Two-stage pressure relief valve

Claim 1

A sealed lubricated rotary rock bit comprising: a bit body having at least one leg extending downwardly therefrom, said leg having a journal pin for rotatively supporting a rolling cutter, a seal between said rolling cutter and said journal pin, a lubricant reservoir disposed in said bit body communicating with the bearing area formed between said cutter and said journal pin, said reservoir further having a pressure compensator disposed therein, said pressure compensator comprising a resilient membrane separating the reservoir into a lubricant region and a drilling fluid region, a first valve means for relieving relatively low excess differential lubricant pressure from within said reservoir around said resilient membrane to an exterior of said bit body, said pressure relief valve means comprising a cover cap, said cover cap forming an annular radially disposed flange portion having a first radial surface exposed to the exterior of said bit, a second substantially parallel radial surface of said annular flange overlapping an annular seat formed in a wall of said body forming said reservoir, a flexible flange portion formed by said resilient membrane, said flexible flange of said membrane having a first substantially radial surface positioned adjacent said second radial surface of said annular cover cap flange, a second substantially radial surface of said flexible flange of said resilient membrane forming an annular valve face that is positioned adjacent said annular seat formed in a wall of said body forming said reservoir, said flexible flange of said resilient membrane being sandwiched between said second radial surface of said annular cover cap flange and said annular seat in said body, said annular radially disposed flange of said cover cap forms one or more cavities in said second radial surface, said cavities serve to receive a portion of said flexible flange of said resilient membrane diverted into said one or more cavities formed in said flange of said cover cap by said relatively low excess differential lubricant pressure from within said reservoir, said low excess differential pressure thus being relieved past said valve face of said resilient membrane and said annular seat formed in said wall of said body forming said reservoir adjacent said one or more cavities, and a second valve means for relieving relatively high excess differential pressure within said reservoir around said resilient membrane to the exterior of said bit body, said second pressure relief valve means comprising a means for biasing said annular flange of said cover cap toward said annular seat compressing said annular valve face formed by said second radial surface of said flexible flange of said resilient membrane against said annular seat formed in said wall of said body to prevent flow of lubricant from within said reservoir to the exterior thereof until said relatively high differential pressure within said reservoir lifts said cover cap against said cover cap biasing means thereby relieving said relatively high excess pressure to the exterior of the bit when said differential pressure exceeds a pre-selected value.