Abstract:
A downhole tool ( 10′ ) comprises a body ( 12′ ) having a longitudinal axis and a body through-bore ( 14′ ), a slot ( 34′ ) communicating the outside of the body with the body through-bore. A sleeve actuator ( 30′ ) mandrel also has a bore and is selectively axially slidable in the body through-bore. A hollow bar ( 36′ ) is slidable with a radial component in the slot. At least two levers ( 210 ), each pivoted to the mandrel (about a first axis perpendicular the longitudinal axis) extend into the hollow bar and are pivoted thereto (about a second axis parallel the first axis). The levers lie in an actuation plane perpendicular the first and second axes and containing the slot. The levers are disposed at an inclined angle with respect to the longitudinal axis so that longitudinal motion of the mandrel translates into radial movement of the bar.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of, and priority to, application number 0704484.5, which was filed in the United Kingdom on Mar. 8, 2007, which application is incorporated herein by reference as if reproduced in full below. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       FIELD OF THE INVENTION  
       [0003]    The present invention relates to a downhole tool, in particular an under-reamer. 
       BACKGROUND 
       [0004]    Our pending international application publication number WO2006/072761 (the entire disclosure of which is incorporated herein by reference) discloses a downhole tool comprising:
       a body having a longitudinal axis and a body through-bore, a slot communicating the outside of the body with the body through-bore;   a sleeve actuator mandrel having a sleeve actuator mandrel through-bore and being selectively axially slidable in the body through-bore;   a flange on the sleeve actuator mandrel extending into said slot and having one of ribs and channels formed on its sides and inclined at an acute angle to the longitudinal axis; and   a hollow bar slidable with a radial component in the slots, the other of channels and ribs being formed on the bar and corresponding with, and engaged in, said one of said ribs and channels of the flange.       
 
         [0009]    A result of this arrangement is that the actuating surfaces of the tool, namely the interengaging ribs and channels, are isolated from the drilling fluid. Seals may be provided between the sleeve actuator mandrel and the body beyond both ends of the slot and define, between them and seals around the bars in the slots, a chamber enclosing lubricating oil. In this event, the mutually engaging surfaces are primarily within the confines of the oil chamber, where they are not only protected from contamination by drilling fluid and debris, but also they are washed in lubricant to facilitate their movement and to reduce wear. 
         [0010]    An object of the present invention is to provide an alternative arrangement that has the same benefits of this arrangement. 
         [0011]    U.S. Pat. No. 4,865,137 discloses an under-reamer in which cutter arms are pivoted in the body of the tool and a pivot link pivoted to the arm is urged by a hydraulically actuated control piston to pivot each arm outwardly. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0012]    In accordance with a first aspect of the present invention, there is provided a downhole tool comprising:
       a body having a longitudinal axis and a body through-bore, a slot communicating the outside of the body with the body through-bore;   a sleeve actuator mandrel having a sleeve actuator mandrel through-bore and being selectively axially slidable in the body through-bore;   a hollow bar slidable with a radial component in the slot; and   at least two levers, each pivoted to said sleeve actuator mandrel about a first axis perpendicular to a line parallel to said longitudinal axis, and each lever extending into said hollow bar and pivoted thereto about a second axis parallel said first axis, wherein an actuation plane of movement of said hollow bar on pivoting of the levers is perpendicular said first and second axes and contains said slot, and the first and second pivot axes at the intersection thereof with said actuation plane define a parallelogram.       
 
         [0017]    Thus, when the sleeve actuator mandrel is actuated to move along said longitudinal axis from a deactuated to an actuated position thereof, said levers are pivoted about their first axes to increase the radial position of said second axes with respect to said longitudinal axis, and whereby said bar slides with a radial component in said slot 
         [0018]    A return mechanism is provided to guarantee that the bars return to their deactuated position when this is selected. Usually, the strongest mechanism is utilized to actuate tools, because this will generally involve contact with the hole bore (to start cutting, for example, with an under-reamer), whereas retraction is generally not opposed. On the other hand, when components get worn or contorted by their interaction with the bore hole, they may be difficult or impossible to withdraw. 
         [0019]    This might be very problematic with an under-reamer where, to get the tool out through a narrow casing above the reamer, the reamer must be withdrawn (deactuated). Consequently, the levers may be captivated by pivot pins forming said first and second pivots between the levers and the sleeve actuator mandrel and the hollow bar respectively. Thus the sleeve actuator mandrel cannot return to its deactuated position without withdrawing the hollow bar into its slot. 
         [0020]    Said sleeve actuator mandrel may comprise a separate mandrel and sleeve actuator, the sleeve actuator having an actuator through-bore and being axially slidable in the body through-bore between a tool actuated position and a tool deactuated position, the mandrel having a mandrel through-bore and being selectively axially slidable in the body through-bore between a tool actuated position, an interlock position and a sleeve-lock position; wherein:
       an extension of the mandrel is a close sliding fit inside a first end of the sleeve actuator;   said first end captivates a lock element;   said body has an internal groove positioned so that, when said sleeve actuator is in said tool deactuated position, said lock element is aligned with said groove and held in engagement therein by said extension while the mandrel is between its interlock and sleeve-lock positions; and   said mandrel has an external recess positioned so that, when said mandrel is in said interlock position, said lock element is aligned with said recess, whereupon movement of the mandrel towards said tool actuated position releases said lock element from said groove permitting said sleeve actuator to be moved by the mandrel to said tool actuated position, said mandrel and sleeve actuator being locked together by the body holding said lock element in said recess between said interlock and tool actuated positions of the mandrel.       
 
         [0025]    Put another way, first means may lock the sleeve actuator with respect to the body in said tool deactuated position and while said mandrel is between said interlock and sleeve-lock positions; and
       second means may lock the sleeve actuator with respect to the mandrel and while said mandrel is between said interlock and tool actuated positions.       
 
         [0027]    Separating the mandrel from the sleeve actuator permits them to move independently when for some stroke movements of the mandrel which is needed for switching between actuation mode and deactuation mode of the tool. Generally, a strong return spring is utilized and, by connecting the mandrel with the sleeve actuator during some movements thereof, the return spring for the mandrel can also serve as the return spring for the bars. Since it is normal to provide signalling in the form of pressure pulses, at least when the tool is actuated, then, by connecting the mandrel to the tool actuator, signalling by the mandrel equates to signalling by the tool, at least when they are interconnected. 
         [0028]    Said sleeve actuator mandrel may have a port therethrough which aligns with a jet in the body when the sleeve actuator mandrel is in its tool actuated position, whereupon the through-bore of the sleeve actuator is in fluid communication with said jet, and whereby drilling fluid under pressure in said mandrel through-bore is directed onto the well bore in the region of said bar. 
         [0029]    Indeed, the applications disclosed herein are not limited to under-reamers. Adjustable stabilisers could benefit from the invention. 
         [0030]    Seals between said sleeve actuator mandrel and body beyond both ends of said slot define, between them, and a bar seal around the bar in the slot, a chamber enclosing lubricating oil. 
         [0031]    The levers may be pivoted to a flange connectable to the mandrel sleeve actuator. Preferably there are more than two of said levers in parallel. The levers are captivated by pivot pins forming said first and second pivots between the levers and the sleeve actuator mandrel and the hollow bar respectively. Said pivot pins are captured in blind bores in said hollow arms, said blind bores being formed by elements inserted in said hollow arms. Said elements may be welded in said arms. This is desirable because said seal around the arm which seals the arm in said slot is preferably in the same region as the pivots between said levers and arms. That is to say, a projection of said pivot pins in the direction of said second pivot axes preferably intersects said bar seal. Therefore, should the pivot pins be located in through bores of the arms, the pins would interfere with operation of the seal. 
         [0032]    There are a plurality, possibly three, of said bars, slots and flanges spaced around the longitudinal axis of the tool. 
         [0033]    Where the tool is an under-reamer, said bars are provided with cutting elements to effect under-reaming when the tool is actuated in a well bore having a pilot hole receiving the tool. 
         [0034]    Said body is thickened in the region of said slots and bars to support said bars. The body may have fins ahead of said slots having dimensions to match said pilot hole and bear against its surface and stabilise the tool, in use, said fins being provided with a hardened wear surface to minimise wear. 
         [0035]    Alternatively, the tool may be an adjustable stabiliser, said bars being provided with hardened wear surfaces to minimise wear of the bars, in use. 
         [0036]    Furthermore, the tool may be an azimuth controller, in which one or more bars in one or more slots are arranged asymmetrically around the longitudinal axis of the tool. The tool may also comprise one or more static blades. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    Embodiments of the invention are further described hereinafter, by way of example only, with reference to the accompanying drawings, in which: 
           [0038]      FIGS. 1   a, b  and  c  are side sections through an under-reamer in accordance with the invention in WO2006/072761 in sleeve lock, interlock and tool actuated positions respectively; 
           [0039]      FIGS. 2   a   1   , a   2   , b  and  c  are views of a variation of the tool shown in  FIGS. 1   a  to  c,  in corresponding positions, but also in greater detail; 
           [0040]      FIGS. 3   a  and  b  are sections along the lines A-A and B-B in  FIGS. 1   a  and  1   c  respectively; 
           [0041]      FIGS. 4   a  to  d  are, respectively two side views, in the directions of Arrows A and B in  FIG. 4   d , a section on the line A-A in  FIG. 4   a , and an end view in the direction of the Arrow D in  FIG. 4   b , of a tool in accordance with the teachings herein, in a deactuated position thereof; 
           [0042]      FIGS. 5   a  to  d  are, respectively two side views, in the directions of Arrows A and B in  FIG. 5   d , a section on the line B-B in  FIG. 5   a , and an end view in the direction of the Arrow D in  FIG. 5   b , of the tool shown in  FIGS. 4   a  to  d,  but in an actuated position thereof; and 
           [0043]      FIGS. 6   a  to  f  are, respectively a side view, and end view in the direction of Arrow B in  FIG. 6   a , a section on the line C-C in  FIG. 6   b , a section on the line D-D in  FIG. 6   a , a section on the line E-E in  FIG. 6   a , and a perspective, transparent view of an arm and lever mechanism for the tool shown in  FIGS. 4 and 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    In  FIGS. 1 to 3  of the drawings, an under-reamer  10  comprises a body  12  having a through-bore  14  along a longitudinal axis  50  of the tool  10 . A mandrel  16  actuates the tool  10  and is a component of an actuation mechanism  18 , only one end of which is shown in the drawings. The actuation mechanism  18  is connected at its end  18   a  to end  12   a  of the body  12  by a standard screw thread connection  20   a.  The other end  12   b  of the tool  10  comprises a female connection  20   b.    
         [0045]    The actuation mechanism  18  forms no part of the invention and may be in the form disclosed in WO-A-00/53886, U.S. Pat. No. 5,483,987, U.S. Pat. No. 6,289,999 (the entire disclosures of which are incorporated herein by reference), or any suitable means. Connected to the end of the mandrel  16  is mandrel end  22 , which, conveniently, is screw threaded to the mandrel  16 . However, in suitable circumstances end  22  may be integral with the mandrel  16  and henceforth is considered a part of the mandrel  16 . In the drawings, mandrel  16 , and its end  22 , is shown in three positions. In  FIG. 1   a,  it is shown in a sleeve-lock position. In  FIG. 1   b,  it has moved axially rightwardly in the drawings to an interlock position and, in  FIG. 1   c,  it has moved further rightwardly to a tool actuated position. The above positions are described further below. 
         [0046]    The tool  10  further comprises a sleeve actuator  30  which also has a sleeve through-bore  32 . Therefore, it can be seen that a clear passage comprising mandrel through-bore  24 , sleeve through-bore  32 , and body through-bore  14  through the tool  10  permits unimpeded passage of drilling fluid to a drill bit (not shown) connected to the tool  10 . 
         [0047]    Neither end  12   a, b  of the tool  10  is necessarily nearer the drill bit. However, for reasons explained further below, in the present arrangement, end  12   a  of the tool  10  is preferably arranged nearest the drill bit. 
         [0048]    The body  12  is provided with three axially disposed, circumferentially spaced slots  34   a, b, c,  only  34   a  of which is visible in  FIGS. 1   a  to  1   c.  Each slot receives a radially slidable cutter bar  36   a, b, c.  Although radial, there is no reason why the axis of the slots  34  should not be inclined to the radial. The top surface  38  of each cutter bar is provided with cutting elements, further details of which are not given herein. Suitable form of cutting elements will be known to those skilled in the art. One arrangement is shown in U.S. Pat. No. 6,732,817 (the full disclosure of which is herein incorporated by reference). Each cutter bar  36  is hollow, with an interior space or pocket  46 . The interior sides  40   a, b  (which sides are parallel the longitudinal axis  50 ) are formed with ribs  42  which are inclined with respect to the axis  50 . 
         [0049]    The actuator sleeve  30  is provided with three flanges  44   a, b, c  which are received within the pockets  46  of the hollow bars  36 . The flanges  44  are each provided with channels  48  which are also inclined with respect to the longitudinal axis  50  and which cooperate with the ribs  42  in the sides  40   a, b  of the pocket  46 . Indeed, the channels  48  define ribs between them, as do the ribs  42  define channels between them. 
         [0050]    With reference to  FIGS. 3   a  and  b,  the actuator sleeve  30  has, on its external surface, three open sections  52   a, b, c.  On assembly of the tool  10 , these sections are aligned with the slots  34   a, b, c  respectively. Each bar  36  with its corresponding flange  44  is then inserted through the slots  34  until a dovetailed base of the flanges  44  abut the open sections  52 . The actuator sleeve  30  is also provided with three dovetail sections  56   a, b, c  disposed between each open section  52   a, b, c.  When correctly aligned, the sleeve  30  is rotated through 60° about the longitudinal axis  50 . An hexagonal section of a nose  31  at second end  67  of the sleeve actuator  30  is adapted to receive a tool for this purpose. Dovetails  58  on the dovetailed sections  56  of the sleeve actuator  30  then lock with corresponding dovetails  60  on the dovetailed base of the flanges  44 . In this way, the flanges  44  are locked to, and become an integral part of, the actuator sleeve  30 . However, it is required to ensure that the sleeve  30 , in operation, does not rotate about axis  50  relative to the slots  34 , otherwise this will disengage the dovetails  58 ,  60 . For this purpose, a drilling  64  ( 64 ′ in  FIG. 2   a   2 ) in the body  12  is adapted to receive a pin (not shown) adapted to slide in a longitudinal groove  63  on the surface of the sleeve  30 . Thus the sleeve  30  is constrained rotationally about the longitudinal axis  50  but is free to move axially. 
         [0051]    When the actuator sleeve  30  does move axially, as it does between the positions shown in  FIGS. 1   b  and  1   c,  the ribs/channels  42 , 48  on the flanges  44  and inside the bars  36  interact to radially displace the bars  36  from a stowed, deactuated position (as shown in  FIGS. 1   a  and  b ), and where the bars are within the confines of the slots  34 , to an actuated position as shown in  FIG. 1   c.  Here, the bars  36  can bear against and cut the well bore (not shown). 
         [0052]    The actuator sleeve  30  is controlled by the mandrel  16 . The mandrel end  22  has a cylindrical extension  62  which is a close sliding fit in sleeve  30  at its first end  65 . On the end  65  are formed a number of pockets  66  which each receive a lock element in the form of a ball  68 . A shoulder  70  is provided in the body  12  and the lock elements  68 , sitting on the cylindrical surface of the extension  62 , prevent the sleeve  30  from moving rightwardly by engaging the shoulder  70 . The sleeve is therefore in a sleeve-lock position because the lock elements  68  prevent any rightward movement of the sleeve  30 , while the flanges  44  are at their leftmost position, in which the bars  36  fully withdrawn into the slots  34 . 
         [0053]    In this position, the mandrel  16  is free to move between the positions shown in  FIG. 1   a  and the position shown in  FIG. 1   b  without affecting the position of the sleeve  30 . However, when the mandrel  16  is moved rightwardly to an interlock position as shown in  FIG. 1   b,  recesses  72  on the surface of the mandrel extension  62  align with the lock elements  68 . They are consequently released from engagement with the shoulder  70 . Now, further rightward movement of the mandrel moves the actuator sleeve  30  rightwardly in the drawing to actuate the bars  36 . 
         [0054]    Between the interlock position shown in  FIG. 1   b  and the tool actuated position shown in  FIG. 1   c,  the internal cylindrical surface  74  of the body  12  locks the lock elements  68  in the recess  72  of the mandrel. Thus, the mandrel is locked to the actuator sleeve  30 . Consequently, when the mandrel returns leftwardly in the drawings from the  FIG. 1   c  position, the actuator sleeve  30  is constrained to follow it. 
         [0055]    This arrangement is also shown in greater detail in  FIGS. 2   a  to  c.  A difference, however, between the embodiment shown in  FIGS. 1   a  to  c  is that, here, the shoulder  70  is replaced by a circumferential groove  70 ′. 
         [0056]    A circumferential gallery  82  is provided around the body bore  14 , adjacent the ends of the slots  34 . Each slot  34  has an associated jet  84   a, b, c  (only jet  84   a  being visible in the drawings). The jets  84  communicate with the gallery  82 . The gallery  82  is sealed to the external surface of the sleeve  30  by seals  86   a, b.  The sleeve  30  is provided with a number of apertures or ports  88 . These put the sleeve bore  32  in fluid communication with its external surface. In the deactuated position of the actuator sleeve  30  ( FIGS. 1   a  and  2   a   1 ), the apertures  88  are sealed by seals  86   a  and further seals  86   c  in the body bore  14 . However, when the actuator sleeve  13  moves into its actuated position as shown in  FIGS. 1   c  and  2   c,  the ports  88  communicate with the gallery  82  so that drilling fluid under pressure in the actuator sleeve bore can escape to the outside through the ports  88 , gallery  82  and jets  84 . In issuing from the jets  84 , the drilling fluid serves to clear debris caused by the action of the cutters  36  against the well bore. 
         [0057]    Each slot  34  is not rectangular in section but has rounded ends  34   d,    34   e.  The bars  36  are correspondingly rounded at their ends and a circumferential groove  90  is formed around the entire periphery of each bar in which a seal (not shown) is disposed. 
         [0058]    At its second end  67 , the sleeve  30  is received within a liner  92  of the body  12 . The liner  92  is sealed to the body  12  by seal  94  and the end  67  is sealed to the liner  92  by seal  96 . Thus, between the seals  86   b,  seals  94 , 96 , and seals  90  around the bars  36 , an oil chamber  102  is defined. This can be filled with lubricating oil through a tapping  98  and longitudinal groove  100  in liner  92 . In use and after filling, tapping  98  is plugged by means not shown. 
         [0059]    Thus the interacting surfaces of the flanges  44  and bars  36  (that is to say, the ribs/channels  42 , 48 ), as well as the external surfaces of the bars  36  against the slots  34 , and the sliding of the sleeve actuator  30  in the body through-bore  14 , are all facilitated by the lubrication. This serves to reduce wear. Also, drilling fluid, particularly that in the annulus surrounding the tool  10  inside the well-bore, is isolated from these components so that the risk of jamming by hard particles carried by the drilling fluid is reduced. 
         [0060]    However, it will be appreciated that the volume of the chamber  102  changes as the radial position of the bars  36  changes, not to mention the axial position of the sleeve actuator  30 . Therefore, several longitudinally arranged drillings  104  are spaced around the circumference of the end  65  of the sleeve actuator  30 . These are positioned both to avoid the ports  88  and the pockets  66  and therefore should not strictly be visible in the drawings. However, they are shown in  FIGS. 2   a   1   , b  and  c  for illustrative purposes. 
         [0061]    Drillings  104  connect the chamber  102  with the annulus  106  in actuation mechanism  18  and surrounding mandrel  16 . The pressure in the annulus  106  is released by a bladder arrangement  108 , further details of which are not given as its essential structure is well understood in the art. 
         [0062]    The drillings not only relieve pressure in the chamber  102  but also serve to damp movement of the sleeve actuator  30 . They also supply the interlock arrangement  72 , 68 , 70  with lubricant to facilitate its action as well. 
         [0063]    Beyond the pressure relief bladder arrangement  108 , a mandrel return spring  110  is visible. Although not shown completely, spring  110  acts between bladder  108  fixed in the body of mechanism  18  and a shoulder on the mandrel  16 , urging it leftwardly in the drawings (see  FIG. 2   a   1 ). 
         [0064]    As mentioned above, the direction of orientation in a well bore of the tool  10  is not absolutely determined by its structure: it will operate in either direction; at least, it will if the actuation mechanism  18  operates on fluid pressure. However, it is preferred that it be arranged with the end  12   a  closest to the drill bit for three reasons. The first is that the jets  84  are more effective being directed immediately at the cutting interface between the cutters  36  and the well bore. Secondly, in the event that the bars  36  (or one of them), jam in their slots  34  and the normal deactuation force applied by the mandrel return spring is inadequate to overcome the jamming, then pulling the tool  10  up against the under edge of the casing (not shown) is considered more likely to nudge the jammed bar(s) back into the slots  34  than from the other direction. Thirdly, in the event of jamming, it would be possible to drop a ball down the well bore so that it closes the end of nose  31  of the sleeve actuator  30 . Then, hydraulic pressure above the actuator can supplement the force applied by the mandrel return spring  110 . 
         [0065]    It is to be noted that there are shown in the drawings three circumferentially spaced bar/flange/slot combinations around the tool. This is for illustrative purposes. The invention includes the possibility of more or less. The possibility of a tool with just one bar exists in the application of an azimuth controller, where it is desired to deflect the drill-string to one side of the well bore so that the azimuth of a motor assembly in the string may be adjusted. 
         [0066]    In the case of a stabiliser, the bars  36  are not provided with cutting elements, as shown, but with hardened wear surfaces. 
         [0067]    The body  12  is provided with thickened regions  114  to support the slots  34  and bars  36 . From another perspective, the tool has thinned regions, where the extra thickness of the body is not required! 
         [0068]    In the case of the under-reamer, the thickened regions  114  ahead (in the drilling direction) of the slots  34  have an enlarged diameter surface  116  which is provided with hardened wear elements. In use, the tool here bears against the pilot hole formed by the drill bit on the end of the drill string (not shown) and stabilises the under-reamer keeping it central with respect to the pilot hole. 
         [0069]    Turning to  FIGS. 4 to 6 , an under-reamer  10 ′ is shown of similar overall construction to the under-reamer  10  of  FIGS. 1 to 3 . Like parts are given the same reference number, except with an apostrophe. Thus, with reference to  FIG. 4   c,  the under-reamer  10 ′ comprises a body  12 ′ having a through-bore  14 ′ and including a sleeve actuator  30 ′. The mandrel is not visible in these drawings. 
         [0070]    Slots  34   a ′ are provided in thickened regions  114 ′ of the body  12 ′. Hollow arms  36 ′ slide in the slots  34   a ′. A flange  44 ′ is similarly connected with the sleeve actuator  30 ′ by corresponding inter-engaging dovetails  58 ′,  60 ′. However, the flange  44 ′ mounts a series of parallel levers  210  pivoted in a line to the flange  44 ′ about pivot pins  212 . The axes of the pivot pins  212  are substantially perpendicular to the longitudinal axis  50  of the tool (or at least, perpendicular to a line (not shown) parallel the longitudinal axis  50 ) and also substantially perpendicular to respective ones of the radial planes  50   a, b, c  that contain the longitudinal axis  50 , and which also contain the respective slot  34 ′ a, b, c  of the respective bar  36 ′. 
         [0071]    The levers  210  are also pivoted about pivot pins  214  to the bars  36 ′. In  FIG. 4   c , the sleeve actuator  30 ′ is shown in its tool-de-actuated position. Here, the levers  210  are at a minimum inclination with respect to the longitudinal axis  50 . This inclination is of about 25°. When the tool is actuated, however, the sleeve actuator  30 ′ moves from the position shown in  FIG. 4   c  to that shown in  FIG. 5   c.  Here, the levers  210  have been pivoted in an anti-clockwise direction about their axes  212  to adopt almost an orthogonal position with respect to the longitudinal axis  50 . Given that this results in a radial extension of the ends of the levers connected to pivot pins  214 , the bars  36 ′ are pushed out of the slots  34 ′ a, b, c,  in this movement, as can be seen in  FIG. 5   d.  This occurs, of course, because the bars  36 ′ are unable to move axially in the slots  34 ′ a, b, c  and therefore can only move radially. An advantage of this arrangement is that, when the bars  36 ′ are extended to their maximum extension, and therefore most liable to suffer damage from contact with the bore wall and the like, the maximum refraction force is imposed on the arms  36 ′ when the actuator sleeve  30 ′ begins to move from the tool-actuated position shown in  FIG. 5   c  towards the tool-de-actuated position of  FIG. 4   c.  Moreover, at the first stages of this movement, there is little axial component of the forces on the bar  36 ′, and therefore less risk of the bar jamming in the slot  34 ′ a, b, c.  When the actuator sleeve  30 ′ approaches the position in  FIG. 4   c  then, while the geometry becomes unfavourable for further withdrawal of the bars  36 ′, nevertheless, by the time this position is reached, the bars have been withdrawn to a significant extent. 
         [0072]    Turning to  FIGS. 6   a  to  f,  the structure of the bars  36 ′, flange  44 ′, and the levers  210  is more evident. Flange  44 ′ is a saddle shaped component with a hollow interior  44 ′ a,  forming seats or pockets for the levers  210 . Pivot pins  212  pass through apertures  216  in the side of the flange  44 ′, as well as through bores  218  in the ends of the levers  210 . 
         [0073]    The other ends of the levers  210  likewise have eyes  220  receiving their pivot pins  214 . These pins are journalled in carrier elements  222  which are welded along line  224  to the inside of the pocket  46 ′ of the arm  36 ′. This enables the exterior of the arm  36 ′ to be unbroken. The benefit of this is that the axial projection of the pivot pins  214  coincides with the region of the outside surface of the bars  36 ′ where circumferential seal  90  is located. If eyes  228  which support the pins  214  penetrated to the surface, they may compromise the seals  90 . Consequently, the bores  228  are “blind”. It is to be noted that the levers  210  are all substantially parallel. Moreover, the quadrilateral  250  defined by the axes of the pivots  212 , 214  where they intersect the plane  50   a, b, c  of actuation of the bars  36 ′, is a parallelogram. This ensures that the surface of the bars  36 ′ maintain a constant orientation with respect to the bore wall. The parallelogram lies in the plane of actuation of the bar  36 ′, which in the drawings comprises a respective one of the radial planes  501 ,  b, c.    
         [0074]    However, apart from the simplicity of the design, there is no absolute reason why the bars  36 ′ and slots  34 ′ a, b, c  must be radial (in the sense that movement of the bars in the slots must be orthogonally radial with respect to the longitudinal axis  50 ), or even in an actuation plane that is parallel the longitudinal axis. Similarly, the levers do not necessarily need to be the same length or define a parallelogram. On the contrary, there are several alternative possibilities although these are not preferred as they add considerable complication to the design without necessarily providing any obvious benefit. 
         [0075]    Thus, the actuation plane could be inclined to some degree with respect to the longitudinal axis. This would result in an helically arranged bar  36 ′. In this event, some sliding connection between the flange  44 ′ and the actuator sleeve  30 ′ would be required, or some rotation of the sleeve  30 ′ must be provided, to enable the movement to occur. The slot would also have to have a helical form. 
         [0076]    The actuation plane may be parallel the longitudinal axis, but spaced from it, so that the slots  34 ′ a  have a somewhat tangential orientation, rather than a radial one. 
         [0077]    The slot  34 ′ a  in side section is rectangular in the embodiments described above. However, it could be a parallelogram itself, whereby movement of the bars  36 ′ is not radial but also axial to some extent. This might provide a useful feature if the inclination of the slot was upwardly oriented with respect to the borehole in which the tool is employed. Then, should the tool jam, knocking the extended arms onto the bottom of a casing or narrower bore through which the tool is to be retrieved, will have the effect of knocking the arms back into their slots. This might be deemed desirable in some cases. 
         [0078]    The levers  210  need not be the same length. In this case the arms  36 ′ move in an arc, rather than in a straight line. 
         [0079]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. 
         [0080]    Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0081]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. 
         [0082]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
         [0083]    All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
         [0084]    Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0085]    The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.