Schlagvorrichtung und Schlagverfahren

A jar apparatus (jar) 10 includes a plurality of pistons 16 that are secured together to form a piston assembly 23. Piston assembly 23 is movable axially in response to an actuating pressure differential between the jar bore 17 and the portion of borehole 19 that surrounds jar 10. Biasing spring 28 biases piston assembly 23 to return to a closed position when the actuating pressure differential is reduced by a predetermined amount. Piston assembly 23 forces a hammer member 32 and a movable anvil portion 30A axially to cock the jar in response to the applied actuating pressure differential. When the pressure differential is reduced, biasing spring 28 moves movable anvil portion 30A against anvil support portion 30B. Detent assembly 47 temporarily limits the velocity of hammer member 32 towards movable anvil portion 30A after the pressure differential is reduced. Compressed nitrogen 64 in nitrogen chamber 31 biases hammer member towards movable anvil portion 30A to deliver a jar impact.