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CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of Great Britain patent application serial number GB 0330070.4, filed on Dec. 27, 2003, which is herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a downhole tool. In particular, but not exclusively, the present invention relates to a tool for generating a force downhole and to a method of generating a force downhole. 
   2. Description of the Related Art 
   As is well known in the oil and gas exploration and production industry, access to subterranean hydrocarbon bearing formations is achieved by drilling a borehole to a desired depth and casing\lining the borehole with tubing. Strings of smaller diameter tubing and downhole tools are often located within the casing\liner for performing desired downhole functions. These tubing strings and tools may require to be fixed relative to the casing\liner, and this is typically achieved using dedicated downhole locks, which may include locking dogs that are radially movable to engage a recess in a wall of the casing\liner. 
   Downhole tools or tubing strings including such locks are typically run into the casing\liner with the locking dogs in a retracted position, to allow passage of the string through the tubing. Once the string has been located in the desired position, the lock is activated to engage the locking dogs in the recess. Examples of existing locks include the Otis Engineering lock, commercially available under the X-LINE trade mark, and the Baker Oil Tools lock, commercially available under the SUR-SET trade mark. These locks are of a “jar up to release” type, where a force is exerted on the lock, via a fishing neck, in an upward direction (along the borehole towards the surface) to release the lock. 
   Locks of this type suffer from the disadvantage that the direction of release of the lock is the same as the direction of flow of well fluids through the borehole. Accordingly, it has been found that there is a tendency for the fishing neck to vibrate and creep upwardly, especially in a severe or heavy flow situation, which can cause premature release. 
   Alternative locks are of a “jar down to release” type where a force is exerted in a downward direction to release the lock. In locks of this type, flow of well fluids does not cause premature release and in fact tends to further energise the lock, and these locks are often selected for this reason. One such lock is commercially available from the applicant under the UNISET QX trade mark. 
   However, it is generally preferred to exert an upward jarring force to release a lock in the downhole environment, for reasons including that it is safer to exert a large force by jarring up compared to jarring down and, furthermore, an upward jarring can be performed using wireline\slickline. As is known in the art, wireline\slickline offers advantages in terms of speed of tool\tubing deployment and recovery. 
   It is among the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention, there is provided a tool for generating a force downhole, the tool comprising:
         a longitudinally movable activating member, and   a longitudinally movable driven member operatively associated with the activating member such that on translation of the activating member in one axial direction, the driven member is translated in an opposite axial direction.       

   The invention therefore provides a tool where movement of the activating member in one direction can be used to generate a movement of the driven member in an opposite direction. Thus by coupling the downhole tool to, for example, a downhole component, a downward movement of the component or part of the component can be generated by applying an upwardly directed force on the activating member, or vice-versa. It will be understood that references herein to upward and downward directions are made relative to a borehole in which the downhole tool is to be located, upward referring to a direction along the borehole towards an upper end of the borehole and downward to a direction along the borehole towards a lower or deeper end of the borehole. 
   Preferably, the downhole tool is adapted to be located and suspended in a borehole on a wireline or slickline. As is well known in the art, wireline\slickline offers advantages in the speed of tool deployment and recovery. Where it is desired to exert an upwardly directed force on the activating member, it may be preferred to deploy the tool on wireline\slickline, as this is suitable for exerting an upwardly directed force, and allows relatively quick deployment\recovery of the tool compared to other methods. Alternatively, the downhole tool may be adapted to be located and suspended in a borehole on coiled tubing or the like. Coiled tubing also offers advantages in speed of tool deployment and recovery when compared to conventional sectional tubing, and where it is desired to exert a downwardly directed force on the activating member it may be preferred to deploy the tool on coiled tubing. 
   Preferably also, the activating member is adapted to be translated in an upward direction corresponding to said one axial direction to thereby translate the driven member in a downward direction corresponding to said opposite axial direction. Thus the activating member may be adapted to be translated on exertion of a pulling force on the activating member, to generate a pushing force on the driven member. The tool may thus have a particular utility for releasing a downhole lock of the type which is released by a downward movement, as the tool allows this action to be achieved through an upward jarring, with the advantages discussed above. Alternatively, the activating member may be adapted to be translated in a downward direction corresponding said one axial direction, to thereby translate the driven member in an upward direction corresponding to said opposite axial direction. Thus the activating member may be adapted to be translated on exertion of a pushing force on the activating member, to generate a pulling force on the driven member. 
   The activating member may be movable in a first direction corresponding to said one axial direction and a second direction corresponding to said opposite axial direction, to cause a corresponding movement of the driven member in the second and the first axial directions, respectively. Alternatively, the activating member may be operatively associated with the driven member such that on translation of the activating member in said one axial direction, the driven member is translated in the opposite direction, and on translation of the activating member in said opposite direction, the driven member remains axially stationary. Thus repeated movements of the activating member in said one axial direction and then said opposite axial direction may facilitate successive translations of the driven member in said opposite direction, to progressively translate the driven member to a desired position. The tool may thus be arranged to selectively translate the driven member in response to translation of the activating member only in a selected axial direction. The tool may further comprise a mechanism for allowing selective translation of the driven member. 
   The tool may be movable between retracted and extended positions and may be adapted to be located in a borehole in a selected one of said positions, for subsequent movement towards the other one of said positions downhole. Where the activating member is adapted to be translated in an upward direction, the tool may be adapted to be located in a borehole in the retracted position. Where the activating member is adapted to be translated in a downward direction, the tool may be adapted to be located in a borehole in the extended position. The activating member and the driven member may each be movable between retracted and extended positions to define said corresponding positions of the tool. 
   Preferably, the tool further comprises a rotary member by which the activating member may be operatively associated with the driven member. The rotary member may be coupled to the activating member and adapted to be rotated on translation of the activating member in at least one axial direction. The rotary member may also be coupled to the driven member, and may be adapted to translate the driven member in an opposite axial direction on rotation. Thus translation of the activating member may rotate the rotary member, to thereby translate the driven member. 
   The tool may further comprise a clutch for selectively transferring rotation of the rotary member to the driven member, to selectively translate the driven member. 
   The rotary member may take the form of a threaded member such as a threaded shaft or screw, translation of the activating member rotating the threaded member about an axis thereof, which axis may be substantially parallel to axes of one or both of the activating and driven members. The threaded member may comprise first and second sets of threads or threaded portions of opposite hand (rotational orientation), one of the first and second threads associated with the activating member and the other with the driven member. This may facilitate translation of the driven member in an opposite direction to the activating member when the rotary member is rotated by the activating member. 
   Alternatively, the rotary member may be arranged for rotation about an axis substantially perpendicular to axes of one or both of the activating and driven members, and may take the form of a wheel, roller, drum, arm, plate or the like which may be located between and coupled to the activating and driven members. 
   Alternatively, the activating member may be operatively associated with the driven member by fluidly coupling the activating member to the driven member. The tool may further comprise a piston assembly by which the activating member may be fluidly coupled to the activating member. The piston assembly may comprise an activating piston coupled to the activating member and a driven piston coupled to the driven member. The activating and driven pistons may be fluidly coupled and may be arranged such that translation of the activating member is adapted to translate the activating piston, thereby supplying fluid to the driven piston to translate the driven piston and thus translate the driven member. The piston assembly may be arranged to evacuate fluid from an activating piston cylinder on translation of the activating member in said one direction and to direct said evacuated fluid into a driven piston cylinder to translate the driven member in said opposite direction. 
   The tool may be arranged to provide a mechanical advantage in the movement of the driven member relative to the activating member. Thus the tool may be arranged to generate a force on the driven member greater than a force applied on the activating member, which, in one embodiment, may be achieved by arranging the driven member to be translated a smaller axial distance than the activating member, or vice-versa. The tool may be arranged to generate a force on the driven member in a ratio of 2:1, 3:1, 4:1 or greater relative to the force exerted on the activating member. The driven member may therefore be geared relative to the activating member. 
   The tool may further comprise at least one, preferably a plurality of drive transfer members for transferring drive between the activating member and the driven member. Where the tool comprises a rotary member, the tool may further comprise at least one drive transfer member for transferring drive between the activating member and the rotary member, and at least one drive transfer member for transferring drive between the rotary member and the driven member. The drive transfer member may take the form of a ball, pin, key, tooth, dog, follower or the like. The drive transfer member may be fixed relative to the activating member and\or the driven member for movement therewith. Thus movement of the drive transfer member independently of the respective activating\driven member may be prevented. 
   Preferably, the activating member is restrained against rotation and may be restrained against rotation relative to a body of the tool in which the activating member is mounted. The activating member may be restrained against rotation by a locking member which may permit axial movement, but prevent rotation of the activating member. The locking member may comprise a tongue, latch, arm, leg, finger or other protrusion and may be coupled to the activating member and movable within a groove, slot, channel or the like in a body of the tool, or vice-versa. The driven member may similarly be restrained against rotation. Alternatively, the driven member may be adapted to be rotated and may be threaded such that rotation of the driven member is adapted to translate the driven member axially. The driven member may be adapted to be rotated by the rotary member. 
   According to a second aspect of the present invention, there is provided a tool for generating a force downhole, the tool comprising:
         an activating member;   a rotary member coupled to the activating member and adapted to be rotated on translation of the activating member in at least one axial direction; and   a driven member coupled to the rotary member and adapted to be translated in an opposite axial direction on rotation of the rotary member.       

   Accordingly, translation of the activating member causes a rotation of the rotary member, which in turn causes a translation of the driven member. Furthermore, exertion of a pull force on the activating member generates a push force on the driven member and vice-versa. 
   Further features of the tool are defined in relation to the first aspect of the invention. 
   According to a third aspect of the present invention, there is provided a method of generating a force downhole, the method comprising the steps of:
         providing a downhole tool comprising a longitudinally movable activating member and a longitudinally movable driven member operatively associated with the activating member;   locating the tool downhole; and   translating the activating member in one axial direction to thereby translate the driven member in an opposite axial direction.       

   The method may further comprise coupling the downhole tool to a wireline, slickline, coiled tubing or the like and running the downhole tool into a borehole before exerting a force on the activating member of the downhole tool through the wireline or the like. 
   The method may be a method of generating a downwardly directed force downhole, and may comprise exerting an upwardly directed force on the activating member. Through the operative association between the activating member and the driven member, a downwardly directed force may thereby be exerted on the driven member. Alternatively, the method may be a method of generating an upwardly directed force and may comprise exerting a downwardly directed force on the activating member to thereby exert an upwardly directed force on the driven member. 
   The method may be a method of generating a plurality of discrete downhole movements and this may be achieved by repeated translations of the activating member. Thus the activating member may be moved a number of times in a selected one axial direction, or may be moved in more than one axial direction. For example, the activating member may be moved in a first axial direction, to thereby translate the driven member in said opposite axial direction and may subsequently be moved in said opposite axial direction, to thereby translate the driven member in said one axial direction. Accordingly, the activating and driven members may be moved between a plurality of positions, and may, for example, be moved between retracted and extended positions, or vice-versa. 
   In one embodiment of the invention, the driven member may only be moved in response to translation of the activating member in a selected one axial direction. Furthermore, the plurality of movements of the activating member in said one axial direction may be carried out to progressively move the driven member towards a desired axial position. 
   Preferably the method further comprises operatively associating the activating member with the driven member by a rotary member, the method further comprising translating the activating member in said one axial direction to rotate the rotary member such that the rotary member translates the driven member in said opposite axial direction. This may be achieved by coupling the rotary member between the activating and driven members. 
   The method may further comprise translating the activating member a greater axial distance than the driven member, to generate a driving force on the driven member larger than a force exerted on the activating member. This may be achieved by gearing the driven member relative to the activating member. 
   According to a fourth aspect of the present invention, there is provided a method of generating a push force downhole in response to an applied pull force, the method comprising the steps of: 
   locating a downhole tool in a borehole; 
   restraining a body of the tool against movement; 
   exerting an axial pull on an activating member of the tool to translate the activating member relative to the tool body, 
   rotating a rotary member of the tool; and 
   exerting an axial push on a driven member of the tool to translate the driven member relative to the tool body. 
   The method may comprise operatively associating the activating member with the driven member such that translation of the activating member rotates the rotary member to thereby translate the driven member. 
   According to a fifth aspect of the present invention, there is provided a method of releasing a downhole lock, the method comprising the steps of: 
   coupling a downhole tool to the lock; 
   exerting an axial pull on an activating member of the tool to rotate a rotary member of the tool such that the rotary member exerts an axial push on a driven member of the tool; and 
   arranging the driven member to transfer the axial push to the lock to release the lock. 
   The method may comprise arranging the driven member to transfer the axial push to part of the lock to translate said part and release the lock, and may comprise bringing the driven member into abutment and\or coupling the driven member to the lock\lock part. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a perspective, partial sectional view of the downhole tool in accordance with an embodiment of the present invention, shown in a retracted, running-in position; 
       FIG. 2  is a longitudinal half-sectional view of the downhole tool of  FIG. 1  shown located downhole engaged with a downhole component and in the retracted position of  FIG. 1 ; 
       FIG. 3  is a view of the downhole tool of  FIG. 1  following reference to an extended position; 
       FIG. 3A  is a schematic view of the downhole tool in use, showing a wireline and a jar coupled to the tool; 
       FIGS. 4 and 5  are partial sectional perspective and side views, respectively, of a downhole tool in accordance with an alternative embodiment of the present invention, shown in a retracted, running-n position. 
       FIG. 6  is a view of the downhole tool of  FIGS. 4 and 5  following movement to an extended position; 
       FIGS. 7 and 8  are partial sectional perspective and side views, respectively, of a downhole tool in accordance with an alternative embodiment of the present invention, shown in a retracted, running-in position; and 
       FIG. 9  is a view of the downhole tool of  FIGS. 7 and 8  following movement to an extended position. 
   

   DETAILED DESCRIPTION 
   Referring firstly to  FIG. 1 , there is shown a perspective, partial sectional view of a downhole tool in accordance with an embodiment of the present invention, the tool shown in  FIG. 1  in a retracted, running-in position and indicated generally by reference numeral  10 . 
   As will be described in more detailed below, the downhole tool  10  has a particular utility for releasing a downhole lock, such as a lock  12 , which is shown in  FIG. 2 . In  FIG. 2 , the downhole tool  10  is shown in longitudinal half-section following engagement with the downhole lock  12 , and is in the retracted, running-in position. 
   The downhole tool  10  generally comprises an activating member  14  and a driven member  16  operatively associated with the activating member  14  such that on translation of the activating member  14  in one axial direction (indicated by the arrow A), the driven member  16  is translated in an opposite axial direction (indicated by the arrow B), to release the lock  12  as shown in  FIG. 3 . The activating member  14  and the driven member  16  are thus moved between retracted positions (FIGS.  1 \ 2 ) and extended positions ( FIG. 3 ), to release the lock  12 . 
   The downhole lock  12  is shown in  FIG. 2  located and locked within a section of downhole tubing  18 , which may comprise a section of casing, liner, production tubing or the like. The lock  12  is itself provided at the upper end of a string of tubing or a tool string  15 , shown in the schematic view of  FIG. 3A , and serves for locating and suspending the string within the tubing  18 . 
   In brief, the downhole lock  12  includes a body  22  with a fish-neck sleeve  24  connected to an upper end of the body  22 , and a connecting sub  26  coupled to a lower end  20  of the body  22 . An inner mandrel  28  is mounted within the body  22  for axial movement between the lock position ( FIG. 2 ), and a release position ( FIG. 3 ). 
   The body  22  includes a number of ports  30  in which locking dogs  32  are radially movably mounted, and the mandrel  28  includes a recessed portion  34  and a shoulder portion  36 , and is run into and located within the casing  18  in the release position of  FIG. 3 . In this position, the inner mandrel  28  is held downwardly by mandrel locking dogs  35 , compressing a return spring  38 , and the locking dogs  32  are radially retracted in the mandrel recessed portion  34 . 
   The lock  12  is activated by releasing the inner mandrel  28  and de-supporting the mandrel dogs  35 , such that the mandrel  28  is moved to an upper position ( FIG. 2 ) by the spring  38 . The mandrel shoulder portion  36  then urges the dogs  32  radially outwardly to engage a recess  40  in a wall of the casing  12 , locking the string to the tubing  18 . 
   Considering the downhole tool  10  in more detail, the activating member  14  is mounted for axial movement within a body  42  of the tool and is biased towards a retracted position ( FIG. 2 ) by a spring  44 . The tool  10  also includes a rotary member  46  coupled to the activating member  14  and the driven member  16 . In the illustrated embodiment, the rotary member  46  takes the form of a wheel or drum having two flanges  48 , and is mounted on a shaft  50  for rotation about an axis perpendicular to a main axis of the tool  10 . 
   The activating member  14  is connected to the drum  46  between the flanges  48  at an off-centre location by a connecting arm  52 , and a similar arm  54  connects the driven member  16  to the drum  46  at a location spaced 180 degrees from the connection point of the arm  52 . 
   The driven member  16  takes the form of a pusher including a hollow shaft  56  which is coupled to the connecting arm  54  by a threaded bolt  58 , and the shaft  56  carries an activating collar  60  at a lower end. 
   The tool  10  also includes a fishing assembly  62  having a number of resilient fingers  64  that engage a fish-neck  66  on the fish-neck sleeve  24 , as shown in  FIGS. 2 and 3 . The fingers  62  are located around a locking mandrel  72 , which is moved to support the fingers  62  to couple the tool  10  to the lock  12 , as will be described below. 
   The method of connecting the downhole tool  10  to the lock  12  and subsequently releasing the lock  12  will now be described. 
   The downhole tool  10  is run into the borehole on a wireline  17  shown in  FIG. 3A  (or alternatively slickline, coiled tubing or the like) which is coupled to a jar  19 , the jar  19  coupled to the activating member  14  by a cross-over  68 . As is known in the art, a jar is used to generate a relatively large force in a downhole environment. A jar, such as the jar  19 , is typically hydraulic, and is “set” by a number of separate activating forces exerted on the jar, such as through the wireline  17 . When sufficient force is stored in the jar  19 , the jar releases, exerting a large force in the tool  10 . However, it will be understood that the tool  10  may be activated without the need for a jar, for example, by direct activation through the wireline  17 . 
   In the running position of  FIG. 1 , the activating member  14  is held against axial movement relative to the body  42  by shear pins  70 . The tool  10  is brought into engagement with the lock  12  by snapping the fingers  64  into the fish-neck  66  and then moving the locking mandrel  71  to support the fingers  64 . A pulling force is then exerted on the connector  68  through the jar  19  to shear the pins  70  and translate the activating member  14  upwardly, compressing the spring  44 . 
   This movement causes the connecting arm  52  to rotate the drum  46  in the direction of the arrow C ( FIG. 2 ). This rotation causes the drum  46  to exert a pushing force on the connecting arm  54  and thus on the bolt  58  and hollow shaft  56 . A ratchet mechanism  59  between the bolt  58  and the shaft  56  facilitates translation of the shaft  56  downwardly (to the right in the Figures), to translate the activating collar  60  from the position of  FIG. 2  towards the position of  FIG. 3 . The ratchet  59  permits the desired movement of the shaft  56  to be achieved progressively, as the ratchet mechanism  59  prevents return movement of the shaft  56  upwardly (to the left in the Figures) when the crossover  68  is released and the spring  44  urges the bolt  58  back to the position of  FIG. 2 . Thus a number of cycles of movement of the bolt  58  is required to release the lock. 
   Movement of the shaft  56  to the  FIG. 3  position carries the lock inner mandrel  28  downwardly, compressing the spring  38  and de-supporting the locking dogs  32 . The locking dogs  32  can thus be disengaged from the recess  40  by upward movement of the lock  12 , and the lock  12  can then be returned to surface. 
   It will therefore be understood that the downhole lock  12 , which is of the type that is released in response to an applied downward force, can thus be released by application of an upwardly directed force by using the downhole tool  10 . 
   Turning now to  FIGS. 4 and 5 , there are shown partial sectional perspective and side views, respectively, of a downhole tool in accordance with an alternative embodiment of the present invention, the downhole tool indicated generally by reference numeral  110 . The tool  110  is shown in  FIGS. 4 and 5  in a retracted, running-in position corresponding to that of the tool  10  shown in  FIGS. 1 and 2 . 
   It will be understood that the tool  110  is suitable for releasing a lock such as the downhole lock  12  of  FIGS. 2 and 3 , and is connected to the lock in a similar fashion, but that the lock and other components have been omitted from the Figures, for ease of illustration. Furthermore, like components of the downhole tool  110  with the downhole tool  10  of  FIGS. 1 to 3  share the same reference numerals, incremented by 100. 
   The downhole tool  110  includes an activating member in the form of a driver or sleeve  114 , which is axially movably mounted in a body  142  of the tool. A driven member in the form of a pusher or sleeve  116  is also mounted for axial movement within the body  142 , and a rotary member  146  is coupled to the driver  114  and pusher  116 . 
   The rotary member  146  comprises a screw having threaded portions  174 ,  176  of opposite hand (rotational orientation), and is mounted for rotation within the body  142  by a bearing  178 . 
   The driver  114  carries a number of roller bearings  180  which are movable within a groove  182  formed in the body  142 . In this fashion, the activating sleeve  114  is axially movable with respect to the body  142 , but is held against rotation. In a similar fashion, the pusher  116  carries a number of roller bearings  184  mounted for movement within a groove  186 . 
   The tool  110  also includes a plurality of drive transfer members in the form of balls  188  and  190  for transferring drive between the driver  114  and the screw  146 , and between the screw  146  and the pusher  116 , respectively. Each ball  188 ,  190  is mounted within a respective aperture  192 ,  194  in the driver  114  and the pusher  116 . In this way, the balls  188  and  190  are rotatable within their apertures  192 ,  194  and axially movable with the driver and pusher, respectively, but are captive and thus held against rotation around an inner circumference of the tool body  142 . 
   Following engagement with a lock, an upwardly directed pull force is exerted on the driver  114 , translating the driver upwardly and carrying the bearings  180  within the groove  182 . As the drive transfer balls  188  are held captive in the driver apertures  192 , the balls  188  are translated with the driver  114 , as shown in  FIG. 6 . This movement of the balls  188  imparts a rotation on the threaded portion  174  of the screw  146  in the direction of the arrow D ( FIG. 4 ). 
   As the screw threaded portion  176  is of opposite hand to the portion  174 , rotation of the screw  146  in the direction D imparts a downwardly directed force on the drive transfer balls  190 . As the balls  190  are held captive in the pusher apertures  194 , this movement carries the pusher  116  axially downwardly carrying the roller bearings  184  within the groove  186 , to translate the balls  190  to the position of  FIG. 6 . This movement brings the tool  110  to the extended position with an activating collar  160  moving downwardly to release the lock. 
   Turning now to  FIGS. 7 and 8 , there are shown partial sectional perspective and side views, respectively, of a downhole tool in accordance with a further alternative embodiment of the present invention. The downhole tool is indicated generally by reference numeral  210  and shown in  FIGS. 7 and 8  in a retracted, running-in position. 
   Like components of the downhole tool  210  with the tool  10  of  FIGS. 1 to 3  share the same reference numerals incremented by 200, and with the downhole tool  110  of  FIGS. 4 to 6  incremented by 100. 
   The downhole tool  210  is again suitable for releasing a lock such as the lock  12  of  FIGS. 2 and 3 , but is shown without the lock and other components, for ease of illustration. 
   The downhole tool  210  includes an activating member in the form of a driver or sleeve  214  and a rotary member  246  in the form of a threaded shaft or driver screw having a series of axially spaced threads  196   a ,  196   b ,  196   c . The driver  214  includes a roller bearing  280  mounted for movement in a groove  282 , for restraining the driver  214  against rotation, and a number of drive transfer members in the form of captive driver pins  288  (two shown,  288   a ,  288   b ) associated with each set of threads  196   a ,  196   b  and  196   c . The tool  210  also includes a driven member or pusher screw  216  which is threaded at  298  and is rotated and axially translated on movement of the driver  214 , as will be described below. 
   The driver screw  246  is mounted in the tool body  242  by a bearing  278 , and the tool includes a drive transfer assembly  299  comprising a rotatable drive transfer sleeve or pusher  211 , and a number of drive transfer members in the form of pusher pins  290 , which are mounted in apertures in the drive transfer sleeve  211 . The driver screw  246  is coupled to the drive transfer sleeve  211  by a clutch  213 , for selectively rotating the drive transfer sleeve  211  on translation of the driver  214 . 
   The tool  210  is operated as follows. After engagement with a downhole lock, a pulling force is exerted on the driver  214 . This translates the driver  214  upwardly carrying the driver pins, which thereby rotate the driver screw  246  through interaction with their respective threads  196 . 
   The driver screw  246  is thus rotated in the direction of the arrow E, and through the clutch  213 , rotates the drive transfer sleeve  211 . This in turn rotates the captive driver pins  290 , which translate the pusher  216  axially downwardly through their interaction with the threads  298 . 
   The threads  196  and  298  are arranged such that there is a smaller axial translation of the pusher  216  relative to the driver  214 , thereby providing a mechanical advantage in movement of the pusher  216  relative to the driver  214 , in a ratio of 2:1, 3:1, 4:1 or greater. This ratio depends upon the relative geometry of the threads  196  on the driver screw  246  and the threads  298  on the pusher  216 . Thus a relatively large movement of the driver  214  produces a relatively small movement of the pusher  216 . However, the pulling force exerted on the driver  214  is smaller than the resultant pushing force which is generated and exerted on the pusher  216 . 
   On movement of the tool  210  to the extended position of  FIG. 9 , the spring  244  is compressed and, when the pulling force on the driver  214  is released, the sleeve is returned to the retracted position of  FIGS. 7 and 8 . 
   This causes a corresponding rotation of the driver screw  246  in the direction of the arrow F. However, the clutch  213  is disengaged on rotation of the driver screw  246  in this direction, such that the rotation is not transmitted to the drive transfer sleeve  211 . Accordingly, the pusher  216  is not rotated and remains axially stationary. On exerting a renewed pulling force on the driver  214 , the pusher  216  is again translated axially downwardly a small distance, and repeated such movements of the driver  214  progressively move the pusher  216  towards an extended position, shown in  FIG. 9 . 
   Various modifications may be made to the foregoing within the scope of the present invention. 
   For example, the downhole tool may have other uses. In particular, the tool may be used for setting a downhole lock, that is, for locating and activating a lock. This may be achieved by, for example, operating the tool in reverse. Thus, either of the tools  10 ,  110  may be coupled to the lock  12  at surface with the tool in the extended position, and the tool and lock run into a borehole to a desired location. A pushing force may then be exerted on the respective activating member  14 ,  114  to thereby exert a pulling force on the driven member  16 ,  116 . This may allow the lock inner mandrel  28  to move upwardly to the locking position of  FIG. 2 . It will be understood that the tool may equally be used to release the lock by reconnecting the tool to the lock and operating the tool as described above. 
   The tool  210  may equally be used to set a lock, by providing a clutch which transfers drive when the screw  246  is rotated in the opposite direction (F), following coupling of the tool to the lock in the extended position of  FIG. 9 . The clutch may be adapted to selectively transfer rotation to the drive transfer sleeve  211  in either direction, for example, by setting the clutch at surface or by providing a control signal to the tool from surface. 
   It will also be understood that the tool may have many further uses in the downhole environment, for releasing and or setting a number of different tools, or indeed for performing a range of downhole functions. In particular, the tool may have a use with any downhole tool, component or part thereof which is released, set\activated or actuated by a longitudinal movement, and may be used for operating valves; sliding sleeves; perforating guns; packers or the like. 
   The downhole tool may be adapted to be located and suspended in a borehole on coiled tubing or the like, which may be used to exert a downwardly or upwardly directed force. A downward force may be exerted through a wireline, if the tool is anchored relative to the borehole. 
   The rotary member may be arranged for rotation about any suitable axis or axes, and may take the form of a roller, arm, plate or the like. 
   The activating member may be operatively associated with the driven member by fluidly coupling the activating member to the driven member. The tool may further comprise a piston assembly by which the activating member may be fluidly coupled to the activating member. The piston assembly may comprise an activating piston coupled to the activating member and a driven piston coupled to the driven member. The activating and driven pistons may be fluidly coupled and may be arranged such that translation of the activating member is adapted to translate the activating piston, thereby supplying fluid to the driven piston to translate the driven piston and thus translate the driven member. The piston assembly may be arranged to evacuate fluid from an activating piston cylinder on translation of the activating member in said one direction and to direct said evacuated fluid into a driven piston cylinder to translate the driven member in said opposite direction.

Summary:
A tool for generating a force downhole comprises a body, a longitudinally movable activating member mounted to the body, and a longitudinally movable driven member also mounted to the body. The driven member is operatively associated with the activating member such that on translation of the activating member in one axial direction, the driven member is translated in an opposite axial direction. The tool may be utilised to convert a pulling action, applied by a spoolable member, to a pushing action, useful in disengaging a downhole lock.