An arrangement by a hydraulic jar device, especially for use in underground wells, where the jar device is installed in a pipe string led down into the well, and designed so that e.g. a stuck object in the well may be loosened or broken up by upward or downward percussions from the jar device. The jar device is actuated by increasing the flow of drill fluid. Alternatively, the device is actuated by compression. A valve ( 27, 37, 38 ) closes off the flow of drill fluid, whereby a percussion cycle is initiated. The valve is designed so as also to seal during strong lateral accelerations.

CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is the National Stage of International Application No. PCT/NO01/00513, filed Dec. 28, 2001 and published under PCT Article 21(2) in English, and claims priority of Norway Application No. 20010059, filed on Jan. 5, 2001. The aforementioned related patent application is herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention regards an arrangement by a hydraulic jar device, especially for use in underground wells, where the jar device is installed in a pipe string led down into the well, and designed so that e.g. a stuck object in the well may be loosened or broken up by upward or downward percussions from the jar device, where the jar device comprises a casing member, a connector sleeve, a jar, the casing member and connector sleeve each having separate longitudinal through bores, while the jar has a bore such that hydraulic liquid may pass in the jar device, and where the jar device is provided with a piston associated with a valve designed to close and open a bore during the percussion cycle, the piston valve being designed, respectively, to be closed by the inflow of hydraulic liquid and be opened by a tension spring tensioned during the percussion cycle, when the spring force of the tension spring exceeds the pressure from the inflowing hydraulic liquid, to enable the piston to displace the jar relative to the casing member in order to carry out the percussion. 
   2. Background of the Related Art 
   Such jar devices are often used in connection with anchoring of valves, measurement equipment and other equipment downhole. The jar device is provided in a pipe string, e.g. a drill pipe string or coiled tubing, and equipment to be placed in the well is fitted to the lower end of the jar device. As mentioned, the jar device is provided with a passage such that inflowing liquid may pass before the jar device is actuated for percussion. The equipment to be placed in the well may be equipped with grippers, spring bosses or something else that will grip e.g. grooves or seating areas in the wall of the well. In order to ensure that the equipment does not come loose, it is often provided with a locking device to be actuated when at least one shear pin is broken off. In those cases where the pipe string is not able to transfer sufficient force to break off at least one shear pin, it may be broken by means of the jar device. Moreover, the jar device is often used purely as a measure, so that the equipment may be loosened, were it to get stuck. 
   Such hydraulic jar devices are often pre-tensioned by means of an external spring over the jar device. Alternatively, a long drill string or coiled tubing may constitute the spring element. The percussion is carried out by impact areas on the jar device being moved apart, whereupon the pre-tensioned spring sends the impact areas back towards each other. As mentioned, the jar device comprises a hydraulic piston provided with a passage and an associated valve. The valve is normally open, so that liquid may pass through the piston of the jar device when not actuated for percussion. When the jar device is to be actuated for percussion, increasing the flow rate of the inflowing hydraulic liquid closes the passage, so that the valve is closed at the time in question during the percussion cycle. Alternatively, pushing the jar into the casing member may in one embodiment actuate the device. At this, the piston and also the impact areas of the jar device will be displaced relative to each other during the preparation for the percussion. At the same time, the spring is tensioned further as a result of the movement in the jar device. The piston valve is opened when, during the percussion cycle, the jar device has been brought to the extreme position in question, to allow the liquid to flow through the piston again. The hydraulic force against the piston will then suddenly decrease, and the external, associated spring over the jar device will send the impact areas against each other in order to carry out the percussion, whereupon the percussion cycle is repeated. 
   The use of a spring that can be pre-tensioned from the outside in order to drive the percussion in the jar device is known. It is further known to design the spring so as to allow it to be pre-tensioned either by pulling the pipe string in the direction away from the jar device or pushing the pipe string in the direction towards the jar device. The magnitude of the impact force may be varied through the pre-tensioning of the spring. When the pre-tensioned spring over the jar device is in a neutral position, hydraulic liquid may be passed through the pipe string without actuating the jar device. The jar device is actuated for percussive movement by a pressure increase in the hydraulic liquid contained in the jar device; this will result in cyclic closing and opening of the piston valve, so that the jar device prepares and performs the percussion in the percussion cycle by displacing the relevant components of the jar device, whereupon the procedure is repeated for new percussions. In one embodiment, the jar device may, as mentioned above, be actuated through the jar being pushed into the casing member. 
   In many of the known jar devices, see e.g. U.S. Pat. Nos. 4,807,709, 3,570,611, 3,379,261 and 3,361,220, the weight of the equipment hanging from the jar device is often sufficient to actuate the piston valve, so as to close the passage for the hydraulic liquid, thereby actuating the percussion effect. This means that it is not possible to circulate liquid through the pipe string when the jar device is being run into or out of the well. If prolonged circulation is required, the percussion effect may damage the equipment. The hydraulic parts of the jar device such as the piston and valve components, will become worn during operation and therefore require regular replacement. Upon lengthy operation requiring circulation of liquid, parts of the jar device may wear significantly before the jar device comes to be used in the required operations. This may result in a reduced percussion effect and faulty operation. However these are conditions that have essentially been remedied by the jar device according to NO patent 304 199. Here, an efficient, reliable and robust hydraulic jar device of the above-mentioned type has been provided through relatively simple and reasonable means. Furthermore, circulation of liquid such as drill fluid through the jar device is possible without this being actuated upon pre-tensioning of the spring, and it is possible to initiate the percussion effect by increasing the pressure of the inflowing volume of liquid, as the piston valve can not close until there is an increase in pressure in the inflowing liquid. 
   However the known jar devices, especially jar devices with upward percussions, suffer from a shortcoming in that the impact areas in question are provided on the outside of the jar device. Consequently, the percussion effect may be limited by influences from the outside of the jar device, e.g. by contaminants depositing between the impact areas. Another shortcoming of known jar devices is that the hydraulic liquid can close the piston valve before the impact areas has reached full impact against each other during the final period of the percussion cycle. This means that the liquid over such a prematurely closed piston valve will brake the percussion and give a reduced percussion effect. 
   SUMMARY OF THE INVENTION 
   In consequence, one object of the invention is to provide a jar device of the above-mentioned type, where these shortcomings of previous jar devices have been remedied. Another object is to provide a jar device of the simplest and most reliable construction possible. These and any other objects have been realised in the manner that appears from the characterising part of the present independent claim. In accordance with the invention, a piston valve is constructed in a manner such that the sealing body of the valve, which in a preferred embodiment is a ball, is guided via a precise valve guide towards a valve seat where the valve body is supported radially by the valve guide, also in the closed position. The valve is thereby safeguarded against inadvertent opening, e.g. upon the jar device being subjected to great lateral acceleration. A bore in the piston is kept open with clear passage for the hydraulic liquid through the piston, at least until the percussion that during the percussion cycle is triggered by the valve opening, has been completed. Thus the percussion will not be braked by trapped hydraulic liquid and as a result give a reduced percussion effect. Other beneficial features of the invention appear from the claims and the rest of the specification. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, which exists in two embodiments of which one describes an upward striking jar device and one describes a downward striking jar device, will in the following be explained in greater detail with reference to the drawings, in which: 
       FIG. 1  is a longitudinal section of the present upward striking jar device, comprising a casing member, a lower connector sleeve and a jar, where the lower end of the jar is equipped with a movable piston provided in a longitudinal through bore in the casing member, and which is associated with a valve in the form of a valve ball, an intermediate seating area and a lower valve body, where the jar device is in a non-actuatable position with clear passage for hydraulic liquid through the casing member, the jar and the connector sleeve; 
       FIG. 2  is a longitudinal section of the midsection of the jar device, on a larger scale, where the impact collar has been brought to a stop against the end socket. In this position, the jar device is ready to commence a percussion cycle, but still has a clear passage for hydraulic liquid through the casing member, the jar and the connector sleeve; 
       FIG. 3  shows the same longitudinal section, where the valve body is displaced as a result of an increased volumetric flow of liquid, so that the ball closes against the seating area. The liquid pressure against the piston and the ball displaces the piston downwards while the piston tensions a lower tension spring by means of the valve body; 
       FIG. 4  shows the same longitudinal section, where the piston and the valve body are displaced fully in the tensioning direction as a result of the liquid pressure, so that the valve body abuts the connector sleeve. The liquid pressure against the ball tensions the tension spring by means of the valve body; 
       FIG. 5  shows the same longitudinal section, but here the valve ball has been lifted off the seating area to allow liquid to flow through the piston, whereby the jar is free to be displaced in the direction of percussion; 
       FIG. 6  shows various sections through the jar device at lines A—A, B—B in  FIG. 2 ; 
       FIGS. 7   a–b  show a longitudinal section of the present downward striking jar device, comprising a casing member, a jar and a connector piece, where the casing member comprises a longitudinal through bore equipped with a movable piston provided in a through piston bore, and which is associated with a valve in the form of a seating area, an upper valve ball and a lower valve body over and under the seating area, respectively, where the jar device is in a non-actuated position with clear passage for hydraulic liquid through the casing member, the jar and the connector sleeve; 
       FIGS. 8   a–b  show the same longitudinal sections as  FIGS. 7   a–b , but here the jar device is compressed further, and the piston is moved to a position in the percussion cycle in which the ball is brought into sealing contact against the collar of the piston. Compressed oil is flowing to the upper side of the piston, initiating the downward piston movement; 
       FIGS. 9   a–b  show the same longitudinal section as  FIGS. 7   a–b , but here the piston and valve body have been moved to a lower extreme position during the percussion cycle, while a tension spring associated with the valve body is tensioned, making the jar device ready for a percussion; 
       FIGS. 10   a–b  show the same longitudinal section as  FIGS. 7   a–b , but here the percussion has been triggered by the valve body having lifted the valve ball off the seating area as the spring tension in the associated tension spring tensioned during the percussion cycle exceeds the pressure from the inflowing hydraulic liquid; and 
       FIGS. 11   a–c  show various sections through the jar device at is the cutting lines A—A, B—B and C—C in  FIGS. 8   a–b.    
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   First of all, an embodiment is described with reference to  FIGS. 1 to 6 , in which the jar device is designed to strike upwards. 
   The present jar device designed to strike upwards comprises a tubular casing member  1  having a longitudinal through bore  2  so as to allow passage of hydraulic liquid through the casing member  1 . The lower end of the casing member  1  is connected to a connector sleeve  3  with a longitudinal through bore  4  for passage of hydraulic liquid. The connection between the casing member  1  and the connector sleeve  3  may for instance be constituted by a threaded connection  5  formed internally of the casing member bore  2 , and which is made pressure tight in an appropriate manner. With this, the lower end of the jar device may be coupled to the tool, pipe string etc. (not shown) in question by means of e.g. a lower male threaded connection  6  on the connector sleeve  3 . 
   The upper end of the casing member  1  is such that a jar  7  may be displaced upwards relative to the casing member  1  when the jar device is actuated for percussion effect by an increase in the rate of flow of the inflowing hydraulic liquid. In order to facilitate the axial displacement of the jar  7 , the casing member  1  is provided with an axially split end socket  8 . The casing member  1  and the end socket  8  are fixed to each other by means of e.g. a threaded connection  9  that is located internally of the upper end of the casing member bore  2 , and which is pressure tight. Further, a lower section  10  of the jar  7  is during the percussion cycle movably guided into a longitudinal through bore in the end socket  8 . The lower jar section  10  is in sliding abutment against an upper end socket section  11  made pressure tight by an appropriate seal  12  and a lower end socket section  13  made pressure tight by e.g. a compression packing  14 , respectively. Furthermore, a seal  15  has been provided to seal against pressure between the casing member bore  2  and the lower end socket section  13 . 
   In addition, the jar  7  has an upper bore  16  provided with a female threaded connection  17 , so as to allow the jar device to be coupled to a drill string, coiled tubing etc. (not shown) in a pressure tight manner. The upper jar bore  16  changes into a longitudinal bore  18  that ends up in a vertical gateway  19  at a distance above the lower section  10  of the jar, so that hydraulic liquid may pass through the jar  7  and further out into the casing member bore  2 , as shown in  FIG. 1 . 
   Furthermore, the jar  7  includes an external, projecting flange-like impact collar  20 . With this, the lower, wider section of the impact collar  20  forms an upward facing impact area  21  designed to impact against a downward facing impact area  22  in a midsection of the end socket  8  on the casing member  1 . The upward facing impact area  21  on the impact collar  20  is located in an annulus  23  formed by a recess in the end socket  8  between the downward facing end socket impact area  22  and the lower end socket section  13 , respectively. The impact collar  20  further has dimensions that allow the lower, wider section of the impact collar  20  to abut the inner wall of the annulus  23  in a sliding manner. As is apparent from  FIG. 1 , the impact areas  21 ,  22  on the jar  7  and the end socket  8  are spaced apart when the jar device is in an inactive state. 
   The impact collar  20  is further provided with at least one vertical passage  24  that extends from the underside of the impact collar  20  and up to an associated passage  25  in the upper section  11  of the end socket. The passage  25  ends in a gateway  26 . This means that the hydraulic liquid in the annulus  23  between the lower section  10  of the jar and the end socket  8  has an outlet from the jar device via this at least one passage  24  in the impact collar  20 , together with passage  25  and gateway  26  in the end socket  8 . 
   In addition, the jar device comprises a piston  27  that, among other things, makes it possible to move the jar  7  when the jar device has been actuated by an increase in the liquid flow of inflowing hydraulic liquid. The piston  27  is fixed to the lower end of the jar  7  underneath the gateway  19  by the end of the jar bore  18 , and this fixing is achieved by e.g. a threaded connection  28 . A lower section  29  of the piston  27  is in sliding abutment against the inner wall of the casing member bore  2  during the percussion cycle, and is pressure sealed by e.g. an upper compression packing  30  and a lower, relatively wide seal  31 . An upper section of the piston  27  has cross section that is a little smaller than that of the casing member bore  2 , so as to allow the formation of an annulus  32  on the outside of and above the upper piston section  27  for the passage of hydraulic liquid. At least one gateway  33  leads from the piston annulus  32  and into a lower bore  34  positioned centrally in the lower section  29  of the piston. The piston bore  34  has an upper section, the valve guide  34 ′, the diameter of which is slightly larger than that of the midsection of the piston bore  34 , and the midsection becomes a lower section that slopes out towards a lower piston area  35 . The piston has an upper piston area  36 , and the upper end of the lower piston section  29  will likewise form an intermediate piston area. 
   The passage for hydraulic liquid through the piston bore  34  may be shut off by a valve consisting of a (valve) ball  37 , an intermediate seating area  38  and a lower valve body  39 . The ball  37  is located in an upper section of the piston bore  34 , and has approximately the same diameter as the valve guide  34 ′. The seating area  38  is formed in the transition zone between the upper section and midsection of the piston bore  34 . The seating area  38  further has a form that causes the ball  37  to seal against it during the relevant periods of the percussion cycle. The valve body  39  has an upper section that runs into the piston bore  34  and a lower section that runs on the outside of the piston  27 , down towards an upper end face  40  of the connector sleeve  3 . An upper seating area on the valve body  39  will normally abut the lower side of the valve ball  37 . Otherwise, the lower section of the valve body  39  has a cross section that is slightly larger than that of the upper section. The transition zone between these sections of the valve body  39  slopes in a similar manner to the lower section of the piston bore  34 , and is provided with upward facing fins  41 . The fins  41  on the valve body abut the lower, outward sloping section of the piston bore  34 , partly when the jar device is not in the actuated state and partly when the percussion has been triggered following opening of the valve  37 ,  38 ,  39  in the piston bore  34 , as can be seen from  FIGS. 1 ,  2  and  5 . 
   The valve body  39  is equipped with a sliding valve  42  that is movable in a recess  43  at the bottom end of the valve body  39 , as shown in  FIGS. 1 and 2 . Furthermore, the sliding valve  42  is associated with a lower tension spring  46  that is tensioned during the percussion cycle when preparing for the impact between the impact area  21  of impact collar  20  and the impact area  22  of the jar  7 , respectively. As can be seen from  FIG. 3 , the tensioning of the lower tension spring  46  takes place via the valve body  39  when the piston  27  is displaced downwards in the casing member bore  2  during the relevant period of the percussion cycle. Otherwise, the lower tension spring  46  extends between a lower abutment surface  45  on the sliding valve  42  and an outward facing abutment surface  45 ′ in a recess by the upper end of connector  3 . 
   In order to make the valve body  39  retain the ball  37  at an upper limit of travel, clear of the seating area  38 , partly when the jar device is not in the actuated state and partly during the relevant periods of the percussion cycle, the valve body is provided with a valve spring  47 . The valve spring  47  extends between a lower end face on the fins  41  of the valve body  39  and an upper abutment surface  44  on the sliding valve  42 . The valve body  39  further has at least one gateway  48  that enables hydraulic liquid to pass from the casing member bore  2  into a bore  49  in the bottom end of the valve body  39  and then out of this, among other thing to the bore  4  in the connector  3 . 
   The special design of the piston valve  37 ,  38 ,  39  ensures that the valve does not inadvertently close off the passage of the piston bore  34  before the impact area  21  of the impact collar  20  has reached full impact against the impact area  22  is of the end socket  8 . Consequently, it will not be possible for hydraulic liquid to become trapped on the upper side of the piston  27 , as such premature closing would have braked the piston  27  and given a reduced percussion effect during the percussion cycle. 
   In the following, the principle of operation of the jar device will be explained with reference to the drawings. 
   In a non-actuated state of the jar device, the impact area  21  of the impact collar  20  is, as shown in  FIG. 2 , located in the immediate vicinity of the impact area  22  of the jar  7 . The valve spring  47  and the valve body  39  further lift the ball  37  off the seating area  38 , to leave the piston valve open. With this, hydraulic liquid has a clear passage via the bores, the bore and gateway ( 19 ), respectively, of the respective components of the jar device. Furthermore, the jar device is held in this non-actuated state and is also subjected to an upward force from an pre-tensioned spring (not shown) positioned in a suitable location in the pipe string. 
   When increasing the flow of hydraulic liquid, the bottom valve body  39  and thereby the ball  37  are displaced downward by the valve spring  47  being compressed, as can be seen from  FIG. 3 . The ball  37  closes off the passage of liquid through the piston  27  by sealing against the seating area  38 . The hydrostatic pressure, which among other things acts on the upper surface  36  of the piston  27  and the ball  37 , displaces the piston  27  and the jar  7  to a lower limit of travel just before the percussion is triggered by the opening of the piston valve, see  FIG. 4 . At this lower limit of travel, the spring tension of the lower tension spring  46  against the sliding valve  42  has reached a value exceeding the pressure from the inflowing hydraulic liquid against the ball  37 . Consequently, the spring tension will, via the sliding valve  42  and the valve body  39 , displace the ball  37  from the seating area  38  to re-open the valve, see  FIG. 5 . Alternatively, the piston valve may open when the lower end of the sliding valve  42  abuts the upper end face of the connector  3 . By the latter alternative, a continued inflow of liquid will contribute to the valve body  39  lifting the ball  37  off the seating area  38  in order to open the valve. 
   The pressure drop that results from the opening of the piston valve ( 37 ,  38 ,  39 ) allows liquid again to flow through the piston bore  34 . With this, the spring tension from the tension spring  46  will displace the sliding valve  42  upwards while the valve body  39  displaces the ball  37  off the seating area  38  and the piston  27  upwards in the casing member  1 , whereby the spring tension from the pre-tensioned spring (not shown) will be conductive to the impact area  21  of the impact collar  20  being led to impact against the impact area  22  of the end socket  8  by the upper end of the casing member  1 . 
   The piston  27  at the end of the jar  7  and the ball  37  in the piston bore  34  must be provided with piston areas that can cause the piston valve to be closed and opened in the manner intended. Likewise, the spring tension in the valve and tension springs  46 ,  47  must be selected according to the pressure conditions in the hydraulic liquid being fed to the jar device. In the embodiment shown, the closing and opening of the valve in the piston  27  is controlled by a valve ball  37  and a valve body  39 , i.e. two separate parts. These may however be made up from one single part, which will be a combined unit of these with an upper portion adapted to seal against the seating area  38  of the piston bore  34 . 
   The following describes, with reference to  FIGS. 7 to 11 , an embodiment in which the jar device is designed to strike downwards. 
   The present jar device for downward percussion comprises a tubular casing member  1  with a longitudinal through bore  2  for allowing hydraulic liquid to pass through the casing member  1 . The upper end of the casing member  1  is connected to a connector sleeve  3  in an appropriate manner, e.g. by means of a pressure tight threaded connection  5  formed internally of the bore  2 . The upper end of the jar device may thereby in a suitable manner be coupled to a pipe string (not shown), e.g. by means of a pressure tight threaded connection  6  located internally of an upper bore  16  in the upper connector sleeve  3 . A lower bore  4  extends further down through the connector sleeve  3  as a continuation of the upper bore  16 , to allow hydraulic liquid from the pipe string to pass through the upper connector sleeve  3 . 
   The lower end of the casing member  1  is designed such that the casing member  1  may be displaced externally along a jar  7 . The jar  7  has an external impact area  109 , preferably extending at right angles to the jar  7  around its entire periphery. Over the impact area  109 , the jar  7  has an upper section  110  extending upwards in the casing member bore  2 . The external diameter of the upper jar section  110  is considerably smaller than both the external diameter of the jar  7  under the impact area  109  and the diameter of the casing member bore  2 . The upper end of the jar section  110  is provided with a sleeve  111  fixed to the upper jar section  110  e.g. by means of a threaded connection  112 , so that the area of an upper abutment surface  113  by the upper end of the upper jar section  110  may be increased. The external diameter of the jar sleeve  111  is a little smaller than the diameter of the casing member bore  2 , to allow hydraulic liquid to flow past an end face  114  of the jar sleeve  111 . The jar  7  and the upper jar section  110  have a longitudinal through bore  18  that allows hydraulic liquid to pass through the jar  7 . In addition, the jar  7  is coupled to the relevant tool, pipe string etc. in a pressure tight manner by means of e.g. a lower, male threaded connection  116 . 
   In order to enable the casing member  1  to be displaced along the upper jar section  110 , the lower end of the casing member  1  is provided with an end socket  117 . The casing member  1  and the end socket  117  are fixed to each other, e.g. by means of a threaded, pressure tight connection  118 . Moreover, the end socket  117  is designed to abut the outer periphery of the upper jar section  110  in a sliding manner when the jar  7  is displaced along it during the percussion cycles. The end socket  117  may be provided with internal, longitudinal grooves that are complementary to grooves in the outer periphery of the upper jar section  110 , whereby interrotation between the jar  7  and the end socket  117  is prevented. The end socket  117  is made pressure tight against the upper jar section  110  by means of e.g. an upper compression packing  119  and a lower, relatively wide seal  120 . Furthermore, the lower end of the end socket  117  is provided with an impact area  121  that is located above the impact area  109  of the jar  7 , and which is designed to impact against the impact area  109  of the jar  7  during the percussion cycle of the jar device. 
   Below a lower section  123  of the connector sleeve  3 , the casing member is equipped with a piston  27  that causes the casing member  1  to be movable up along the upper section  110  of the jar in advance of each single percussion of the jar device. The lower end of the connector section  123  is provided with a recess  124  having a fit such that an upper section of a longitudinal valve guide  34 ′ that, together with a bore  34 , constitutes a through bore in the piston  27 , may locate in the recess  124 , partly when the jar device is not actuated for percussive motion and partly during periods of the percussion cycle, such as shown in  FIGS. 7   a  and  8   a . The lower end of the lower bore of the connector sleeve  3  is fitted with an end piece  125  where hydraulic liquid may pass from bore  4  to at least valve guide  34 ′ via a plurality of orifices  126  running at an angle down through a transition zone between the wall of the recess  124  in the connector sleeve section  123  and the end piece  125 . 
   A midsection of the piston bore  34 ,  34 ′ is provided with a shoulder  130  projecting into the piston bore  34 ,  34 ′. A valve ball  37  is placed in the valve guide  34 ′ above the shoulder  130 . The shoulder  130  has an upper seating area  38  that allows the ball  37  to seal against the piston shoulder section  130  in advance of each percussion during the percussion cycle. The seating area  38  of the shoulder  130  and the ball  37  will thereby form a valve that may close and re-open, respectively, the passage for the hydraulic liquid in the piston bore  34 ,  34 ′ during the respective periods of the percussion cycle. The ball  37  otherwise has a diameter essentially corresponding to the diameter of the valve guide  34 ′, see  FIG. 11   b , whereby is achieved accurate and safe control of the ball  37  towards the seating area  38  during closing. The valve mechanism  37 ,  34 ′,  38  is relatively insensitive to lateral accelerations. Hydraulic liquid may pass by the ball  37  via a plurality of passages  129  running externally of the valve guide  34 ′ over the shoulder  130 , partly when the jar device is not actuated for percussive motion and partly during periods of the percussion cycle, as shown in  FIGS. 7   a  and  10   a . As compared with other types of valve bodies, a ball  37  has a relatively small mass and thereby a low mass moment of inertia. A low mass moment of inertia will, together with the favourable fluid flow resistance of a ball  37 , cause the jar device to be able to work at a higher percussion frequency than jar devices according to prior art. 
   The outside of the piston  27  is designed so as to allow it to slidingly abut the inner wall of the casing member bore  2  during the percussion cycle, and the piston  27  is pressure tight against the casing member bore  2  through a central compression packing  30  and relatively wide, upper and lower seals  31 ,  133 , respectively. Moreover, the piston  27  is provided with at least one upper bore  135  extending essentially vertically down from the upper end face of the piston and further into the passage  129 . This at least one bore  135  allows hydraulic liquid to be controlled to an annulus  151  over the top surface  27 ′ of the piston  27 , and may allow hydraulic liquid that is undesirably located in the same annulus  151 , to escape via the bore  135  and further out through the passages  129  in the piston  27 . 
   The jar device also comprises a displacement piece  136  that extends between the lower end of the piston  27  and the upper abutment surface  113  of the jar section  110  with the associated jar sleeve  111 . The displacement piece  136  causes the casing member  1  to be movable up along the jar section  110  when the piston  27  is displaced downwards relative to the casing member  1  in advance of the percussion of each percussion cycle. The displacement piece  136  has an external diameter that is considerably smaller than the diameter of the casing member bore  2 , and also a longitudinal through bore  137  for passage of hydraulic liquid through the displacement piece  136 . The upper end of the displacement piece  136  has been guided into an enlargement of the lower section of the piston bore  34 . The lower end of the displacement piece  136  has an enlarged section  138  abutting the upper abutment surface  113  of the upper jar section  110  and the associated jar sleeve  111 . 
   The upper section of the displacement piece  136  has a plurality of longitudinal elongated slots  139  that allow hydraulic liquid to pass from the bore  137  and out into the annulus  152  between the displacement piece  136  and the casing member bore  2 . Further, there is a valve body  39  in the casing member bore  2 , associated with the piston  27 . An upper section  141  of the valve body  39  has been carried upwards in the piston bore  34 . The external diameter of the upper valve body section  141  is a little smaller than the opening through the shoulder  130  of the piston  27 , so as not to impede the passage of liquid. The upper end of the valve body section  141  has a seating area that will normally abut the ball  37 . Likewise, the lower end of the end piece  125  has, at the outlet of the connector bore  4 , a corresponding seating area that may abut the upper side of the ball  37 , as shown in  FIGS. 7   a  and  8   a.    
   A lower section  142  of the valve body  39  extends downwards in the upper end of the bore  137  of the displacement piece  136 , and the external diameter of the lower valve body section  142  is formed so as to allow the formation of a passage  143  for the hydraulic liquid between the lower valve body section  142  and the displacement piece  136 . The lower valve body section  142  is furthermore equipped with fins  144  carried out through the elongated slots  139  at the upper end of the displacement piece  136 . Side faces on the fins  144  of the valve body  39  slidingly abut adjacent faces in the elongated slots  139  of the displacement piece  136 , and end faces  153  on the fins  144  slidingly abut the inside wall of the bore  2  of the casing member  1 . Consequently, the valve body  39  may be displaced relative to the displacement piece  136  during the percussion cycle, as shown in  FIGS. 8   a  and  9   a . The fins  144  have an upper abutment surface  145  for the lower end face  154  of the piston  27 , and a lower abutment surface  45  for a tension spring  46  associated with the valve body  39 . 
   The tension spring  46  enables the valve  39  in the piston  27  to be opened in order to trigger each percussion during the percussion cycle, i.e. by displacing the ball  37  up from the seating area  38  on the piston shoulder  130 . The tension spring  46  is positioned in the annulus between the exterior face of the displacement piece  136  and the inside wall of the casing member bore  2 . The tension spring  46  further extends between the lower abutment surface  45  on the fins  144  of the valve body  39  and an upper abutment surface  149  on a shoulder  148  that projects into the casing member bore  2  by an area near the place where the upper abutment surface  113  of the jar section  110  with the associated jar sleeve  111  will be when the jar device is not actuated for percussive motion. The bore through the shoulder  148  has a fit that allows hydraulic liquid to flow past it unimpeded in the casing member bore  2 . The tension spring  146  is otherwise designed in a manner such that the tension spring  46  will only be compressed in order be tensioned by the valve body  39  when the ball  37  is placed sealingly in the shoulder  130  of the piston  27  and the hydraulic pressure over the ball  37  in the jar device exceeds a predetermined value, while the tension spring  46  will only open the valve in the piston  27  when the tension spring  46  has reached another predetermined higher value that exceeds the hydraulic pressure applied to the jar device. 
   Selecting an appropriate length for the displacement piece  136  and position for the seating area  38  on the shoulder  130  for the ball  37 , as well as the distance between the lower abutment surface  45  on the fins  144  of the valve body  39  and the seating area  38  for the ball  37 , can ensure that the valve  37 ,  38  in the valve guide  34 ′ does not in an undesirable manner close before the impact area  121  of the end socket  117  has reached full impact against the impact area  109  of the jar  7 . Thus it is made certain that hydraulic liquid located over the ball  37  during this phase of the percussion cycle is not able to force the ball against the seating area  38  of the shoulder  130  so as to close the valve and trap hydraulic liquid. This avoids such potentially trapped liquid on the underside of the piston  27  braking the piston stroke and giving a reduced percussion effect during the percussion cycle. 
   In the following, a brief explanation will be given of the principle of operation of the downward striking jar device, with reference to the drawings. 
   In a non-actuated state of the jar device, the impact area  121  of the end socket  117  is, as shown in  FIGS. 7   a  and  7   b , located a small distance above the impact area  109  of the jar  7 . Further, the valve body  39  lifts the ball  37  off the seating area  38  of the shoulder  130 , so that the piston valve is open and the ball  37  abuts the seating area of the end piece  125  at the lower end of the connector  3 . This leaves a clear passage for hydraulic liquid via the bores and passages of the respective components of the jar device. The jar device is maintained in this non-actuated state by force from at least one pre-tensioned spring (not shown) or similar positioned at a suitable place in the pipe string. 
   The jar device is actuated by further compression of the tool, see  FIG. 8 . The lower jar  7  moves the piston  27  upwards in the bore  2  relative to the ball  37  via the displacement piece  136 , so that the seating area  38  of the piston seals against the ball  37 . The ball  37  will in this phase of the percussion cycle be held in place in the seating area  38  by the end piece  125  of the connector  3 . At the same time, the hydrostatic pressure exerts a force against the ball  37  which forces it against the seating area  38  of the piston  27 . During this phase of the percussion cycle, liquid flows through the relatively narrow bore  135  to the upper side of the piston  27 . By the pressurised liquid filling the annulus  151  relatively slowly, the acceleration of the piston  27  at the starting moment is reduced, so that the ball  37  remains in sealing contact with the seating area  38 . 
   The hydrostatic pressure displaces the piston  27  and the ball  27  downwards in the bore  2  of the casing  1 , see  FIG. 9 , where the piston  27  is at/near its lower limit of travel. At the same time, the valve body  39  under the ball  37  is compressing the tension spring  46 , and the jar device is extended by the piston  27  moving the end socket and casing member  1  upwards relative to the jar  7 , via the displacement piece  136 . This extension will, together with the increase in hydrostatic pressure upstream of the jar device, cause a tensioning of the spring packet above (not shown). 
   During the further displacement of the piston  27  along with the valve body  39  in the casing member bore  2 , the hydrostatic pressure keeps the valve closed until the tension spring  46  reaches a spring tension that exceeds the hydrostatic pressure, as can be seen from  FIGS. 9   a  and  9   b.    
   When this spring tension is achieved, the tension spring  46  will displace the ball  37  off the seating area  38  on the valve shoulder  130 , via the valve body  39 , so that the piston valve is re-opened to trigger the percussion, see  FIG. 10 . Alternatively, the valve body  39  will push the ball  37  off the seat  38  if the tension spring  46  reaches the bottom. 
   The pressure drop at the opening of the valve in the piston  27  means that the liquid may again flow through piston bore  34 . At this, the spring tension in the tension spring  46  will displace the valve body  39  and the piston  27  abutting the upper abutment surface  145  on the fins  144  of the valve body  39 , back into the casing member bore  2 . At the same time, the movement of the piston  27  causes the impact area  121  of the end socket  117  to impact on the impact area  109  of the jar  7  by means of the force from the pre-tensioned spring (not shown) in the pipe string. 
   The length of among other things the displacement piece  136  relative to the seating area  38  for the ball  37  in the piston  27  will furthermore cause the valve in the piston  27  to remain open until the impact area  121  of the end socket has impacted on the impact area  109  of the jar  7 . The hydraulic liquid may if required have a possibility of passing through passage  135  at the upper end of the piston  27 . 
   Those skilled in the art will appreciate that the piston  27  in the casing member bore  2  and the ball  37  in the valve guide  34 ′ must be provided with piston areas that cause the piston valve to be closed and opened in the manner intended. Likewise, the spring tension of the tension spring  46  must be selected on the basis of the pressure conditions in the hydraulic liquid flowing into the jar device. In the embodiment shown, a ball  37  and a valve body  39 , i.e. two separate parts, control the closing and opening of the valve in the piston  27 . These may however be made up of one single part, which will be a combined unit of these with an upper section adapted for sealing against the seating area  38  on the shoulder  130  in the piston bore  34 . 
   During the percussion cycle, there will also be an approximately constant flow of drill fluid through the jar device by the piston  27  being displaced downwards, thus displacing fluid from the underside of the piston  27  during that part of the percussion cycle where the ball  37  shuts off the flow in the piston  27 .