tup mounted between the moving platform and the bottom wall of the jack hammer are two elastic ropes 22 24 each having an in line shock absorber 26 28 the jack hammer is shown in a vertical orientation with the chisel lowermost the most common in use orientation the ropes are high duty high energy storage ropes each rope is made up of several hundred strands per cross section of natural rubber able to undergo strain crystallisation an inelastic textile braid is provided as a sheath around the strands this is fitted with the rope in an extended condition and is such that the rope cannot relax to its untensioned condition rather it is held by the braid at an extension of 60 beyond its untensioned length in the operation of the jack hammer the body 20 is lifted against the force of gravity and against the tensile force developed by the elastic ropes 22 24 at a defined point the body is permitted to move from its retracted position to its impact position in which it strikes the chisel 8 we have determined that when the jack hammer is upright with the chisel lowermost approximately 75 of the energy transferred to the chisel comes from the elastic ropes and approximately 25 comes from gravity the invention concerns the manner in which the body is reciprocated under the action of.