Abstract:
A substantially constant force actuator that is applicable to centralizers, anchors and tractors for use in wells and is applicable to lifting devices such as jacks and load supporting devices. One or more sets of linkage arms are angularly movable by the force of one or more force transmitting members from a minimum angle with the force transmitting members at maximum spacing to a maximum angle with the force transmitting members at minimum spacing to impart a substantially constant force to an object or surface, with the direction of the force being substantially perpendicular to the direction of relative linear movement of the force transmitting members. With the linkage arms at their minimum angles, movement control elements on at least one of the force transmitting members react with guide surfaces of the linkage arms to achieve angular linkage movement and to develop a substantially constant force during angular linkage movement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Application No. 60/364,189, filed Mar. 13, 2002, which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to mechanisms that employ a force applied in one direction to lift or support a load in a direction perpendicular to the direction of the applied force. Such mechanisms find application in many fields and may be employed, for example, in tools for use in wells or pipes, such as centralizers, calipers, anchoring devices, and tractors. The invention is particularly applicable to the field of tractors for conveying logging and service tools in deviated or horizontal oil and gas wells, or in pipelines, where such tools may not be readily conveyed by the force of gravity. The invention may also be employed in jacking devices.  
           [0004]    2. Description of Related Art  
           [0005]    After an oil or gas well is drilled, it is often necessary to log the well with various measuring instruments. This is usually done with wireline logging tools lowered inside the well on a logging cable. Similarly, pipelines may require inspection and, therefore, the movement of various measuring tools along the pipe.  
           [0006]    Some logging tools can operate properly only if they are positioned at the center of the well or pipe. This is usually done with centralizers. All centralizers operate on the same general principle. Equally spaced, multiple bow springs or linkages of various kinds are extended radially from a central hub toward the wellbore or pipe wall. These springs or linkages come into contact with the wellbore or pipe wall and exert radial forces on it which tend to move the body of the tool away from the wall. Since the bow springs and linkages are usually symmetric with respect to the central hub, they tend to position the tool at the center of the well. Hence, the radial forces exerted by these devices are often referred to as centralizing forces.  
           [0007]    Centralizers usually remain open throughout their operation. In other words, their linkages are always biased toward the wellbore wall and they always remain in contact with the wellbore wall. Most centralizers are designed such that they can operate in a large range of wellbore sizes. As the centralizers expand or contract radially to accommodate changes in the size of the wellbore, their centralizing forces may vary. In wells that are nearly vertical, the variation in radial force is not a problem because the radial component of the tool weight is small and even weak centralizers can cope with it. In addition, the centralizing force and the frictional drag resulting from it are such a small fraction of the total tension on the logging cable that its variability can be neglected for all practical purposes.  
           [0008]    Wells that have horizontal or highly deviated sections may, however, present problems. In a horizontal section of the well, the centralizer must be strong enough to lift the entire weight of the tool off the wellbore wall. On the one hand, the minimum level of the centralizing force must be made equal to the weight of the tool to ensure proper operation in all wellbore sizes. On the other hand, in a different wellbore size, the force exerted by the centralizer may be excessive, causing extra frictional drag that impairs the motion of the tools along the well. This situation has led to the development of constant force centralizers, which have been previously disclosed and are commercially available. The present invention, however, presents a new approach to constructing such a constant force centralizer.  
           [0009]    Similar to centralizers, calipers extend arms or linkages from the tool body toward the wellbore wall. One difference between centralizers and calipers is that the arms of a caliper may be individually activated and may not open the same amount. Another difference is that caliper arms are usually selectively opened and closed into the tool body by some mechanical means. Thus, the arms of a caliper do not necessarily remain in contact with the wellbore wall at all times.  
           [0010]    Various measuring instruments are often mounted on the caliper arms. In order to ensure the proper operation of some of these measuring instruments, it is often necessary to maintain a certain range of the magnitude of the radial force with which the caliper arms are pressed toward the wellbore wall. This requirement is sometimes difficult to achieve in horizontal sections of the well and variable wellbore sizes. The reason is that, like centralizers, the mechanical advantage of caliper linkages varies with wellbore size. Thus, the mechanical devices responsible for opening and closing the caliper must provide variable force output. This usually leads to poor efficiency of the mechanical device and its under-utilization in a large range of wellbore sizes. It is, therefore, beneficial to develop caliper linkage mechanisms that apply virtually constant radial forces given a constant mechanical input from the actuation device. The present invention provides such a mechanism.  
           [0011]    Horizontal and highly deviated wells present yet another problem. Logging tools cannot be effectively conveyed into such wells by the force of gravity. This has led to the development of alternative conveyance methods. One such method is based on the use of a downhole tractor that pulls or pushes logging tools along the well.  
           [0012]    Downhole tractors, such as those described in U.S. Pat. Nos. 5,954,131 and 6,179,055 B1, use various radially expandable mechanisms to force wheels or anchoring devices against the wellbore wall. Independent of the principle by which the motion with respect to the wellbore wall is achieved, the traction force that a tractor can generate is directly proportional to the radial force applied by the mechanism. Similar to centralizers and calipers, downhole tractors are designed to operate in a wide range of wellbore sizes. Like centralizers, they also have the problem of radial force variability as a function of wellbore size. Typically, for a given expansion mechanism, the traction force diminishes with wellbore size. It is advantageous if the radial force that a tractor generates is constant. However, no satisfactory solution to this problem has thusfar been disclosed.  
           [0013]    Some tractors use several sets of different size linkages to provide a relatively constant traction force in a wide range of wellbore sizes. These mechanisms must, however, be replaced at the surface, which is very inconvenient. In addition, some wells are drilled with a variety of wellbore sizes that no single mechanism can handle. The present invention provides a mechanism that may be used with all known tractoring concepts to achieve a constant radial force and, therefore, consistent traction over a very wide range of wellbore sizes.  
           [0014]    Centralizers, calipers, and tractors all rely on radially expandable mechanisms to perform their functions. These mechanisms may be either active or passive. The active mechanisms are powered by hydraulic or electric actuators. They are normally closed and are activated only during service. The passive mechanisms usually rely on springs to generate the outward radial force. While passive constant force mechanisms are commercially available, no active constant force mechanism has been disclosed. The present invention may be used either as a passive or an active mechanism that is capable of producing a substantially constant radial force.  
           [0015]    The prior art that is relevant to the principle of operation of the invention discloses either the construction of constant force centralizers or the use of wedges in centralizing devices. For example, U.S. Pat. No. 4,615,386 discloses a centralizer that has approximately constant radial forces through a range of wellbore sizes. The constancy of the force is achieved by a combination of two springs with different characteristics. The sum of the two spring forces remains approximately constant over a wide range of movement of the centralizer arms. The advantage of this approach lies in its simplicity. The disadvantage is that it can only be used for centralizers, but not for calipers and anchoring devices that require selective opening and closing of the arms. Another disadvantage is that this operating principle requires the centralizer to be quite long, which may be undesirable in some instances. Similarly, U.S. Pat. Nos. 4,557,327 and 4,830,105 teach centralizing devices that achieve a virtually constant centralizing force by combining at least two springs of different kinds. The advantages and disadvantages of these devices are similar to those discussed above. U.S. Pat. No. 5,005,642 discloses a logging tool centralizer that achieves a lower degree of variability of the centralizing force by moving the attachment points of the centralizing arms at the opposite side of the tool body. Thus, the angle between the centralizer arm and the tool body can never become zero, which is the condition that makes inoperable most other centralizing devices that rely only on axial actuation. The disadvantage of this approach is that it does not solve the problem completely, as the radial force still varies with the wellbore size. It also makes construction of the device difficult, especially when it is desirable to use more than two centralizing arms.  
           [0016]    In all patents discussed above, the radial expansion of the centralizer is achieved by a mechanism that consists of two arms that are joined together at one of their ends and are attached to moving hubs at their other ends. When the distance between the hubs changes, the attachment point of the two arms moves in or out in the radial direction. Another approach to achieving a radially expandable device is based on the use of tapered surfaces or wedges. Centralizers built on this principle are disclosed in U.S. Pat. Nos. 5,348,091 and 5,934,378. A radially expandable well drilling tool is disclosed in U.S. Pat. No. 4,693,328. The principle of radial expansion is again based on moving parts sliding over inclined surfaces (wedges). The advantage of this concept is that the forces generated can be substantial. A major disadvantage is the relatively limited range of radial expansion.  
           [0017]    The present invention overcomes the disadvantages of both types of radially expandable mechanisms discussed above by kinematically combining these mechanisms into a single device that accomplishes new and novel results in a manner that is different from either of the devices.  
         BRIEF SUMMARY OF THE INVENTION  
         [0018]    In one aspect of the present invention, a constant force actuator mechanism is provided that may be used with all known wellbore tractoring concepts to achieve a substantially constant radial force and, therefore, consistent traction in a very wide range of wellbore sizes.  
           [0019]    In another aspect of the invention, a constant force actuator mechanism is provided that may be utilized either as a passive or as an active mechanism that is capable of producing a substantially constant radial force for application to opposed surfaces.  
           [0020]    In a further aspect of the present invention, a constant force actuator mechanism is provided that may be effectively utilized as the operational component of a centralizer, a caliper, an anchoring device, a lifting jack, or other force transmitting devices, and may be energized by springs, hydraulic motors, pneumatic motors, mechanical energizing devices, and the like.  
           [0021]    Briefly, the present invention is a mechanism that uses a force applied in a first linear direction to lift or support a load, or transmit a force, in a second linear direction that is substantially perpendicular to the first linear direction. Devices and mechanisms constructed in accordance with the principles of the present invention are constructed in such manner that the force that is required to support the load is of practically constant magnitude and is independent of the position of the load in the second linear direction. In particular, the invention relates to logging tools or other devices for wells that are conveyed along the inside surfaces of a wellbore or a pipe, or between spaced surfaces. The invention can conveniently take the form of a centralizer, a caliper, an anchoring device, or a tractor mechanism for use in wells, or may take the form of a lifting or load supporting device when embodied in jacks and other lifting or load supporting devices. The function of the present invention is to apply or react radial forces against the internal cylindrical wall of a wellbore or circular conduit, such as a pipe, for centralizing objects within the wellbore or pipe, to provide an anchoring function, or to provide mechanical resistance enabling the efficient operation of internal traction devices for conveying objects such as logging tools. When used as a centralizer for logging tools, a plurality of radially movable actuating linkages embodying the present invention maintain the logging tools at the center of the wellbore and thus enhance the accuracy of the logging process. When used as a caliper, the invention extends arms or other linkages toward the wellbore wall and exerts a controlled radial force on the wall surface. When used as an anchoring device, the invention can apply or react radial forces that generate enough friction against a wellbore or pipe wall to prevent any sliding at the points of contact between the anchoring device and the wall surface of the wellbore or pipe. The latter is needed for the construction and operation of downhole tractor tools, which are often used to convey other tools along wells that have horizontal or highly deviated sections. A major advantage of the present invention is that the magnitudes of the radial forces that it applies to the wellbore wall are virtually constant and independent of the wellbore size.  
           [0022]    The main elements of the invention are force transmitting members or hubs, wheels, axles, and at least a pair of linkage arms with built-in wedges or with guide surfaces of predetermined geometry defined by the linkage arms. For purposes of the present invention the terms “force transmitting members” or “hubs” are each intended to mean members of any desired configuration, that are relatively linearly movable, with one or both of the members movable and, if desired, one of the members stationary. The linkage arms, the force transmitting members or hubs, and the wheels are joined by the axles to form a linkage that can expand or contract radially as the distance between the hubs changes in the axial direction. The linkage arms are joined together by a pivot member or axle at one of their ends, which allows only angular motion of the linkage arms to occur. At their second ends, the linkage arms are attached to separate hubs by axles or pivots that can both rotate and slide within an elongate slot in the hub body. The wheels or rollers, which define movement control elements, are rotatably mounted onto the hubs and, when in contact with the guide surfaces of the linkage arms, roll on the force transmitting guide surfaces of wedges or guide surfaces that are built into the linkage arms, formed on the linkage arms, or attached to the linkage arms. Although wheels or rollers are shown as force transmitting elements of the hubs or force transmitting members, structures other than wheels or rollers may be employed within the spirit and scope of the present invention to transmit forces from the hubs to the guide surfaces of the wedges or linkage arms. The force transmitting guide surfaces are of predetermined geometry so as to react with the force transmitting surfaces of the wheels or rollers and develop resultant force vectors on the linkage arms that are angulated with respect to the direction of linear motion of one or both of the hubs. These angulated force vectors cause pivotal movement of the linkage arms even when the linkages are fully retracted. This feature permits ease of starting motion of the linkages from their retracted positions.  
           [0023]    The invention combines two separate principles to generate the required radial expansion. At small angles between the arms and the hubs, the radial force is created by the wheels, which roll on the force transmitting surfaces of the wedges or linkage arms. At larger angles, the expansion movement of the linkages is created on the principle of a triangular three-bar linkage. A transition between the two principles occurs at a pre-selected intermediate angle of the linkage arms between the fully retracted and fully extended positions. By combining these two principles and by the selection, placement and shape of the force transmitting guide surfaces of the wedge members it is possible to achieve substantially constant input axial force, which is the major advantage of the present invention and which is distinct as compared with other similar devices. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings in which:  
         [0025]    FIGS.  1 A- 1 F are elevation views of a first illustrative embodiment of a constant force actuator according to the invention showing various positions of the constant force actuator from a closed or retracted position, shown in FIG. 1A, to a completely open or extended position shown in FIG. 1F;  
         [0026]    [0026]FIG. 2 is a force versus movement diagram illustrating the axial force required for support of a radial load and illustrating small angle linkage movement with the wedge of the actuator and larger angle linkage movement after the linkage has separated from the force transmitting surface of the wedge;  
         [0027]    [0027]FIG. 3 is a sectional view of a spring urged centralizer embodiment of the present invention applicable for use in wells and for other centralizing applications and incorporating symmetrical opposed linkages with roller engaging wedges on all linkage arms;  
         [0028]    [0028]FIG. 4 is a sectional view of a spring urged centralizer embodiment of the present invention having asymmetric linkages having wheel or roller engaging wedges only on upper linkage arm sections;  
         [0029]    [0029]FIG. 5 is a sectional view of a spring urged centralizer embodiment having asymmetric linkages oppositely arranged;  
         [0030]    [0030]FIG. 6 is an isometric illustration showing an embodiment of the present invention as a downhole tractor tool grip;  
         [0031]    [0031]FIG. 7A is a sectional view of the upper portion of a downhole tractor tool grip embodying the principles of the present invention;  
         [0032]    [0032]FIG. 7B is a sectional view of the intermediate portion of the downhole tractor tool grip of FIG. 7A;  
         [0033]    [0033]FIG. 7C is a sectional view of the lower portion of the downhole tractor tool grip of FIGS. 7A and 7B;  
         [0034]    [0034]FIG. 8 is a sectional view of a downhole tractor mechanism embodying the principles of the present invention and including powered tractor wheels for driving engagement with opposed surfaces or opposite sides of a wellbore;  
         [0035]    [0035]FIG. 9 is a sectional view of a downhole tractor mechanism constructed according to the present invention and including powered tracks for driving engagement with opposed surfaces or with opposite sides of a wellbore;  
         [0036]    [0036]FIG. 10 is a sectional view of a downhole tractor mechanism constructed according to the present invention and having rollers and rotating hubs for driving engagement with opposed surfaces or with opposite sides of a wellbore;  
         [0037]    [0037]FIG. 11 is a sectional view showing an object raising and lowering jack mechanism embodying the principles of the present invention and having manual actuation of opposed linkages by a rotary jack screw; and  
         [0038]    [0038]FIG. 12 is a partial sectional and partial elevation view illustrating a load lifting scissors mechanism having a set of scissors arms defining interacting linkages with wedges and force transmitting rollers for substantially constant force scissors actuation. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]    Illustrative embodiments of the invention are described below. It will be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0040]    Referring now to FIGS.  1 A- 1 F, the basic principles of the present invention are shown by way of operational illustrations, with the substantially constant force linkage of the apparatus being shown in its closed or fully retracted condition in FIG. 1A and at various stages of movement to a fully open or fully extended condition shown in FIG. 1F. The major elements and the principle of operation of the invention are schematically illustrated in FIGS.  1 A- 1 F. Two linkage arms  2 , with wedges  4  that are integral parts of the linkage arms, are joined together at their first ends by an axle or pivot  6 . The axle  6  may also join other elements to the linkage arms depending on the desired function of the device constructed. For illustration purposes, FIGS.  1 A- 1 F show a wheel or roller  8  also mounted onto axle  6 , which implies that in this case, the invention would be used as a centralizer with the wheels  8  disposed for contact with opposed surfaces or for contact with opposite walls of a wellbore. The second ends of the linkage arms  2  are attached to hubs  10  with pivot pins  12 , which slide and rotate inside elongate slots  14  in the hubs  10 . Wheels  16  are mounted with axles  18  into brackets  20 , which are parts of hubs  10 . The function of the wheels  16  is to roll on the guide surfaces  22  of the wedges  4  and to react with the guide surfaces  22  to impart vectored forces to the linkage arms  2  and achieve linkage arm movement. The hubs  10  are restricted to move only linearly with respect to each other by other force transmitting elements or devices (not shown in FIGS.  1 A- 1 F). All of these elements of the invention are combined to form a linkage, designated by the numeral  25 .  
         [0041]    FIGS.  1 A- 1 F show the position of linkage  25  at various degrees of radial expansion. FIG. 1A shows linkage  25  in its closed or fully retracted position, when the angle between the arms and the hubs is zero (the angle being designated by the letter a in FIGS.  1 B- 1 F). Note that in this position, wheels  16  contact the wedge surfaces  22  close to their top ends. Also note, that the pivot pins  12  are at the front ends of their respective elongate slots  14 .  
         [0042]    Now, imagine that the hubs  10  are displaced towards each other by axial forces designated by F a  in FIGS.  1 A- 1 F. This causes the wheels  16  to roll downwards on the guide surfaces  22  of the wedges  4 , thus developing a force having a vector that is oriented for pushing the linkage arms upward, rotating them about their pivot pins  12 . The arms  2  slide and pivot at their second ends during linkage movement, which leads to the configuration shown in FIG. 1B. Note that the angle α between the arms  2  and the straight line connecting the hubs  10  increases from its zero value in FIG. 1A to some positive value in FIG. 1B. In this situation, pins  12  are in some intermediate position in the elongate slots  14 . The pivot pins  12  are free to move axially, and thus cannot support any axial load. However, they prevent the second ends of the linkage arms  2  from moving in the radial direction. All of these interactions force the first ends of the linkage arms  2  and the wheel  8  to move outwardly in the radial direction for radial extension of the linkage  25 . When the wheel  8  comes into contact with the wellbore wall, it begins to exert radial force on it, moving the hubs  10 , away from the wall and toward the center of the wellbore, thus creating a centralizing effect.  
         [0043]    Further radial expansion of linkage  25  based on the rolling of wheels  16  on guide surfaces  22  is shown in FIGS. 1C and 1D. As seen in these Figures, angle α continues to increase and wheel  8  continues to move out in the radial direction. FIGS.  1 A- 1 D illustrate the first kinematic principle used in the invention, which is based on the interaction between the guide surfaces  22  of the wedges  4  and the force transmitting wheels or rollers  16 . Note that in FIG. 1D, the wheels  16  have reached the very bottom end of the wedge surfaces  22 . This situation indicates that the amount of radial expansion based on this first kinematic principle has already been exhausted. Also note that the pivot pins  12  have reached the rear ends of the elongate slots  14 . This position of pins  12  and wheels  16  is the transitional point between the two kinematic principles used in the invention. For this reason, the linkage arm angle in FIG. 1D is designated by α t  (transition). At angles smaller than α t , the radial expansion of the linkage is caused by the wedges, while at angles larger than α t , the radial expansion of the linkage is caused by the equivalent of a three-bar mechanism.  
         [0044]    The second kinematic principle on which the invention is based is illustrated in FIGS.  1 D- 1 F. The two linkage arms  2  and the hubs  10  form a triangular three-bar mechanism with the hubs  10  representing a bar with variable length. As the distance between the hubs  10  decreases, the triangle changes shape with its tip moving further outward in the radial direction. Note that the wedges  4  do not take any part in this motion, because, as shown in FIGS. 1E and 1F, the guide surfaces  22  of the wedges  4  have lifted off wheels or rollers  16 .  
         [0045]    Now imagine that a downward radial force F r  has acted through the whole expansion process. Also imagine that the magnitude of the axial force F a  that is necessary to overcome F r  and to continue the expansion has been recorded and represented graphically. An illustration of such a graphical representation is shown in FIG. 2. The exact magnitudes of the numbers and the shapes of the curves represented in FIG. 2 will vary depending on the location of the wedge  4  on the linkage arms  2  and the radius of curvature of the wedge guide surface  22 . However, FIG. 2 is a sufficient illustration of the advantage of combining two separate kinematics principles in one mechanism. In FIG. 2, the curve indicated by F a  (no wedge) illustrates the magnitude of the axial force F a  that would be required to overcome F r  if only the second kinematic principle of the three-bar linkage were used. As seen from the chart of FIG. 2, in this case F a  rises sharply at small values of α. This means that the three-bar linkage, on which many existing devices are based, has real difficulties in supporting radial loads at small angles. In fact, at α equal to zero, the axial force required to support the load would be infinitely large, which means that no practical device can be constructed to operate in this range. The second curve on the chart of FIG. 2 represents possible values of F a  if two kinematic principles are combined, as suggested in the present invention. It can be seen that the sharp increase of F a  at small angles α is avoided and that F a  remains fairly constant within a large range of values of the angle α. It should be noted that FIG. 2 is by no means exhaustive of the possible values of F a  that can he achieved by the present invention. As indicated earlier, by varying the location of the wedge  4  on the arm  2  and by varying the radius of curvature of the wedge  4  and the geometry of the guide surface  22 , it is possible to achieve almost any shape of curve dependent on the function demanded from the particular embodiment of the invention.  
         [0046]    Various embodiments of the invention are discussed in more detail in FIGS.  3 - 12 . FIG. 3 represents one embodiment of the invention as a tool centralizer. A minimum of three linkages  25  (only two opposing linkages are shown in FIG. 3) are combined together by common hubs  10 . The hubs  10  slide on a mandrel  24 . Integral with the mandrel  24  is a hub stop  26 , which limits the linear motion of the hubs  10  on the mandrel  24 . The mandrel  24  is also connected to upper head  28  and lower head  30 , which are used to connect the centralizer to other tools and devices in the tool string (the details of the connections to other tools are not essential for the present invention and are not shown in FIG. 3). The mandrel  24  may also have wires  32  going through it for electrical communication with other tools in the tool string. The axial force that causes the centralizer to expand radially and to position the other tools in the tool string at the center of the wellbore is provided by springs  34 . As seen from the embodiment of the invention shown in FIG. 3, only one type of spring is necessary for the construction of a centralizer with a relatively constant centralizing force.  
         [0047]    The linkage  25  used for the construction of various devices does not need to be symmetric. Two devices that are constructed with asymmetric linkages, which still operate on the principles disclosed above, are shown in FIGS. 4 and 5. In these figures only one of the arms that are used to build the linkage has a wedge. Alternatively, wedges with guide surfaces of different geometry could be put on arms that have unequal lengths.  
         [0048]    All embodiments of the invention discussed above represent tool string centralizers. Constant force centralizers can be achieved by means other than those discussed above. The present invention represents a new method by which such centralizers can be constructed.  
         [0049]    The advantages of the invention, however, are far greater in devices that have the ability to selectively open and close their linkages in and out of the tool body. The reason is that such “active” devices usually have only axial linear actuators available for opening and closing the linkages into the tool as opposed to elements used in centralizers, which have a radial force component. Examples of devices that require selective opening and closing of linkages are calipers and downhole tractor tools. An embodiment of the invention used as a grip in a downhole tractor tool is shown in FIGS. 6 and 7A- 7 C. FIG. 6 is a three dimensional view of a tractor tool grip, which is constructed using the constant force actuator principles discussed above. The tractor tool grip has two main functions. The first is to selectively open and close the linkages and centralize the tool in the wellbore when necessary. In this respect, the tractor grip is not much different from the centralizers shown in FIGS.  3 - 5 . The difference is that the grip is not continuously open and that it is powered by hydraulic or electromechanical actuators, which allow the selective opening or closing. The second function of the tractor grip is to selectively anchor the tool with respect to the well wall. In the embodiment shown in FIG. 6, this is achieved by the installation of cams  42  at the tips of linkages  25  and a device for selectively locking the geometry of the linkage (not shown in FIG. 6). The principle on which the cams  42  selectively anchor the tool with respect to the well wall and the physics of tractoring have been disclosed in U.S. Pat. Nos. 5,954,131 and 6,179,055, and in co-pending U.S. patent application Ser. No. 09/921,825, incorporated herein by reference. Since these are not essential for the operation of the proposed invention they are not discussed here in detail.  
         [0050]    As seen in FIG. 6, the tractor grip consists of three symmetrical linkages  25 . Similar to the description provided with regard to FIG. 1, each linkage consists of two arms  2 , which are joined together at their first ends by an axle  6 . The axle  6  also joins other elements of the grip such as the wheels  8  and the bi-directional cam  42 , which is responsible for the tractoring action. The three upper arms  2  in FIG. 6 are attached to hub  10  which can slide with respect to the grip body  44 . This is also similar to the description given in FIG. 1. However, the three bottom arms  2  are not attached to a moving hub, but are instead mounted onto a stationary hub  40 , which is an integral part of the grip body  44 . This demonstrates the flexibility of the invention. As explained earlier, the only requirement for the invention to work is that the hubs  10  can move with respect to each other in the axial direction. It is not necessary, however, that both hubs can move with respect to the tool body. FIG. 6 also shows other elements of the invention such as wedges  4 , wedge guide surfaces  22 , wheels  16 , pivot pins  12 , and slots  14 . Note that the grip in FIG. 6 is shown in its fully opened or extended state. The moving hub  10  and the stationary hub  40  are touching, which is seen from the proximity of the wheels  16 . Also note that the pins  12  are at the bottom end of slots  14 , which indicates that the second kinematic principle of the invention is active. FIG. 6 also shows that the wedge guide surface  22  can also be made flat (infinite radius of curvature) to achieve the desired force characteristics.  
         [0051]    The basic elements of the invention, shown in FIG. 6 can be combined with other linkages to construct more complex mechanisms. While the invention has been described with respect only to its basic set of elements, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein.  
         [0052]    FIGS.  7 A- 7 C are cross sectional views of the downhole tractor grip embodiment shown in FIG. 6. FIG. 7B is a continuation of FIG. 7A, and FIG. 7C is a continuation of FIG. 7B. The linkages  25  of the tractor grip shown in FIGS.  7 A- 7 C are shown in their fully open position. Note that wheels  16  are away from the wedge guide surfaces  22 . In addition to the elements of the embodiment discussed earlier, FIG. 7B also shows the actuator  60  that provides the axial force necessary for the selective opening and closing of the linkages  25  in and out of the tool body, as well as parts of the hydraulic control circuits necessary for the operation of the grip. In this particular embodiment, the axial force is generated by a hydraulic actuator  60 , which consists of piston  62 , spring  64 , and dynamic seals  66  and  68 . The piston  62  of the actuator  60  can move up or down as chamber  70  is connected to or disconnected from a source of high pressure hydraulic fluid (not shown in FIGS.  7 A- 7 C). Piston  62  is attached to the moving hub  10  with a screw  72  and thus, the motion of the actuator forces hub  10  to move with respect to hub  40 . Other elements of the embodiment shown in FIGS.  7 A- 7 C are a high pressure accumulator, designated with the general numeral  80 , and the two hydraulic cartridges  85  and  90 , which control the opening and closing of linkages  25  and control the tractioning process. Since the high pressure accumulator  80  and the hydraulic cartridges  85  and  90  are peripheral to the operation of the invention, and since they have been disclosed in co-pending U.S. patent application Ser. No. 09/921,825, they are not discussed in detail here. All other elements of the invention shown in FIGS.  7 A- 7 C have the same numerical designations and the same functions as those discussed with regard to previous figures.  
         [0053]    Those skilled in the art will appreciate that traction mechanisms other than cams can be combined with the invention. Thus, the invention can improve the operation of virtually every downhole tractor tool, independent of the principle upon which the traction of the tractor is generated. Examples of the usage of different traction devices in conjunction with the invention are schematically shown in FIGS. 8, 9, and  10 .  
         [0054]    [0054]FIG. 8 represents a downhole tractor tool in which the traction is generated by powered drive wheels  100  mounted at the tips of linkages  25 . Similar to the asymmetric linkage design shown in FIG. 4, the tractor tool shown in FIG. 8 has arms  2  equipped with wedges  4  only on the bottom side of each linkage  25 . The two top arms  102  can only pivot with respect to the stationary hub  104 , which is an integral part of the tool body  106 . Arms  102  also house drive trains (not shown), which transmit rotary motion from a motor (not shown) inside the tool body  106  to the drive wheels  100 . The moving hub  10 , arms  2 , wedges  4 , wheels  16 , pins  12 , and slots  14  all function as described in connection with FIG. 1. FIG. 8 also shows schematically one type of actuator  110  that can be used to selectively open and close linkages  25 . In this embodiment, the actuator  110  consists of a motor  112 , which drives a ball screw  114 . As the ball screw  114  turns, a ball nut  116  travels up or down. The ball nut  116  transmits its linear motion to the hub  10  through a spring  118 , which provides the flexibility of linkages  25  necessary when the tractor tool encounters small variations in wellbore size or other obstacles.  
         [0055]    [0055]FIG. 9 is a schematic representation of another traction mechanism that can be used with the invention. In this case, tracks  120  are mounted at the tips of symmetric linkages  25 . The tracks are attached to linkages  25  with pivot pins  6  that can slide and pivot in slots  124  in the tracks  120 . At their upper ends the tracks  120  are attached to arms  130  which, similar to arms  102  in FIG. 8, house mechanical elements (not shown) for transmitting rotary motion from a motor (not shown) in the tool body  44  to the drive sprockets  122  of the tracks  120 . At their lower ends tracks  120  are attached to another set of arms  132 , which enable the tractor tool to go through changes in wellbore size and other obstacles. Arms  132  are attached to the tool body  44  with pins  134  that slide in slots  136 . FIG. 9 also shows a moving hub  10  and a stationary hub  40 , which have exactly the same functions as those described in connection with FIG. 6. The actuator  140 , shown in FIG. 9, operates on a different principle from the actuator  110  shown in FIG. 8. The actuator  140  consists of a hydraulic piston  142 , which is an integral part of the moving hub  10 . This illustrates the flexibility of the invention and the fact that it will work with a variety of actuators that operate on different principles. The type of actuator used does not affect how the invention achieves its expansion.  
         [0056]    [0056]FIG. 10 is a schematic illustration of yet another embodiment of the present invention having the form of a downhole traction system. In this case, roller assemblies  151  that consist of rollers  152  are mounted on inclined axles  154  at the tips of linkages  25 . Traction is achieved by rotating the moving hub  10  and the stationary hub  160  with respect to a central mandrel  164  of the tool body  44 . The direction of rotation is indicated by the rotational movement arrow  162  in FIG. 10. As the whole set of linkages  25  rotates, the tractor tool achieves a corkscrew motion along the internal wall of a wellbore. The rotary motion of the tractor mechanism is generated by a motor and a gear train (not shown) that are inside the tool body  44 . The rotary motion is then transmitted to hub  160 . Note that hub  160  is only free to rotate with respect to the central mandrel  164  but is prevented from sliding with respect to the tool body  44  by a ledge  166 , which is defined by an enlarged section of the central mandrel  164 . The other hub  10  can both rotate and translate with respect to the central mandrel  164  as indicated by arrows  172  and  168 . When hub  10  slides up or down on the central mandrel  164 , linkages  25  expand or contract radially. Similar to the embodiments discussed earlier, the translation of hub  10  up or down is achieved by a linear actuator, designated by the numeral  170 . In FIG. 10, the actuator is shown as a hydraulic piston  174  that is an integral part of hub  10 . As explained earlier, actuators operating in accordance with other principles can also be constructed without departing from the spirit and scope of the present invention.  
         [0057]    In all the embodiments discussed so far, the invention was combined with other mechanisms to construct various downhole tools to be operated in wells and pipelines. However, the invention is not limited to these embodiments. In general, the invention can improve the operation of any device that is designed to support a load in one direction by the application of a force in a second direction perpendicular to the first direction. Two such embodiments are shown in FIGS. 11 and 12. FIG. 11 illustrates an embodiment of the present invention which functions as a load lifting jack device, such as a jack for raising and lowering an automotive vehicle. In FIG. 11, one symmetric linkage  25  is attached to a base  180 , while another linkage  25  is attached to the lifting fixture  182 . The two force transmitting members or hubs  10  and  190  function exactly as described in connection with FIG. 1 as they move with respect to one another in the axial direction. The axial actuator in this case is a screw-nut mechanism, with a driven nut  184  being a part of hub  10 . The screw  186  is threaded into nut  184  and can be rotated with respect to hub  190  with a crank handle  192 . The linear motion of screw  186  with respect to hub  190  is prevented by the stop  188  and the bearing assembly  194 . Most existing car jacks that use triangular kinematic mechanisms are very difficult to start when they are fully contracted. The present invention overcomes this problem. As explained with regard to FIGS. 1 and 2, the axial force that the invention requires is substantially constant. Thus, the rotational force that must be applied to the crank handle  192  in order to lift the load is also constant and thus the jack is easy to start from its contracted position.  
         [0058]    Another embodiment of the invention that can be used to lift a load in one direction by the application of a force in a perpendicular direction is shown in FIG. 12. In FIG. 12, an actuator  200  that generates the force F a  is used to lift the load  202 , which exerts a downward force F r . As seen in the figure, arm  2  can be extended beyond the location of the pivot or axle  6  that joins the two linkage arms  2  in pivotal assembly. This does not change the principle upon which the invention operates and again demonstrates the flexibility of the invention. The addition of extra linkages  204  joined at pins  206  and  208  does not change the principle of operation of the invention. Those skilled in the art will readily appreciate that a great variety of mechanisms and devices for a variety of industrial applications can be constructed within the scope of the present invention.  
         [0059]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.