Fail-safe servo control valve

Claim 1

A fail-safe fluid control system comprising: a. a valve housing including a fixed sleeve member having a plurality of passages and lands, b. first and second fluid passageways connected to said housing for inlet and return of fluid, respectively, c. an actuating cylinder means, d. third and fourth fluid passageways connected between said housing and said cylinder means for selected inlet and return of fluid therebetween, e. first and second valve means normally in a centered position movable within said fixed sleeve member and normally movable relative to each other for selectively controlling fluid flow from said first and second passageways to said third and fourth passageways, f. said first valve means being annular and having a chamber therein and having a plurality of passageways and a plurality of lands and said second valve means being slidable within said first valve means and including a center land and a plurality of other lands with spaces between defining a plurality of passageways, g. resilient means continuously urging said first valve means into a predetermined normal centered operating position relative to said housing, h. said first and second valve means being movable as a unit overcoming said resilient means when sticking occurs, and i. said second valve means in said centered position having outer lands thereof blocking said third and fourth fluid passageways, said first valve means in said centered position having lands thereof spaced from lands on said fixed sleeve member forming passageways to provide said first passageway communication with said spaces on opposite sides of said center land of said second valve means between said outer blocking lands and to provide said second passageway communication with spaces between other of said lands of said second valve means and said other blocking lands, whereby any short circuit paths between said first passageway and said second passageway are effectively blocked.