Downhole jarring tool

1. A jarring tool comprising:

a first lower sub end;

a second upper sub end;

an extensible joint connecting the first and second sub ends, the joint comprising a first inner latch piece connected to the lower sub end, and second outer latch piece connected to the upper sub end, and a stationary restraining collar;

wherein the joint, in a latched position, has the outer latch piece latched to the inner latch piece and the inner and outer latch pieces restrained from unlatching by the restraining collar;

wherein the joint, under tensile force, unlatches into an unlatched position by the inner latch piece pulling the outer latch piece through the restraining collar into a position where the inner and outer latch pieces are free to separate; and

wherein an impact force is generated from the tensile force when the joint unlatches and reaches a maximum extension.

2. The tool of claim 1 , wherein the joint relatches into a latched position by the inner latch piece pushing the outer latch piece back through the restraining collar into a position where the inner and outer latch pieces are free to relatch.

3. The tool of claim 1 , wherein the outer latch piece comprises a collet device.

4. The tool of claim 3 , wherein outer latch piece comprises a collet having a plurality of fingers with nubs along distal ends that contact a lip on the inner latch piece when being moved into the latched or unlatch positions through the restraining collar.

5. The tool of claim 4 , wherein the collet is biased toward the inner latch piece by a coil spring.

6. The tool of claim 1 , further comprising:

a lower shaft interconnecting the inner latch piece to the lower sub end; and

a lower stop slidably receiving the lower shaft

wherein the impact force at maximum extension results from contact between the lower shaft and the lower stop.

7. The tool of claim 1 , further comprising:

an upper sub housing connected to the upper sub end;

a lower sub housing;

a center connector connecting the upper sub housing and the lower sub housing;

an upper shaft slidably received through the center connector and connecting to the upper latch piece; and

a plurality of springs biasing the upper shaft away from the center connector.

8. The tool of claim 7 , wherein the restraining collar is attached in a fixed relationship to the lower sub housing.

9. The tool of claim 7 , wherein the plurality of springs comprise a plurality of spring washers.

10. The tool of claim 9 , further comprising a coil spring abutting the plurality of spring washers and a spring cage partially surrounding the coil spring.

11. The tool of claim 1 , further comprising:

a central passage defined through the extensible joint and through the upper and lower sub ends; and

an electrical conductor carried within the central passage.

12. A down hole jarring apparatus comprising;

an upper housing and a lower housing joined at proximal ends by a center connector;

an upper sub end connected to a distal end of the upper housing;

a lower stop connected to a distal end of the lower housing

a lower shaft slidingly engaged through the lower stop, the lower shaft attaching on a distal end to a lower sub and having a shoulder limiting the distal travel of the lower shaft through the lower stop;

an upper shaft slidingly engaged through the center connector and biased toward the upper sub end through the upper housing by a plurality of spring washers;

an upper latch piece connected to a proximal end of the upper shaft;

a lower latch piece connected to a proximal end of the lower shaft; and

a release sleeve affixed to an inside wall of the lower housing;

wherein the downhole jarring apparatus has a first latched position and a second unlatched position and unlatches under a relatively high tensile force but relatches under a relatively low compressive strength;

wherein, in the latched position, the lower shaft is substantially retracted into the lower housing and the lower latch piece is latch to the upper latch piece, the upper and lower latch pieces being retained in a latched position by the release sleeve; and

wherein, in the latched position, under the relative high tensile force, the inner and outer latch pieces are displaced toward the lower stop a distance sufficient to clear the release sleeve and the latch pieces unlatch and separate allowing the lower shaft to slide through the lower stop until the lower shaft shoulder contacts the lower stop resulting in an impact jar.

13. The apparatus of claim 12 , wherein in the unlatched position, the relatively low compressive force drives the lower latch piece against the upper latch piece through the release sleeve a sufficient distance to allow the latch pieces to relatch.

14. The apparatus of claim 12 , further comprising a biasing spring that biases the upper latch piece toward the lower latch piece, the biasing spring force being overcome to relatch the apparatus.

15. The apparatus of claim 14 , wherein the biasing spring has a lower spring rate than the spring washers.

16. The apparatus of claim 12 , further comprising a retaining spring and spring cage retaining the spring washers in a fixed position relative to the upper shaft.

17. The apparatus of claim 12 , wherein a central passage that contains at least one conductor is defined through the apparatus by the upper and lower housings, the upper and lower shafts, the upper and lower latch pieces, the center connector, the lower stop, and the upper and lower sub ends.

18. An apparatus comprising:

an inner latch shaft with a beveled proximal end and a distal end attaching to a lower connector;

an outer latch collet; and

a release sleeve attached to a tool body;

wherein, the release sleeve retains the collet against the beveled distal end of the inner latch shaft creating a latched condition and the inner latch shaft displaces the collet from the release sleeve releasing the collet from the latch shaft creating an unlatched condition.

19. The apparatus of claim 18 , further comprising a spring loaded upper shaft attached to the collet and biasing the collet against movement in the direction of the lower connector.

20. The apparatus of claim 19 , wherein the collet is slidably engaged over a predetermined and limited range to the upper shaft, the collet being interfitted to the upper shaft with a spring allowing the collet to displace toward the upper shaft through the release sleeve in response to pressure from the inner latch shaft to allow the collet and the inner latch shaft to relatch.