The combination valve is intended for holding the piston of the machine tool in its extreme upper position. Regulating screw 5 of the combination valve is adjusted so that the pressure in the lower end of cylinder 1, due to the weight of piston 2, cannot overcome the force exerted by spring 4. In this case, valve spool 3 is in its left-hand position, disconnecting chambers a and d. Check valve member 6 is held on its seat by its spring, fluid discharge from the lower end of cylinder 1 is blocked off and piston 2 remains in its upper position until fluid is admitted into the upper end of cylinder 1. Then piston 2 begins to move downward, the fluid pressure in the lower end of cylinder 1 increases, valve spool 3 is shifted to the right, overcoming the resistance of spring 4 and causing discharge of fluid to the tank. To raise the piston, the flow of fluid is reversed, fluid being directed to the lower end of cylinder 1, opening check valve 6. Piston 2 moves upward and fluid from the upper end of the cylinder drains to the tank.
$4182$CHP,Dr$