Abstract:
A constant force device has at least a first non-constant axial force driving the first set of arms and a second non-constant axial force driving the second set of arms, where the two sets of arms are offset from one another by 90°. Each of the non-constant axial forces is converted to a radially extending force by the interaction of a force guide and actuator. The force guide is attached to the inner mandrel of the constant force device and is shaped to produce an essentially constant radially extending force through the entire range of motion of the arms. Typically each arm of the pair of arms has a pivoting arm and a telescoping arm where the joint between the pivoting arm and telescoping arm has one or more wheels to reduce friction as the constant force device moves through the tubular. Generally the first pair of arms is opposed to and overlaps by some distance the second pair of arms where the second pair of arms is 90° offset from the first pair of arms. Additional features may include friction reducing members at the joint between the telescoping arm and the pivoting arm, an extension lock, extension limiters, and rotating force guides.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 62/144,657 that was filed on Apr. 8, 2015. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to a device that may apply a vectored force radially outward from a central axis. Such vectored force may be applied in multiple directions at once where the application of the vectored force is to maintain the central axis of the device relatively aligned with the central axis of the tubular through which the device passes. 
       BACKGROUND 
       [0003]    Once a hydrocarbon bearing well has been drilled it is usually necessary to perform several tests upon the well, for instance to determine the integrity of the casing after it has been installed, to determine for instance the quality of the cementing job, or to determine the presence and locations of any hydrocarbons adjacent to the well. Such testing is usually done with a set of instruments referred to as a logging tool. In most instances the logging tool is lowered into the well on a cable, where the cable may include a power and/or data line. Logging tools may be transported through any tubular structure including pipelines and refineries. 
         [0004]    Certain types of logging tools work best when they are centrally positioned within the tubular structure being tested. In order to centrally position the logging tool within the tubular, a centralizer may be used. Centralizers typically use a set of springs, such as bow springs, to apply force radially outward from a central axis. Provided that the force is applied equally in all directions and that there is sufficient force to overcome any bias due to the weight of the logging tool, the logging tool will remain more or less centralized within the wellbore, whether open hole or cased hole. Unfortunately the diameter of the wellbore varies as the tool progresses through the wellbore. Variations in diameter may be due to other tools or equipment located in the wellbore or to different sizes of casing installed as the well progresses from the surface to the well&#39;s final depth. Other variations in the well diameter may be due to changes in the well&#39;s direction causing the casing to become ovalized as the tubular bends through turns. Unfortunately the force applied to different sizes of tubulars by a standard centralizer varies such that a centralizer may have sufficient force to keep a logging tool centralized in one size of tubular, but when the logging tool is in a smaller diameter tubular such force is excessive, causing damage to the centralizer or even preventing the centralizer from progressing through the well. On the other hand while the force applied may be sufficient to keep a logging tool centralized in one size of tubular, in a larger diameter tubular such force is inadequate allowing the logging tool to substantially deviate from the center of the tubular. 
         [0005]    In order to address such concerns many variations of constant force centralizers have been developed. There are several constant force centralizers available in the market, but there is very little information showing quantitative force values vs. casing size. Ideally, each constant force centralizer would have a force chart similar to the force chart shown in  FIG. 1 . 
         [0006]    Though customers seem to have a clear need for a constant force centralizer, such requests do not appear to include a definition of “constant.” The understanding is that clients just need a device that keeps their tools centralized in a wide range of environments. 
       SUMMARY 
       [0007]    A constant force centralizer is envisioned where a first non-constant axial force drives the first set of arm assemblies and at least a second non-constant axial force drives the second set arm assemblies where the two sets of arm assemblies are offset from one another by 90°. Typically the non-constant axial forces are provided by some type of biasing device usually a spring or compressed gas but other types of biasing devices may be used. A force guide may be permanently affixed, rotatably attached, or otherwise mounted on the central mandrel of the constant force centralizer. Each of the non-constant axial forces is converted to a radially extending force by an interaction of a three guide and actuator. The force guide is shaped to produce an essentially constant radially extending force through the entire range of motion of the arm assemblies. Preferably the radially extending force is maintained throughout each arm assembly&#39;s travel within about ten percent of the maximum radially extending force. Typically, each arm assembly is comprised of a pivoting arm and telescopic section. Typically a wheel is positioned at the joint of the pivoting arm and the telescopic section to reduce friction as the constant force centralizer moves through the tubular. In the collapsed condition where the pivoting arm and wheel are relatively close to the mandrel of the constant force centralizer the telescoping arm is in its substantially shortest state whereas in the extended condition where the pivoting arm and wheel are at their maximum distance from the mandrel the telescoping arm is in its longest state. The telescoping arm is generally necessary in order to allow the constant force centralizer to reverse direction when moving from a larger diameter tubular to a smaller diameter tubular. A portion of the telescoping arm will interact with the tubular to force the pivoting arm and wheel to retract to at least a semi-collapsed condition. By utilizing a telescoping arm in place of a solid arm, the overall length of the constant force centralizer is shorter than would otherwise be possible, and this is considered beneficial for many logging tool embodiments. 
         [0008]    It is envisioned that two pairs of arm assemblies will usually be used in a constant force centralizer. The pairs of arm assemblies are typically arranged such that a first end of the first pair and a first end of the second pair of arm assemblies extend toward each other from a first end of a mandrel and from an opposing second end of the mandrel. Generally the first pair of arm assemblies is allowed to collapse into a nested position with the second pair of arm assemblies. When fully collapsed, opposing pairs of pivoting arms overlap by some distance. The overlap and telescoping arms generally allows the tool to be shorter than a standard tool not having overlapping arms. 
         [0009]    Ovalized casing is encountered occasionally, and centralization in such conditions can be difficult. Constant force centralizers offered to date have linked arms providing lateral arm movement that is symmetric in all directions. In round casing and in vertical wells, this arrangement is adequate. However, in deviated wells where the casing is ovalized, these centralizers may not perform well. In a current embodiment typically the arms that are offset from one another at some angle, typically 90°, allow for the offset arms to provide non-symmetric arm movement in at least two directions providing centralization even in non-symmetric or ovalized wellbores or tubulars. In certain situations it has been found that non-symmetric constant force is necessary such that the tool is held in an eccentric condition within the well. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0011]      FIG. 1  depicts a calculated and measured force curve of an embodiment of the invention. 
           [0012]      FIG. 2  depicts a calculated force curve of an alternate embodiment of the invention. 
           [0013]      FIG. 3  depicts a side view of an embodiment of the invention in its extended condition. 
           [0014]      FIG. 4  depicts an end view of an embodiment of the invention. 
           [0015]      FIG. 5  depicts a side view of an embodiment of the invention in its retracted condition. 
           [0016]      FIG. 6  depicts an end view of an embodiment of the invention in a partially extended condition in an oval tubular. 
           [0017]      FIG. 7  depicts an extended joint of an embodiment of the invention. 
           [0018]      FIG. 8  depicts a side view of an alternate embodiment of the invention in its extended condition. 
           [0019]      FIG. 9  depicts a side view of an alternate embodiment of the invention having a rotatable force guide in an extended condition of a constant force centralizer. 
           [0020]      FIG. 10  depicts an orthogonal view of a rotatable force guide. 
           [0021]      FIG. 11  depicts a side view of an alternate embodiment of the invention having an extension limiter in a limited extension condition. 
           [0022]      FIG. 12  depicts a side view of an alternate embodiment of the invention having an extension lock in a retracted and locked condition. 
           [0023]      FIG. 13  depicts a close-up of the area A from  FIG. 12 . 
       
    
    
     DESCRIPTION 
       [0024]      FIG. 1  depicts a graph of the measured force curve  10  versus the predicted force curve  14  of an embodiment of the present invention. The measured force curve  10  is a poly fit of the measured points  12  while the predicted force curve is a based upon a computer simulation. A perfectly flat, linear response was the original design goal, but in order to keep the mechanisms relatively simple, a slight “curve”, as depicted by the predicted force curve  14  and the measured force curve  10  was thought to be acceptable. 
         [0025]      FIG. 2  depicts a graph of the predicted force curve  20  of an alternate embodiment of the present invention. While other force ranges may be used, the predicted force curve  20  utilizes a force range of from about 40 pounds of force at the minimum diameter of just over three inches increasing to about 43 pounds of force at the mid-range diameter of 8 inches then decreasing again to about 40 pounds of force at the maximum diameter of about thirteen inches. Such a force range has less than a 10% variation across the range of applied force from the minimum diameter to the maximum diameter. 
         [0026]      FIG. 3  is a side depiction of an embodiment of a constant force centralizer  50  providing a substantially constant radially outward force throughout a predetermined range of tubular diameters. The constant force centralizer  50  has an inner mandrel  52 , beginning with the right side of the constant force centralizer  50 , and at least one axial biasing device such as axial biasing device  54 . A collar  58  is fitted to the mandrel  52  in such a manner that its position is fixed relative to the mandrel  52 . The collar  58  may be threaded, pinned, welded, or formed as an integral part of inner mandrel  52 , or connected by any other means known to the inner mandrel  52 . The axial biasing device  54  typically surrounds inner mandrel  52  and abuts collar  58 . The axial biasing device  54  also abuts a movable sleeve  62 . Typically the movable sleeve  62  is circumferential about an exterior surface of inner mandrel  52 . The movable sleeve  62  is generally only axially movable. A first end  70  and  72  of pivotal force arms  64  and  68  is attached to movable sleeve  62 . Each pivotal force arm  64  and  68  has a recess  74  and  76 . Within each recess  74  and  76  is an actuator  80  and  82 , such as a roller. Each recess  74  and  76  is sized such that when pivotal force arms  64  and  68  are in the retracted position, lying flat against inner mandrel  52 , most of the force guides  84  and  88  that extend beyond the exterior surface of inner mandrel  52  are contained within each recess  74  and  76 . The force guides  84  and  88  are fixed to the inner mandrel  52  and maybe threaded on, pinned on, or formed as an integral part of the inner mandrel  52 . It is generally the interaction between the force guides  88  and  84  with the corresponding actuators  80  and  82  that describes the constancy of the force curves such as the curves in  FIGS. 1 and 2 . Each force guide  88  and  84  will have a surface such as surfaces  90  and  92 . Generally the surfaces  90  and  92  are linear surfaces at some angle α relative to the axis of mandrel  52  where the angle α provides a reasonably flat force curve. The angle α in  FIG. 3  is 47°. 
         [0027]    Continuing with the left side of the constant force centralizer  50 , the constant force centralizer  50  has at least one axial biasing device such as axial biasing device  56 . A collar  100  is fixed onto a second end  102  of inner mandrel  52 . The collar  100  is typically threaded onto inner mandrel  52  but may be pinned, welded, or formed as an integral part of inner mandrel  52 . The axial biasing device  56  typically surrounds inner mandrel  52  and abuts collar  100 . The axial biasing device  56  also abuts a movable sleeve  104 . Typically the movable sleeve  104  is circumferential about an exterior surface of inner mandrel  52 . The movable sleeve  104  is generally only axially movable. A second end  106  and  108  of telescopic arms  110  and  112  is attached to movable sleeve  104 . 
         [0028]    A first end  114  and  116  of telescopic arms  110  and  112  is pivotally connected to a second end  120  and  122  of pivotal force arms  64  and  68 . Generally, at the pivotal connection where first end  114  and second end  122  as well as first end  116  and second end  120  are connected, a wheel, such as wheel  124  and  126 , a roller, a skid, or other friction reducer is attached. Generally it is at wheels  124  and  126  that the constant force is applied to the casing or other tubular in a direction perpendicular to the long axis of the constant force centralizer  50 . 
         [0029]    When the constant force centralizer is in a tubular with sufficiently small diameter, each of the pivotal force arms  64 ,  68 , and telescopic arms  110 , and  112  will be in a collapsed position such that wheels  124  and  126  are at a minimal radial distance from inner mandrel  52 . With wheels  124  and  126  at their minimal radial distance from inner mandrel  52 , axial biasing device  54  is at maximum compression thereby applying the maximum normal force against movable sleeve  62 . The force applied by axial biasing device  54  is transferred to the movable sleeve  62 . The force applied by axial biasing device  54  is not necessarily constant. Movable sleeve  62  in turn transfers the force to pivotal force arms  64  and  68 . Subsequent movement of pivotal force arm  64  is guided by actuator  80  acting on surface  90  causing end  122  to move in a direction substantially perpendicular to the axis of the mandrel  52 . The dimensions of pivotal force arm  64 , actuator  80 , force guide  88 , movable sleeve  62 , collar  58  and biasing device  54  are chosen so that the force from the biasing device  54  is transferred to the wheel  124  in such manner that that force of wheel  124  against the tubular remains reasonably constant as the diameter of the tubular changes. Movement of pivotal force arm  68  is guided by actuator  82  acting on surface  92  causing end  120  to move in a direction substantially perpendicular to the axis of the mandrel  52 . The dimensions of pivotal force arm  68 , actuator  82 , force guide  84 , movable sleeve  62 , collar  58  and biasing device  54  are chosen so that the force from the biasing device  54  is transferred to the wheel  126  in such manner that that force of wheel  126  against the tubular remains reasonably constant as the diameter of the tubular changes. 
         [0030]    When the constant force centralizer  50  moves from a large diameter tubular to a smaller diameter tubular the force vectors are reversed such that the wheels  124  and  126  are forced inward exerting force through the pivotal arm  64  and  68  to the actuators  80  and  82  were the forces are redirected by the interaction of the actuators  80  and  82  with force guides  84  and  88  into movable sleeve  62  and ultimately into axial biasing device  54 . 
         [0031]    As indicated in  FIG. 2 , a current embodiment of the tool produces  40  lbs of radial force at the wheels  124  and  126  at the joint between pivoting force arms  64  and  68  and telescoping arms  110  and  112 . The force response is adjustable with the nominal radial force either increased or decreased. Such increases or decreases may be adjusted where biasing devices  54  and  56  may be replaced with springs, gas chambers, etc. having proportionally higher or lower force rates. If needed, shims can add compression to the biasing devices  54  and  56  and thereby increase the axial force. A flatter force response curve, see  FIGS. 1 and 2 , is achievable if a more complex shape is machined into the force guides  84  and  88 . 
         [0032]    As further indicated in  FIG. 3 , the force guides  84  and  88  are generally fixed to the inner mandrel  52  of the constant force centralizer  50 . In a current embodiment. the force guides  84  and  88  have an angle α where the angle α is about 47° relative to the axis of the inner mandrel  52  achieving a substantially constant force response as indicated in  FIG. 2 . The force guide angle and/or shape controls the shape of the force response curves such as the force response curves in  FIGS. 1 and 2 . 
         [0033]      FIG. 4  is an end view of the fully collapsed constant force centralizer  50 . In the embodiment of the constant force centralizer depicted an outside diameter of 3.5 inches was chosen as the nominal diameter of the centralizer. A smaller outer diameter can be achieved in the centralizer design by scaling down the size of the components. 
         [0034]    In general, for wireline tools, shorter tools are preferred. As tools become shorter, their overall weight is reduced. In one embodiment of the 3.5 inch diameter constant force centralizer  100 , the total tool weight is less than 40 pounds. With this in mind, as indicated below, several unique design features were employed to minimize the tool length. As shown in  FIG. 5 , one embodiment of the constant force centralizer  100  has a length of 26.8 inches. The relatively short tool length of the design is a result of offsetting the pivoting force arms  102 ,  104 , and  106 . The pivoting force arm  102  is attached to movable sleeve  108  while pivoting force arm  104  is attached to movable sleeve  110  by pin  112  and pivoting force arm  106  and is attached to movable sleeve  110  by pin  114 . The pivoting force arm  102  is connected to telescoping arm  122  at the joint  132 . Also at joint  132  are wheels  116  and  117 . Telescoping arm  122  has a first portion  124 , connected to pivoting force arm  102  at joint  132 , and a second portion  126 . Second portion  126  is attached to movable sleeve  110  via pin  134 . In this embodiment the second portion  126  slides within the first portion  124 . The other two pivoting force arms  104  and  106  seen in  FIG. 5  are each rotated 90° around the central axis of the constant force centralizer  100 . For ease of reference only pivoting force arm  104  will be further described. As described previously pivoting force arm  104  is attached to movable sleeve  110  by pin  112 . The pivoting force arm  104  is connected to telescoping arm  128  at the joint (not shown) where wheel  118  is attached to the constant force centralizer  100 . 
         [0035]    As can be seen in  FIG. 5  the movement of the arms occurs in two planes (not shown). The two planes are perpendicular to each other and both planes contain the axis of the constant force centralizer  100 . Additionally each of the wheels  116  and  118  are offset by some axial distance D. The distance D may vary depending upon whether the arms are fully extended or fully collapsed or at some point in between. 
         [0036]    When the arms are fully open the wheels  116  and  118  are axially offset by 1.35 inches. When the arms are fully closed the wheels  116  and  118  are axially offset by 2.1 inches. With offset wheels, the centralizer  100  can traverse radial upsets in the tubular more easily, and erratic tool movement is minimized. 
         [0037]    Generally by having the telescoping arms  122  and  128  attached to their respective pivoting force arms  102  and  104  the respective axial biasing devices  140  and  142  operate to apply force to their associated wheels  116  and  118  independently. 
         [0038]    The wheels  116 ,  118 ,  117  and  120 , at each of the joints between the pivoting force arms  102 ,  104 , and  106  and the telescoping arms  122 ,  128 , and  130  are free to rotate even when the tool is completely closed to its minimum outside diameter. In the embodiment of the constant force referred to in  FIG. 2  the constant force centralizer is designed to open from about 3.5 inches to about 12.7″ which is the inner diameter of typical casing that has an outer diameter of 13⅜ inches. As shown in  FIG. 2 , about 40 pounds of centralizing force is active across that entire range. 
         [0039]    In an embodiment of the current invention of the constant force centralizer  100  from  FIG. 5  as further depicted in  FIG. 6  the pivoting force arm  102  is paired with the pivoting force arm  107  on the opposite side of the constant force centralizer  100 . The opposing pivoting force arms  102  and  107  move symmetrically with one another. The telescoping arm  128  is paired with the telescoping arm  130  on the opposite side of the constant force centralizer  100 . The opposing telescoping arms  128  and  130  move symmetrically with one another. 
         [0040]    The telescoping arms  128  and  130  arms are generally orthogonal to the pivoting force arms  102  and  107 . The pivoting force arms  102  and  107  are typically coupled to each other such that the axial biasing device  140  drives both of the pivoting force arms  102  and  107 . While the telescoping force arms  128  and  130  may be linked to the same movable sleeve  108  as the pivoting force arms  102  and  107  the telescoping mechanism does not allow force to be applied by movable sleeve  108  to the telescoping force arms  128  and  130 . In oval holes, conventional wisdom suggests that one pair of arms, either the pivoting force arms  102  and  107  or the telescoping arms  128  and  130 , will naturally align with the “long axis” of the hole. In  FIG. 6  the long axis of the tubular  136  is depicted as being 12.4 inches as shown by reference numeral  133  while the short axis of the tubular  136  is depicted as being 11.3 inches as shown by reference numeral  135 . The position of the constant force centralizer as depicted in  FIG. 6  is preferable and is likely to maintain good tool centralization in oval holes. 
         [0041]    Several features of the constant force centralizer are intended to minimize rolling friction. The wheels  150  and  152 , as depicted in  FIG. 7 , in an embodiment of the constant force centralizer are preferably as large in diameter as possible, here 1.3 inches in diameter as shown by reference numeral  154 , without exceeding the desired 3.5 inch constant force centralizer outside diameter. Maximizing the wheel diameter allows each wheel  150  and  152  to last longer and roll more smoothly across irregularities in the tubular. Typically, each joint between the telescoping arm  158  and the pivoting arm  156  arm has two wheels  150  and  152  at the joint. Each wheel rolls independently on ball bearings  160 . 
         [0042]    As shown in  FIG. 8 , when an embodiment of the constant force centralizer  200  is fully open, the pivoting force arms  202  and  204  are at an angle β relative to the axis of the constant force centralizer  200 . In this instance angle β is 30°. The telescoping arms  206  and  208  are at an angle Ω relative to the axis of the constant force centralizer  200 . In this instance angle Ω is 35°. Generally, it is desired to have the angles β and Ω as shallow as possible in order to help the constant force centralizer  200  slide through any restrictions that may exist within the tubular. 
         [0043]      FIG. 9  is a depiction an alternative embodiment of the constant force centralizer  300 . In some instances it has been found desirable to allow the inner mandrel  302  to remain fixed to the wireline or other transporting device while allowing the components of the centralizer assembly including the rotatable force guide  304 , pivoting force arms  306 , telescoping arms  308 , first axial biasing device  312 , first movable sleeve  316 , second axial biasing device  314 , second movable sleeve  318 , and other associated portions of the centralizer assembly to rotate around the inner mandrel  302 . By allowing the centralizer assembly to rotate around the inner mandrel  302  the wireline (not shown) avoids becoming twisted thereby avoiding any torque build up on account of constant force centralizer  300 . 
         [0044]      FIG. 10  is a depiction of the rotatable force guide  304  from  FIG. 9 . The rotatable force guide  304  typically consists of a first-half  356  and a second half  358 . Each half  356  and  358  has a semicircular section such as  366  and semicircular section  364  each half  356  and  358  also has at least a portion of the force guide  304  attached to the semicircular sections  366  and  364 . The upper force guide includes a relatively linear surface  370  set at an angle α to the central axis of the inner mandrel  302  of the constant force centralizer  300 . The upper force guide also includes a means to pivotally attach a limiting arm (not shown) such as providing a slot  372  for a wrist pin (not shown). The lower force guide includes a relatively linear surface  368  set at an angle α to the central axis of the inner mandrel  302  of the constant force centralizer  300 . The lower force guide also includes a means to pivotally attach a limiting arm (not shown) such as providing a slot  374  for a wrist pin (not shown). 
         [0045]    In the embodiment shown the rotatable force guide  304  is applied to the inner mandrel  302  by placing each half  356  and  358  such that the semicircular portions  364  and  366  surround the inner mandrel  302 . Then using bolts such as bolts  362  and  360  to fix each half  356  and  358  in place around inner mandrel  302 . It is envisioned that any known means of manufacturing a rotatable force guide could be used for instance in some instances the force guide  304  could be machined out of a solid piece of material and then slid onto the mandrel  302  from one end. 
         [0046]      FIG. 11  is a depiction an alternative embodiment of the constant force centralizer  400 . In some instances it has been found desirable to limit the outward translation of the pivoting force arm  402  in turn limiting the outward translation of the telescoping force arm  404  and wheel  406 . Such a limitation may be useful in, for instance, circumstances where the constant force centralizer  400  may pass through very large openings such as when it passes through a blowout preventer which might cause damage to the constant force centralizer  400 . 
         [0047]    One such extension limiter may use a link  410  attached to the inner mandrel  412  or as is shown in  FIG. 11  a first end of link  410  is attached to the rotatable force guide  414  by wrist pin  416  within slot  472 . A second end of link  410  is attached to pivoting force arm  402  by wrist pin  418  within slot  420 . Wrist pin  418  is configured such that it may slide within slot  420  depending upon the extension position of wheel  406  as wheel  406  moves towards inner mandrel  412  wrist pin  418  will move towards wheel  406  within slot  420 . However as wheel  406  moves away from inner mandrel  412  wrist pin  418  moves within slot  420  towards movable sleeve  422 . Eventually wrist pin  418  reaches the end of slot  420  closest to movable sleeve  422  whereupon wheel  406  is prevented from moving any further radially outward from inner mandrel  412 . 
         [0048]      FIG. 12  is a depiction of an alternative embodiment of a portion of a constant force centralizer  500 . In most instances it has been found to be preferable to restrict any expansion of the force pivoting arms  502 ,  504 , and  506  as well as the associated telescoping arms  508 ,  510 , and  512  until at least the constant force centralizer  500  has been deployed into the tubular or wellbore. Preferably a lock will maintain the pivoting force arms and telescoping arms in the retracted position until some predetermined parameter is reached. For instance a pressure actuated retaining pin  520  may be used where the pressure actuated retaining pin  520  is designed to protrude from the force guide  522  when the constant force centralizer  500  is below some preset pressures such as atmospheric pressure. The portion of the pressure actuated retaining pin  520  that protrudes from the force guide  522  engages pivoting force arm  502  and prevents it from opening. When the constant force centralizer  500  enters the tubular the external pressure may be increased such that at some predictable point the pressure will be sufficient to force the pressure actuated retaining pin  520  inward into its recess within the force guide  522 . With the pressure actuated retaining pin  520  moved inward the pivoting force arm  502  is released so that the wheel  524  may move radially outward to engage the tubular at the predetermined force level. 
         [0049]      FIG. 13  is section A from  FIG. 12 .  FIG. 13  depicts force guide  522  having the pressure actuated retaining pin  520  within recess  524 . In the embodiment shown in  FIG. 13  a pressure actuated retaining pin is utilized. In other instances the retaining pin could be actuated by temperature, elapsed time, a sacrificial wear pin, by a chemical reaction, or by an electrical signal. A portion  526  of the pressure actuated retaining pin  520  extends from force guide  522  into a port  528  within pivoting force arm  502 . The pressure actuated retaining pin  520  and recess  524  form a chamber  530  sufficient to allow the pressure actuated retaining pin  522  to move into the recess  524  within force guide  522  upon the application of sufficient force to port  528  and acting upon the portion of the pressure actuated retaining pin  522  that extends into port  528 . The pressure actuated retaining pin  522  may be held outwardly extended by the force exerted upon the pressure actuated retaining pin  522  by the pivoting force arm  502  and/or may have any other means known in the industry for securing the pressure actuated retaining pin  522 . 
         [0050]    While the embodiments are described with reference to various implementations and exploitations, it will be understood that these embodiments are illustrative and that the scope of the inventive subject matter is not limited to them. Many variations, modifications, additions and improvements are possible. Variations are likely to be beneficial when employed in tools such as calipers, anchoring devices, eccentering devices, and downhole tractors. 
         [0051]    While the embodiments shown are described with the intention of maintaining a substantially constant radial force across the full operating range of the device, it is understood that, if desired, the mechanism can be modified to achieve different radial forces within different size tubulars. 
         [0052]    Plural instances may be provided for components, operations or structures described herein as a single instance. In general, structures and functionality presented as separate components in the exemplary configurations may be implemented as a combined structure or component. Similarly, structures and functionality presented as a single component may be implemented as separate components. These and other variations, modifications, additions, and improvements may fall within the scope of the inventive subject matter.