US Patent 8206058 Manhole cover hinge assembly

A manhole cover assembly includes a frame defining an access opening, first and second mounting posts extending from the frame, and a cover for covering the access opening. The cover includes first and second mounting recesses that pivotally receive the first and second mounting posts respectively, such that said cover is movable between an open position and a closed position by rotating the recesses about the posts. At least one of the recesses includes a locking groove that aligns with the post when the cover is moved to an open position to lock the cover in the open position.