs  140  biasing the spool to the right of the figure. The valve housing  290  defines an elongated generally cylindrical spool passage  292  that receives the spool  346 . The inner surface of the passage  292  has annular recesses  362 ,  364 , and  366  (commonly referred to as “galleys”), in which the passage has a slightly enlarged inner diameter. The valve housing  290  also includes side passages or fluid ports  348 ,  350 ,  352 , and  354  that are open to the spool passage  292 . When the valve housing  290  is secured onto the control housing  280 , these ports align with openings of fluid passages in the housing  280 . The ports  348  and  352  are in fluid communication with the main galley  144  of the valve system  133 . The ports  350  and  354  are in fluid communication with the inlet control galley  134 . The ports  348 ,  350 , and  352  are located within the annular recesses  362 ,  364 , and  366 , respectively. The port  354  is located aftward of the second end surface  138  of the spool  346 . The port  354  permits fluid within the inlet galley  134  to impart a pressure force against the end surface  138 , which tends to move the spool  346  toward its second and third position ranges (to the left in  FIG. 6 ). The housing  290  further includes the aforementioned vents  294 ,  305 , and  307 . The port  305  is non-functional in this configuration. It exists only because it is desirable to have identical designs for the clamp elements  304 , and because a vent is desired within the forward clamp element. On the aft end of the valve housing  290 , a plug  374  and an O-ring seal are provided to prevent fluid on the second end surface  138  of the spool  346  from flowing out to the annulus  40  through the vent  305 . 
     As described above, the first end surface  139  of the spool  346  is in contact with a set of springs  140  that bias the spool  346  aftward, or to the right in  FIG. 6 . In a preferred embodiment, Belleville springs are stacked in 30 sets in series, each set containing three springs in parallel. This configuration provides a desired spring rate and resultant deflection. The spool  346  has three “landings”  356 ,  358 , and  360 . These landings comprise larger diameter portions that effect a fluid seal of the spool passage  292 , as known in the art. In other words, each landing slides within the passage and prevents fluid on one side of the landing from flowing to the other side of the landing. The spool  346  also includes a locking feature to lock the spool in its third position range, in which the inlet control valve  136  is closed at high pressure. In the illustrated embodiment, the locking feature comprises a deactivation cam  368 , described in further detail below. 
     As explained above, the spool  346  has first, second, and third position ranges. In the first and third ranges, the inlet control valve  136  provides a flow path for fluid from the main galley  144  of the valve system to vent into the annulus  40 , and prevents fluid within the inlet galley  134  from flowing through the valve  136  into the main galley  144 . In the second range, the valve  136  provides a flow path for fluid within the inlet galley  134  to flow into the main galley  144 , and prevents fluid within the main galley  144  from flowing through the valve  136  into the annulus  40 . 
     In  FIG. 6 , the spool  346  is shown in its first position range, shifted to the right. In this position, fluid from the main galley  144  flows through the fluid port  348 , past the forward end of the landing  356 , through the spool passage  292 , and out to the annulus  40  through the vent  307 . The spool  346  occupies this position when the pressure in the inlet galley  134  is below a lower shut-off threshold (e.g., 800 psid). As the pressure in the galley  134  rises, the fluid pressure force acting on the second end surface  138  of the spool  346  increases and pushes the spool to the left in  FIG. 6 , until the fluid pressure force is equalized by the spring force from the springs  140 . When the pressure in the inlet galley  134  exceeds the lower shut-off threshold, the spool  346  moves to the left in  FIG. 6  until it occupies a position within its second range. In this position, the landing  356  blocks flow between the port  348  and the vent  307 , and permits flow between the ports  348  and  350 . Fluid now flows from the inlet control galley  134  through the port  350 , the spool passage  292 , the port  348 , and into the main galley  144 . Fluid within the galley  144  is prevented from flowing through the valve  136  into the annulus  40 . When the pressure in the inlet galley  134  exceeds an upper shut-off threshold (e.g., 2100 psid), the spool  346  moves further left in  FIG. 6  until it occupies a position within its third range. In this position, the landing  358  blocks flow through the port  350  but permits flow between the port  352  and the vent  294 . Fluid flows from the main galley  144  through the port  352 , the spool passage  292 , the vent  294 , into the annulus  40 . 
     A spring adjustment screw  370  is preferably provided to adjust the compression of the springs  140 . In the illustrated embodiment, the screw  370  is accessible via a recess  372  in the control housing  280 , which is also shown in  FIG. 4 . Adjustment of the screw  370  permits the shut-off threshold pressures of the inlet control valve  136  to be adjusted. 
     As shown in  FIG. 6 , the landings  356 ,  358 , and  360  include “centering grooves”  376 . The grooves  376  comprise circumferential grooves oriented generally perpendicular to the spool passage  292 . The grooves  376  reduce leakage across the landings by providing a series of expansions and contractions in the leak path. Also, the grooves effectively equalize pressure around the circumference of the landing. During operation, fluid within the valve tends to push the spool against the side of the spool passage. By equalizing the pressure around the landings, the centering grooves cause the spool to remain more accurately centered within the spool passage. As a result, less energy is required to move the spool, and the valve operates more efficiently and reliably. Further, the centering function reduces leakage. The concentric relationship between the landings and the spool passage minimizes the largest width of the leak path. The grooves  376  also provide a region for small particles to deposit, which further prevents jamming of the spool within the spool passage. Any number of centering grooves can be provided on each of the landings of the spool  346 . In the preferred embodiment, the grooves have a depth between 0.010 and 0.030 inches, and a width between 0.010 and 0.020 inches. 
       FIGS. 7 and 8  further illustrate the deactivation cam  368  of the spool  346  of the inlet control valve  136 . The cam  368  forms a portion of the spool  346  and is preferably axially fixed, but rotationally free, with respect to the remainder of the spool. The cam  368  comprises a large diameter portion  378  having a first portion  382  and a second portion  384  separated by an annular cam path recess  380 . The peripheral surface of the first portion  382  includes at least one slot  386  oriented parallel to the spool passage  292  and extending into the recess  380 . In the preferred embodiment, four slots  386  are provided in the peripheral surface of the first portion  382  and are spaced at 90° intervals (with respect to the longitudinal axis of the spool  346 ) around the circumference of the cam  368 . Each slot  386  is sized and configured to receive a slot engagement portion of the valve housing  290 . At least one slot engagement portion is provided within the spool passage  292 . The slot engagement portion extends radially inward from an inner surface of the spool passage  292 . Preferably, there are two slot engagement portions, on opposite sides of the spool passage separated by 180°. In the preferred embodiment, the slot engagement portions comprise pins  300  ( FIG. 4 ) received within side walls of the valve housing  290 . 
     The cam path recess  380  of the deactivation cam  368  is defined partially by a first annular sidewall  388  and a second annular sidewall  390 . The sidewalls  388  and  390  include a plurality of cam surfaces  392  and valleys  394 . As used herein, a “valley” refers to a region of the sidewall in which one of the slot engagement portions can become restrained within when the slot engagement portion bears against the sidewall  388  or  390 . The cam surfaces  392  are angled with respect to the axis of the spool  346 . In the preferred embodiment, the cam surfaces  392  are oriented at angles of about 60° with respect to the axis of the spool  346 . The valleys  394  are configured to receive the slot engagement portions, such as the pins  300 . When the pins  300  are not received within the slots  386 , the cam  368  can freely rotate about the longitudinal axis of the spool passage  292 . In a less preferred embodiment, the spool  346 , including the deactivation cam  368 , is rotatable about its longitudinal axis within the spool passage  292 . 
     When the spool  346  is in its first position range, as defined above, the pins  300  are received within the slots  386  of the deactivation cam  368 , preventing the cam from rotating. In the first position range, the pins  300  are positioned near the first ends  396  of the slots  386 . As the spool  346  moves to its second position range, the cam  368  moves toward the springs  140  ( FIG. 6 ) and the cam path recess  380  moves closer to the pins. However, the pins  300  remain within the slots  386 . When the spool  346  moves to the lower endpoint of its third position range (i.e., when the pressure in the inlet galley  134  reaches the lower shut-off threshold pressure, as explained above), the pins  300  are still within the slots  386 . As the pressure within the inlet galley  134  continues to rise, the pins  300  eventually enter the cam path recess  380 , at which point the cam  386  becomes free to rotate. When the pressure in the inlet galley  134  reaches an upper cam activation pressure (e.g., 2500 psid), which is above the upper shut-off threshold pressure (e.g., 2100 psid), cam surfaces  392  of the first sidewall  388  bear against the pins  300 . This causes the cam  368  to rotate in a first direction (so that the labeled slot  396  moves upward in  FIG. 7 ) until each pin  300  is nestled in a valley  394  of the first sidewall  388 . In a preferred embodiment, the cam surfaces  392  are configured similarly, such that the spool  346  rotates 22.5°. If the pressure in the inlet galley  134  increases beyond the upper cam activation pressure, the pins  300  nestled within the valleys  394  of the first sidewall  388  prevent the spool  346  from moving further toward the springs  140 . 
     With the cam  368  in this rotated position, the pins  300  are no longer aligned with the slots  386 . If the fluid within the inlet galley  134  (or in the passage  44 —it will be understood that the pressure within the passage  44  is very closely equal to the pressure in the galley  134 ) is depressurized only once, the pins  300  will not re-enter the slots  386 . Rather, the pins  300  are now restrained within the cam path recess  380 . In this locked position of the valve  136 , the spool  346  is in its third position range, such that the fluid within the valve system  133  is free to vent to the annulus  40 . In this position, the tractor is in a failsafe mode, i.e., a mode in which the gripper assemblies are depressurized and retracted from the borehole surface  42 . A significant advantage of this failsafe mode is that equipment connected to the tractor can undertake activities without risking damage to the gripper assemblies. For example, perforation guns can be operated with the gripper assemblies assured of being retracted, thus preventing or minimizing any possible damage to the gripper assemblies. Also, with the gripper assemblies assured of being retracted, they cannot cause the perforation guns to be erroneously moved. The failsafe mode also makes it possible to pull the tractor out of the borehole in case of an emergency. 
     After the cam surfaces  392  of the first sidewall  388  bear against the pins  300  for the first time and cause the cam  368  to initially rotate in the first direction, a subsequent first depressurization of the fluid within the inlet galley  134  below a lower cam-activation pressure (which is above the upper shut-off threshold) causes the deactivation cam  368  to move to the right in  FIG. 7 , so that cam surfaces  392  of the second sidewall  390  bear against the pins  300 . This causes the cam  368  to rotate further in the first direction, until each pin  300  is nestled within a valley  394  of the second sidewall  390 . In the preferred embodiment, the cam surfaces  392  of the second sidewall  390  are configured so that the cam rotates another 22.5°. At this point, the cam has rotated a total of 45° from the time the spool  346  was last in its first or second position ranges. The spool  346  is still restrained within its third position range. If the fluid in the inlet galley  134  is further depressurized, the pins  300  nestled within the valleys  394  of the second sidewall  390  will prevent the spool  346  from moving into its second (or “operating”) position range. 
     Thus, as described above, a single pressure spike of the fluid in the inlet galley  134  to the upper cam activation pressure causes the entry control valve  136  to move to its locked position, in which the gripper assemblies are assured of being retracted. 
     The deactivation cam  368  is preferably configured so that, in order to move the spool  346  back into its second or first position ranges, it is necessary to again pressurize the fluid within the inlet galley  134 . In the illustrated embodiment, this repressurization must occur after the pressure was first lowered from the upper cam activation threshold to the lower cam activation threshold. With the pins  300  restrained within the cam path recess  380  and nestled within valleys  394  of the second sidewall  390 , a repressurization of the fluid within the inlet galley  134  to the upper cam activation pressure causes the spool  346  to move to the left in  FIG. 7 , so that the pins  300  again bear against cam surfaces  392  of the first sidewall  388 . The cam  368  again rotates in the first direction (again, preferably 22.5°, such that the cam will have rotated a total of 67.5° since the spool  346  was last in its first or second position ranges) until each pin is again nestled within a valley  394  of the first sidewall  388 . Then, a subsequent second depressurization of the fluid within the inlet galley  134  causes the spool  346  to move to the right in  FIG. 7 . When the pressure decreases to the lower cam activation level, each pin  300  bears against a partial cam surface  398  just “above” (see  FIG. 7 ) one of the slots  386 . As the pressure in the galley  134  continues to drop, the pins  300  slide along the cam surfaces  398  such that the cam rotates another 22.5° in the first direction. At this point, the cam  368  will have rotated a total of 90° since the spool  346  was last in its first or second position ranges. This causes the pins  300  to reenter the slots  386 , although each pin is now in a different slot than before. The reengagement of the pins  300  within the slots  386  prevents the cam  368  from rotating further and permits the spool  346  to move into its second and first position ranges. 
     The spool  346  of the inlet control valve  136  can have variable diameter sections to allow some degree of throttling of the fluid into the tractor. This configuration provides some control over the pressure drop and speed of the tractor. In one embodiment, the landings of the spool  346  include notches, such as the notches  438  shown in  FIG. 11  and described below. Thus, it will be understood that, in industry parlance, the valve  136  is commonly referred to as a “four-way valve,” as it has a throttling position. 
     If desired, the cam  368  could be made to be completely rigid with respect to the remainder of the spool. However, such a configuration would require more force to rotate the cam and is thus less desirable than the preferred configuration described above. 
     Propulsion Control and Gripper Control Valves 
     The propulsion control valve  146  and the gripper control valve  148  function similarly. They are both piloted by fluid pressure on both sides. In a preferred embodiment, the valves  146  and  148  are configured substantially identically. Thus, only the propulsion control valve  146  is herein described. 
     Preferably, the propulsion control valve  146  almost has a “critically lapped spool design.” A critically lapped valve has no “center” position (or third position), which would allow the valve to be closed. In this case, a closed propulsion control valve would render the tractor non-operational. Instead, the valve  146  is preferably “overlapped,” which assures that fluid flows to only one of the chambers  196  and  198  ( FIG. 3 ). An overlapped design also keeps leakage to a minimum. In contrast, an “under lapped” design would allow fluid to simultaneously flow to both of the chambers  196  and  198 . Preferably, the valve  146  is not under lapped. 
       FIG. 9  is a longitudinal sectional view of the preferred embodiment of the control assembly  102 , with the aft end shown on the left and the forward end on the right.  FIG. 9  shows the propulsion control valve  146  in cross-section. The valve  146  is located toward the forward end of the control housing  280 .  FIG. 10  is an exploded view of the valve  146  as depicted in  FIG. 9 . In the preferred embodiment, the valve  146  functions as a two-position spool valve with detents that tend to retain the spool within one of its two main positions. In reality, it is a three-position valve with a center (blocked) position. However, the spool resides within its center position for only about 0.005 inches of a total spool stroke of 0.35 inches, which makes the center position relatively insignificant. In the illustrated embodiment, the valve  146  includes a valve housing  410  having an internal cylindrical spool passage  412 . Plugs  414  with O-rings seal the ends of the spool passage  412 . The valve housing  410  includes two vents  416  and  418 . Two clamp elements  440  secure the ends of the valve housing  410  to the control housing  280  via bolts  426 . 
     In the illustrated embodiment, the valve housing  410  includes fluid ports  430 ,  422 ,  420 ,  424 , and  432 , which align with openings of fluid passages within the control housing  280 . The ports  430  and  432  provide pilot pressures that control the position of the spool  400 . The ports  430  and  432  fluidly communicate with chambers  204  and  206 , respectively. Fluid from the chamber  204  flows through the port  430  into the spool passage  412  and imparts a pressure force against the end surface  188  of the spool  400 . Fluid from the chamber  206  flows through the port  432  into the spool passage  412  and imparts a pressure force against the end surface  190  of the spool  400 . The ports  422 ,  420 , and  424  fluidly communicate with the chamber  198 , the main galley  144 , and the chamber  196 , respectively. 
     Near the ends of the valve housing  410 , the inner surface of the spool passage  412  includes two grooves  442 . Each groove  442  is preferably circular and sized to receive a resilient stop  434 ,  436 . The stops  434  and  436  perform a detent function; they tend to retain the spool  400  in one of its two main positions. Each stop  434 ,  436  preferably defines an inner diameter and is positioned at least partially within the groove  442 . Each stop  434 ,  436  has a relaxed position in which it has a first inner diameter and in which at least an inner radial portion of the stop is positioned outside of the groove  442 . Each stop  434 ,  436  also has a deflected position in which it has a second inner diameter larger than the first inner diameter. Preferably, in its deflected position, substantially all of the stop is in the groove  442 . In a preferred embodiment, each stop  434 ,  436  comprises an expandable ring-shaped spring. However, various other configurations are possible. For example, each stop could alternatively comprise a plurality of (e.g., three) circumferentially separated stop portions that extend radially inward from the inner surface of the spool passage  412 . 
     The valve  146  includes a spool  400  having four landings  402 ,  404 ,  406 , and  408 . In the preferred embodiment, each of the two ends of each of the outer landings  402  and  408  have a radially tapered section followed by a generally constant diameter section that intersects the bottom of the taper. The tapered section has a tapered peripheral or radial surface  428 . The tapered or conical surfaces  428  operate in conjunction with the stops  434 ,  436  to provide the detent function. The tapered surfaces  428  also function to prevent the stops  434 ,  436  from falling out or being washed out of the grooves  442 . In their relaxed positions, each stop  434 ,  436  is configured to bear against or be in very close proximity to one of the tapered peripheral surfaces  428  of the landings  402  and  408 , while being immediately radially outside of the reduced constant diameter section that intersects the bottom of the taper. It is this reduced diameter section that retains the stop from inadvertently being removed from the groove  442 . The resilient stops are configured so that the landings  402  and  408  cannot move across the stops until the net longitudinal movement force on the spool  400  (from the fluid pressure on the end surfaces  188  and  190 ) reaches a threshold at which the tapered surfaces  428  of the landings cause the stops to move to their deflected positions. In their deflected positions, the stops  434 ,  436  permit the landings  402  and  408  to move across the stops. As used in this context, the terms “longitudinal” and “axial” refer to the longitudinal axis of the spool  400 . Preferably, the shifting threshold of the valve  146  is relatively low, preferably between 250 and 800 psid. 
     As described above, the spool  400  of the propulsion control valve  146  has two main positions. The position shown in  FIG. 10  corresponds to the above-described first position (shown in  FIG. 3 ). In this position, fluid flows from the main galley  144  through the port  420 , the spool passage  412 , the port  424 , and into the chamber  196 . Simultaneously, fluid in the chamber  198  flows through the port  422 , the spool passage  412 , the vent  416 , and into the annulus  40 . As the fluid pressure forces against the end surfaces  188  and  190  fluctuate, the stops  434  and  436  bear against tapered surfaces  428  of the landings  402  and  408 , respectively, to maintain the spool  400  in the position shown in  FIG. 10 . When the pressure differential acting on the end surfaces  188  and  190  (the force acting on end surface  190  being larger) reaches a threshold, the pressure force on the spool  400  exceeds the retaining forces of the stops  434 ,  436 . The tapered surfaces  428  force the stops to move to their deflected positions, such that the spool  400  is permitted to shift to its second main position (to the left in  FIGS. 3 and 10 ). After the spool  400  shifts, the stops  434 ,  436  move back to their relaxed positions and bear against or come in close proximity to the tapered surfaces  428  on the opposite sides of the landings  402  and  408 . The spool  400  is thus maintained in its second position by the stops&#39; contact with or close proximity to the tapered surface. The spool is prevented from moving away from the stop by the spool ends bearing against or being in close proximity to the end plugs  414 . In the second position of the spool, fluid flows from the main galley  144  through the port  420 , the spool passage  412 , the port  422 , and into the chamber  198 . Simultaneously, fluid in the chamber  196  flows through the port  424 , the spool passage  412 , the vent  418 , and into the annulus  40 . The spool  400  will not shift back to its first position until the pressure differential acting on the end surfaces  188  and  190  (the force acting on end surface  188  being larger) reaches the aforementioned threshold necessary to again overcome the retaining forces of the stops  434 ,  436 . 
     The landings of the spool  400  preferably include centering grooves  326 , similar to those of the inlet control valve spool  346  described above. In the illustrated embodiment, the center landings  404  and  406  each include three centering grooves, and the outer landings  402  and  408  each include two centering grooves. Any number of centering grooves can be provided on each landing. 
     The center landings  404  and  406  preferably include a plurality of notches  438  (preferably between 3 and 8) at each end. The notches  438  permit a small amount of fluid flow past the landings when the landings are almost in a completely closed position with respect to a fluid port. The notches  438  help to reduce hydraulic shock caused by the sudden flow of fluid into a valve (commonly referred to as “hammer”). Thus, the notches help decrease wear on the valves. The skilled artisan will understand that notches can be included on some or all of the landings of the valves of the tractor  100 . The notches  438  are preferably V-shaped.  FIG. 11  shows an exemplary notch  438 , having an axial length L extending inward from the edge of the landing, a width W at the edge of the landing, and a depth D. In one embodiment, L is about 0.055-0.070 inches, W is about 0.115-0.150 inches, and D is about 0.058-0.070 inches. Preferably, the positions of the notches  438  are carefully controlled, as the notches provide the lapping function of the valve  146 . 
     As mentioned above, the gripper control valve  148  is preferably configured substantially identically to the propulsion control valve  146 . One difference is that, in the valve  148 , the fluid ports analogous to the fluid ports  430 ,  422 ,  424 , and  432  of the valve  146  are in fluid communication with the chambers  220 ,  206 ,  204 , and  222 , respectively. Also, the gripper control valve  148  can be significantly smaller than the propulsion control valve  146 , because the flow through the valve  148  can be significantly less. 
     In a preferred embodiment, the stops  434 ,  436  of the propulsion control valve  146  have about twice the detent force of analogous stops within the gripper control valve  148 . In one embodiment, only one stop is provided within the valve  148 , as opposed to two in the valve  146 . Also, it is possible to use stops of differing stiffness or grooves  442  of differing diameter to adjust the detent force, keeping in mind the goal of ensuring that upon the completion of the strokes of the propulsion cylinders the gripper assemblies switch between their actuated and retracted positions before the valve  146  switches positions. It will also be understood that the detent force can be modified by adjusting the angles of the tapered sections  428  of the spools. 
     Cycle Valves 
     In the preferred embodiment, the cycle valves  150  and  152  are configured substantially identically. Thus, only the aft cycle valve  150  is herein described. 
       FIG. 12  shows a longitudinal sectional view of the aft cycle valve  150 , according to a preferred embodiment, with the aft end shown on the left and the forward end shown on the right. With reference to the inlet control valve  136  and the propulsion control valve  146  described above, the cycle valve  150  includes a generally similarly configured valve housing  444 . The housing  444  has an internal cylindrical spool passage  445  and includes vents  446  and  448 . The housing  444  also includes fluid ports  450 ,  452 , and  454  that fluidly communicate with the chamber  198 , the main galley  144 , and the chamber  220 , respectively. The valve  150  includes a spool  456  with landings  458 ,  460 , and  462  as shown. One or more of the landings preferably include centering grooves  376  as described above. The spool  456  has end surfaces  228  and  230 . The end surface  228  is in fluid communication with the fluid in the chamber  198 , via the port  450 . A spring, and more preferably a set of springs  232  (preferably Belleville springs), bears against the end surface  230 , such that the springs bias the spool  456  to the left in  FIG. 12 . 
     As explained above, the spool  456  of the valve  150  has a first position and a second position. The spool  456  is shown in its first position in  FIG. 12 . In this position, fluid within the chamber  220  flows through the port  454  and the spool passage  445 , within the springs  232 , through the vent  448 , and out into the annulus  40 . The fluid from the chamber  198  imparts a pressure force against the end surface  228 , which tends to push the spool  456  to its second position (to the right in  FIG. 12 ). When the fluid pressure force on the end surface  228  exceeds an actuation threshold, the spool  456  moves such that the landing  462  blocks the flow of fluid between the port  454  and the vent  448 , and permits flow between the ports  452  and  454 . When the spool  456  is in its second position, fluid within the main galley  144  flows through the port  452 , the spool passage  445 , the port  454 , and into the chamber  220 . Preferably, the actuation threshold of the valve  150  is between 800 and 1500 psid, or possibly even as high as 2000 psid. The vent  446  is non-operational. It exists only because of a preference that all of the valve housings have the same configuration, to keep manufacturing costs down. 
     As mentioned above, the forward cycle valve  152  is preferably configured substantially identically to the aft cycle valve  150 . One difference is that, in the valve  152 , the fluid ports analogous to the fluid ports  450  and  454  of the valve  150  are in fluid communication with the chambers  196  and  222 , respectively. If desired, the valves  150  and  152  can be provided with screws to permit adjustment of the spring forces of the springs. Such screws can compensate for variance in manufacturing tolerances. 
     Pressure Reduction Valves 
     In a preferred embodiment, the pressure reduction valves  244  and  246  are configured substantially identically. Thus, only the aft pressure reduction valve  244  is herein described. 
       FIG. 13  shows a longitudinal sectional view of the aft pressure reduction valve  244 , according to a preferred embodiment, with the aft end shown on the right and the forward end shown on the left. The valve  244  includes a valve housing  330  configured generally similarly to those of the valves described above. The housing  330  has an inner cylindrical spool passage  332  with an annular recess  478 . The housing  330  also includes two vents  334 , as well as fluid ports  477  and  479  that fluidly communicate with the chambers  248  and  204 , respectively. Each of the ports  477  and  479  is aligned with a fluid passage opening  344  in the aft transition housing  282  ( FIG. 4 ). The port  477  is open to the annular recess  478  of the valve  244 . The valve housing  330  is secured via clamp elements  336  and bolts to the aft transition housing  282 . 
     The valve  244  includes a spool  458  comprising a first spool portion  460  and a second spool portion  462 . The second spool portion  462  is preferably a spring guide. The spool portion  460  includes landings  470 ,  472 , and  474  as shown. In some embodiments, one or more of the landings include centering grooves as described above. The spool portion  460  also includes a center-drilled passage  482  and a side passage  480 . The passage  482  extends from the aft end of the spool portion  460  to the longitudinal position (in this context, the term “longitudinal” refers to the axis of the spool passage) of the side passage  480 . The spool portion  460  is configured so that in normal operation the side passage  480  is positioned within the annular recess  478  of the spool passage  332 . The side passage  480  is fluidly open to the center-drilled passage  482  so that fluid within the chamber  248  can flow into the passage  482 . The fluid within the center-drilled passage  482  imparts a pressure force against the surface  254 , which tends to push the spool  458  to the left in  FIG. 13 . As referred to herein, the surface  254  can include the aft end surface of the spool portion  460 , outside of the passage  482 . 
     The spool portion  462  has a flange  484  that defines an annular surface  256 . A spring  258  is positioned between the surface  256  and an end plug  476 . The spring  258  biases the spool portion  462  to the right in  FIG. 13 . In the illustrated embodiment, the spring  258  comprises a coil spring (only one coil is shown in  FIG. 13 ) coiled around an elongated portion of the spool portion  462 . In the preferred embodiment, there is always a clearance between a flange  484  of the spool portion  462  and an annular step  486  formed within the spool passage  332 . 
     The spool portions  460  and  462  have opposing end surfaces with partially tapered and preferably partially conical ball-receiving recesses  466  and  468 , respectively. A ball  464  is interposed between the spool portions  460  and  462 , partially within the ball-receiving recesses  466  and  468 . Preferably, the recesses  466  and  468  are configured to only partially receive the ball  464 , so that the ball makes contact with both spool portions. The presence of the ball  464  and the ball-receiving recesses  466  and  468  results in improved alignment of the spool  458  within the spool passage  332 , which in turn results in reduced leakage and more efficient operation. 
     As explained above, the spool  458  of the valve  244  has first, second, and third positions. The spool  458  is shown in its first position in  FIG. 13 . In this position, fluid within the chamber  204  flows through the port  479  across the forward end of the landing  472 , and through the spool passage  332 , the port  477 , and into the chamber  248 . When the fluid pressure force on the surface  254  exceeds an actuation threshold, the spool  458  moves to its second position (shifted partially to the left in  FIG. 13 ). In this position, the landing  472  blocks fluid flow between the ports  477  and  479 , which stops the flow into the aft gripper assembly  104  ( FIG. 3 ). This spool will normally be in the second position when the gripper assembly is actuated. If the pressure in the chamber  248  is further increased, such as by an external friction force on the gripper assembly, the spool shifts further left to its third position. In the third position, excess pressure in the chamber  248  bleeds past the aft end of the landing  472  through the aft vent  334  into the annulus  40 . The forward vent  334  accommodates volume changes on the left side of the landing  470  as the spool moves to the left. 
     As mentioned above, the forward pressure reduction valve  246  is preferably configured substantially identically to the aft pressure reduction valve  244 . One difference is that, in the valve  246 , the fluid ports analogous to the fluid ports  477  and  479  of the valve  244  are in fluid communication with the chambers  260  and  206 , respectively. 
     Shaft Configuration and Manufacturing Process 
     With reference to  FIG. 2 , a process for manufacturing the shafts  118  and  124  of the tractor  100  is herein described. 
     As explained above in the Background section, prior art shafts designed for downhole tools used in drilling and intervention applications have been formed from more flexible materials, such as copper beryllium (CuBe), in order to facilitate turning at sharper angles in the bore of a well. Due to the various constraints of CuBe and other materials, prior art individually gun-drilled shaft portions have been attached to one another by electron beam welding, a very expensive process. The geometry of prior art shafts (e.g., larger internal passages necessitated by drilling mud) and the constraints of softer materials like CuBe have limited the possible length of gun-drilled passages and required a relatively large number of gun-drilled shaft portions. 
     In one aspect, the present invention provides a shaft design and manufacturing method for a tractor to be used primarily for intervention. In contrast to drilling, intervention applications are typically undertaken in cased boreholes and do not require the ability to negotiate sharp turns. In contrast to drilling tools, which typically use drilling mud having larger solid particles, an intervention tractor can use an operating fluid such as clean brine, and thus does not require as large an internal flow passage for fluid to the downhole equipment and valve system. Accordingly, a preferred embodiment of a tractor of the present invention includes a shaft with a relatively smaller internal flow passage for fluid to the downhole equipment and valve system. Also, the shaft is preferably formed from a stronger, more rigid material. The combination of a smaller diameter flow passage, which leaves more space for gun-drilled passages, and a stronger material of the shaft makes it possible to gun-drill longer passages. This in turn allows for fewer shaft portions. In a preferred embodiment of the invention, each shaft  118  and  124  ( FIG. 2 ) includes only two shaft portions and an end flange. 
       FIG. 14  shows a preferred embodiment of the forward shaft  124  of the tractor of the invention. In this embodiment, the tractor includes only a single forward propulsion cylinder  112  enclosing a single piston. The forward gripper assembly is not shown for clarity, but would typically be located generally at position  490 . Attached to the forward end of the shaft  124  is a tool joint assembly  129  for attachment to downhole equipment. The assembly  129  includes an internal bore for the passage  44  for operating fluid to the downhole equipment. The aft end of the shaft  124  is welded to a flange  488  for connection to the forward end of the control assembly  102  ( FIG. 2 ). The shaft  124  preferably includes a first shaft portion  494  and a second shaft portion  496 . The shaft portions are preferably brazed together, as described below. The braze joint is located, for example, at about the position  492 . The braze joint is enclosed by the cylinder  112 . 
       FIG. 15  shows the forward end of a preferred embodiment of the first shaft portion  494  of  FIG. 14 . Preferably, the end surfaces of the first shaft portion  494  and the second shaft portion  496  are configured to mate with each other. The illustrated forward end of the first shaft portion  494  comprises a male connection, while a conforming aft end of the second shaft portion  496  is female. The shaft portion  494  includes an elongated end portion  498  having a reduced width (which may include non-circular configurations) or diameter (for circular configurations). The portion  498  has a peripheral surface  500  and an end surface  502 , and is preferably about one inch long. A connecting annular surface  504  is formed between the end portion  498  and the remainder of the shaft portion  494 . In the illustrated embodiment, the end surface  502  and the connecting surface  504  are generally flat and perpendicular to the longitudinal axis of the first shaft portion  494 . However, other configurations are possible, such as tapered surfaces. 
     A “mating surface” of the first shaft portion  494  comprises the surfaces  502 ,  500 , and  504 . The second shaft portion  494  preferably has a “mating surface” that mates with that of the first shaft portion  494 . Other mating surface configurations are possible, giving due consideration to the goal of forming a strong joint that is capable of withstanding combined tensile, shear, and bending loads experienced downhole. At the outside diameter of the shaft portion  494 , an edge  506  is formed between the connecting surface  504  and the remainder of the shaft portion  494 . The illustrated edge  506  is circular and forms an outer interface between the first and second shaft portions when they are attached together. Bores  508  form fluid passages within the shaft portion  494  (for the flow to the gripper assemblies and propulsion chambers), while a larger center bore forms the main passage  44  ( FIG. 3 ). In the illustrated embodiment, the outside diameter of the end portion  498  interrupts the passages. 
     Preferably, a stress-relief groove  510  is formed proximate the mating surface of the first shaft portion  494 . The groove  510  provides a stress concentration point to reduce the stresses felt at the outside diameter of the joint between the first and second shaft portions. Thus, the groove  510  further reduces the risk of failure at the joint by taking the stress away from the outside diameter of the shaft, where stresses are typically at a maximum. Preferably, the groove  510  extends along the entire or substantially the entire circumference of the outer diameter of the shaft portion  494 . The groove  510  is preferably circular. The longitudinal position, as well as the width and depth, of the groove  510  can vary, keeping in mind the goal of pulling stress away from the outermost edge of the brazed connection. The groove  510  is desirably positioned within 0.060 inches of the edge  506 . Preferably, the groove  510  has a width between 0.080 and 0.120 inches, and a depth between 0.050 and 0.060 inches. 
     In the preferred embodiment, the mating surfaces of the first and second shaft portions are silver brazed together. The silver braze connection is formed by placing a brazing shim on the end surface  502  and then mating together the mating surfaces of the first and second shaft portions. The connected shafts are then heated to melt the brazing shim. The brazing shim contains silver alloy which, when melted, flows along the mating surfaces of the shaft portions by capillary action. Advantageously, the silver generally does not flow into the bores  508  or the passage  44  it remains substantially along the mating surfaces. Since the heat will normally be applied from the exterior surfaces of the shaft portions, the surface  502  will be heated last. Thus, the surfaces  500  and  504  will be slightly hotter than the surface  502 . This ensures that when the brazing shim melts at the surface  502  it will flow to the warmer surfaces  500  and  504  and remain in liquid form to effect a better connection. The emergence of excess silver at the external interface  506  signals that the silver has fused completely through the mating surfaces. Preferably, the shaft portions  494  and  496  are formed from stainless steel, such as 17-4PH steel, a high-strength corrosion-resistant steel that is readily brazed. Furthermore, in the H-1150 condition, the strength is sufficient and is not significantly affected by the silver braze process. In experimental testing, silver braze joints of the illustrated configuration have withstood multiply administered tension loads greater than 100,000 pounds. 
       FIG. 16  is a longitudinal sectional view of the braze joint of the shaft  124  of  FIG. 14 . Preferably, the piston  184  is fitted over the interface  506  between the first and second shaft portions  494  and  496 . Advantageously, the piston  184  provides additional strength to the joint, reducing the risk of failure.  FIG. 16  also illustrates a preferred embodiment of a piston  184 , which comprises two ring-shaped compression clamps  514  and  516 , a spacer ring  518 , and a locking assembly  521 . The compression clamps  514  and  516  each apply a radial inward compression force onto the shaft  124 . The compression clamps rigidly lock onto the shaft and, along with the spacer ring  518  described below, provide the majority of the piston&#39;s resistance to moving with respect to the shaft  124 . In the illustrated embodiment, each compression clamp comprises a pair of ring-shaped clamp members with tapered annular surfaces that interact with one another to produce the compression force. For example, the clamp  514  includes an inner clamp member  530  and an outer clamp member  532 . The members  530  and  532  have inclined annular surfaces that mate with one another. As the members  530  and  532  are forced axially together with respect to the shaft axis, the axial force is converted into a radial inward compression force that locks the compression clamp  514  onto the shaft. The compression clamp  516  is preferably configured substantially similarly to the compression clamp  514 . In a preferred embodiment, the clamps  514  and  516  comprise Ringfeder® clamps, available from Ringfeder Corporation of Westwood, N.J., U.S.A. 
     The spacer ring  518  is not a necessary element of the illustrated piston  184 . However, the spacer ring advantageously provides additional resistance to axial movement or sliding of the compression clamps  514  and  516  with respect to the shaft  124 . The spacer ring, preferably a two-piece part to facilitate installation, includes an annular lip  520  on its inner surface. The lip  520  is sized and adapted to fit within the stress-relief groove  510  of the first shaft portion  494  of the shaft. The reception of the lip  520  within the groove  510  resists axial sliding of the spacer ring  518 , and thus of the entire piston  184 , with respect to the shaft  124 . Another advantage of the groove  510  and the spacer ring  518  is that the groove provides a convenient method for locating and properly positioning the piston  184  during assembly of the shaft  124 . 
     The locking assembly  521  imparts an axial compression force onto each pair of clamp members of the compression clamps  514  and  516 . The clamps  514  and  516  convert the axial compression force of the locking assembly  521  into the aforementioned radial inward compression force onto the shaft  124 . In the illustrated embodiment, the locking assembly  521  comprises a pair of ring-shaped locking members  522  and  524 , which are clamped axially together by one or more bolts  526  extending through holes in the member  522  and into threaded holes in the member  524 . As the locking members  522  and  524  are clamped together, they increase the radial compression force of the compression clamps  514  and  516 . The locking assembly  521  also comprises a majority of the volume of the piston  184 . Preferably, the locking assembly  521  extends radially to the inner surface  523  of the propulsion cylinder  112 . Seals  528  are provided within recesses in the peripheral surface of the locking member  524 . The seals  528  effect a fluid seal between the piston  184  and the inner surface  523  of the cylinder  112 . Also, at least one seal  531  is provided between the piston  184  and the shaft  124 . The seals  528  and  531  may comprise O-ring type or lip type seals. It will be understood that seals can alternatively or additionally be positioned within recesses in the peripheral surface of the locking member  522 . Seals  529  are also provided within recesses at the ends of the cylinder  112  adjacent the shaft  124  to prevent leakage of fluid from within the cylinder to the annulus  40 . The aforementioned Ringfeder Corporation sells locking assemblies. However, in the preferred embodiment, the locking assembly  521  is custom sized and shaped. 
     It will be understood that each of the shafts  118  and  124  ( FIG. 2 ) may comprise any number of shaft portions silver brazed together, preferably configured as shown in  FIGS. 15 and 16 . Also, some or all of the joints can be strengthened by positioning the pistons so as to enclose the interfaces of the joints, as shown in  FIG. 16 . Also, some or all of the pistons of the shafts can comprise compression clamps (preferably with spacer rings) and locking assemblies, as shown in  FIG. 16 . 
     Hydraulically Controlled Reverser Valve 
       FIG. 17  illustrates a valve system  540  for a tractor according to an alternative embodiment of the invention. As explained below, the valve system  540  permits the direction of travel of the tractor to be controlled. With the exception of a number of modifications discussed below, the valve system  540  is configured substantially similarly to the valve system  133  shown in  FIG. 3 . Elements of the valve system  540  are labeled with the reference numbers of analogous elements of the valve system  133 . The valve system  540  includes a propulsion control valve  146 , gripper control valve  148 , aft cycle valve  150 , forward cycle valve  152 , aft pressure reduction valve  244 , and forward pressure reduction valve  246 , all configured similarly to corresponding elements of the valve system  133 . However, the inlet galley  541  and the inlet control valve  542  of the valve system  540  are configured differently than the inlet galley  134  and inlet control valve  136  of the valve system  133 . The valve system  540  also includes a hydraulically controlled reverser valve  550 , as well as fluid chambers  564  and  566 , described below. 
     The inlet galley  541  of the valve system  540  extends to the inlet control valve  542  and the reverser valve  550 . The inlet control valve  542  preferably comprises a spool valve. The valve spool has a first position (shown in  FIG. 17 ) in which fluid is prevented from entering the remainder of the valve system  540 , and a second position (shifted vertically downward in  FIG. 17 ) in which fluid does enter the remainder of the valve system. In the first position of the spool, the valve  542  provides a flow path (represented by arrow  549 ) for fluid within the main galley  144  to flow into the annulus  40 . In the first position of the spool, fluid within the inlet galley  541  is prevented from flowing through the valve  542  into the main galley  144 . In the second position of the spool, the valve  542  provides a flow path (represented by arrow  548 ) for fluid within the inlet galley  541  to flow into the main galley  144 . In the second position of the spool, fluid within the main galley  144  is prevented from flowing through the valve  542  into the annulus  40 . 
     The inlet control valve  542  is piloted by the fluid pressure within the inlet galley  541 . The spool has a surface  544  exposed to fluid within the inlet galley  541 . At least one spring  546  biases the spool in a direction opposite to the fluid pressure force received by the surface  544 . In this respect, the operation of the valve  542  is effectively similar to that of the cycle valves  150  and  152  and the pressure reduction valves  244  and  246 . The valve spool of the valve  542  moves to its second position when the pressure in the inlet galley  541  exceeds a threshold determined by the characteristics of the at least one spring  546 . Thus, the valve  542  effectively has an “off” position (as shown in  FIG. 17 ) and an “on” position (shifted vertically downward in  FIG. 17 ). 
     The reverser valve  550  controls the direction that the tractor travels within the passage or borehole. The valve  550  permits the sequence of operations for forward motion of the tractor (to the right in  FIG. 13 ) to be modified so that the actuation and retraction of the gripper assemblies are reversed. During the operational cycle of the valves associated with forward motion of the tractor (described above), fluid is distributed to and from the gripper assemblies and to and from the chambers of the propulsion cylinders according to a specific sequence. At certain stages of the sequence, the aft gripper assembly is actuated and the forward gripper assembly is retracted. At other stages of the sequence, the aft gripper assembly is retracted and the forward gripper assembly is actuated. If this operational sequence is modified so that each gripper assembly is actuated during stages when it was previously retracted, and so that each gripper assembly is retracted during stages when it was previously actuated, the tractor will travel backward (to the left in  FIG. 13 ). The reverser valve  550  accomplishes this task. 
     In the illustrated embodiment, the reverser valve  550  communicates with the chambers  204  and  206 . Unlike in the valve system  133 , the chambers  204  and  206  do not extend to the pressure reduction valves. The reverser valve  550  also communicates with the chambers  564  and  566 . The chamber  564  extends from the valve  550  to the aft pressure reduction valve  244 . The chamber  566  extends from the valve  550  to the forward pressure reduction valve  246 . The valves  244  and  246  communicate with the chambers  564  and  566 , respectively, in the same manner that the valves  244  and  246  communicate with the chambers  204  and  206  in the valve system  133  ( FIG. 13 ). 
     In the preferred embodiment, the reverser valve  550  comprises a two-position spool valve. The valve spool has a first position (shown in  FIG. 17 ) in which the tractor travels forward, and a second position (shifted to the right in  FIG. 17 ) in which the tractor travels backward. In the first position of the spool, the valve  550  provides a flow path (represented by arrow  560 ) for fluid within the chamber  206  to flow into the chamber  564 . In the first position of the spool, the valve  550  also provides a flow path (represented by arrow  562 ) for fluid within the chamber  566  to flow into the chamber  206 . In the second position of the spool, the valve  550  provides a flow path (represented by arrow  558 ) for fluid within the chamber  204  to flow into the chamber  566 . In the second position of the spool, the valve  550  also provides a flow path (represented by arrow  556 ) for fluid within the chamber  564  to flow into the chamber  206 . 
     In the illustrated embodiment, the fluid pressure in the inlet galley  541  controls the position of the spool of the reverser valve  550 . The spool has a surface  552  exposed to the fluid from the inlet galley  541 . The surface  552  receives a pressure force that tends to move the spool to its second position. At least one spring  554  biases the spool toward its first position and opposes the pressure force on the surface  552 . Thus, the spool shifts to its second position, to effect backward travel of the tractor, when the fluid within the inlet galley  541  exceeds a shifting threshold pressure determined by the characteristics of the at least one spring  554 . Preferably, the shifting threshold pressure (e.g., 2000 psid) required to move the spool of the reverser valve  550  to its second position is greater than the threshold pressure (e.g., 800 psid) required to move the spool of the inlet control valve  542  to its second position. The skilled artisan will understand that the greater the variance between these threshold pressures, the easier it will be to open the inlet control valve  542  (i.e., to move the spool to its second position) without inadvertently reversing the direction of tractor motion. 
     In the preferred embodiment, the reverser valve  550  includes a locking feature, schematically represented by a latch  568 , which locks the spool in its second (or first) position. Preferably, the locking feature comprises a cam such as the deactivation cam  368  ( FIGS. 5-8 ) described above. In this embodiment, in order to shift and lock the spool within its second (or first) position, it is necessary to increase the pressure in the inlet galley  541  above the upper cam-activation threshold of the cam (e.g., 2000 psid). In order to unlock the spool, it is necessary to (1) reduce the pressure below the lower cam-activation threshold of the cam (e.g., 1000 psid), (2) increase the pressure back above the upper cam-activation threshold, and (3) reduce the pressure below the shifting threshold of the valve  550 . Refer to the discussion of the deactivation cam  368  above. 
     Thus, the illustrated reverser valve  550  provides a convenient means for reversing the direction of the tractor, while preserving an all-hydraulic design for the valve system of the tractor. 
     An alternative embodiment of a tractor of the invention includes a hydraulically controlled reverser valve configured to be actuated only once. When the reverser valve is actuated, the tractor will walk backward out of the passage or borehole. A preferred configuration of the valve system of this embodiment is herein described with reference to  FIG. 17 . The valve system is substantially identical to that shown in  FIG. 17 , with the following exceptions. First, the reverser valve  550  is modified so that the toggle feature  568  and the spring  554  are removed. Second, a burst disc or rupture disc device is provided in the pilot line that extends from the inlet galley  541  to the end surface  552  of the spool of the reverser valve  550 . The burst disc is configured to burst or open when the pressure in the inlet galley  541  reaches a burst pressure of the disc. 
     It will be understood that this configuration is useful if the tractor gets stuck in the borehole or if any downhole equipment of the BHA needs assistance in being removed, the reverser valve can be actuated. In this configuration, the tractor will normally be inserted into a borehole with the reverser valve  550  in its first position (the position shown in  FIG. 17 ). The burst disc prevents fluid within the inlet galley  541  from exerting a pressure force on the spool of the valve  550 . When it is desirable to reverse the direction of tractor motion, the pressure in the inlet galley  541  can be increased to the burst pressure of the burst disc. The burst disc will then burst or open to allow the fluid pressure within the inlet galley to move the spool of the valve  550  to its second position (shifted to the right in  FIG. 17 ). Since the spring  554  is removed from this design, the valve  550  will not change its position. Optionally, stops or detents can be provided to prevent inadvertent shifting of the spool, such as the stops  434 ,  436  illustrated in  FIG. 10 . The burst pressure of the burst disc is preferably between 2500 and 7000 psid, and more preferably about 3200 psid. Preferably, the burst pressure of the disc is greater than the shifting threshold of the inlet control valve  542 . 
     Electrically Controlled Reverser Valve 
       FIG. 18  illustrates a valve system  570  for a tractor according to another alternative embodiment of the invention. Like the valve system  540  of  FIG. 17 , the valve system  570  permits the direction of travel of the tractor to be controlled. With the exception of a number of modifications discussed below, the valve system  570  is configured substantially similarly to the valve system  540 . Elements of the valve system  570  are labeled with the reference numbers of analogous elements of the valve system  540 . However, the inlet galley  574  of the valve system  570  is different than the inlet galley  541  of the valve system  540 . Also, the reverser valve  550  is controlled differently. 
     The inlet galley  574  of the valve system  570  does not extend to the reverser valve, as in the valve system  540 . This is because the reverser valve  550  of the system  570  is not piloted by fluid pressure. Instead, a motor  572  controls the position of the spool of the reverser valve. In a preferred configuration, the output shaft of the motor  572  is coupled to a leadscrew, and a traversing nut is threadingly engaged with the leadscrew. The nut is coupled to the spool of the reverser valve  550 , preferably via a flexible stem. As the leadscrew rotates with the motor output, the nut traverses the leadscrew and thereby moves the spool. The position of the spool can be controlled by controlling the amount of rotation of the motor output shaft. An assembly for controlling the position of a valve spool with a motor, within a tractor, is illustrated and described in U.S. Pat. No. 6,347,674. 
     Preferably, the motor  572  is controlled by electronic signals sent from a remote location (such as from ground surface equipment) or even from a programmable logic controller on the tractor itself. 
     It will be understood that the position of the spool of the reverser valve  550  can alternatively be controlled via solenoids or other electronic means. 
     Electrical Control of Fluid Entry 
       FIG. 19  illustrates a valve system  574  for a tractor according to yet another alternative embodiment of the invention. As explained below, the valve system  574  provides electronic control of whether the tractor is “on” or “off.” With the exception of a number of modifications discussed below, the valve system  574  is configured substantially similarly to the valve system  133  shown in  FIG. 3 . Elements of the valve system  574  are labeled with the reference numbers of analogous elements of the valve system  133 . 
     The valve system  574  includes an inlet galley  578 , a pair of inlet control valves  576  and  577 , and a fluid chamber  582 . The inlet galley  578  extends to both of the valves  576  and  577 . The chamber  582  extends between the valves  576  and  577 . Preferably, the valve  576  comprises a spool valve. The valve  576  is controlled by a motor  580 , and can be configured similarly to the reverser valve  550  of the valve system  570  ( FIG. 18 ). It will be understood that the position of the spool can alternatively be controlled via solenoids or other electronic means. The spool of the valve  576  has a first “closed” position (shown in  FIG. 19 ) in which the valve  576  provides a flow path (represented by arrow  586 ) for fluid within the chamber  582  to flow into the annulus  40 , and in which fluid within the inlet galley  578  is prevented from flowing through the valve  576  into the chamber  582 . The spool of the valve  576  also has a second “open” position (shifted vertically downward in  FIG. 19 ) in which the valve  576  provides a flow path (represented by arrow  584 ) for fluid within the inlet galley  578  to flow into the chamber  582 , and in which fluid within the chamber  582  is prevented from flowing through the valve  576  into the annulus  40 . 
     The valve  577  preferably comprises a spool valve and is preferably configured substantially similarly to the valves  542  of  FIGS. 17 and 18 . The spool of the valve  577  has a first “closed” position (shown in  FIG. 19 ) in which the valve  577  provides a flow path (represented by arrow  590 ) for fluid within the main galley  144  to flow into the annulus  40 , and in which fluid within the chamber  582  is prevented from flowing into the main galley  144 . The spool of the valve  577  also has a second “open” position (shifted vertically downward in  FIG. 19 ) in which the valve  577  provides a flow path (represented by arrow  588 ) for fluid within the chamber  582  to flow into the main galley  144 , and in which fluid within the main galley  144  is prevented from flowing through the valve  577  into the annulus  40 . 
     The pair of inlet control valves  576  and  577  operate to control the flow of fluid into the remainder of the valve system  574 . The hydraulically controlled valve  577  shifts to its “open” position only when the fluid in the inlet galley  578  exceeds the threshold pressure associated with the valve  577 . Regardless of the position of the valve  576 , when the valve  577  is closed the fluid within the main galley  144  flows through the valve  577  into the annulus  40 . Thus, when the pressure in the inlet galley  578  is below the threshold associated with the valve  577 , the tractor is “off.” In other words, the valve  577  is a failsafe valve to deactivate the tractor in case of control system failure. The electrically controlled valve  576  provides additional control. When the valve  576  is closed, the tractor is “off,” regardless of the position of the valve  577 . Even if the valve  577  is open when the valve  576  is closed, fluid within the main galley  144  flows through the valve  577 , the chamber  582 , the valve  576 , and into the annulus  40 . The tractor is “on” only when both the valves  576  and  577  are open. In such a condition, fluid within the inlet galley  578  flows through the valve  576 , the chamber  58 , the valve  577 , and into the main galley  144 . Thus, fluid flows into the remainder of the valve system  574  only when (1) the pressure in the inlet galley  578  exceeds the threshold associated with the valve  577  and (2) the valve  576  is shuttled to its “open” position. 
     Electrical Control of Fluid Entry and Reverse Motion 
       FIG. 20  illustrates a valve system  592  for a tractor according to yet another alternative embodiment of the invention. The valve system  592  comprises a combination of the valve systems  570  ( FIG. 18) and 574  ( FIG. 19 ). The valve system  592  includes a pair of inlet control valves  576  and  577 , configured similarly to analogous valves of the valve system  570 . In particular, the valve  576  is electrically controlled and the valve  577  is hydraulically controlled. The valve system  592  also includes an electrically controlled reverser valve  550 , configured similarly to the analogous valve of the valve system  574 . Thus, the valve system  592  permits electrical control of (1) the on/off state of the tractor and (2) the direction of tractor motion. 
     Gripper Assemblies 
     As mentioned above, the gripper assemblies  104  and  106  are preferably configured in accordance with a design illustrated and described in a U.S. patent application Ser. No. 10/004,963, entitled “GRIPPER ASSEMBLY FOR DOWNHOLE TRACTORS,” filed on Dec. 3, 2001, now U.S. Pat. No. 6,715,559.  FIGS. 21-34  illustrate a preferred configuration of such a gripper assembly. Below is a brief description of the configuration and operation of the illustrated gripper assembly. For a more detailed description, please refer to the above-referenced application. 
     In a preferred embodiment, the gripper assemblies  104  and  106  are substantially identical. Thus, the gripper assembly configuration shown in  FIGS. 21-34  describes both assemblies  104  and  106 . In  FIG. 21 , the gripper assembly is shown with its aft end on the left and its forward end on the right. The gripper assembly includes an elongated mandrel  600 , a cylinder  602  engaged on the mandrel, toe supports  608  and  610 , a tubular piston rod  604 , a slider element  606 , and three flexible toes or beams  612 . The mandrel  600  surrounds and is free to slide longitudinally with respect to the shafts  118  and  124  ( FIG. 2 ) of the tractor. When used for non-drilling applications, the mandrel  600  is preferably also free to rotate with respect to the shafts (i.e., there are no splines that prevent rotation). This is because it is generally not necessary to transmit torque to the borehole wall for non-drilling applications. The ends  614  and  616  of the toes  612  are pivotally secured to the toe supports  608  and  610 , respectively. The cylinder  602  and the toe support  608  are fixed with respect to the mandrel  600 , while the toe support  610  is free to slide longitudinally along the mandrel. The piston rod  604  and the slider element  606  are fixed with respect to each other and are together slidably engaged on the mandrel  600 . The cylinder  602  encloses an annular piston (not shown) that is fixed with respect to the piston rod  604  and slider element  606  and also slidably engaged on the mandrel  600 . The piston is biased in the aft direction by a return spring (not shown) that is also enclosed within the cylinder  602 . 
     With reference to  FIGS. 21-25 , the central region of each toe  612  has a recess  624  ( FIG. 24 ) formed in the inner radial surface of the toe. The recess  624  is formed between two axial sidewalls  618  of the toe  612 . The recess  624  includes two rollers  626  on axles  628  secured within the sidewalls  618 . The slider element  606  includes three pairs of ramps  630 , each pair aligned with one of the toes  612 . The ramps  630  are radially interior of the toes  612 . As the slider element  606  slides forward, each roller  626  rolls up one of the ramps  630 , causing the central regions of the toes  612  to bend radially outward to grip onto a borehole surface. As the slider element  606  slides aftward, the rollers  626  roll down the ramps  630 , causing the toes  612  to relax back to the position shown in  FIGS. 21 and 22 . 
     The gripper assembly is actuated by pressurized operating fluid supplied to the cylinder  602 , on the aft side of the enclosed piston. The pressurized fluid causes the piston, piston rod  604 , and the slider element  606  to slide forward against the force of the return spring. As explained above, this causes the rollers  626  to roll up the ramps  630  and deflect the toes  612  radially outward. The toe support  610  freely slides aftward to accommodate the deflection of the toes  612 . The gripper assembly is retracted by reducing the pressure aft of the piston, which causes the return spring to push the piston, piston rod  604 , and slider element  606  aftward. The rollers  626  roll down the ramps  630 , allowing the toes  612  to relax. 
       FIGS. 22-29  illustrate the design of the toes  612 , toe supports  608  and  610 , and the slider element  606 . The ends  614  and  616  of the toes  612  include elongated slots  607  and  609 , respectively. The slots receive axles  611  secured to the toe supports  608  and  610 . The slots  607  and  609  reduce potentially dangerous compression loads in the toes  612  when the toes experience external forces (e.g., sliding friction against the borehole surface).  FIGS. 22-25  show a toe  612  in a normal position with respect to the (retracted) slider element  606  and toe supports  114  and  116 , as the toe will shift forward due to gravity.  FIGS. 26-29  show the toe  612  in a shifted position, which occurs when the toe experiences an aftwardly directed external force. As shown in  FIGS. 24 and 28 , as the toes  612  shift axially between these positions, the aft rollers  626  remain between the ramps  630  without rolling up the aft ramps. In other words, external forces applied to the toes do not cause the gripper assembly to self-energize. 
     As shown in  FIGS. 30 and 31 , each toe  612  includes four spacer tabs  620  that extend radially inward from the toe&#39;s sidewalls  618 . Two spacer tabs  620  are positioned on each sidewall  618 , one tab near each end of the sidewall. The spacer tabs  620  are configured to bear against the slider element  606  when the toes  612  are relaxed. Also, as shown in  FIG. 32 , when the toes  612  are relaxed the rollers  626  do not contact the slider element  606 . Thus, when the toes  612  are relaxed, the spacer tabs  620  absorb radial loads between the toes and the slider element  606  and also prevent undesired loading of the rollers  626  and roller axles  628 . 
     As shown in  FIGS. 33 and 34 , each toe  612  includes four alignment tabs  622  that, like the spacer tabs  620 , extend radially inward from the toe&#39;s sidewalls  618 . A pair of alignment tabs  622  is provided for each of the ramp/roller combinations, one tab on each sidewall  618 . Each pair of alignment tabs  622  straddles one of the ramps  630  and thus maintains the alignment between the roller  626  and the ramp. The alignment tabs  622  prevent the rollers  626  from sliding off of the sides of the ramps  630 , particularly when the rollers are near the radial outward ends or tips of the ramps. 
     With reference to  FIG. 33 , each ramp  630  of the slider element  606  is configured to have a relatively steeper initial inclined surface  632  followed by a relatively shallower inclined surface  634 . This causes the toes  612  to deflect radially outward at an initially high rate, followed by a low rate of deflection. Advantageously, during actuation of the gripper assembly, the toes  612  quickly approach the borehole surface. Before the toes  612  contact the borehole, the rate of expansion is slowed as the rollers roll along the shallower surfaces  634 , to permit a degree of fine tuning of the radial expansion. 
     The gripper assemblies  104  and  106  are preferably formed of CuBe, but other materials can be employed. For example, the flexible toes can be formed of Titanium, and the mandrel can be formed of steel. 
     It will be understood that the tractor  100  can be utilized with any of a variety of different types of gripper assemblies. For example, U.S. Pat. No. 6,464,003 discloses a compatible gripper assembly in which toggles are utilized to radially expand flexible toes that grip a passage surface. Many compatible gripper designs comprise packerfeet. For example, U.S. Pat. No. 6,003,606 to Moore et al. discloses packerfeet that include borehole engagement bladders. Another reference, U.S. Pat. No. 6,347,674, discloses one packerfoot design having bladders strengthened by attached flexible toes and another packerfoot design in which the bladders and toes are not attached. Yet another reference, U.S. Pat. No. 6,431,291, discloses an improved packerfoot design. 
     Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. Further, the various features of this invention can be used alone, or in combination with other features of this invention other than as expressly described above. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.