gas pressure in the isolator is predetermined by the known weight to be suspended in addition the stroke of the isolator i e the amount of elongation of the tool between fig2 a 2c and 3 a 3 c should accommodate the amount of movement of the impacted drive pipe 27 if not any residual forces will be transferred upwardly to the platform in operation nothing happens to the isolator 23 until impact and the hanging position is shown in fig2 a 2c upon impact the outer tubular structure moves downwardly equal to the amount the drive pipe 27 is moved upon impact when this occurs the chamber above the valve 36 becomes smaller in this direction of movement the valve 36 is closed to compress the fluid within the chamber this action absorbs the energy caused by the impact and prevents any impact energy from being transferred above the isolator 23 the operator then lowers the drill string to restore the stroke going from the position shown in fig3 a 3c back to the position shown in fig2 a 2c the cushion sub 33 operates from its initial position shown in fig1 a 10c in this position the downward movement of the hanging assembly lowers the inner structure of the cushion sub 33 this movement causes the valve to compress the fluid in the chamber beneath the valve to absorb the energy of the impact of the bit.