is initially rigged up with downcrowding cable 47 and take up cable 51 taut spring means 53 will be under some tension the position of the boom 15 with respect to the platform 13 will remain constant during drilling kelly 21 should be hoisted by the crane winch to an initial position with kelly head 23 a considerable distance above the kelly bushing 20 drum 27 should be in the upper position with the hydraulic cylinders 35 extended the maximum distance as the rotary table 17 rotates the kelly 21 it begins advancing into the earth to assist the advancement the downcrowding circuit may be used as shown in fig3 this is performed by placing control valve 71 into downcrowding position 71a fluid will be supplied through lines 75 and 83 and returned from hydraulic cylinder 41 by lines 79 and 73 locking brake 39 once the pump pressure reaches 300 psi the accumulator 85 will begin filling and its piston 89 will move to the right once the pressure reaches 500 psi sequence valve 95 will allow fluid to pass through line 101 to the drum hydraulic cylinders 35 pressure in line 101 will be sensed by check valve 81 allowing fluid to return from.