US3685598A

US3685598A - Mechanical jar having an adjustable tripping load

Info

Publication number: US3685598A
Application number: US82285A
Authority: US
Inventors: Benjamin P Nutter
Original assignee: Schlumberger Technology Corp
Current assignee: Schlumberger Technology Corp
Priority date: 1970-10-20
Filing date: 1970-10-20
Publication date: 1972-08-22
Anticipated expiration: 1989-08-22

Abstract

A well jar apparatus having an adjustable tripping force includes a mandrel telescopically disposed within a housing, a tripping mechanism including a latch means that is held in gripping engagement with the mandrel by locking surfaces on the housing, the locking surfaces being released by longitudinal movement, a resilient structure capable of longitudinal deflection and reacting between the housing and the latch means to oppose longitudinal movement with a force that is a function of the amount of longitudinal movement required to position the locking surfaces for release of the latch means, and selectively adjustable means for changing the amount that the resilient structure need be deflected before the locking surfaces are in such position, thereby to adjust the tripping force for the jar.

Description

/N VEN TOR Benjamin P. Nu tter Patented Aug. 22, 1972 3 Sheets-Sheet 1 MM? AORNEV United States Patent ns1 3,685,598 Nutter [451 Aug. 22, 1972 [541 MECHANICAL JAR HAv-INGAN ADJUSTABLE TRIPPING LOAD Siracusa..... ..175/303 X Primary Examiner-David H. Brown Attorney-Emest R. Archambeau, Jr., Stewart F. Moore, David L. Moseley, Edward M. Roney and Wil liam R; Sherman [57] ABSTRACT A well jar apparatus having an adjustable tripping force includes a mandrel telescopically disposed within a housing, a tripping mechanism including a latch means that is held in gripping engagement with the mandrel by locking surfaces onthe housing, the locking surfaces being released by longitudinal movement, a resilient structure capable of longitudinal deflection and reacting between the housing and the latch means to oppose longitudinal movement with a force that is a function of the amount of longitudinal movement required to position the locking surfaces for release of the latch means, and selectively adjustable means for changing the amount that the resilient structure need be deflected before the locking surfaces are in such position, thereby to adjust the tripping force for the jar. i

8 Claim, 7 Drawing Figures i 35T` n '63 fr -f-55 mglff- .2, f 57 /1 F/G. 2B j FIG 2 C Benjamn P. Nutter /NVENTOR ATTORNEY Patented Aug. 22, 1972 5 Sheets-Sheet 5 m F u E l....i G 5 9 5 F ,dwwdwwdl s,,fEEE

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Benjamin P. Nutter /N VEN TOR A TTORNEV MECHANICAL JAR HAVING AN ADJUSTABLE TRIPPING LOAD This invention relates generally to jars used in extricating stuck objects or fish from well bores, and more particularly to a new and improved mechanical drilling jar that is constructed and arranged whereby the tripping load, that is to say the longitudinal force that must be applied to the jar before it will hit, can be selectively adjusted.

A drilling jar may typically comprise the combina tion of a mandrel that is telescopically disposed within a housing, the mandrel and housing being adapted for connection in the drill string. A tripping mechanism between the housing and the mandrel can be engaged or cocked and tends to prevent relative longitudinal movement, but can be released by the application of a predetermined amount of longitudinal force. Upon release, the mandrel and housing can telescope freely to bring a hammer surface on one member against an anvil surface on the other member in a violent manner to deliver a jarring blow to an object that is stuck in the well. The cocking and releasing process is repeated by manipulating the drill pipe at the surface as many times as is necessary to jar the stuck object loose.

It is highly desirable to be able to selectively adjust the above-mentioned amount of longitudinal force in accordance with the conditions of the well as well as the limitations of the drilling rig. For example, the drill pipe weight plus the tripping force that is normally recommended for the jar may be wellwithin the hook load capacity for the rig, however, the well is thereafter deviated so that there is a substantial amount of frictional drag between the drill pipe and a portion f the well bore wall. Since the drag load must also be overcome in order to trip the jar, the total hook load that would be required at the surface might not provide for the factor of safety that is necessarily required. Accordingly, it is desirable to be able to adjust the jar tripping load downward to keep the total load requirements within acceptable values.

Moreover, it is quite common for the driller to run a number of heavy drill collars above the jar to provide a substantial mass that can be'accelerated to provide an adequate jarring action in case the drill collars below the jar become stuck. Thus, it is desirable to be able to selectively adjust the jar tripping load, taking into ac-v count the weight of drill collars above the jar to prevent possible damage to the jar or the stuck fish during jarring operations.

ln sum, there is a significant need to be able to tailor the jar tripping load to the well conditions and to the capabilities and limitations of the rig and associated equipment.

It is therefore the general object of the present invention to provide a new and improved well jar having an adjustable tripping load.

This and other objects are attained in accordance with the concepts of the present invention through the provision of a well jar apparatus including mandrel and housing members that are telescopically disposed and have impact or jar surfaces that can be brought together to deliver a jarring blow to an object that is stuck in a well bore. A tripping mechanism is disposed between the members and includes laterally movable latch means and a releasable locking means that normally prevents lateral movement of the latch means, the locking means being positioned for release of the latch means by longitudinal movement of the locking means with respect thereto. A resilient means capable of longitudinal deflection reacts between the housing member and the latch means to resist or oppose such longitudinal movement with a reaction force that is a function of the amount of such longitudinal movement, and a selectively adjustable meansis provided for adjusting the amount that the resilient means must be deflected before the locking means is in position to release the latch means,thereby correspondingly adjusting the tripping load for the jar.

The present` invention has other objects, features and advantages which will become more clearly apparent in connectionV with the following detailed description of an embodiment taken in conjunction with the appended drawings in which:

` FIG. l is a somewhat schematic viewvof a drill string and jar in a borehole;

FIGS. 2A-2C are longitudinal sectional views ofthe tripping mechanism and associated structure in accordance with the present invention;

FIG. 3 is an isometric view of the tripping sleeve;

FIG. 4 is a cross-section on line 4-4 of F IG. 2B; and

FIG. 5 is a longitudinal half-section view to illustrate further detail of the keying system used in the tripping mechanism.

Referring initially to FIG. 1, a borehole l0 is being drilled through earth formations by using conventional rotary drilling techniques. A drill bit l1 isvattached to the lower end of a drill string which normallyincludes a number of heavy drill collars 12 to provide the necessary weight on the bit. A jar 13, -constructed in accordance with the principles of this invention, can be selectively coupled at various locations in the drill string, but is preferably located at a point near the upper end of the drill collar string and below the drill pipe 14. Under certain borehole conditions, such as soft or highly permeable formations, particularly when encountered in deviated wells, there is the risk that the bit and/or the drill collars may become stuck. When such sticking occurs for any reason, the jar 13 can be operated to free the stuck tools.

The jar 13 includes a tubular housing 20 that is telescopically disposed over a tubular mandrel 2l, the housing having a threaded box portion 22 and the mandrel 2l having asimilar portion 23 for coupling the jar in the drill string. The mandrel 2l has a central bore 24 which continues the drilling fluid circulation path through the drill string. The housing 20 is movable to some extent in both longitudinal directions along the mandrel 21 from a central or cocked position, upward movement causing one hammer provided by an inwardly extending shoulder surface 25 to strike an anvil provided by an outwardly extending shoulder 26 on the mandrel, downward movement causing another hammer surface 27 to strike a second anvil sur-v face 28. Thus the jar 13 is adapted to hit in both directions, upward and downward. The impact force in either direction is transmitted by the mandrel 2l to the stuck tools therebelow. The housing 20 and the mandrel 21 have coengaging splines 29 for transmitting rotation through the jar during drilling operations. A seal packing 30 prevents drilling fluids from entering in between the housing 20 and mandrel 21 at their lower A disposed in between upper and lower

resilient structures

33 and 34 and functions to restrain longitudinal movement of the housing 20 in either direction from the cocked or intermediate position to enable energy to bestored in the drill string before the jar hits so that a blow of considerable magnitude can be struck. Once the tripping mechanism 32 releases however, movement of the housing 20 along the mandrel 2l is unrestricted as the hammers approach the respective anvils. After a blow is struck,the housing 20 is moved by the drill pipe 14 in the opposite direction, and the tripping mechanism 32is automatically recocked in the intermediate position. The jarring action is repeated until the stuck tools are driven loose.

As shown in greater detail inFIG. 2B, the tripping mechanism 32 includes an expansible and contractible sleeve member 35 that is normally contracted around the mandrel 2l, ythe sleeve member having internal threads 36 that mesh with companion external threads 37-formed on a reduced diameter section of the man drel. As disclosed and claimedy in copending application, Ser. No. 82,284-, yKisling, filed concurrently herewith and assigned to the assignee of the present invention, the

threads

36 and 37 have a stub form providv'ing

inclined wall surfaces

39 and 40. When the sleeve member 35 and the mandrel-21 are forced in opposite longitudinal directions the inclined-wall surfaces provide radial force components tending toexpand the sleeve member outwardly. Such outward expansion can occur due to the provision of slots 41 and 42 `as shown in FIG. 3, extending in alternating longitudinal directions for less than the `,full length of the sleeve a pluralityv of segments, each having its ends joined to an end of an adjacent segment. In the cocked position however, the sleeve member 35 is locked in contracted condition around the mandrel 21 by locking surfaces 43 (FIG. 2B) provided by threads 44 on a locking sleeve 45, the surfaces slidably engaging like surfaces 46 provided by threads 47 fonned on the exterior of the sleeve member 35. As disclosed and claimed in copending application, Ser. No. 82,287, Berryman, filed concurrently herewith and assigned to the assignee o'f this invention, the release grooves 48 formed between the locking surfaces 43 are adapted to receive the threads 47 on the sleeve member 35 upon relative longitudinal movement of the locking sleeve 45 and the member 35, thus permitting expansion of the'sleeve member to a condition where the

threads

36 and 37 are disengaged. The

locking surfaces

43 and 46 extend parallel to `the axis of the sleeve member 35 and thus positively prevent any outward expansion of the sleeve member during the longitudinal movement required before 'disengagement can'occur. The release grooves 48 have inclined wall surfaces 49 andv 50 in order to provide an inward camming action in the presence of longitudinal force on the sleeve member 3S when the threads. 47 are meshed with the release grooves 48.

The upper end of the locking sleeve 45 abuts against an inwardly extending shoulder l on the housing20, whereas the lower end is engaged by abushing 52 that is disposed above an inwardly extending shoulder 53 (FIG. 2C) on the housing to fix the locking sleeve longitudinally with respect to the housing. Each end of the locking sleeve 45 vis also slotted at cireumferentially spaced locations to receive'outwardly extending

projections

54 and 55 formed on depending portions '.56 and 57 of upper and lower` drive rings58 and 59 which are keyed to the mandrel `21 in a manner and for purposes to be more fully described below.

Annular thickened portions 60 and 6l of the mandrel 21 are providedabove and below the threads 37, the portions having larger lateraldimensions vthan the height of the threads, but are considerably long with respect to the lead of the threads. The sleeve member 35 has similar inwardly thickened

portions

62 and 63 which engage above andbelow the

respective portions

60 and 61 on the mandrel`2l. Further, the crests ofthe.

threads 37 are formed on a smaller diameter than the member 35, such slots dividin'gme s1eve member mm size through the sleeve threads 36 is larger than the transverse dimension of the'portions 60vand 61.` Thus, when the sleeve member 35 is-permitted to expand and move upwardly, for example, along the mandrel 21, `the portion 62 will n'de along the youter surface 64 orthe mandrel 21, while the 'lowerl portion 63will ride first over the outer surface of the'lower mandrel portion 61r and Athen along the'crests of the threads 37. The crests of the sleeve member threads 36 ywill slide along lthe upper mandrel portion 60, with the result being that the respective -sleevemember and mandrel threads 36 and f 37 are positively held out of engagement as the sleeve member 35 moves relatively along the mandrel 21 in either direction. l

It will be apparent that vin orderA for the'sleeve v member 35 to be released from the mandrel 2l.to enableunrestricted telescoping movement, the housing 20 and the locking sleeve 45 must moveeither .upwardly or downwardly relative to the sleeve member 35. Such relative movement is restrained however by the 1

resilient structures

33 and 34. Since each structurev is made of identical, oppositely disposed parts, the upper structure will be discussed in detail with the appropriate numbers referred to in the drawings, the lower resilient structure 34 having parts numbered in the same manner but with the prime notation. AThe upperA resilient structure 33 is shown in FIG. 2A as including an outer tube 68, an intermediate tube 69 and an inner tube 70 all concentrically disposed and made of metallic materials. The intermediate tube 69 has thickened

sections

71 and 72 at its ends, the lower section providing an upwardly facing shoulder 73 and the upper section providing a downwardly facing shoulder 74. The lower end of the outer tube 68 abuts against the shoulder73, whereas the upper end of the inner tube 70 abuts the shoulder 74. Thus it will be appreciated that oppositely directed longitudinal forces applied to the outer tube 68 and the innerjtube 70 will place these tubes in compression and theintermediate tube 69 in tension. Thetotal deflection ofthe com` posite structure of course will be the sum of the respective elongation and foreshortenings of the individual tubes, so that the tubes provide a resilient structure in the nature of a spring having a high modulus of elasticity. The upper end of the outer tube 68 and the lower end of the inner tube 70 extend beyond the adjacent ends of the intermediate tube 69.

The drive rings 58 and 59 are located between the sleeve member 35 and the respective

resilient structures

33 and 34, each drive ring having an internal annular recess 76 (FIG. 2B) that receives a washer 77. The washer 77 has a concave annular groove 78 that receives a rounded end of a thrust sleeve 79. The other end of each thrust sleeve 79 is also rounded and is received within an annular groove 80 in the end of the sleeve member 32. Each of the thrust sleeves 79 is also made to be radially expansible and contractible by slots 81 (FIG. 5) extending in alternating longitudinal directions for less than the full length thereof in the same manner as the slots in the sleeve member 35 shown in FIG. 4. The provision of the slotted thrust sleeves 79 having the same capability for expansion as the sleeve member 35, together with the rounded end surface construction thereof, substantially prevents any inward component of force that resists expansion of the ends of the sleeve member 35 such as might occur for example, between two sliding surfaces were the thrust rings 58 and 59 to engage the ends of the sleeve member 35 directly.

Each of the

resilient structures

33 and 34 can be a justably positioned within the housing in order to selectively adjust the required tripping force in accordance with the present invention. Tripping force adjustment is accomplished by providing upper and lower jack mandrels 85 and 86, each having an end portion adapted to engage the outermost tube 68 of the respective resilient structures 33, Each jack mandrel is threaded to the housing at 87 so that relative rotation will cause feeding thereof along the housing 20. Suitable windows 88 are provided in the wall of the housing 20 adjacent to the distal end section of each jack mandrel, the windows being normally closed by plugs 89. Removal of the plugs 89 provides access openings through which a suitable tool (not shown).can be inserted and engaged in circumferentially spaced recesses 90 and used to rotate the jack mandrel with respect to the housing 20. Such rotation causes the threads 87 to feed the jack mandrel along the housing in one direction or the other, depending upon the direction of rotation, to correspondingly change the amount the

resilient structures

33 and 34 must be foreshortened in order to position the release grooves 48 opposite the outer sleeve member threads 47. Since the required tripping force is a direct function of the amount of longitudinal deflection of the

resilient structures

33 and 34, the tripping force can be adjusted as desired. A stop plug 91 having a tang 92 that projects into one of a plurality of circumferentially spaced external grooves 93 on the jack mandrel is of course removed during rotation and is then engaged to lock the jack mandrel in the selected longitudinal position by preventing rotation thereof.

To prevent relative rotation of the various parts of the tripping mechanism 32 and thereby maintain the proper alignment of the threads and the locking surfaces during operation as well as the slots in the sleeve member 35 and thrust sleeves 79 for proper expansion during operation, a system of keys is used as shown more clearly in FIGS. 4 and 5. The sleeve member 35 can have, for example, sixteen slots (eight opening in each direction as shown in FIG. 3) with the mandrel 2l and the release sleeve 45 each having eight radially aligned keyways provided by longitudinally extending

grooves

95 and 96. In this example, each of the thrust sleeves 79 also has eight slots 81 extending in each direction. Thus the upper thrust sleeve can have its downwardly opening slots longitudinally aligned with the upwardly opening slots in the sleeve member 35, and a series of elongated keys 100 are provided, each extending into the aligned slots. The inner edge of each key 100 rides in a respective mandrel keyway 95, whereas the outeredge is received by the radially aligned keyway 96 in the locking sleeve 45. A separate set of shorter keys 101 ride in the same mandrel keyways 95 and extend into slots of the lower thrust sleeve so that the upwardly opening slots in the lower thrust sleeve are longitudinally aligned with the downwardly opening slots 42 in sleeve member 35.

Also upper and lower sets of

short keys

102 and 103 are received in radially extending slots 104 and 10S cut in the drive rings 58 and 59 and ride in the mandrel keyways 96 to rotatively position the drive rings and thus the locking sleeve 44 with respect to the mandrel 21 and the sleeve member 35. Accordingly, the

release threads

36 and 37 and the locking

surfacesv

43 and 46 are fixed rotationally for the proper meshing relationship, and the slots that enable expansion of the sleeve member 35 and the thrust sleeves 79 are properly aligned for opening and closing movement during such expansion.

In operation, the jar 13 in its fully open or extended condition is connected in the drill string at a point near the upper end'of the string of drill collars 12 but below several of the very uppermost collars, and lowered into the borehole for drilling operations. During drilling, rotary motion is transmitted through the jar 13 by virtue of the interengagement of the splines 29, and during normal operations the jar is always in tension. In case the tools below the jar 13 become stuck, the jar is operated as follows. The drill pipe is lowered until a' restraint is seen on the rig weight indicator which means that the tripping mechanism 32 is in the cocked position as shown in FIG. 2B. To hit upwardly, an upward strain is applied to the drill pipe 14 at the top of the borehole, causing the string to stretch and store energy. The upward force will tend to pull the housing 20 upwardly relative to the mandrel 2l, however such upward movement is resisted by the resilient structure 34 disposed between the lower jack mandrel 86 and the lower drive ring 59, which is held against upward movement by the sleeve member 33 and the thrust sleeve 79. The upward force on the sleeve member 35 and the

threads

36 and 37, due to their inclined wall surfaces of the latter, will tend to cause expansion of the sleeve member; however the top or crest surfaces 43 and 46 lock the sleeve member in gripping engagement with the mandrel 21. As the lower resilient structure 34 deflects or foreshortens, the housing 20 and thus the locking sleeve 45 move upwardly relative to the sleeve member 35 to a point where the release grooves 48 are positioned laterally opposite the threads 47. At this point, which corresponds to a predetermined tripping or release force, the inclined walls of the

threads

36 and 37 will cause the sleeve member 35 to expand outwardly and suddenly release the gripping engagement with the mandrel 21. The housing together with the drill collars l2above the housing will be accelerated upwardly, causing the hammer surface 25 to strike the the threads 37 it will contract into gripping engagement with the mandrel 2l, such contraction being assisted by the reaction force of the deflected structure 34 which is translated to radial inward force components bythe inclined wall surfaces 49 of the release grooves 48. The

mandrel portions

60 and 61 coact with the internal configuration of the sleeve member 35 and function in such a manner that there is only one longitudinal position where the sleeve member can contract, thus minimizing thread wear. f

. J arring blows can be applied repeatedly in an upward direction, or downward jarring can be effected by slacking off the weight of the drill pipe onto the jar housing 20. Thesarne coaction vof parts will occur as described above except involving the uppervresilient Although the disclosed embodiment of the present invention is illustrated in connection with a two-way hitting jar (both upward and downward), it' will be appreciated that the jar could be readily modified toan embodiment that hits only in onedirection, upwardly for example, by eliminating the upper resilient struc-A ture, jack mandrel and associated structure and correspondingly shortening the housing and the mandrel.l A`

Although the present invention'is illustrated in connection with a drilling jar, of course the concepts are equally applicable to jars used infishing, whipstocking andrcoring. v.

Since certain changes or modifications maybe made in the disclosed embodiment without departing from the inventive concepts involved, it is Vthe aim of the appended claims to cover all such changes andmodifications falling within the true spirit and present invention.

I claim:

1. A well jar apparatus comprisingzinner and outer members telescopically related and yhaving impact surfaces that can be engaged by-telescoping motion to deliver a jarring blow to an objectv that is stuck in a well means being positioned for release of said latch means by longitudinal movement; resilient means capable of being foreshortened, by said longitudinal movement and reacting between one of said members and said structure 33, drive ring 58 and thrust sleeve 79 releaseofthe sleeve member enabling the hammer surface Y 27 to strike the anvil ksurface 28 in a violent manner.

f The tripping force for the jar 13 is adjusted. to a predetermined value at the surface vprior to running'y the jar into `the well bore in the following manner. lf for example it is desired to adjust the tripping force downwardly, the lower jack mandrel 86 is rotated in the proper direction in order to feed it downwardly somewhat along the housing 20. The effect is that with Y thetripping mechanism 32 in the cocked position and an upward strain on the housing 20, the locking threads j 44 on the locking sleeve 45 will not be initially aligned opposite the companion threads 47 on the outside of the sleeve member 35 as shown in FIG. 2B, but rather will be disposed upwardly somewhat with respect thereto. Of course the locking surfaces 43 and 46 will still be in sliding contact, however a lesser amount of upward movement ofthe locking sleeve relative to the sleeve member 35 is necessary to dispose the release grooves 48 opposite the threads 47 and thereby enable expansion of the sleeve member. Correspondingly, a lesser amount of longitudinal deflection of the lower resilient structure 34 is necessary to release the tripping mechanism 32. Since the tripping force for the jar 13 is a direct function of the amount that theresilient structurev 34 must be deflected in i order to position the release grooves48 opposite the threads 47 ,it will be apparent that tripping force would be less, A range of tripping forces is thus available depending upon the longitudinal position of the jackmandrels 85 and 86, and as an aid to adjustment, suitable latch means for resisting said longitudinal movement with a reaction force that is a function of the amount of saidlongitudinal movement; and selectively adjustable means cooperable with said resilient means for adjusting the amount that said resilient means must be foreshortened before said locking means isv in position to release said latch means, thereby adjusting the tripping load for said jar apparatus.l

2. The well jar apparatus of claim lf wherein lsaid selectively adjustable means'includes structure movable longitudinally along said one member for adjusting' the longitudinal position of -said resilient means with respect to said one member.

v3. The well jar apparatusy of claim 2 wherein said structure includes a jack mandrel having threaded engagement with said one Amember whereby rotation of said jack mandrel will feed said jack mandrel along said one member.

4. The well jar apparatus of claim 3 further including means providing access to said jack mandrel from the exterior of said members to facilitate rotational adjustment thereof.

5. A well jar apparatus comprising: inner and outer members telescopically related and having impactsurfaces that can be engaged by telescoping motion to deliver a jarring blow to an object that is stuck in a well bore; tripping means disposed between said members including laterally movable latch means having gripping engagement with said inner member, and releasable locking means movable with said outer member for normally preventing lateral movement of ment with respect thereto; resilient means capable of l being foreshortened by said longitudinal movement scope of the and reacting between said outer member and said latch means for opposing said longitudinal movement with a reaction force that is a function of the amount of said longitudinal movement; and selectively adjustable means for adjusting the position of said resilient means within said outer member to change the amount that said resilient means must be foreshortened before said locking means is in position to release said latch means and enable lateral movement thereof, thereby to adjust the tripping load for said jar apparatus.

6. The well jar apparatus of claim wherein said selectively adjustable means includes a mandrel movable longitudinally along said outer member, and axial v cam means responsive to rotation of said mandrel for feeding said mandrel along said outer member in either direction, depending upon the direction of rotation thereof.

7. The well jar apparatus of claim 6 further including means for fixing said mandrel against rotation in a selected longitudinal position thereof.

8. The well jar apparatus of claim 7 further including means extending through the wall of said outer member for providing access to mandrel to facilitate rotational adjustment thereof.

s v=|= :r

Claims

1. A well jar apparatus comprising: inner and outer members telescopically related and having impact surfaces that can be engaged by telescoping motion to deliver a jarring blow to an object that is stuck in a well bore; tripping means disposed on said members and including latch means and releasable locking means for preventing release of said latch means, said locking means being positioned for release of said latch means by longitudinal movement; resilient means capable of being foreshortened by said longitudinal movement and reacting between one of said members and said latch means for resisting said longitudinal movement with a reaction force that is a function of the amount of said longitudinal movement; and selectively adjustable means cooperable with said resilient means for adjusting the amount that said resilient means must be foreshortened before said locking means is in position to release said latch means, thereby adjusting the tripping load for said jar apparatus.

2. The well jar apparatus of claim 1 wherein said selectively adjustable means includes structure movable longitudinally along said one member for adjusting the longitudinal position of said resilient means with respect to said one member.

3. The well jar apparatus of claim 2 wherein said structure includes a jack mandrel having threaded engagement with said one member whereby rotation of said jack mandrel will feed said jack mandrel along said one member.

5. A well jar apparatus comprising: inner and outer members telescopically related and having impact surfaces that can be engaged by telescoping motion to deliver a jarring blow to an object that is stuck in a welL bore; tripping means disposed between said members including laterally movable latch means having gripping engagement with said inner member, and releasable locking means movable with said outer member for normally preventing lateral movement of said latch means, said locking means being positioned for release of said latch means by longitudinal movement with respect thereto; resilient means capable of being foreshortened by said longitudinal movement and reacting between said outer member and said latch means for opposing said longitudinal movement with a reaction force that is a function of the amount of said longitudinal movement; and selectively adjustable means for adjusting the position of said resilient means within said outer member to change the amount that said resilient means must be foreshortened before said locking means is in position to release said latch means and enable lateral movement thereof, thereby to adjust the tripping load for said jar apparatus.

6. The well jar apparatus of claim 5 wherein said selectively adjustable means includes a mandrel movable longitudinally along said outer member, and axial cam means responsive to rotation of said mandrel for feeding said mandrel along said outer member in either direction, depending upon the direction of rotation thereof.