Controllable check valve

Claim 1

A controllable check valve, for a hydraulic system, comprising, in combination, a relatively elongated body having means defining a valve zone, fluid inlet means and fluid outlet means, said body being formed with a frustoconical valve seat between said inlet and outlet means; a valve member displaceable in said body and formed with a frustoconical valve seating surface sealingly engageable with said valve seat; and means operatively associated with said valve member and maintaining the volume of fluid in said valve zone constant in all operating positions of said valve member; said last named means comprising a pair of stems extending in opposite longitudinal directions from said valve seating surface and guidingly engaged in said valve body to guide said valve member; said stems extending through said valve zone, in a longitudinal direction, through at least the extent of the maximum possible valve stroke in a respective direction; said valve body being formed with a woRking zone completely separated from said valve zone; a piston movable in said working zone and engaged with one of said stems; and means for applying fluid pressure to an outer surface of said piston to move said valve member from a closed position to an open position; means biasing said valve member to the closed position; and a bore extending centrally through said valve member and through said stems and communicating, at one end, with the space beneath said piston and, at the other end, with a fluid return line.